Scenario development during commissioning operations on the National Spherical Torus Experiment Upgrade. (20th February 2018)

• Record Type: Journal Article
• Authors: Battaglia, D.J.; Boyer, M.D.; Gerhardt, S.; Mueller, D.; Myers, C.E.; Guttenfelder, W.; Menard, J.E.; Sabbagh, S.A.; Scotti, F.; Bedoya, F.; Bell, R.E.; Berkery, J.W.; Diallo, A.; Ferraro, N.; Kaye, S.M.; Jaworski, M.A.; LeBlanc, B.P.; Ono, M.; Park, J.-K.; Podesta, M.; Raman, R.; Soukhanovskii, V.
• Other Names: collab.
• Is Part Of: Nuclear fusion. Volume 58:Number 4(2018:Apr.)
• Publication Date: 2018-02-20
• Subjects: spherical tokamak -- integrated scenarios -- start-up -- wall conditioning -- real-time control -- error field correction -- NSTX-U; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aaa6e0
• Languages: English
• ISSNs: 0029-5515

Abstract

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 ( B T0 = 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 ( t pulse > 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 ( I p ). H-mode discharges achieved similar levels of stored energy, confinement (H98y, 2 > 1) and stability ( β N / β N-nowall > 1) compared to NSTX discharges for I p ⩽ 1 MA. High-performance H-mode scenarios require an L–H transition early in the I p ramp-up phase in order to obtain low internal inductance ( l i ) 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.