S. C. Luckhardt

Experimental exploration of profile control in the Princeton Beta Experiment‐Modified (PBX‐M) tokamak

The experimental program of the Princeton Beta Experiment‐Modified (PBX‐M) tokamak [Phys. Fluids B2, 1271 (1990)] is directed toward tailoring plasma profiles to achieve greater stability and confinement and to gain access to the second stability region. Modification of the current density profile has been achieved with lower‐hybrid current drive (LHCD), leading to a regime free of global magnetohydrodynamic modes, while raising the value of q(0) above unity. The diffusion of the fast electrons produced by LHCD has been examined using two‐dimensional hard x‐ray imaging. Ion Bernstein waves (IBW) have been used for ion heating: a preliminary analysis shows that ion heating was spatially localized and in agreement with theoretical calculations. Divertor biasing has modified the electric field inside the last closed surface, resulting in the formation of a transport barrier, which in turn has reduced the threshold power of neutral beam injection (NBI) for H‐mode transition by 25%.

Particle confinement and the anomalous Doppler instability during combined inductive and lower‐hybrid current drive

Increases in average density have been observed during lower‐hybrid current‐drive experiments (LHCD) on the Versator II tokamak [Phys. Rev. Lett. 48, 152 (1982)], and on other LHCD experiments where inductive‐Ohmic and lower‐hybrid current drive are combined. In the present experiment it is found that this density increase is the result of an improvement in global particle confinement time τp in comparison to purely Ohmic discharges with similar plasma parameters. Furthermore, it is found that the improved confinement deteriorates when the anomalous Doppler relaxation instability of the anisotropic high‐energy electron tail becomes unstable during LHCD, and the loss rate of tail electrons increases. However, energetic electron losses alone are at least an order of magnitude too small to explain this deterioration in particle confinement.

A quasilinear Fokker-Planck code for the numerical solution of the lower-hybrid current-drive problem in the presence of electron cyclotron heating

Abstract The finite element library PDE-PROTRAN is applied for the numerical solution of a quasilinear two-dimensional Fokker-Planck equation to study the cross-effect of electron cyclotron heating and lower-hybrid current drive in tokamak plasmas. Results show that the current drive efficiency of EC waves interacting with the LH waves is higher than that of LH waves alone. The results complement previously published results on this problem. In particular, a representation of the hot population produced by EC waves, better than what has been previously published, is obtained.

The anomalous Doppler instability during lower‐hybrid current drive

The anomalous Doppler instability is investigated for two‐dimensional velocity distributions f(v∥,v⊥) appropriate for lower‐hybrid current drive. In this case, in contrast to the runaway electron problem, this mode becomes unstable at high density when the ratio ω2pe/ ω2ce is sufficiently large. Increasing the maximum velocity v2 of the rf (radio‐frequency) plateau, v1≤v∥ ≤v2, is also a destabilizing influence. In this paper expressions are given for the growth rate of the electrostatic waves obeying the dispersion relation ω=ωpe (1+ω2pe/ ω2ce)−1/2 k∥/k, employing both analytic expressions for f(v∥,v⊥) and numerical solutions for f from the two‐dimensional Fokker–Planck equation. The analytical and numerical results are in close agreement.

Experimental measurements of lower-hybrid wave propagation in the versator II tokamak using microwave scattering

A series of 139 GHz microwave scattering experiments has been performed on the Versator II tokamak (B. Richards, Ph.D. thesis, Massachusetts Institute of Technology, 1981) to study the propagation of externally launched 0.8 GHz lower‐hybrid waves. During lower‐hybrid current drive, the launched waves are found to follow a highly directional resonance cone in the outer portion of the plasma. Wave power is also detected near the center of the plasma, and evidence of wave absorption is seen. Scattering of lower‐hybrid waves in k space by density fluctuations appears to be a weak effect, although measurable frequency broadening by density fluctuations is found, Δω/ω=3×10−4. In the detectable range (2.5<N∥<5.5) the N∥ spectra inferred from the scattering measurements are quite similar above and below the current drive density limit. Numerical modeling of these experiments using ray tracing is also presented.

Lower-hybrid RF current drive and electron-cyclotron heating on the Versator II tokamak

ABSTRACT Lower-hybrid current drive experiments (LHCD) have been carried out on the Versator II tokamak in which RF injection for pulse lengths longer than the plasma L/R time has generated large increases in the net toroidal current. Incremental increases, ΔI/I, of more than 35% have been obtained. These current rises are strongly dependent on the relative phasing between waveguides, Δφ, with largest current rise occurring when waves are launched in the direction of the electron ohmic drift, Δφ=-90°. In typical cases, Thomson scattering measurements during RF drive show a decrease in the central electron temperature from 240eV±45eV to 120eV±20eV due to the spontaneous reduction in ohmic heating power during RF drive. The identification of these RF effects as the generation of current by lower-hybrid waves has been previously reported.1,2,

Detection of lower‐hybrid waves on Versator II using microwave scattering

An experiment using 139 GHz microwave scattering to detect 0.8 GHz lower‐hybrid waves is described. A simple analysis is presented to relate scattered signal to wave power. Radial scans and Nz spectra are presented as well as evidence of wave absorption during current‐drive.

Particle confinement improvement during 2.45 GHz lower‐hybrid current‐drive experiments

Particle confinement behavior during 2.45 GHz lower‐hybrid current drive has been investigated on the Versator II tokamak [Phys. Fluids 29, 1985 (1986)]. It is found that during combined Ohmic and rf current drive the global particle confinement time τp increases by up to a factor of 2 compared to purely Ohmically driven discharges, as observed in earlier 800 MHz experiments at lower densities ne ≤6×1012 cm−3. In the present experiments, τp increases have been observed at densities up to ne =2×1013 cm−3.

Particle transport simulation of lower‐hybrid current drive experiments on the Versator II tokamak

The one‐dimensional particle transport equation has been solved numerically to simulate temporal and spatial evolutions of density behavior observed during 800MHz and 2.45GHz LHCD experiments. In order to fit the 800MHz profiles, the inward pinch velocity has to be increased several fold. However, for the 2.45GHz case, the reduction of the diffusive loss near the periphery seems to be needed.

SUPPRESSION OF PLASMA FORMATION IN WAVEGUIDE ANTENNAS FOR LOWER HYBRID HEATING

A magnetic field has been applied to the 800 MHz four-waveguide lower-hybrid heating grill on the Versator II tokamak to suppress plasma formation and RF breakdown in the evacuated waveguides during tokamak operation. Without an auxiliary magnetic field, the incident power density is limited typically to 0.13 kW/cm/sup 2/. When the imposed magnetic field in the antenna is above the electron-cyclotron resonance value, the grill operates without RF breakdown or plasma formation at an incident power level of 0.42 kW/cm/sup 2/, limited only by the available RF power. A power density of 0.94 kW/cm/sup 2/ has been achieved when all the RF power was injected into one waveguide, again with no evidence of plasma formation.