FY2022 Research Milestones

R(22-1): Pedestal structure prediction based on non-ideal MHD and gyrokinetics

Description: The EPED model for pedestal structure is based on two key constraints: (1) ideal MHD peeling-ballooning modes limit the maximum achievable pressure, (2) kinetic ballooning modes limit the maximum pressure gradient (modeled using ideal MHD infinite-n ballooning modes as a proxy). However, EPED has been unsuccessful in predicting pedestal structure in spherical tokamaks like NSTX and MAST. Recent analysis has highlighted a few ways in which the assumptions within EPED (based on ideal MHD) are insufficient to represent NSTX pedestal structure. First, new MHD simulations of peeling-ballooning (P-B) stability predict that resistivity significantly increases P-B growth rates and alters the current and pressure gradient stability boundaries. Second, linear gyrokinetic analysis has found that a variety of NSTX H-mode pedestals are within 10% of pressure gradient thresholds for KBM, even for cases where the EPED “boundary critical pedestal” transport KBM constraint fails to reproduce pedestal width scaling (such as lithtiated, low-recycling, wide-pedestal ELM-free cases). Following the EPED approach, this milestone will combine the non-ideal MHD simulations and gyrokinetic simulations to produce generalized pedestal structure predictions. Using model equilibrium and parameterized profile fits, the scaling of pedestal height vs. width will be mapped out for both ideal and non-ideal peeling-ballooning stability to investigate the difference resistivity and other non-ideal effects make. New profile parameterizations will be tested to enable evaluation of the wide-pedestal cases. Gyrokinetic stability will also be used to map out a similar relationship based on KBM thresholds. Together, the two approaches will be combined to uniquely predict pedestal height and width, and to illustrate how non-ideal MHD and kinetic effects alter pedestal structure predictions for NSTX/-U beyond those predicted using ideal MHD in EPED.

R(22-2): NBI+RF synergy projections to NSTX-U and impact of modified fast ion distribution function on AE stability

Description: The synergy between NBI and RF injection can result in a large distortion of the original NBI distribution function by pulling the tail to very high energies (E>>100 keV) or by causing increased prompt losses of NB ions that are displaced to loss orbits by wave-particle interactions with the RF field. The modification of the NBI distribution by RF can then lead to different stability properties for Alfvénic modes, making RF a candidate actuator to affect the mode stability. ThIs Milestone has two components. (a) The expected modifications of the NBI distribution on NSTX-U as a function of the injected RF spectrum will first be assessed via a combination of multiple modeling tools. The full wave code TORIC will be employed to provide the wave field components and the perpendicular wave vector to evaluate the RF quasi-linear diffusion coefficients, which can then be used to modify the NBI energetic particle distribution. This modeling exercise will expand the range of tools mostly tested for ICRH on other devices (JET, ASDEX-U) to the medium/high harmonic fast wave scheme envisioned for NSTX-U. (b) The impact of the modified NBI distribution on Alfvénic instabilities will be assessed through stability codes such as NOVA/NOVA-K, using the ORBIT and NUBEAM+kick model. This modeling exercise would provide initial guidance on the expected effects of the NBI+RF synergy for NSTX-U scenarios, and it would serve as a starting point to design dedicated experiments in FY23+. The RF-NBI simulation framework development and validation on NSTX-U can be invaluable moving forward to ITER scenarios where the two auxiliary heating mechanisms will be similarly important.

R(22-3): Advance liquid metal PFC concept designs for NSTX-U

Description: Liquid metal plasma-facing components present a transformative opportunity for enhanced power exhaust and possibly improved energy confinement in future fusion devices, e.g. fusion pilot plants. Central to their consideration for this use is near-term deployment and evaluation in high power tokamaks. In this activity we propose to advance design calculation for two concepts in NSTX-U: a lithium vapor box (LVB), and a fast-flowing liquid lithium coolant in a capillary porous system (CPSF). We would extend plasma and neutral transport calculations for the LVB, and also initiate a conceptual design of a CPSF system for NSTX-U. For the Lithium Vapor Box, specific tasks will include performing SOLPS calculations for the highest expected NSTX-U PFC heat flux equilibrium (~100 MW/m2). This work will involve modeling a no radiation case to compare with 0D estimates, adding in Li and C radiation with temperature dependent evaporative fluxes, comparing C and W PFCs in terms of sputtering, and adding baffles as a way of retaining impurities in the divertor. COMSOL and ANSYS calculations for a CPSF in an NSTX-U will be performed to assess surface temperature vs flow velocity, and MHD pumping costs, as a function of surface heat flux profiles. The porous layer structure will be designed to optimize heat transfer and requirements for deployed insulators will be evaluated. We will assess whether higher temperature CPSF solutions may connect to LVB concepts.

R(22-4): High Field Spherical Tokamak Studies on ST40

Description: ST40 is an operating compact high field ST that has achieved auxiliary heated plasmas with toroidal magnetic fields more than twice that of any major ST in the world (i.e., 2.1T) with an aim to reach 3T operation in FY22 to advance the company’s goal of achieving ~10 keV ion temperatures. Uniquely, ST40 is a privately funded mainline magnetic fusion facility. The fast pace and lean staffing of this private venture is unlike typical government sponsored research, which, for example, requires years of prior planning, preparation, and reviews to implement hardware changes or revise experimental run campaigns. In contrast, ST40 experimental run campaign plans and hardware modifications may be adjusted within days if there is evidence that such changes will advance the company’s goals. This different mode of operation requires a paradigm shift in approach for National Laboratory staff to fruitfully collaborate on ST40. Although this new way of operating offers challenges, it also offers tremendous opportunities. For example, high plasma currents and high ion temperatures were recently achieved in ST40 due to the expert guidance of US research and operations staff. The FY22 Research Milestone for the ST40 collaboration consists of the following elements: (1) Continue to provide operations and experimental support to enable the extension of operating parameters toward the highest possible plasma performance, (2) Conduct expert on-site evaluation and modification of existing X-ray spectroscopy, CHERS, and Thomson scattering diagnostics and continue IRTV diagnostic anlaysis and preperation to enable measurements before the end of Program 2.2 operation (targeting end of March 2022); All present NSTX-U diagnostic hardware may be temporarily repurposed to permit the immediate augmentation of existing ST40 diagnostics to achieve this goal. Initial site visits of each National Laboratory diagnostic experts should take place no later than November 30, 2021. (3) Submit a minimum of two first author manuscripts by the end of FY22 that report experimental ST40 data and analysis to peer reviewed journals for publication. (4) Aim to present initial plasma physics results at a major scientific conference before the end of CY22.