Advancing understanding and predictive capability for fast-ion driven instabilities and associated anomalous transport in NSTX-U

An experimental effort is proposed to investigate fast-ion driven instabilities and associated anomalous transport through experiments in the National Spherical Torus eXperiment - Upgrade (NSTX-U) and, in collaboration with the NSTX-U Team, to validate the physics models of leading codes that simulate the processes involved: the Gyrokinetic Toroidal Code (GTC) [Lin Science 1998] and the Hybrid MHD code (HYM) [Belova PRL 2015]. Fast ions from neutral beams excite instabilities, or modes, across a broad range of frequencies that cause fast-ion transport, critically impacting heating and current drive, and also cause performance limiting energy transport. GTC is a leading code for low frequency modes (ω/ωci << 1), while HYM is the leading code for high frequency modes (~ 0.1 ≤ ω/ωci < 1). GTC has been recently extended to simulate high frequency modes, as well, and thus a closely linked theoretical effort is proposed for verification of GTC for high frequency modes with ~ 0.1 ≤ ω/ωci < 1 by benchmarking with HYM and linear theory. This verification effort lays a critical foundation for experimental validation of GTC for high frequency modes. The experimental effort, led by UCLA PI Dr. Neal A. Crocker, and the theoretical effort (led by UCI PI Prof. Zhihong Lin, author of GTC) will be performed in close collaboration with Dr. Elena Belova of PPPL, the author of HYM. Dr. Crocker will be the Lead PI.

The objectives of the proposed research are to: (1) Experimentally advance understanding of anomalous fast-ion and energy transport by fast-ion driven instabilities across a broad range of frequencies in spherical tokamaks in the new parameter regime opened up by NSTX-U capabilities (higher IP, BT, β, beam power, off-axis injection) and (2)Advance predictive capability by (a)experimental validation of physics models in GTC and HYM for simulating fast-ion driven instabilities and their fast-ion and energy transport, and (b) verification of GTC with HYM and linear theory in the high frequency regime.

The proposed research also has an educational and fusion workforce development objective. Graduate students at both UCLA and UCI will participate, in fulfillment of the research requirements for a Ph.D. Thesis.

The proposed research exploits the unique capabilities of NSTX-U (higher PNB and BT, and new tangential beam) to advance the frontiers of ST research. It addresses multiple high priority research topics of the NSTX-U program, which seeks to extend the investigation of the favorable scaling of confinement with reduced collisionality in the ST configuration [Kaye NF 2007, Valovic NF 2011] to lower collisionality and to develop high-performance steady-state non-inductively sustained regimes.

One topic is the impact of fast-ion driven mode-induced energy transport on the temperature profiles. High frequency CAE and GAE activity correlated with Te profile broadening in NSTX [Stutman PRL 2009, Crocker PPCF 2011], increasing as PNBincreased. Te profiles also broadened as BT increased [Kaye NF, 2007], raising a question about the role of CAEs and GAEs. The correlation of Te profile broadening with BT is believed to play a critical role in the favorable ST scaling of confinement with collisionality [Kaye NF 2013]. The tangential beam can suppress GAEs [Fredrickson PRL 2017], and possibly CAEs, offering a valuable opportunity to test their role in Te profile broadening. Recently, low frequency modes have also been implicated in energy transport, raising questions about their impacts on temperature profile.

Fast-ion transport is also a high priority topic in the NSTX-U program. Low frequency modes, including fishbones and TAEs, have long been known to cause significant fast-ion transport, impacting development of advanced tokamak scenarios. Recently, it has emerged that CAEs and GAEs can also cause fast-ion transport, with relatively unexplored consequences. Understanding of fast-ion transport, along with a validated predictive capability, are critical to developing strategies to mitigate, or exploit, those effects to achieve high-performance steady-state non-inductively sustained regimes.

Advances in understanding and predictive capability for CAEs and GAEs also contribute to the physics basis for diagnosis of fast ions in burning plasmas. CAEs, GAEs and the closely coherent ion cyclotron emission (ICE), can provide useful information about the fast-ion distribution, and can be detected via non-invasive diagnostics that are robust in a burning plasma environment [K.G. McClements, NF 2015].