EP H-mode on NSTX

Devon Battaglia

Princeton Plasma Physics Laboratory

Thursday, September 30, 2021

3:00pm

Virtual

PSFC Seminars

Abstract: Spherical tokamaks, such as the National Spherical Torus Experiment (NSTX), leverage low aspect ratio to achieve large bootstrap current fraction (fBS ≥ 0.5) at large Greenwald density fraction (f_GW > 0.7) with potentially beneficial stability and confinement properties for future steady-state reactor concepts.  For example, the critical pressure gradient for kinetic ballooning mode (KBM) instabilities is reduced at low aspect ratio leading to H-mode pedestals that can be wider compared to conventional aspect ratio tokamaks.  It was observed on NSTX that at sufficiently low collisionality the entire pressure profile is limited by the KBM below the resistive kink-peeling stability limit leading to a robustly ELM-free regime. At the lowest collisionalities, the discharges transitioned to the ELM-free Enhanced Pedestal (EP) H-mode regime with a significant increase in the edge ion temperature and rotation gradients, a beneficial decrease in the impurity accumulation relative to standard ELM-free regimes and the achievement of the largest normalized energy confinement on NSTX (H_98y,2 > 1.3). EP H-mode is the result of a positive feedback interaction between the neoclassical ion energy transport and instability-driven (such as KBM) particle transport.  The unique ELM-free regimes at low aspect ratio expand the physics basis needed to optimize steady-state reactor designs that require simultaneous achievement of high energy confinement and bootstrap current without ELMs.

Bio: Dr. Devon Battaglia is a Research Physicist at the Princeton Plasma Physics Laboratory (PPPL) where he leads both the Physics Operations Group and the Advanced Scenarios and Control Science Group at the NSTX-U experiment. His current research activities include H-mode pedestal transport, plasma initiation physics and non-inductive scenario development in spherical tokamaks. Dr. Battaglia earned a B.S. (Engineering Physics) and B.A. (Music) from Lehigh University and a Ph.D. (Engineering Physics) from the University of Wisconsin-Madison before joining Oak Ridge National Laboratory under a DOE Postdoctoral Fellowship.  He is a member of the ITPA Integrated Operational Scenarios (IOS) Topical Group, Leader of the US BPO Integrated Scenarios Topical Science Group and is leading the FY22 Joint Research Target (JRT) on “Intrinsically non-ELMing enhanced-confinement regimes.”