核融合、太空、雷射電漿

P02 Dual-self similarity in phase-space of magnetized plasma turbulence (Eiichirou Kawamori, Yu-Ting Lin)

Eiichirou Kawamori kawamori@isaps.ncku.edu.tw

The universality hidden in the turbulence of collisionless magnetized plasmas exemplified by space, astrophysical, and magnetically confined fusion plasmas has been of long-time paramount interest. Entropy cascade akin to the Kolmogorov energy cascade in fluid turbulence has been theoretically proposed universal dynamics. Here, we show corroborating evidence of the existence of the entropy cascade by the first-ever direct visualization of entropy distribution in the phase-space of turbulence in laboratory experiments. This measurement confirms the scaling laws predicted by the gyrokinetic theory with the dual self-similarity hypothesis, which reflects the interplay between the position- and velocity-spaces of ions by perpendicular non-linear phase-mixing. This verification is crucial for understanding turbulent heating in the solar corona, solar wind, accretion disks, magnetically confined fusion plasmas.

Zongmau Lee happyfat332@gmail.com

A laminar flow is an ordered form of energy containers in plasma systems, hence tractable energy form to control or to convert to the other forms in plasmas. In this study, we investigate properties of out flow of ions from a magnetic mirror in a laboratory experiment with a detailed measurement of axial profiles of plasma parameters for varied mirror ratios of the magnetic mirror. The experimental results reveal that the speed of the ion out flow is increased with the mirror ratio. This is attributed to enhancement of an ambipolar axial electric field accompanied by the increased mirror ratio. We also confirmed that the ion out flow satisfies the Bernoulli’s theorem and the continuity equation consistently. Accordingly, the ion out flow speed can be increased up to the speed corresponding to unity Mach number in the downstream of the magnetic mirror.

陳奕傑 cyijie727@gmail.com

Quasi-linear theory along with the linear and weak nonlinear theories constitutes the most fundamental theories of the plasma wave physics, which can describe evolution of the equilibrium velocity distribution functions of the particles as consequences of wave-particle interactions. One of the well-known phenomena described by the quasi-linear theory is a plateau formation after bump-on-tail instability (BOTI). Despite its importance in wide spectrum of the plasma physics, few experiments have exhibited clear verification of the quasi-linear theory until now. This fact stems from some difficulties in experimental realization of set ups which meet requirements of the quasilinear diffusion. For example, for BOTI setting, low energy electron beams (~ a few times the bulk electron temperature T_e) having sufficient intensity are necessary. In addition, high frequency electrostatic wave measurement and measurement of electron velocity distribution function (f_e (v)) with satisfactory resolution are inevitable for the verification. This lack of the experimental verification becomes our motivation to this research.

The purpose of this research is verification of the quasi-linear theory by means of experiment. Our idea for producing a BOTI condition is to inject a low- energy electron beam (~ 2−10T_e) into target magnetized plasmas. The beam energy should range between 10 eV to 100 eV for T_e ~ 5 eV−10 eV of our plasma. For the purpose, we have developed a low energy electron beam injector. Our beam injector consists of a filament covered with a small casing and a biasing circuit. The beam injector is immersed into the target plasma to produce a bump in f_e (v). The intensity and the energy of the electron beam are controllable. We measure f_e (v) by a Langmuir probe technique. High frequency electrostatic waves in the plasma are diagnosed by receivers (sets of monopole antennas) immersed into the plasma. We observed excitation of high frequency electrostatic waves when the electron beam was injected and a bump was created in f_e (v). The dispersion (ω-k) relation of the waves measured by the receivers shows a good agreement with the theoretical dispersion relation of Langmuir waves (LWs). The phase velocity of the LWs and the velocity of the electron beam is consistent with the prediction by the BOTI theory. The evolution of f_e (v) along the beam propagation direction (z-direction) clearly demonstrates a formation of a plateau at the velocity slightly slower than that of the injected beam, indicating that validity of the quasilinear theory.

In summary, we developed a low energy electron beam injector for verification of the quasi-linear theory. Evidences of the wave-particle interaction after BOTI was measured consistent with the prediction of the quasi-linear theory. The waves generated by BOTI is LWs. In the next step, we employ this electron beam injector for generation of a new state of LWs, called Langmuir wave supercontinuum, which has been predicted by theory and a numerical simulation.

郭名翔 jackismy40@gmail.com

We are studying a rotating plasma disk generated by two head-on rotating plasma jets colliding each other. The rotating plasma disk can be used to simulate Milky Way, Stars and Starforming regions, which are large-scale and difficult to study. We use a 1-kJ pulsed power system driving a conical-wire array which consists of four 20 µm tungsten wires, to generate plasma jet. The 1-kJ pulsed-power system consists of 20 capacitors with a total capacitance of 5 µF and is charged to 20 kV. When the system discharges, it generates a pulsed current of ~ 135 kA with a rise time of ~ 1592 ns. The plasma jet is generated by the conical-wire array with a speed of ~ 90 km/s. Further, the rotating plasma jet is generated by a twisted-conicalwire array. Furthermore, a pair of conical-wire array sitting against each other, which is called a bi-conical-wire array, will be used to generate counter-propagating plasma jets. When two plasma jets collide with other head-to-head, a rotating/non-rotating plasma disk is formed. We are using an Nd-YAG Q-switch laser with pulse width of ~ 5 ns in 532 nm for taking the timeresolved shadowgraph and schlieren images. The same laser is used for interferometry for measuring the plasma disk density. On the other hand, we are taking the time-integrated images in visible light from the top and from the side of the plasma jet and the time-integrated soft x-ray image from the side. Therefore, we will use these measurements to study the angular momentum of rotating plasma disk from two head-on colliding plasma jets. This work is supported by the Ministry of Science and Technology (MOST), Taiwan, under Award Number 105-2112-M-006-014-MY3 and 109-2112-M-006-011.

Chih Hsien Liu roger206125@gmail.com

We are building a high-voltage pulse generator. It will be used to convert a high-power 500-V DC voltage to 10-kHz AC voltage. Then, the voltage can be raised to 2-KV by using a transformer. In the future, high-power, high-voltage power sources can be used to generate plasma. It can be used as a high-power high-voltage power supply if combing with a rectifier. To convert the DC to AC voltage, the H-bridge pulse generator configuration is used. The 10-kHz controlling signal required for the pulse generator is provided by an Arduino board. The design and the status of the high-voltage pulse generator are given.

張博宇 pchang@mail.ncku.edu.tw

High-energy-density plasma is the regime where pressure is larger than 1 Mbar, i.e., ~ one million of atmosphere. It is the regime where many astrophysical phenomena take place. Inertial confinement fusions (ICF) also occur in this regime. To reach the regime, material needs to be compressed in a short period of time. The 1-kJ pulsed-power system in Pulsed-Plasma Laboratory is used for compressing materials into the HEDP regime. The 1-kJ pulsed-power system, the Pulsed-power Generator for Space science (PGS), is capable of providing a pulsed current with a peak current of ~135 kA and a rise time of ~1.6 us. A suite of diagnostics has been integrated into the system. It includes a plasma-jet velocity measurement system, time-integrated side-view, top-view cameras in visible light, a time-integrated x-ray pinhole camera, a time-resolved shadowgraphy, and a time-resolved Schlieren imaging system. The plasma can be compressed to near the HEDP regime by using either theta pinch or z pinch driven by the PGS machine. The PGS machine is and will be used to study the plasma jets for experimentally simulating the solar wind and supersonic flows in astrophysical objects. It is also used to develop the EUV light source by compressing the plasma to a high temperature.