NSTX/NSTX-U theory, modeling and analysis results

Abstract

The mission of the spherical tokamak NSTX-U is to explore the physics that drives core and pedestal transport and stability at high- and low collisionality, as part of the development of the spherical tokamak (ST) concept towards a compact, low-cost ST-based pilot plant. NSTX-U will ultimately operate at up to 2 MA and 1 T with up to 12 MW of neutral beam injection power for 5 s. NSTX-U will operate in a regime where electromagnetic instabilities are expected to dominate transport, and beam-heated NSTX-U plasmas will explore a portion of energetic particle parameter space that is relevant for both -heated conventional and low aspect ratio burning plasmas. NSTX-U will also develop the physics understanding and control tools to ramp-up and sustain high performance plasmas in a fully-noninductive fashion. NSTX-U began research operations in 2016, but a failure of a divertor magnetic field coil after ten weeks of operation resulted in the suspension of operations and initiation of recovery activities. During this period, there has been considerable work in the area of analysis, theory and modeling of data from both NSTX and NSTX-U, with a goal of understanding the underlying physics to develop predictive models that can be used for high-confidence projections for both ST and higher aspect ratio regimes. These studies have addressed issues in thermal plasma transport, macrostability, energetic particlet-driven instabilities at ion-cyclotron frequencies and below, and edge and divertor physics.

Hybrid simulations of sub-cyclotron compressional and global Alfvén eigenmode stability in spherical tokamaks

“…Hence, these conditions place constraints on the beam parameters (λ 0 , v 0 /v A , as well as Δλ for co-CAEs) and mode properties for a mode to be driven unstable by a neutral beam distribution. Similarly to our previous analysis, equations (9) and (10) demonstrate that cntr-GAEs and co-GAEs have complementary instability conditions resulting from opposing signs of in the resonances driving these modes. Consequently, co-GAE excitation is favored when λ 0 is small, while cntr-GAEs prefer large λ 0 .…”

Physics-guided machine learning approaches to predict the ideal stability properties of fusion plasmas

“…Disruption of the plasma current leading to thermal and current quenches, halo currents [53,54], and heat and electromagnetic forces on the device structure were tolerated in NSTX experiments as the energy in the disruptions was not high enough to cause damage to the device. A reduced kinetic model for resistive wall mode stability based upon theoretical understanding has already been included in DECAF [2]. Here we will briefly describe the kinetic model and its underlying ideal stability components.…”

“…The DCON stability code was developed to calculate A c c e p t e d M a n u s c r i p t the fluid δW terms and was previously used for thousands of calculations spanning the NSTX operating space [7]. Analysis of these thousands of calculations formed the basis of an analytical model for the fluid δW terms that was incorporated into the DECAF code [2]. Improvements to that model now by means of machine learning is examined in this paper.…”

Benchmarking the Scrape-Off-Layer Fast Ion (SOLFI) particle tracer code in a collisionless magnetic mirror with electrostatic potential drop

“…Spherical Tokamaks (STs) are an attractive design option for a fusion reactor that promises significant cost reduction because of its compact size and denser confined plasmas [1][2][3][4]. However, there are still open questions regarding ST physics, particularly regarding the physics of fast ions that are produced through fusion reactions or neutral beam injection (NBI) [5,6].…”