Exploration of the equilibrium operating space for NSTX-Upgrade

Abstract

This paper explores a range of high-performance equilibrium scenarios achievable with neutral beam heating in the NSTX-Upgrade device (Menard J.E. 2012 Nucl. Fusion 52 083015). NSTX-Upgrade is a substantial upgrade to the existing NSTX device (Ono M. et al 2000 Nucl. Fusion 40 557), with significantly higher toroidal field and solenoid capabilities, and three additional neutral beam sources with significantly larger current-drive efficiency. Equilibria are computed with free-boundary TRANSP, allowing a self-consistent calculation of the non-inductive current-drive sources, the plasma equilibrium and poloidal-field coil currents, using the realistic device geometry. The thermal profiles are taken from a variety of existing NSTX discharges, and different assumptions for the thermal confinement scalings are utilized. The no-wall and ideal-wall n = 1 stability limits are computed with the DCON code. The central and minimum safety factors are quite sensitive to many parameters: they generally increase with large outer plasma-wall gaps and higher density, but can have either trend with the confinement enhancement factor. In scenarios with strong central beam current drive, the inclusion of non-classical fast-ion diffusion raises qmin, decreases the pressure peaking, and generally improves the global stability, at the expense of a reduction in the non-inductive current-drive fraction; cases with less beam current drive are largely insensitive to additional fast-ion diffusion. The non-inductive current level is quite sensitive to the underlying confinement and profile assumptions. For instance, for BT = 1.0 T and Pinj = 12.6 MW, the non-inductive current level varies from 875 kA with ITER-98y,2 thermal confinement scaling and narrow thermal profiles to 1325 kA for an ST specific scaling expression and broad profiles. Scenarios are presented which can be sustained for 8–10 s, or (20–30) τCR, at βN = 3.8–4.5. The value of qmin can be controlled at either fixed non-inductive fraction of 100% or fixed plasma current, by varying which beam sources are used, opening the possibility for feedback control of the current profile. In terms of quantities like collisionality, neutron emission, non-inductive fraction, or stored energy, these scenarios represent a significant performance extension compared with NSTX and other present spherical torii.

Full wave simulations of fast wave efficiency and power losses in the scrape-off layer of tokamak plasmas in mid/high harmonic and minority heating regimes*

“…Nonetheless, AORSA simulations, presented in this work, show similar results between these two cases (see section 2 and subsection 3.1). In order to further verify the effect of the magnetic field pitch angle has on the behavior of the RF electric field and the associated power lost in the SOL, different NSTX-U magnetic equilibria have been generated by the stand-alone free-boundary equilibrium ISOLVER code (some information on this code can be found in [32]) for B T,0 = 1 T and I p = 0.5, 1.0, 1.6, and 2 MA. Figure 10 shows the amplitude of the RF electric field for these cases with n φ = −21 and n ant = 5.0 × 10 18 m −3 , a density at which the FW can propagate in the SOL region and large RF field together with high SOL power losses have been found in the "standard" NSTX-U with B T,0 = 1 T and I p = 1 MA (see Figure 5 in [24] Table 1.…”

Progress in simulating turbulent electron thermal transport in NSTX

“…Consistent with the interpretation of stochastic transport, there is negligible particle, ion thermal or momentum transport as ions are much heavier than electrons (v ||,i <<v ||,e ). The magnetic fluctuations are strongly localized to the outboard side and calculations using a synthetic diagnostic approach predict measurable phase shifts from a polarimetry diagnostic [69,70] to be installed on NSTX-Upgrade [58,59]. The density perturbations exhibit narrow radial corrugations (also distinct from ITG, TEM), which is directly coupled to the rational surface separation ( r rat =1/nq =1/k s) of the dominant toroidal modes in the nonlinear spectra.…”

A megawatt-level 28 GHz heating system for the National Spherical Torus Experiment Upgrade

“…GENRAY-ADJ [10] numerical simulations were run for two I p = 1.2 MA, B T (0) = 1 T, 100% NI NSTX-U NBI H-mode cases, one with broad n e and T e profiles and the other with narrow profiles [11] [Fig. 8].…”

“…4). Microwave power will be transmitted to NSTX-U via a 50 mm diameter, low-loss HE 11 corrugated circular waveguide. Either an existing modular insulated-gate bipolar transistor power supply will be used as the main power supply for the gyrotron or a new similar power supply will be purchased.…”