Scenario development during commissioning operations on the National Spherical Torus Experiment Upgrade

Abstract

The National Spherical Torus Experiment Upgrade (NSTX-U) will advance the physics basis required for achieving steady-state, high-beta, and high-confinement conditions in a tokamak by accessing high toroidal fields (1 T) and plasma currents (1.0–2.0 MA) in a low aspect ratio geometry (A = 1.6–1.8) with flexible auxiliary heating systems (12 MW NBI, 6 MW HHFW). This paper describes the progress in the development of L- and H-mode discharge scenarios and the commissioning of operational tools in the first ten weeks of operation that enable the scientific mission of NSTX-U. Vacuum field calculations completed prior to operations supported the rapid development and optimization of inductive breakdown at different values of ohmic solenoid current. The toroidal magnetic field (BT0 = 0.65 T) exceeded the maximum values achieved on NSTX and novel long-pulse L-mode discharges with regular sawtooth activity exceeded the longest pulses produced on NSTX (tpulse > 1.8 s). The increased flux of the central solenoid facilitated the development of stationary L-mode discharges over a range of density and plasma current (Ip). H-mode discharges achieved similar levels of stored energy, confinement (H98y,2 > 1) and stability (βN/βN-nowall > 1) compared to NSTX discharges for Ip ⩽ 1 MA. High-performance H-mode scenarios require an L–H transition early in the Ip ramp-up phase in order to obtain low internal inductance (li) throughout the discharge, which is conducive to maintaining vertical stability at high elongation (κ > 2.2) and achieving long periods of MHD quiescent operations. The rapid progress in developing L- and H-mode scenarios in support of the scientific program was enabled by advances in real-time plasma control, efficient error field identification and correction, effective conditioning of the graphite wall and excellent diagnostic availability.

Plasma boundary shape control and real-time equilibrium reconstruction on NSTX-U

“…Due to its importance for operations and scenario development, establishment of reliable feedback control of the plasma position and the shape of the plasma boundary was one of the first commissioning activities during initial plasma operations. By enabling accurate boundary control and repeatable discharge evolution, the system contributed to many milestones during the first NSTX-U campaign, including achieving scenarios of up to 1 MA, 0.65 T, and discharges with a 2 s pulse length [2,3], as well as high beta discharges calculated to exceed the no-wall stability limit [36].…”

“…Therefore, active control of X-point positions and δr sep enables optimization of the discharge shaping for triggering L-H trans ition. Reliably realizing L-H transition early in discharges when neutral beam absorption is low is critical for developing highperformance H-mode discharges on NSTX-U [3].…”

“…The necessary modifications were small, on the order of the requested change in inner gap. This new capability will enable improved reproducibility of the inner gap evolution during the ramp-up phase of discharges, which was found to be critical for achieving reliable H-mode access [3].…”

“…The National Spherical Torus eXperiment Upgrade facility (NSTX-U) [1], which completed its first plasma operation campaign in 2016 [2,3], aims to span between the previous class of spherical torus devices, like NSTX [4] or the megaampere spherical tokamak (MAST) [5], and future facilities planned to study plasma-material interaction [6], nuclear comp onents [7], and demonstration of fusion power production [8,9]. NSTX-U looks to build upon the results of NSTX [10] to improve the physics understanding of several key issues for future devices, including the scaling of electron transport with field and current [11][12][13][14], the physics of fast particles [15][16][17][18], and the achievement and sustainment of non-inductive, high-β scenarios [19][20][21][22][23][24].…”

Compact steady-state tokamak performance dependence on magnet and core physics limits

“…The leading NSTX confinement scaling coefficient is chosen such that the ITER and ST energy confinement times are identical for a reference NSTX scenario defined by A = 1.5, R 0 = 0.86 m, I P = 0.75 MA, B T = 0.5 T , P NBI = 4 MW and f GW = 1.0 consistent with a total β N = 4.4 using the 0D scaling methodology outlined in [26]. Other similar ST scaling expressions are of course possible, and obtaining a more definitive ST confinement scaling at reduced collisionality and higher magnetic field and current is a major research goal for both NSTX Upgrade [26,40–43] and MAST Upgrade [44–46].…”