NSTX-U theory, modeling and analysis results

Abstract

The mission of the low aspect ratio spherical tokamak NSTX-U is to advance the physics basis and technical solutions required for optimizing the configuration of next-step steady-state tokamak fusion devices. NSTX-U will ultimately operate at up to 2 MA of plasma current and 1 T toroidal field on axis for 5 seconds, and has available up to 15 MW of Neutral Beam Injection (NBI) power at different tangency radii and 6 MW of High Harmonic Fast Wave (HHFW) heating. With these capabilities NSTX-U will develop the physics understanding and control tools to ramp-up and sustain high performance fully non-inductive plasmas with large bootstrap fraction and enhanced confinement enabled via the low aspect ratio, high beta configuration. With its unique capabilities, NSTX-U research also supports ITER and other critical fusion development needs. Super-Alfvénic ions in beam-heated NSTX-U plasmas access energetic particle parameter space that is relevant for both -heated conventional and low aspect ratio burning plasmas. NSTX-U can also generate very large target heat fluxes to test conventional and innovative plasma exhaust and plasma facing component (PFC) solutions. This paper summarizes recent analysis, theory and modelling progress to advance the tokamak physics basis in the areas of macrostability and 3D fields, energetic particle stability and fast ion transport, thermal transport and pedestal structure, boundary and plasma material interaction, RF heating, scenario optimization and real-time control.

On the origin of high harmonic fast wave edge losses in NSTX

“…Being limited to low-k HHFW heating regimes, there is little or no danger that this type of resonant surface wave excitation on filaments will pose a problem for future tokamaks such as ITER, or even NSTX-U, which will operate at double the magnetic field strength of NSTX [56]. where φ s is the phasing of strap s. The straps are connected in pairs such that φ s+6 = φ s + π [57].…”

ETG turbulent transport in the Mega Ampere Spherical Tokamak (MAST) pedestal

“…When the temperature gradient a/L T0 is increased by 20%, the growth rates at all scales are also increased by approximately 20%. Note that it is also observed in other machines such as NSTX [7], DIII-D [16], JET [30], etc, that ETG turbulence is sensitive to temperature gradient. The temperature gradient sensitivity The On the Growth rate spectrum at ρtor = 0.94 during the pre-ELM stage.…”

Observation of a new pedestal stability regime in MAST Upgrade H-mode plasmas

“…It has been noted that both the peeling-ballooning and KBM calculations become more challenging at low aspect ratio, due to strong coupling of poloidal modes, and highly shaped geometry [22]. Kinetic ballooning mode calculations in low aspect ratio devices, using both gyrokinetic and high-n MHD proxy calculations, have previously found that fully developed pedestals in these devices are typically near KBM criticality [22,[36][37][38], though in some cases non-local effects need to be accounted for [39]. Similarly, ELM onset is observed to occur near predicted peeling-ballooning mode onset, though in some cases with a lower growth rate threshold than would be expected due to diamagnetic stabilization, with the importance of resistive effects hypothesized as a possible cause [40].…”