FY2022 JRT and Research Milestones

Joint Research Target: Intrinsically non-ELMing Enhanced-confinement regimes

Enhanced energy confinement regimes that exhibit steady operation and intrinsically avoid edge localized mode (ELM) bursts through alternative pedestal transport mechanisms provide a path to high fusion performance without potentially damaging transient ELM heat and particle fluxes to plasma facing components. Coordinated analysis of experiments on multiple tokamaks spanning the operational spaces and measurement capabilities of DIII-D, Alcator C-Mod, and NSTX will advance the physics basis for intrinsically non-ELMing enhanced confinement regimes (Wide Pedestal Quiescent H-Mode, Standard Quiescent H-Mode, Enhanced Pedestal H-mode, I-mode, and other regimes). Experiments will be conducted to explore and expand operating spaces, develop predictive capability, improve core-edge integration, and optimize for future application to burning plasmas. A multi-machine database will be created to support 0-D projections to next-step devices. Analysis and simulation will compare unique aspects of these regimes, such as edge micro- and macro-stability and key features of core and edge turbulence and transport, as well as sourcing and open field-line transport of impurities from divertor targets and main chamber wall. Prospects for application in future burning plasma devices will be examined to identify issues for further experiments and simulation and any new capabilities needed.

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

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.