H. Yuh

Advances in global MHD mode stabilization research on NSTX

Stabilizing modes that limit plasma beta and reduce their deleterious effect on plasma rotation are key goals for the efficient operation of a fusion reactor. Passive stabilization and active control of global kink/ballooning modes and resistive wall modes (RWMs) have been demonstrated on NSTX and research is now advancing towards understanding the stabilization physics and reliably maintaining the high beta plasma for confident extrapolation to ITER and a fusion component test facility based on the spherical torus. Active n = 1 control experiments with an expanded sensor set, combined with low levels of n = 3 field phased to reduce error fields, reduced resonant field amplification and maintained plasma rotation, exceeded normalized beta = 6 and produced record discharge durations limited by magnet system constraints. Details of the observed RWM dynamics during active control show the mode being converted to a rotating kink that stabilizes or saturates and may lead to tearing modes. Discharges with rotation reduced by n = 3 magnetic braking suffer beta collapse at normalized beta = 4.2 approaching the no-wall limit, while normalized beta greater than 5.5 has been reached in these plasmas with n = 1 active control, in agreement with the single-mode RWM theory. Advanced state-space control algorithms proposed for RWM control in ITER theoretically yield significant stabilization improvements. Values of relative phase between the measured n = 1 mode and the applied correction field that experimentally produce stability/instability agree with RWM control modelling. Experimental mode destabilization occurs over a large range of plasma rotation, challenging the notion of a simple scalar critical rotation speed defining marginal stability. Stability calculations including kinetic modifications to the ideal MHD theory are applied to marginally stable experimental equilibria. Plasma rotation and collisionality variations are examined in the calculations. Intermediate rotation levels are less stable, consistent with experimental observations. Trapped ion resonances play a key role in this result. Recent experiments have demonstrated magnetic braking by non-resonant n = 2 fields. The observed rotation damping profile is broader than found for n = 3 fields. Increased ion temperature in the region of maximum braking torque increases the observed rate of rotation damping, consistent with the theory of neoclassical toroidal viscosity at low collisionality.

High Harmonic Fast Wave Heating Efficiency Enhancement and Current Drive at Longer Wavelength on the National Spherical Torus Experiment

High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] for supporting startup and sustainment of the ST plasma. Considerable enhancement of the core heating efficiency (η) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap φ = -90o, kφ = -8 m-1) by increasing the magnetic field from 4.5 kG to 5.5 kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (nonset ∝ ΒΦ× k|| 2/w) away from the antenna face and wall, and hence reducing the propagating surface wave fields. RF waves propagating close to the wall at lower BΦ and k|| can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations

Simulation of microtearing turbulence in national spherical torus experimenta)

Thermal energy confinement times in National Spherical Torus Experiment (NSTX) dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future spherical tokamak (ST) devices at much lower collisionality. Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport (∼98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling....

Momentum transport in electron-dominated NSTX spherical torus plasmas

The National Spherical Torus Experiment (NSTX) operates between 0.35 and 0.55?T, which, when coupled to up to 7?MW of neutral beam injection, leads to central rotation velocities in excess of 300?km?s?1 and E ? B shearing rates up to 1?MHz. This level of E ? B shear can be up to a factor of five greater than typical linear growth rates of long-wavelength ion (e.g. ITG) modes, at least partially suppressing these instabilities. Evidence for this turbulence suppression is that the inferred diffusive ion thermal flux in NSTX H-modes is often at the neoclassical level, and thus these plasmas operate in an electron-dominated transport regime. Analysis of experiments using n = 3 magnetic fields to change plasma rotation indicate that local rotation shear influences local transport coefficients, most notably the ion thermal diffusivity, in a manner consistent with suppression of the low-k turbulence by this rotation shear. The value of the effective momentum diffusivity, as inferred from steady-state momentum balance, is found to be larger than the neoclassical value. Results of perturbative experiments indicate inward pinch velocities of up to 40?m?s?1 and perturbative momentum diffusivities of up to 4?m2?s?1, which are larger by a factor of several than those values inferred from steady-state analysis. The inferred pinch velocity values are consistent with values based on theories in which low-k turbulence drives the inward momentum pinch. Thus, in NSTX while the neoclassical ion energy transport effects can be relatively high and dominate the ion energy transport, the neoclassical momentum transport effects are near zero, meaning that transport of momentum is dominated by any low-k turbulence that exists.

Experimental studies on fast-ion transport by Alfven wave avalanches on the National Spherical Torus Experiment

Fast-ion transport induced by Alfven eigenmodes (AEs) is studied in beam-heated plasmas on the National Spherical Torus Experiment [Ono et al., Nucl. Fusion 40, 557 (2000)] through space, time, and energy resolved measurements of the fast-ion population. Fast-ion losses associated with multiple toroidicity-induced AEs (TAEs), which interact nonlinearly and terminate in avalanches, are characterized. A depletion of the energy range >20 keV, leading to sudden drops of up to 40% in the neutron rate over 1 ms, is observed over a broad spatial range. It is shown that avalanches lead to a relaxation of the fast-ion profile, which in turn reduces the drive for the instabilities. The measured radial eigenmode structure and frequency of TAEs are compared with the predictions from a linear magnetohydrodynamics stability code. The partial disagreement suggests that nonlinearities may compromise a direct comparison between experiment and linear theory.

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

The 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. The 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 BT dependence, with a current dependence that is weaker than that observed at higher aspect ratio. Dimensionless scaling experiments indicate a strong increase in confinement with decreasing collisionality and a weak degradation with beta. The increase in confinement with BT 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 in 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 electron temperature gradient (ETG) modes at the lowest BT. 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 ion temperature gradient/TEM regime up to the ETG regime.

Relationship between onset thresholds, trigger types and rotation shear for the m/n = 2/1 neoclassical tearing mode in a high-β spherical torus

The onset conditions for the m/n = 2/1 neoclassical tearing mode are studied in terms of neoclassical drive, triggering instabilities, and toroidal rotation or rotation shear, in the spherical torus NSTX (Ono M. et al 2000 Nucl. Fusion 40 557). There are three typical onset conditions for these modes, given in order of increasing neoclassical drive required for mode onset: triggering by energetic particle modes, triggering by edge localized modes and cases where the modes appear to grow without a trigger. In all cases, the required drive increases with toroidal rotation shear, implying a stabilizing effect from the shear.

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

Linear gyrokinetic simulations are performed based on a high collisionality NSTX discharge that is part of dimensionless confinement scaling studies. In this discharge, the microtearing mode is predicted to be unstable over a significant region of the plasma (r/a = 0.5–0.8), motivating comprehensive tests to verify the nature of the mode and how it scales with physical parameters. The mode is found to be destabilized with sufficient electron temperature gradient, collisionality, and beta, consistent with previous findings and simple theoretical expectations. Consistent with early slab theories, growth rates peak at a finite ratio of electron-ion collision frequency over mode frequency, νe/i/ω ∼ 1–6. Below this peak, the mode growth rate decreases with reduced collisionality, qualitatively consistent with global confinement observations. Also, in this region, increased effective ionic charge (Zeff) is found to be destabilizing. Experimental electron beta and temperature gradients are two to three times lar...

Modeling fast-ion transport during toroidal Alfvén eigenmode avalanches in National Spherical Torus Experiment

Experiments on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] found strong bursts of toroidal Alfven eigenmode (TAE) activity correlated with abrupt drops in the neutron rate. A fairly complete data set offers the opportunity to benchmark the NOVA [C. Z. Cheng, Phys. Rep. 211, 1 (1992)] and ORBIT [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)] codes in the low aspect ratio tokamak (ST) geometry. The internal structure of TAE was modeled with NOVA and good agreement is found with measurements made with an array of five fixed-frequency reflectometers. The fast-ion transport resulting from these bursts of multiple TAE was then modeled with the ORBIT code. The simulations are reasonably consistent with the observed drop in neutron rate, however, further refinements in both the simulation of the TAE structure and in the modeling of the fast-ion transport are needed. Benchmarking stability codes against present experiments is an important step in developing the...

High spatial sampling global mode structure measurements via multichannel reflectometry in NSTX

Global modes?including kinks and tearing modes (f ~ 400?kHz)?play critical roles in many aspects of plasma performance. Their investigation on NSTX is aided by an array of fixed-frequency quadrature reflectometers used to determine their radial density perturbation structure. The array has been recently upgraded to 16 channels spanning 30?75?GHz (ncutoff = (1.1?6.9) ? 1019?m?3 in O-mode), improving spatial sampling and access to the core of H-mode plasmas. The upgrade has yielded significant new results that advance the understanding of global modes in NSTX. The GAE and CAE structures have been measured for the first time in the core of an NSTX high-power (6?MW) beam-heated H-mode plasma. The CAE structure is strongly core-localized, which has important implications for electron thermal transport. The TAE structure has been measured with greatly improved spatial sampling, and measurements of the TAE phase, the first in NSTX, show strong radial variation near the midplane, indicating radial propagation caused by non-ideal MHD effects. Finally, the tearing mode structure measurements provide unambiguous evidence of coupling to an external kink.