G. Rewoldt

Exploration of Spherical Torus Physics in the NSTX Device

The National Spherical Torus Experiment (NSTX) is being built at the Princeton Plasma Physics Laboratory to test the fusion physics principles for the Spherical Torus (ST) concept at the MA level. The NSTX nominal plasma parameters are R {sub 0} = 85 cm, a = 67 cm, R/a greater than or equal to 1.26, B {sub T} = 3 kG, I {sub p} = 1 MA, q {sub 95} = 14, elongation {kappa} less than or equal to 2.2, triangularity {delta} less than or equal to 0.5, and plasma pulse length of up to 5 sec. The plasma heating/current drive (CD) tools are High Harmonic Fast Wave (HHFW) (6 MW, 5 sec), Neutral Beam Injection (NBI) (5 MW, 80 keV, 5 sec), and Coaxial Helicity Injection (CHI). Theoretical calculations predict that NSTX should provide exciting possibilities for exploring a number of important new physics regimes including very high plasma beta, naturally high plasma elongation, high bootstrap current fraction, absolute magnetic well, and high pressure driven sheared flow. In addition, the NSTX program plans to explore fully noninductive plasma start-up, as well as a dispersive scrape-off layer for heat and particle flux handling.

Nonlinear gyrokinetic theory of toroidal momentum pinch

The turbulent convective flux of the toroidal angular momentum density is derived using the nonlinear toroidal gyrokinetic equation which conserves phase space density and energy [T. S. Hahm, Phys. Fluids, 31, 2670 (1988)]. A novel pinch mechanism is identified which originates from the symmetry breaking due to the magnetic field curvature. A net parallel momentum transfer from the waves to the ion guiding centers is possible when the fluctuation intensity varies on the flux surface, resulting in imperfect cancellation of the curvature drift contribution to the parallel acceleration. This mechanism is inherently a toroidal effect, and complements the k‖ symmetry breaking mechanism due to the mean E×B shear [O. Gurcan et al., Phys. Plasmas 14, 042306 (2007)] which exists in a simpler geometry. In the absence of ion thermal effects, this pinch velocity of the angular momentum density can also be understood as a manifestation of a tendency to homogenize the profile of “magnetically weighted angular momentum ...

Local transport barrier formation and relaxation in reverse-shear plasmas on the Tokamak Fusion Test Reactor

The roles of turbulence stabilization by sheared E×B flow and Shafranov shift gradients are examined for Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] enhanced reverse-shear (ERS) plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E×B and Shafranov shift effects was performed by varying the E×B shear through changes in the toroidal velocity with nearly steady-state pressure profiles. Transport and fluctuation levels increase only when E×B shearing rates are driven below a critical value that is comparable to the fastest line...

Comparison of poloidal velocity measurements to neoclassical theory on the National Spherical Torus Experimenta)

Knowledge of poloidal velocity is necessary for the determination of the radial electric field, which along with its gradient is linked to turbulence suppression and transport barrier formation. Recent measurements of poloidal flow on conventional tokamaks have been reported to be an order of magnitude larger than expected from neoclassical theory. In contrast, poloidal velocity measurements on the NSTX spherical torus [Kaye et al., Phys. Plasmas 8, 1977 (2001)] are near or below neoclassical estimates. A novel charge exchange recombination spectroscopy diagnostic is used, which features active and passive sets of up/down symmetric views to produce line-integrated poloidal velocity measurements that do not need atomic physics corrections. Inversions are used to extract local profiles from line-integrated active and background measurements. Poloidal velocity measurements are compared with neoclassical values computed with the codes NCLASS [Houlberg et al., Phys. Plasmas 4, 3230 (1997)] and GTC-NEO [Wang et...

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.

Energy confinement and thermal transport characteristics of net current free plasmas in the Large Helical Device

The energy confinement and thermal transport characteristics of net current free plasmas in regimes with much smaller gyroradii and collisionality than previously studied have been investigated in the Large Helical Device (LHD). The inward shifted configuration, which is superior from the point of view of neoclassical transport theory, has revealed a systematic confinement improvement over the standard configuration. Energy confinement times are improved over the International Stellarator Scaling 95 by a factor of 1.6 ± 0.2 for an inward shifted configuration. This enhancement is primarily due to the broad temperature profile with a high edge value. A simple dimensional analysis involving LHD and other medium sized heliotrons yields a strongly gyro-Bohm dependence (T E Ω ρ *-3.8 ) of energy confinement times. It should be noted that this result is attributed to a comprehensive treatment of LHD for systematic confinement enhancement and that the medium sized heliotrons have narrow temperature profiles. The core stored energy still indicates a dependence of T E Ω ρ *-2.6 when data only from LIED are processed. The local heat transport analysis of discharges dimensionally similar except for ρ * suggests that the heat conduction coefficient lies between Bohm and gyro-Bohm in the core and changes towards strong gyro-Bohm in the peripheral region. Since the inward shifted configuration has a geometrical feature suppressing neoclassical transport, confinement improvement can be maintained in the collisionless regime where ripple transport is important. The stiffness of the pressure profile coincides with enhanced transport in the peaked density profile obtained by pellet injection.

Measurements of ion temperature fluctuations in the Tokamak Fusion Test Reactor

First of a kind measurements of high-frequency ion temperature microturbulence in fusion-grade plasmas have been made in TFTR. The ion temperature fluctuations and carbon density fluctuations were found to have spectra similar to those of the ion density fluctuations across the plasma radius. The ratio of the relative fluctuation levels, (/T)/(/n), is 2 ? 0.5 from r/a = 0.59 to r/a = 0.99. The fact that this ratio is greater than unity is consistent with the general expectations of ion temperature gradient driven turbulence theory and suggests that ion drift modes dominate trapped electron modes in the turbulent spectrum. The temperature fluctuation spectra were found to exhibit a narrow transition region between distinctive edge and core turbulent modes, as has been seen with ion density fluctuations. The ratio of the relative fluctuation levels is greater than unity across this transition, which suggests that, despite the different modes present, the underlying instability is driven by the ion temperature gradient.

Stabilizing impact of high gradient of β on microturbulence

It is shown here that microturbulence can be stabilized in the presence of steep temperature and density profiles. Indeed in high β plasmas, pressure profile gradients are associated with high |β′|=−∂β/∂ρ, where β=P/(B2/2μ0) and ρ the square root of the toroidal flux normalized to its edge value. It is shown here that high values of |β′| have a stabilizing influence on drift modes. This may form the basis for a positive feedback loop in which high core beta values lead to improved confinement, and to further increase in β. A gyrokinetic electromagnetic flux tube code, GS2 [M. Kotschenreuther, G. Rewoldt, and W. M. Tang, Comput. Phys. Commun. 88, 128 (1995)], is used for analyzing the microstability. In high β spherical tokamak plasmas, high |β′| rather than low aspect ratio is a source of stabilization. Therefore, the effect of high |β′| should be stabilizing in the plasmas of the National Spherical Torus Experiment [Y.-K. Peng, M. G. Bell, R. E. Bell et al., Phys. Plasmas 7, 1681 (2000)].

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 particle and heat transport in JT-60U plasmas with internal transport barrier

The relationship between particle and heat transport in an internal transport barrier (ITB) has been systematically investigated in reversed shear (RS) and high βp mode plasmas of JT-60U. The electron effective diffusivity is well correlated with the ion thermal diffusivity in the ITB region. The ratio of particle flux to electron heat flux, calculated on the basis of the linear stability analysis, shows a similar tendency to an experiment in the RS plasma with a strong ITB. However, the calculated ratio of ion anomalous heat flux to electron heat flux is smaller than the experiment in the ITB region. Helium and carbon are not accumulated inside the ITB even with ion heat transport close to a neoclassical level, but argon is accumulated. The helium diffusivity (DHe) and the ion thermal diffusivity (χi) are 5–15 times higher than the neoclassical level in the high βp mode plasma. In the RS plasma, DHe is reduced from 6–7 times to a 1.4–2 times higher level than the neoclassical level when χi is reduced from 7–18 times to a 1.2–2.6 times higher level than the neoclassical level. The carbon and argon diffusivities estimated assuming the neoclassical inward convection velocity are 4–5 times larger than the neoclassical value, even when χi is close to the neoclassical level. Argon exhaust from the inside of the ITB is demonstrated by applying electron cyclotron heating (ECH) in the high βp mode plasma, where both electron and argon density profiles become flatter. The flattening of the argon density profile is consistent with the reduction of the neoclassical inward convection velocity due to the reduction of the bulk plasma density gradient. In the RS plasma, the density gradient is not decreased by ECH and argon is not exhausted. These results suggest the importance of density gradient control in suppressing impurity accumulation.