AbstractS
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Invited abstracts
Observation of High Frequency Emission from the MST Tokamak Plasma in the Presence of Runaway Electrons
Abdulgder Almagri, University of Wisconsin-Madison
First direct measurement of high frequency Whistler modes activity in the Madison Symmetric Tour (MST) tokamak plasma. Frequencies ranging from 37 MHz up to 5.5 GHz have been measured interior to the plasma edge. These electromagnetic fluctuations above f_ci~1.5 MHz and above f_ce~3.8 GHzm have been directly measured with magnetic probes inserted into the MST plasma to r⁄a~0.8 and digitized at high speed (up to 12.5 GHz). These whistler activities are excited by high energy runaway electrons at low plasma density. These activities occur in discrete frequency bands and in semi-periodic fashion. These bursts are correlated with burst of x-ray emission exhibiting predator-prey cycles. Magnetic fluctuation power spectrum shows strong emission at multipole frequencies lines. A spectrogram of the magnetic fluctuations shows the scaling of mode frequency with the amplitude of the mean magnetic field. The higher frequency modes ramp down with BT faster than the lower frequency modes, consistent with higher order harmonics. Runaway electrons are generated throughout the discharges at plasma density less than 0.03x1019 m3, toroidal magnetic about 0.14 T, and plasma current in the range of 50-60 kA. Whistler mode have been observed when the toroidal magnetic field is as low as 900 gauss. High energy x-ray, few 100 keV, have been observed during the discharge break down stage. These high energy runaway electrons may be responsible for of the secondary runway electrons population that are present throughout low density discharge. The parallel and perpendiculars wave number have been measured as a function of frequency. The phase velocity of all Whistler modes range from 5.0 to 30.0 times the Alfven velocity, VA. The polarization of different frequency modes have been measure with the polarization probe inserted to r⁄a~0.8 . Modes at frequency 37 MHz and the 3.2 GHz are elliptically polarized.
Solar System Storms in the Lab: Creating a Scaled Interplanetary Coronal Mass Ejection
Khalil Bryant, University of Michigan
The Sun, being an active star, undergoes eruptions of magnetized plasma that reach the Earth and cause the aurorae near the poles. These eruptions, called Coronal Mass Ejections (CMEs) send plasma and magnetic fields out into space. CMEs that reach planetary orbits are called Interplanetary Coronal Mass Ejections (ICMEs) and are a source of geomagnetic storms, which can cause major damage to our modern electrical systems with limited warning. To study ICMEs, we devised a scaled experiment using the Big Red Ball (BRB) plasma containment device at the Wisconsin Plasma Physics Laboratory (WiPPL). These experiments inject a compact torus of plasma as an ICME analog through an ambient plasma inside the BRB, which acts as the interplanetary medium (IM). Magnetic and temperature probes provide 3D magnetic field information in time and space, as well as temperature and density as a function of time. Using this information, we can identify features in the compact torus that are consistent with those in real ICMEs.
Two-fluid Gkeyll Simulations of Alfven Wave Reflection From an Alfven Speed Gradient in LAPD
Jason TenBarge, Princeton University
The heating of the solar corona and acceleration of the solar wind is likely driven by Alfvenic turbulence, which requires counter-propagating Alfvenic fluctuations. Alfven waves are observed to be driven from the base of the corona, but the source of inward propagating waves is not yet established. Based on Magnetohydrodynamic (MHD) theories, the leading candidate is reflection from an Alfven speed gradient in the corona. However, prior experimental tests of Alfven wave reflection from a magnetic field gradient in the LArge Plasma Device (LAPD) at UCLA do not agree with the MHD reflection predictions, possibly due to physics beyond MHD. In this talk, we present the Gkeyll simulation framework as general use tool to model LAPD. In this case, we use the Gkeyll two-fluid solvers to explore the role physics beyond MHD may play in the reflection of Alfven waves. We compare Gkeyll simulations to previous LAPD experiments for which reflection was not observed, and we present recent LAPD experiments that do exhibit reflection. In both cases, we find that two-fluid physics well models Alfven wave reflectance in LAPD experiments.
Particle acceleration in diamagnetic cavities: spacecraft observations and planned laboratory experiment in the Big Red Ball
Katariina Nykyri, Embry-Riddle Aeronautical University
Magnetic reconnection is a universal process which results in changes of magnetic topology as well as conversion of magnetic energy to thermal and kinetic energy of the plasma particles. However, the heating and particle acceleration that occurs close to reconnection diffusion regions at the dayside magnetopause is localized to a small area and thus cannot explain particle acceleration and heating at global scale, observed in the Earth's magnetosphere. Recent spacecraft observations have revealed that in the vicinity of cusp-like geometries magnetic reconnection can lead to formation of large-scale magnetic bottle structures (diamagnetic cavities, DMCs) where significant fluxes of electrons and ions can be trapped and energized to high energies. Since cusp-like structures are universal, occurring at planetary magnetospheres and close to surface of magnetized stars we are motivated to study the formation of these structures in laboratory plasma device, Big Red Ball (BRB), at University of Wisconsin Madison. This talk will discuss the formation of DMCs and particles acceleration using multi-spacecraft (Cluster and MMS) observations and discuss the experimental setup at BRB.
Collisionless Magnetic Reconnection in TREX
Cameron Kuchta, UW-Madison
The Terrestrial Reconnection EXperiment (TREX) operates as a nominally cylindrically symmetric experiment, where an initial uniform axial background field is generated and plasma is injected creating a theta-pinch equilibrium. To begin reconnection, a large current is driven through coils creating a reversed field. As the current increases in time, the magnetic pressure near the coils is rapidly reduced allowing plasma to expand radially outward. Since the background field is frozen into the plasma we can measure the resulting magnetosonic wave using our magnetic probes, and through the characterization of the wave speed the initial plasma density profile is inferred. Ringing in the drive circuit creates additional pressure waves which are applied to measure density evolution. In the latter phase of the experiment the reversed field becomes sufficiently large that the total field is reversed and a reconnection current layer is generated. Given recent upgrades to TREX, we identify electron jets in the reconnection outflow, which collisionless reconnection theory predicts to be driven by electron pressure anisotropy. We've developed a pressure anisotropy probe that uses 12 directional Langmuir probes to measure this anisotropy.
Magnetized plasma physics on PUFFIN
Jack Hare, Massachusetts Institute of Technology
The new PUFFIN pulsed-power facility at MIT has finished its design phase, and will move to construction and commissioning in Summer 2023, with first plasma expected by the end of 2023. PUFFIN is unique compared to other 1 MA peak current, University-scale facilities used for fundamental plasma physics studies, because its current pulse lasts for several microseconds, rather than 100s of nanoseconds. With this longer pulse length we will study fundamental processes in quasi-steady-state magnetized plasmas, including magnetic reconnection, magnetized turbulence, shock stability, and heat transport, using a suite of diagnostics with high temporal and spatial resolution, notably b-dot probes, shadowgraphy, laser imaging interferometry, Faraday rotation imaging, and Thomson scattering. I will present recent work on other facilities which motivates the development of PUFFIN, and plans for the initial experiments on this new facility.
Helicon Wave Experiments on the LArge Plasma Device (LAPD)
Josh Larson, University of California, Los Angeles
Helicon waves, also known as fast waves in the lower hybrid range of frequencies or whistler waves, are a proposed means for non-inductive current drive in reactor-grade fusion devices. Helicon waves are not limited by the upper density limit imposed by the lower hybrid resonance that the slow branch, also known as the lower hybrid wave or quasi-electrostatic mode, faces. Hence, these waves are a good candidate for mid-radius current drive required by advanced tokamak scenarios. Work at DIII-D to evaluate the prospect of helicon current drive has been done in the low power (<0.5 kW) regime [R.I. Pinsker et al 2018 Nucl. Fusion 58 106007], and high power (~0.5-1 MW) experiments are currently ongoing [B. Van Compernolle et al 2021 Nucl. Fusion 61 116034]. To further evaluate the operating principles of the traveling wave antenna design used in the experiments on DIII-D a series of experiments are being done on LAPD. A first set of experiments were conducted to study the wave coupling and propagation over a large range of plasma parameters. In these studies, the plasma density (1010 < ne < 1012 cm−3), background magnetic field (0.7 < B0 < 2 kG), and antenna parallel spectrum (2 < npar < 4) were varied. Measurements of the plasma loading on the antenna and wave fields in the plasma were made. Through comparison of the measured phase fronts and wave power deposition we were able to identify the launched modes and demonstrate directional launching from the antenna. Further a study of the effects of background magnetic field misalignment on antenna coupling showed that increased misalignment leads to larger coupling to the slow branch that was apparent in the antenna loading and wave fields. The next set of experiments will investigate the role that edge turbulence plays in wave propagation in the helicon regime. Here we will look specifically at how density filaments can cause wave scattering [A.K. Ram 2016 Phys. Plasmas 23, 022504] and ‘stimulated mode-conversion’ [P.L Andrews 1985 Phys. Rev. Lett. 54] leading to wave power in undesirable locations. Work supported with funding from US DOE and NSF.
Optical Trapping and Transport of Single Particles in Dusty Plasmas with and without Magnetic Field
Pubuduni Ekanayaka, Mississippi State University
Current methods of laser manipulation in dusty plasma mostly focus on manipulating groups of particles suspended in the plasma sheath to probe collective dust-dust interactions. We have introduced a new method of optical trapping and manipulation of single particles in dusty and weakly magnetized dusty plasmas using a universal optical trap (UOT) technique. This study demonstrated the capability of the UOT technique to trap and transport arbitrary particle from strongly absorbing to transparent and size ranging from nano- to micrometer to different locations in the plasma, using balanced optical, Coulomb, and gravitational forces. We used real-time imaging systems to monitor the stability and motion dynamics of the trapped particles and showed the potential of using a single particle as an in situ micro-probe for dusty plasma and magnetized dusty plasma diagnostics at a microscopic level. The work is supported by the DOE Office of Science via the grant DE-SC0021030.
Compressed Current Sheets in the Magnetotail: Importance of the Ambipolar Electric Field
Ami DuBois, U. S. Naval Research Laboratory
Micro-scale features are now being resolved by NASA’s Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate thin, structured current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Our kinetic equilibrium model shows that as a large-scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non- gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects. This work is supported by the US Naval Research Laboratory Base Program.
Electron-ion hybrid instability in laser produced plasmas at high fields
Zachary White, University of Alabama - Huntsville
Wave-Particle Interaction Studies with Relativistic Electrons on DIII-D Tokamak
Alexander Battey, Columbia University
Initial Results from the DIII-D Negative Triangularity Campaign
Kathreen Thome, General Atomics
Negative Triangularity (NT) is a potentially transformative scenario for magnetic fusion energy with a high-performance core, large-major-radius tokamak divertors, and no edge localized modes (ELMs) instability or deleterious transient heat flux levels. Benefits of the NT shape were originally demonstrated on the TCV tokamak and high core performance has been previously achieved on DIII-D, which motivated the installation of graphite-tile armor on the lower outer wall to attain high-power diverted plasmas with strong NT shaping. In early 2023, a dedicated multiple-week experimental campaign was conducted to qualify the NT scenario for future reactors on DIII-D. High confinement (H98y,2≥1), high current (q95<3), and high normalized pressure plasmas (βN>2.5) were achieved during this campaign at high injected power in strongly NT-shaped plasmas with δavg= - 0.5 and a lower outer divertor X-point. Lower measured turbulence levels appear consistent with predictions of trapped electron mode stabilization in NT. These plasmas also demonstrated high normalized density (ne/nGW≤2), particle confinement comparable to energy confinement, and a detached divertor without impurity seeding, all while maintaining a non-ELMing NT-edge with an electron temperature pedestal exceeding that of typical low-confinement (L-mode) plasmas. This reactor-relevant regime is accessed over a wide range of operational space (plasma current, toroidal field, electron density and pressure) in contrast to other high-performance ELM-suppression scenarios that have narrower operating windows. The high confinement mode (H-mode) is predicted to be prevented due to high-n ballooning modes that prevent access to edge second stability, but when the triangularity is weaker than δavg~ -0.15, H-mode and ELMs can be obtained in NT. Further early results on performance, stability, turbulence, transport, and core-edge integration show promise for a NT fusion pilot plant design.
This work was supported in part by the US Department of Energy under the following awards DE-FC02-04ER54698.
Laboratory Study of Residual Energy Generation in Strong Alfvén Wave Interactions
Mel Abler, University of California, Los Angeles
In the MHD inertial range (scales larger than ion-kinetic scales) turbulent fluctuations in the solar wind are often Alfvénic in character, meaning that their magnetic and flow velocity fluctuations are proportional to each other and predominantly perpendicular to the background magnetic field. However, observations of the solar wind have shown that there is a significant difference in the energy in velocity fluctuations and the energy in normalized magnetic field fluctuations. This difference, called the residual energy, should be zero for linear Alfvén waves, but is consistently observed to be negative in the solar wind, with magnetic fluctuations dominating. This work investigates the energy partition in strong three-wave interactions as a building block of the turbulent cascade through an experimental campaign on the LArge Plasma Device (LAPD) in an MHD-like regime relevant to the solar wind. In these experiments, primary (driven) modes are launched from antennas, and the spectrum of secondary modes generated by the strong three-wave interaction is observed. The residual energy present in both the primary and secondary modes is measured using multiple techniques to shed light on how these interactions generate residual energy.
Incorporating Lessons from Formal and Informal Education Research in Public Engagement and Training Programs for the Plasma and Fusion Workforce
Shannon Swilley Greco, Princeton Plasma Physics Laboratory
The American Physical Society’s Division of Plasma Physics recently hosted a mini-conference on “Workforce Development Through Research-Based, Plasma-Focused Science Education and Public Engagement.” The presenters and organizers authored a report on the results of the mini-conference, including several recommendations for community members, leaders, research institutions, and funding agencies. Building community and sharing resources and best practices are central recommendations in the report, especially to "establish the Plasma Network for Engagement and Training (PlasmaNET) as an ecosystem connecting colleagues working in public engagement, education, broadening participation, and workforce development for plasma." In this talk, the authors will address some of these recommendations by sharing resources and model practices to augment the efforts of community members in engaging the public and students.
Speakers: Shannon Swilley Greco, Princeton Plasma Physics Laboratory; Evdokiya Kostadinova, Auburn University
Contributed Talk Abstracts
Towards observing Alfven wave parametric decay in the laboratory: hybrid simulations
Feiyu Li, New Mexico Consortium
A large-amplitude shear Alfven wave may be subject to the parametric decay instability (PDI), producing a backward Alfven wave and a forward ion acoustic wave. This fundamental magnetized plasma wave dynamics may contribute to several space plasma phenomena, such as solar coronal heating and turbulent cascades. Evidence of Alfven wave PDI has been implied by spacecraft observations. To accompany our ongoing experimental efforts towards demonstrating PDI using the Large Plasma Device (LAPD), here we present hybrid fluid-kinetic simulations equipped with many LAPD relevant features to gain insights into the experimental conditions required for exciting PDI. Both one-wave driven standard PDI and two-wave driven seeded PDI (where the pump decay is seeded by a small-amplitude counter-propagating Alfven wave at suitable frequencies) are studied using either quasi-1D or full 3D simulations. The results shed lights on how the Alfven wave amplitude, frequency, source size and wave damping may affect PDI excitation in the laboratory and its potential observable signatures. The 3D hybrid simulation tool should also be attractive for other LAPD-based Alfven wave studies.
Study of magnetic reconnection in ion-scale magnetospheres on the Large Plasma Device
Lucas Rovige, University of California, Los Angeles
Magnetic reconnection is a fundamental process occurring in magnetospheres and other space and astrophysical objects that can lead to topological reorganization of the magnetic fields as well as transfer of magnetic energy to the plasma. In this work, we report on the experimental study of magnetic reconnection in laser-driven ion-scale magnetospheres on the Large Plasma Device (LAPD). In our experiment, we use a high-repetition rate (1 Hz), nanosecond laser to drive a fast moving plasma that expands into the field generated by a pulsed magnetic dipole embedded into a background plasma and magnetic field [1]. The dipole and background fields are oriented to be anti-parallel, so that a magnetic null point naturally occurs. When the laser-plasma expands into the background plasma, it compresses the magnetic field and drives magnetic reconnection at this null point.
Taking advantage of the high-repetition rate, the magnetic and electric fields are measured with magnetic flux and emissive probes, respectively, in a large 3D volume around the reconnection point to characterize its effect on the global structure of the magnetosphere.
[1] D. B. Schaeffer et al. Physics of Plasmas 29, 042901 (2022)
Modeling Experimental Reconnection Processes with Multidimensional Kinetic Simulations: TREX & VPIC
Samuel Greess, University of Wisconsin-Madison
The Terrestrial Reconnection EXperiment (TREX) as the Wisconsin Plasma Physics Laboratory (WiPPL) creates and measures different reconnection geometries in collisionless plasmas, with the aim of understanding the reconnection mechanisms of low-density space environments. Work on TREX is supplemented by kinetic simulations using VPIC, a particle-in-cell code developed at Los Alamos National Laboratory. VPIC simulations of TREX work in tandem with laboratory experiments, such that each provide feedback that shapes the designs and objectives of the other. TREX data and simulations have shown agreements in several facets of the reconnection process: in layer width and magnetic geometries [1] as well as between experimental and simulated reconnection rates as the system size decreases and the reconnection approaches the electron-only regime [2]. The most recent TREX data and simulations make use of the newly constructed drive cylinder geometry; preliminary findings from these two methods will be presented.
[1] Greess et al. JGR Space Physics (2021) 126, e2021JA029316.
[2] Greess et al. POP (2022) 29 (10): 102103.
This work was supported by DOE funds DE-SC0019153, DE-SC0013032, DE-SC0018266, and DE-SC0010463, NASA fund 80NSSC18K1231, and by a fellowship from the Center for Space and Earth Science (CSES) at LANL.
Measurement of the thermal effects in the dispersion relation of the dust acoustic wave in the presence of a magnetic field
Jeremiah Williams, Wittenberg University
A dusty plasma is a four-component plasma system consisting of ions, electrons, neutral particles and charged microparticles. The microparticle component interacts with the other plasma components, acquires a net charge and self-consistently modifies the surrounding plasma medium. The resulting system is notably more complex than the traditional plasma system and supports a wide range of physical phenomena, including a wave mode known dust acoustic wave; a low-frequency, longitudinal mode that propagates through the dust component of the dusty plasma system and is believed to be self-excited by the ions streaming through the dust component. Measurements of the dispersion relation of the dust acoustic wave have revealed that thermal effects can play a significant role and need to be included to accurately model the experimentally measured dispersion relations. While a number of physical mechanisms have been proposed to explain the high temperatures that have been observed experimentally, the physical mechanism responsible remains an open question. One proposed mechanism is an ion-dust streaming instability. This presentation will present measurements made at the MDPX facility where the strength and direction of the magnetic field (relative to the direction of wave propagation) were be varied in a controlled way to test if this mechanism may be (at least partially) responsible for the high dust temperatures that have been observed.
Study of Alfvén wave reflection to address the solar coronal heating problem
Sayak Bose, Princeton Plasma Physics Laboratory
The physics behind the heating of the solar corona and the acceleration of the fast solar wind from coronal holes (predominantly open field regions of the solar corona) is not well understood. Recent observations of large-amplitude counter-propagating Alfvén waves at the base of coronal holes suggest that the outward and the inward waves interact nonlinearly to generate turbulence which heats and accelerates the plasma. However, the mechanism of generating inward Alfvén waves is yet to be fully established. Most theories within the MHD framework invoke partial reflection of outward Alfvén waves from gradients in the Alfvén speed to explain the inward waves. However, to date, no experiment has reported the detection of a reflected Alfvén wave in an experimental arrangement relevant to coronal holes. We have done new experiments to detect a reflected wave from an Alfvén speed gradient under conditions scaled to match coronal holes. The experiments were conducted in the Large Plasma Device at the University of California, Los Angeles. Our results show that the reflected Alfvén wave amplitude increases as the ratio of the wavelength to the gradient scale length increases. The results of the experiments are presented.
Nonlinear dynamics of small-scale Alfvén waves
Alfred Mallet, University of California, Berkeley
We study the nonlinear evolution of very oblique small-scale Alfvén waves with perpendicular lengthscales comparable to the ion inertial length. At these scales, the waves become significantly compressive, unlike in MHD, due to the Hall term in the equations.
We demonstrate that when frequencies are small compared to the ion gyrofrequency and amplitudes small compared to unity, no new nonlinear interaction appears due to the Hall term alone at the lowest non-trivial order. However, at the second non-trivial order, we discover that the Hall physics leads to a slow but resonant nonlinear interaction between co-propagating Alfvén waves, an inherently 3D effect.
Including the effects of finite temperature, finite frequency, and electron inertia, the two-fluid Alfvén wave also becomes dispersive: for oblique waves at low beta as studied here, this can be at a much smaller scale than the ion inertial length. We show that the timescale for one-dimensional steepening of two-fluid Alfvén waves is only significant at these smaller dispersive scales, and also derive an expression for the amplitude of driven harmonics of a primary wave. Importantly, both new effects are absent in gyrokinetics and other commonly used reduced two-fluid models.
Our calculations have relevance for the interpretation of laboratory Alfvén wave experiments, as well as shedding light on the physics of turbulence in the solar corona and inner solar wind, where the dominant nonlinear interaction between counter-propagating waves is suppressed, allowing these new effects to become important.
Stable tokamak operation beyond traditional safety factor and density limits in the Madison Symmetric Torus
Noah C. Hurst, University of Wisconsin - Madison
Tokamak plasmas typically experience disruptions, or discharge-terminating instabilities, when operated with edge safety factor q(a) < 2 or above the Greenwald density limit. The Madison Symmetric Torus (MST) device, when run as a tokamak, has been routinely operated with q(a) as low as 0.6 and with density up to 10 times the Greenwald limit. This capability is thought to be possible due to several unique features of MST: First, it has a close-fitting, thick, conductive wall with a resistive wall time of 0.8 s, which is much longer than the typical discharge duration of 50 ms. This helps to stabilize external kink modes which are often implicated in disruptive behavior. Second, it has a high-bandwidth, high-loop-voltage feedback power supply system which is capable of driving plasma current in very resistive and/or dynamic conditions. In the low-q(a) regime, we discuss measurements of decreased confinement, self-organized q(r) profiles, and irregular fluctuations for 1 < q(a) < 2; and helical structure formation at q(a) = 1 and below. In the high-density regime, we discuss impurity radiation and Ohmic input power scaling with Greenwald fraction > 1, and current-profile collapse for Greenwald fraction ~ 2 and above. Both of these regimes are largely unexplored due to disruptive behavior encountered in other devices. Future research goals and possible implications for fusion technology are discussed. Work supported by U.S. DOE and NSF.
Observing Confined Filamentation Dynamics in Magnetized Low Temperature Plasma with MDPX
Elon Price, Auburn University
Studying complex or “dusty” plasmas enables direct visualization of microscopic plasma processes. As a result, these systems provide valuable insights into the transport, thermal, and collective processes in plasma, from diagnostics development to first principles studies of plasmas, into interdisciplinary areas of research including ranging from energy applications to astrophysics. The Magnetized Dusty Plasma eXperiment (MDPX) is a unique device that can produce steady state, large magnetic fields up to 4 T over an experimental volume that is 50 cm in diameter and over 20 cm long. However, at a magnetic fields larger than ~1 T, plasmas in rf generated, capacity-coupled plasmas exhibit the formation of coherent structures that are generally aligned along the magnetic field direction and that can be stable or mobile that are referred to as “filaments”. These plasma filaments distort the otherwise uniform plasma background and, as a results, cause significant perturbations to the dust. As a result, understanding the morphology and the dynamics of the filaments have become an integral part of the studies using the MDPX device. This talk will discuss observations of confined filaments in capacitively coupled rf-generated Argon plasma. Confinement is achieved by introducing copper rings on the main electrodes, which acts to restrict to motion of the filaments. Through image analysis, filament dynamics will be compared for confined and unconfined experiments. Understanding both the spatial and temporal characteristics will support a first principles theoretical framework for these phenomena.
Building a MagNetUS/PlasmaPy partnership for open plasma science,
Nick Murphy, Center for Astrophysics | Harvard & Smithsonian
PlasmaPy is an open source Python package for plasma science. The overall mission of the PlasmaPy project is to foster the creation of an open source software ecosystem for plasma research and education. In that sense, the mission of PlasmaPy strongly aligns with the mission of MagNetUS. This talk will describe the current and planned capabilities of PlasmaPy, challenges in developing software infrastructure for plasma physics, and how we can forge a partnership between PlasmaPy and MagNetUS for open science. PlasmaPy is developed openly on GitHub, and anyone who follows the code of conduct is welcome to contribute.
Behavior of edge instabilities and turbulence in gas-puff-fueled LAPD discharges
Thomas Look, University of California, Los Angeles
The recent upgrade of the main LAPD plasma source from Barium Oxide to Lanthanum Hexaboride has motivated a new fueling procedure to support higher-density operation. Gas is puffed by piezoelectric values near the anode during the discharge, and the chamber is evacuated between shots. This mode of operation can achieve higher densities and better axial density uniformity than the previous fueling procedure but also has revealed interesting new plasma behaviors. A transition from broadband edge turbulence to a single coherent mode is observed with increase in neutral fueling via gas puffing. We will present experimental data demonstrating this behavior and discuss future experiments to understand the nature of this transition.
Electron Thermal Anisotropy During Electron-Only Reconnection in PHASMA
Earl Scime, West Virginia University
A regime of reconnection, called electron-only reconnection since there is insufficient time or space for ions to couple with reconnection structures, provides an opportunity to study electron heating mechanisms more directly than in fully ion-coupled reconnection. Electron-only reconnection is thought to be important for energy dissipation at kinetic scales in Earth’s turbulent magnetosheath, bow shock and the quiet magnetotail. Here we present direct, in situ, measurements of both perpendicular and parallel electron temperature (relative to the local B) and their spatial distribution in the reconnection plane during electron-only reconnection in a laboratory plasma. Parallel electron heating dominates over perpendicular heating along one separatrix, with the electron temperature anisotropy reaching 1.5. The preferential parallel electron heating and its spatial localization are reproduced in our 2D particle-in-cell (PIC) simulations, which leads us to conclude that electron energization by the parallel reconnection electric field is responsible for the observed electron heating during electron-only reconnection with large guide field, Bg. These observations are consistent with previous experimental studies that were unable to directly measure temperature anisotropies or their spatial distribution during ion-coupled, modest- Bg reconnection. Significant perpendicular electron heating, but smaller than the parallel heating, also appears along both separatrices and increases with distance from the X-point, a phenomenon not reproduced in simulations. This result highlights the necessity of measuring complete, multi-dimensional EVDFs to determine the full energy budget during reconnection in laboratory and magnetosheath plasmas.
Simultaneous axial plasma detachment and radial transport barrier leading to helicon core formation
Saikat Chakraborty Thakur, Auburn University
We show that helicon core formation in rf devices is accompanied by the simultaneous formation of a radial transport barrier and axial plasma detachment; via a self-organized, global transition [1]. Evidence from both Langmuir probes and fast imaging show that the radial extent of the transport barrier is similar to the width of the helicon core. Using spectroscopy and neutral pressure measurements, we simultaneously observe axial plasma detachment, which follow the same hysteresis patterns associated with the radial transport bifurcation. We report dramatic changes in both the mean and fluctuation profiles across this transition. This self-organized global transition is universal, but the transition-threshold depends on the helicon source parameters. 2-D bifurcation diagrams elucidate various regimes of rf plasma source operation (capacitively coupled, inductively coupled, detached helicon mode without a core, attached helicon mode with a core), allowing access to study basic plasma instabilities, turbulence and transport, as well as divertor-relevant plasma detachment in the same device. Spontaneous plasma detachment has serious implications on the relevance of similar rf devices [2, 3] designed to study plasma material interactions. In addition, this also gives us the opportunity to study instabilities, turbulence and transport associated with detached plasmas [4, 5].
[1] L. Cui, et. al., Physics of Plasmas, 23 055704 (2016)
[2] J. Rapp, et. al., Nuclear Fusion, 57 116001 (2017)
[3] S. C. Thakur, et. al., Plasma Sources Science and Technology, 30 055014 (2021)
[4] S. Krasheninnikov, et. al., Nuclear Fusion, 57 102010 (2017)
[5] N. Ohno, et. al., Nuclear Materials and Energy, 19 458 (2019)
Current Status of PHASMA and plasma fluctuations during Magnetic reconnection
Sonu Yadav, West Virginia University
Reconnection describes the process in which magnetic field lines in a plasma break and reconnect, converting magnetic energy to heating and acceleration of particles. In PHASMA (PHAse Space MApping) facility at WVU, reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. To diagnose magnetic reconnection and associated particle heating in PHASMA, laser (incoherent Thomson, LIF), optical, electric, and magnetic probe-based diagnostics are employed. Ion velocity distribution functions (IVDF) and electron energy distribution functions (EEDF) are measured with laser induced fluorescence (LIF) and a retarding field energy analyzer (RFEA), respectively. Preliminary studies of ion heating and electron beam generation during magnetic reconnection will be presented. We will also describe studies of plasma fluctuations during reconnection as measured with high speed photo diodes and Langmuir probes. In addition to this review of physics investigations, we will describe the current status of PHASMA.
Persistence and Evolution of “Staircase” Profiles in a Fluctuating Vortex Array
Fredy Ramirez, University of California, San Diego
A current subject of interest in magnetic fusion energy (MFE) is the E x B staircase, which occurs in the regime of “near marginality”. Near marginal plasmas can sometimes naturally evolve towards an organized critical state of spatially segregated micro-barriers interspersed between sectors of strong avalanche-like transport. A staircase profile of scalar concentration forms in a simple system of stationary, convective cells, set in a fixed array. Here, layering occurs due to the existence of two disparate time scales, the cell turn-over time and the diffusion time. Note that this system is simpler than the E x B shear predator-prey scenario which involves dynamical feedback. It can be argued that the setup is contrived and the cellular array is overly constrained and therefore, unrealistic relative to drift-wave turbulence in tokamaks. The actual physical system will manifest variability and intrinsic jitter. An investigation into layering and staircase dynamics is pursued using a fluctuating vortex array, which intrinsically drives inhomogeneous mixing. By systematically varying the elements of the vortex array, we uncover that scalar staircases form and persist (although steps become less regular) into regimes of increasing Reynolds number (Re). This study focuses on low-modest values of Re, which are comparable to Re found in MFE, where the turbulence is “weak” and the dissipation is due to Landau damping. Furthermore, by tracing the trajectory of the scalar concentration, we find that the scalar flows around vortices forming a flamelet network pattern. The scattering of vortices induces a lower effective speed of scalar concentration front propagation. It’s also found that the effective diffusivity of the perturbed vortex array does not deviate significantly from that of the fixed cellular array. Slight deviations are the result of changes in cell geometric properties.
The effects of electron-cyclotron heating on relativistic-electron plasmas in DIIID
Hari Paul Choudhury, Columbia University
Viscous Boundary Layer For Transonic Equilibrium
Luca Guazzotto, Auburn University
Transonic equilibria have been studied in the past in a number of papers, see e.g. [1-4]. They occur in tokamak geometry when the core region of the plasma has slow (with respect to the poloidal sound speed, Csp = CsBp/B) poloidal rotation, while the edge has supersonic (vp > Csp) rotation, a realistic condition in H-mode tokamak plasmas. In the single-fluid MHD model transonic equilibria are characterized by a radial discontinuity in density, pressure and velocity. By physical intuition, it is expected that including fluid viscosity in the problem would relax the discontinuity to a sharp-gradient layer between the slow- and fast-flowing regions. In this work, we start examining the properties of the viscous boundary layer at the flow transition in transonic equilibria. We observe that the system is similar to a fluid-dynamic viscous flow between two rigid plates. We point out analogies and differences with the classical Blasius solution for viscous boundary layers and investigate the effect of compressibility.
Work supported by the Department of Energy – Fusion Energy Science, grant number DE-SC0014196
[1] L. Guazzotto, R. Betti, J. Manickam, S. Kaye, Physics of Plasmas 11 2, 604-614
[2] L. Guazzotto, R. Betti, Physics of plasmas 12 5, 056107
[3] L. Guazzotto, E. Hameiri, Physics of Plasmas 21 2, 022512
[4] L. Guazzotto, R. Betti, S.C. Jardin, Physics of Plasmas 20 4, 042502
Transport barriers and anomalous diffusion in a strongly magnetized, low temperature plasma in the MDPX device
Edward Thomas, Jr. , Auburn University
A number of studies using the Magnetized Dusty Plasma Experiment (MDPX) device have focused on understanding the physical, thermal, and transport characteristics of the capacitively coupled plasma (CCP) configuration in the presence of strong magnetic fields above 1 Tesla. In the studies presented in this work, a moving probe is inserted into a low temperature argon plasma operating at neutral gas pressures from 5.3 to 16 Pa (40 to 120 mTorr), RF power ~1 W, and magnetic field above 1 Tesla. As the probe is withdrawn from the plasma, an “imprint” of the probe in the form of a channel of diminished visible light emission that persists for 2 to 10 seconds, depending on the neutral pressure. This phenomenon is reproducible over a range of neutral pressures and magnetic fields, but only occurs for magnetic fields, B ≥ 1 Tesla. This presentation will describe the experimental configuration, provide images and videos of this phenomenon, and will discuss some of the initial modeling that is being performed to understand these observations.
This work is supported with funding from the US Department of Energy – Office of Fusion Energy Sciences and the NSF EPSCoR Program.
Development progress of next-generation simulations for magnetically insulated transmission lines
Ayden Kish, University of Rochester, Laboratory for Laser Energetics
“Gateway to Plasma”: Developing a plasma-focused professional development training program in the state of Alabama
Evdokiya Kostadinova, Auburn University
In the past two decades, two assessments on the workforce needs for plasma science and fusion energy have identified major issues with the declining number of plasma faculty, the small number of departments and institutions teaching plasma, and the slow production rate of qualified personnel [1], [2]. Both the recent NAS decadal assessment of plasma science [3] and the FESAC report Powering the Future: Fusion & Plasmas [4] called for emphasis on plasma-specific educational and research programs that also provide opportunities to diverse and less advantaged populations.
In response to the findings and recommendations from these studies, we propose the establishment of a plasma-focused professional development program in the state of Alabama, called “Gateway to Plasma”. This program will be initiated by the NSF EPSCoR project Future Technologies and enabling Plasma Processes (FTPP), which is a collaboration between nine universities (including four HBCUs) and one industry in Alabama. In this presentation we will discuss strategies for establishing the program, including certification process, public engagement, assessment tools, and expected benefits for the state and the broader plasma community. Focus is given on best practices and lessons learned when engaging students, faculty, and staff from HBCUs and MRIs. We include comparisons with other plasma training projects, such as the Minority Serving Institution Faculty Workshop in Plasma Physics conducted at PPPL.
Work supported by NSF EPSCoR FTPP, OIA-2148653.
[1] E. Thomas et al., J. Fusion Energy, vol. 22, no. 2, pp. 139–172, Jun. 2003
[2] FESAC report, “Assessment of the Workforce Development Needs for the Fusion energy Sciences,” 2014.
[3] Plasma Science: Enabling Technology, Sustainability, Security, and Exploration. Washington, D.C.: National Academies Press, 2021.
[4] Report of the Fusion Energy Sciences Advisory Committee, “Powering the Future: Fusion & Plasmas,” 2020. https://arxiv.org/pdf/0710.0856.pdf.
Contributed Poster Abstracts
A new parallel-kinetic-perpendicular-moment model for magnetized lab plasmas
James Juno, Princeton Plasma Physics Laboratory
To facilitate new kinetic models of laboratory plasmas, I will discuss a recent innovation which separates the parallel and perpendicular dynamics starting from the kinetic equation while staying agnostic to the inclusion of effects important to consider in the laboratory, such as geometry of the experiment, the necessary boundary conditions, and other effects such as neutral interactions and collisions. The key component of the derivation lies in a spectral expansion of only the perpendicular degrees of freedom, analogous to spectral methods which have grown in popularity in recent years for gyrokinetics, while retaining the complete dynamics parallel to the magnetic field. We thus leverage our intuition that a magnetized plasma’s motion is different parallel and perpendicular to the magnetic field, while allowing for the treatment of complex phase space dynamics parallel to the magnetic field. This approach also naturally couples to Maxwell’s equations, allowing for the straightforward inclusion of all aspects of the experiment, from vacuum regions to external coils.
Intermittency Studies of Broadband Magnetic Fluctuations
Carlos Cartagena-Sanchez, Xantho Technologies, LLC
Growth of TiO2 dusty microparticles in a magnetized plasma
Bhavesh Ramkorun, Auburn University
Recent research has focused on the spontaneous growth of dust particles from reactive gases in capacitively coupled rf plasmas. For example, carbonaceous and silicate dust growth have been extensively studied over the past two decades. This presentation introduces the growth of titanium dioxide (TiO2) dust particles from titanium-isopropoxide in argon plasmas. The growth of TiO2 particles occurs in three steps: nucleation, coagulation, and agglomeration. These processes are influenced by several forces, including gravitational, electric, ion-drag, neutral-drag, and thermophoretic forces. These forces allow TiO2 particles to grow in spherical shapes, with maximum diameters of approximately 650 nm. The growth rate of these particles is linear, and gravitational force becomes the dominant force once they have accumulated enough mass, causing them to fall out of the plasma in approximately 105 seconds. In this study, a new force, magnetic force, is introduced during particle growth, which affects both the growth rate and the time. A comparison is made between the results with and without the magnetic field. This research expands our understanding of the growth of dust particles in plasmas and provides insight into the effect of magnetic fields on this process. These findings have important implications for a wide range of fields, including materials science, plasma physics, and astrophysics.
Status of the FLARE (Facility of LAboratory Reconnection Experiments) and Future Plans
Peiyun Shi, Princeton Plasma Physics Laboratory
Magnetic reconnection is one ubiquitous plasma physical process responsible for various explosive and energetic phenomena existing throughout the universe. The reconnection distributed in the wide spanning parameter space can be organized in phase diagrams [Ji and Daughton, Phys. Plasma 18, 111207 (2011)], characterized by the Lundquist number S and normalized system size λ. The FLARE (Facility of LAboratory Reconnection Experiments) laboratory experiments target poorly explored multiple X-line reconnection regimes of both large S and λ values, which is closely relevant to reconnection in space and astrophysics plasmas. Multi-scale magnetic reconnection physics coupling global MHD scales and ion and electron kinetic scales can be investigated. Other major scientific questions to be addressed by FLARE will also be discussed. The present status of FLARE engineering and diagnostic developments and future plans to operate FLARE as one user facility will also be presented.
Modulating Plasma Parameters using a Cylindrical Electrode in the ALEXIS device
Jared Powell, Auburn University
The formation of m=1 n=1 helical “density snake” structures within MST tokamak discharges
Brandon Schmall, University of Wisconsin-Madison
“Density snakes” are helical density structures in tokamak discharges which typically form after pellet ablation onto the q=1 surface or spontaneously after sawtooth activity and have been seen within many fusion devices such as JET, Alcator C-mod, EAST, and recently MST. They have been seen within MST’s far infrared interferometry data, as opposed to the typical soft x-ray measurements. A numerical code was developed to identify and characterize these snakes. Their physical structure matches those found in other devices, yet the spontaneous creation of the snake occurs following plasma startup, before sawtooth activity is observed, rather than after. The methodology of this code and the conditions required for snakes to form are discussed, as well as the timing of their formation.
Re-crystallization and instabilities during melting of a 2D dusty plasma cloud
Ravi Kumar, University of Memphis
Melting phenomena of two-dimensional structures are of great interest in condensed matter sciences. Phase transitions in 2D materials have been widely researched for potential applications and novel bulk properties, but due to sub-nano scales, atomic dynamics are near impossible to observe and investigate. Dusty plasmas are a strongly coupled system consisting of highly negatively charged micro-sized dust grains confined over an electrode in a plasma environment, which offers an analogous route to visualize and track individual dynamics of particles undergoing a phase transition. Mono-sized MF particles are used to produce a crystallize 2D dust layer in an RF-generated plasma, confined by a parabolic potential originating from the geometry of the electrode. Melting and re-crystallization of the dust layer are achieved by changing the plasma parameters through varying RF power, which provides an excellent opportunity to study hysteresis behavior. The dust layer is illuminated with vertical and horizontal laser sheets capturing all possible motion of the particles in the x, y, and z-axis simultaneously. It was observed that complete melting only occurred through vertical instabilities in the z-axis, breaking the 2D symmetry of the dust layer. The 3-axis simultaneous recording design of the experiment provides valuable insight into the true nature of phase transitions in 2D dusty plasmas and a leap toward understanding the dynamics of real 2D materials. This work is supported by the Department of Energy (DOE). Grants: DE-SC0021146 and DE- SC0023416.
Validation of plasma response and turbulence simulations across tokamak core magnetic islands
Dmitry Orlov, University of California, San Diego
One of the biggest challenges for long-pulse ELM suppression is the control of the heat and particle fluxes' interaction with plasma-facing surfaces while simultaneously minimizing the impurities influx into the core plasma. Achieving these goals relies on our understanding of the transport in the presence of the 3D perturbation fields changing the magnetic topology of the plasma in the edge and core regions. In this work, we present the results from the recent KSTAR L-mode experiments showing increased turbulence near the X-point of the 2/1 core magnetic island resulting from the locked mode. The observation of localized island transport is consistent with the previously proposed ExB convection mechanism across the island X-point. These experimental results agree with the previous observation in the DIII-D RMP islands.
We use the KSTAR discharge #19118 with a well-diagnosed large 2/1 magnetic island to validate the existing plasma response (M3D-C1, NIMROD) and turbulent transport (GTC) codes. The results of the linear plasma response modeling are in good agreement with ECEI measurements for the width of the resulting magnetic island. The nonlinear turbulent transport simulations initialized from the plasma response solution are performed for both axisymmetric case and with n=1 magnetic island and show the turbulence spreading across the 2/1 magnetic surface. The results of the simulations give confidence that the understanding of the underlying physics of transport in the presence of 3D fields and magnetic islands can be extrapolated to edge islands, giving us insights into RMP ELM suppression mechanisms.
Work supported by US DOE under DE-FG02-05ER54809, DE-SC0020413, DE-SC0020298, DE-SC0021185, DE-SC0018287, and DE-AC02-09CH11466.
Characterization of a wave-launching plasma source using LIF techniques
Jacob McLaughlin, University of Iowa
A novel plasma source has been developed by coupling microwave cavity energy into the electron cyclotron resonance. It is intended for the source to produce plasma with the bulk ion species coming from vaporized calcium. The source currently provides a argon plasma discharge, with densities up to 10^10 cm^-3, flowing in a non-uniform background magnetic field. A conductive, double grid separates the plasma source from the measurement region to confine microwaves in the source and it iselectrically isolated from the background chamber. This allows the grid to be used as a wave launching antenna necessary for future experiments exploring wave behavior in plasma undergoing chaotic orbits and other experiments for validating the field-particle correlation technique (FPC). Presented are laser-induced fluorescence (LIF) measurements of the equilibrium distribution function downstream from the source, the linear response of the ions to the electrostatic perturbation driven by the grid, and novel interferometry techniques utilizing phase-sensitive detection of LIF while sweeping the wave frequency.
Building a roadmap for understanding magnetic reconnection and rapid magnetic reorganization in fusion plasmas using MHD simulations
Dingyun Liu, Princeton University
Fast magnetic reorganization during sawtooth crash events at DIII-D
Will Fox, Princeton Plasma Physics Laboratory
Turing Patterns in Strongly Magnetized Plasmas
Mohamad Menati, Auburn University
Emergence of self-organized patterns in many biological and non-biological systems can be explained through Turing’s activator-inhibitor model. Here, we will show how Turing’s framework can be employed to describe the formation of filamentary structures in a low-pressure electric discharge exposed to a strong magnetic field. It is shown through theoretical investigations that the fluid equations describing a magnetized plasma can be rearranged to take the mathematical form of an activator-inhibitor system. In this approach, electrons can be considered as the activator and the ions as the inhibitor. Numerical simulations based on the equations derived from this approach were able to reproduce the various patterns observed in the experiments. Also, it was shown that due to a density imbalance between electrons and ions in the filamentary patterns and their neighboring depletion regions, a patterned transverse electric field is produced in the bulk of the magnetized plasma. This electric field is responsible for the stability of the filamentary patterns over time scales much longer than the characteristic time scales of the plasma. Based on these theoretical and numerical studies of the phenomenon, a formation mechanism for the emergence of filamentary structures in magnetized plasmas is suggested.
Electron plasma vortices under external strain - A study using 2D Particle-In-Cell simulations
Siddharth Bachoti, Auburn University
Rotating plasmas are found in astrophysical phenomena such as magnetized neutron stars and pulsar electrospheres, as well as in laboratory experiments such as pure electron/ion plasmas trapped in Penning-Malmberg (PM) traps and fusion plasmas. In many of these cases, external velocity shear or velocity strain flows affect the dynamics of the rotating plasmas. Non-neutral (pure electron/ion) plasmas are routinely investigated to study the dynamics of the inviscid fluid vortices due to the isomorphism between the 2D inviscid incompressible Euler equations and the governing dynamics of the electron plasmas at mass-less limit (m_e→0). In the earlier experimental and numerical investigations[1], evolution of a 2D vortex under external non-axisymmetric quadrupole strain has been studied using low density electron plasmas in the PM trap, confined due to an axial magnetic field and a radial electric field created by the 8-segment trap[1].
In the present study, we have numerically investigated pure electron plasmas under experiment-like conditions using 2D2V particle-in-cell (PIC) simulations with an existing PIC code PEC2PIC[2], developed and maintained at the Institute for Plasma Research, India. In the first part, we present results that qualitatively and quantitatively benchmark the code against the experiments[1] and show that in agreement with previous work, there is a critical value of the quadrupole strain (relative to the vorticity of the plasma) beyond which the low density electron plasma vortex is destroyed[1]. In the second part, we have explored the dynamics of high density electron plasma vortices under the effect of external velocity strain flow. We have also investigated the vortex dynamics in the presence of other strain flow geometries and time-varying strains. We report several new findings.
References:
[1] N. C. Hurst et al. Physical Review Letters 117, 235001 (2016).
[2] M. Sengupta and R. Ganesh, Physics of Plasmas 21, 022116 (2014).
A Performance Upgrade to DIII-D to Resolve the Integrated Tokamak Exhaust and Performance Gap for a Fusion Pilot Plant
Richard Buttery, DIII-D National Fusion Facility
The critical challenge to develop a viable concept for a compact fusion pilot plant is to resolve a highly dissipative divertor and its compatibility with a high-performance core. An upgrade to DIII-D is proposed to close gaps on reactor physics regimes in divertor, scape off layer, pedestal and core regions, to test critical physics, pioneer solutions and resolve their mutual compatibility. The key is to raise pressure. This enables high density to be sustained at low collisionality to marry a high dissipation divertor with a high-performance core.
The upgrade is achieved through a rise in shaping, current, volume and RF power, exploiting the natural properties of improved edge performance (pedestal) with highly shaped plasmas to close gaps and push limits. Increases in heating and current systems will then enable development of a range of pulsed and steady state core solutions, with integrated modelling projecting normalized pressure (beta) values up to 5, with unique access to low collisionality, thermalized, peeling limited reactor-like regimes, and short neutral penetration depths into the core to study relevant particle and impurity transport. This will equip DIII-D to study plasma phenomena as they manifest in future power plant regimes.
The resulting increased parallel heat flux and density raise opacity and shorten mean free paths to access reactor relevant physics in the divertor power handling region, where a new modular concept enables a staged divertor program to explore advanced closure schemes and isolate physics mechanisms. New limiter configurations, a tile test facility, and new wall coating techniques and materials are proposed. These combine to enable DIII-D to pioneer integrated core and edge solutions, their materials compatibility and required control, in order to resolve and project the approach for a fusion pilot.
Supported under DOE DE-FC02-04ER54698, DE-AC52-07NA27344, DE-AC05-00OR22725.
Rotating Magnetic Field to emulate a Pulsar's Magnetosphere in BRB
Rene Flores-Garcia, University of Wisconsin-Madison
We are building a rotating magnetic dipole with the intention to emulate the magnetosphere of an obliquely rotating pulsar. We report here on the latest progress in the development of the system. The driver consists of an orthogonal pair of Helmholtz-like drive coils and two tube-based amplifiers for generation of sine and cosine coil currents. These coils will be placed in a dielectric pressure vessel at the center of BRB. We plan to puff gas from outlets on the pressure vessel in the BRB equatorial plane. This will be followed by an integrated test of sustained ~10 ms burst through both channels and then mounting the hardware in BRB for plasma experiments. Currently, the gas puffing system and amplifier tests are in development while the transmission lines and driver coils are being fabricated.
Spectral neutral density measurements in plasmas with varying fractional ionization
Eleanor Williamson , Auburn University
Understanding the transition region between fully ionized and neutral dominated plasmas is important to the study of the magnetosphere of the earth, the corona/chromosphere transition regions of the sun, and detached divertors in fusion devices. Determining the fractional ionization of a plasma requires accurately measuring neutral density. We use an absolutely calibrated spectrometer coupled with results from a collisional radiative model solver to infer neutral density. Results will be shown from benchmarking spectroscopic measurements of neutral density against pressure in an RF generated magnetized plasma column between 0.1 to 2.0 mTorr at 0.1% fractional ionization. Preliminary results show that spectroscopic neutral density measurements agree with pressure-based neutral densities below 0.5 mTorr but do not increase proportionally with pressure measurements above 0.5 mTorr. Results will also be shown from the use of the neutral density diagnostic in a higher fractional ionization experiment ranging between 10% and 70%.
Work supported by USDOE grant (DE-FG02–00ER54610) and NSF EPSCoR program (OIA-1655280)
Modeling magnetized plasmas with Molecular Dynamics Simulations
Vikram Singh Dharodi, Auburn University
When a strongly coupled dusty plasma is placed in external magnetic fields, the interparticle interactions are influenced by both the strength of the Coulomb correlations and the effects due to magnetization of the different species. De- pending on the strength of the external magnetic field, three regimes can be identified: (i) only electrons magnetized, (ii) electrons and ions magnetized, and (iii) electrons, ions, and dust particles magnetized. In laboratory setting, such as those available at the Magnetized Dusty Plasma Experiment (MPDX), where the magnetic field can reach up to 4T, it is possible to achieve direct magnetization of electrons and ions and indirect magnetization of dust parti- cles due to electric fields resulting from charge separation. In this presentation, we discuss the analytical considerations and the computational requirements needed for the development of two-species molecular dynamics simulations of magnetized dusty plasmas. First, we discuss modeling electrons and ions to understand how coherent structure, like filaments, can be formed at high mag- netic fields [1]. Then, we consider modeling ions and dust to understand how the magnetic field leads to anisotropy in the ion shielding of the dust, which in turn, modifies the dust-dust interactions and the resulting structure of a strongly-coupled dusty plasma system.
[1] S Williams, S Thakur, M Menati, and E Thomas Jr. Experimental obser- vations of multiple modes of filamentary structures in the magnetized dusty plasma experiment (mdpx) device. Pop, 29(1):012110, 2022.
This work is supported by NSF EPSCoR program (OIA-2148653) – EPSCoR FTPP
First high-power results from the DIII-D helicon system
Bart Van Compernolle, General Atomics
Helicon current drive, also called fast wave current drive in the lower hybrid range of frequencies, has long been regarded as a promising current drive tool for reactor grade plasmas. A MW-level system [1] at DIII-D will be the first test of this technology in reactor-relevant plasmas, in the sense that full first-pass absorption is expected. A 30-module traveling wave antenna of the comb-line type was installed to launch a highly directive wave at 476 MHz and with n∥ = 3, optimized for DIII-D high-beta target plasmas. First high-power experiments in both L-mode and ELMy H-mode plasmas demonstrated high coupling efficiency, as well as load resilience as the reflected power remained low during L/H transitions and during ELMs. Clear conditioning progress of the helicon system was observed in repeated discharges. To date, up to 0.7 MW has been applied to the input of the vacuum transmission line feeding the antenna. Power modulation experiments observed electron absorption in a relatively low-electron-beta L-mode plasma (not single-pass absorption) near the magnetic axis (ρ < 0.3) in qualitative agreement with ray-tracing calculations. Several dedicated helicon diagnostics obtained data for the first time. A prototype Doppler back scattering diagnostic sensitive to 476 MHz measured spectral broadening correlated with edge turbulence. High-frequency magnetic probe measurements revealed nonlinearly generated sidebands near 476 MHz, separated in frequency by harmonics of the deuterium ion cyclotron frequencies that may result from parametric decay instabilities. Additional novel diagnostics to measure wave propagation and antenna-region density profiles are coming online in 2023.
Work supported by US DOE under DE-FC02-04ER54698, DE-AC05-00OR22725, DE-SC0016154, DE-AC02-09CH11466, DE-NA0003525, DE-SC0020284, DE-SC0020337 and DE-SC0020649.
[1] B. Van Compernolle, et al, Nucl. Fusion 61, 116034 (2021)
Using the DIII-D Tokamak to Study Anomalous Electron Diffusion in the Earth’s Magnetosphere
Jessica Eskew, Auburn University
Energetic electrons (EEs) in magnetized plasmas are sub-populations of particles whose energy is orders of magnitude higher than the bulk electrons. These particles are known to exhibit anomalous diffusion, which results in non-Maxwellian distribution functions. A fundamental question of interest is how features of the magnetic field topology (such as magnetic islands) affect the generation and transport of such particles. EEs are ubiquitous in laboratory conditions (tokamaks and stellarators) and in space plasma (the solar wind and the Earth’s magnetotail). Therefore, appropriate scaling and diagnostics techniques can be adopted to investigate the universal relationships between magnetic topology and anomalous electron transport from lab to space. Data from spacecraft measurements is limited due to their one-dimensional trajectory, while self-consistent 3D simulations are computationally expensive. Therefore, the use of laboratory experiments is sorely needed by the community to fill the gaps in knowledge.
Here we discuss the design of DIII-D experiments that aim to study how the production, trapping, and release of EEs in magnetized plasma is affected by the topology of magnetic island chains. We argue that DIII-D inner-wall limited L-mode discharges with no neutral beam injection can reproduce key features of the plasma environment in the Earth’s magnetosphere. In such discharges, plasma density is kept low and plasma response is minimized, which allows to decouple the role of the vacuum magnetic field in electron transport. In addition, magnetic islands are created and manipulated through coil perturbations while electron cyclotron heating and current drive (ECH/ECCD) pulses are employed to ‘tag’ electrons at specified locations inside and outside island chains at different poloidal planes. Suprathermal electron properties are then determined from electron cyclotron emission (ECE), gamma ray imaging (GRI), and scintillator (hard X-ray) measurements. We discuss main advantages and challenges of the proposed experiments along with some preliminary results.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Awards DE-FC02-04ER54698, DE-SC0023476, DE-SC0023367, DE-SC0023061, DE-FG02-05ER54809.