Confinement and local transport in the National Spherical Torus Experiment (NSTX)

Abstract

NSTX operates at low aspect ratio (R/a∼1.3) and high beta (up to 40%), allowing tests of global confinement and local transport properties that have been established from higher aspect ratio devices. NSTX plasmas are heated by up to 7 MW of deuterium neutral beams with preferential electron heating as expected for ITER. Confinement scaling studies indicate a strong B T dependence, with a current dependence that is weaker than that observed at higher aspect ratio. Dimensionless scaling experiments indicate a strong increase of confinement with decreasing collisionality and a weak degradation with beta. The increase of confinement with B T is due to reduced transport in the electron channel, while the improvement with plasma current is due to reduced transport in the ion channel related to the decrease in the neoclassical transport level. Improved electron confinement has been observed in plasmas with strong reversed magnetic shear, showing the existence of an electron internal transport barrier (eITB). The development of the eITB may be associated with a reduction in the growth of microtearing modes in the plasma core. Perturbative studies show that while L-mode plasmas with reversed magnetic shear and an eITB exhibit slow changes of L T e across the profile after the pellet injection, H-mode plasmas with a monotonic q-profile and no eITB show no change in this parameter after pellet injection, indicating the existence of a critical gradient that may be related to the q-profile. Both linear and non-linear simulations indicate the potential importance of ETG modes at the lowest B T . Localized measurements of high-k fluctuations exhibit a sharp decrease in signal amplitude levels across the L-H transition, associated with a decrease in both ion and electron transport, and a decrease in calculated linear microinstability growth rates across a wide k-range, from the ITG/TEM regime up to the ETG regime.

Simulation Of Microtearing Turbulence In NSTX

“…Above this threshold the transport roughly doubles over the range β e ≈7-13% (0.8-1.5×β e,exp ). While this is qualitatively consistent with NSTX and MAST experimental results [36,40], the degradation is much stronger than the experimentally determined scaling exponent, (B T τ E ) ~β e -0.1 . In all the above scans the transport predictions qualitatively follow the linear stability analysis [11].…”

“…7 implies a confinement time that, for fixed ρ * and β, should scale similar to those deduced from dedicated dimensionless scaling experiments in both NSTX [36][37][38] and MAST [39,40], (Ω i τ E )…”

“…The Eulerian gyrokinetic code GYRO [50][51][52][53] is used for both linear and non-linear simulations which are based on a high collisionality NSTX discharge that is part of energy confinement scaling studies [36,37] toroidal beta β tor =19%). Many of the properties of the linear microtearing mode and scaling for this case are described in [11].…”

“…Dedicated dimensionless scaling studies in both NSTX [36][37][38] and MAST [39,40] find that the normalized energy confinement time exhibits a strong inverse scaling with collisionality, (Ωτ E ) ~ν *e -(0. 8-0.95) .…”

Overview of the physics and engineering design of NSTX upgrade

“…sustainment in the spherical torus/tokamak (ST). In particular, NSTX [3] and MAST [4] observe a strong (nearly inverse) scaling of normalized confinement with ν * . An example of this scaling is show in Figure 1 for NSTX experiments in which the plasma q, β, and ρ * were approximately fixed as the electron collisionality ν * e was varied by a factor of 3.…”