S. Knowlton

High energy X-ray measurements during lower hybrid current drive on the Alcator C tokamak

High energy X-ray emission (E?> 20 keV) from superthermal plasma electrons during lower hybrid current drive on the Alcator C tokamak has been measured using sodium iodide (NaI) scintillation spectroscopy. The X-ray spectra are generally linear on a semi-log plot of count rate versus photon energy and extend out to several hundred kiloelectronvolts. For the range of densities (e?(0.3?0.8) ? 1014 cm?3) over which current drive was performed on Alcator, there was negligible emission before the injection of radiofrequency wave power. The radial profiles of the emission were also measured and indicate that the current carrying high energy electrons exist primarily within the inner half (r/a < 1/2) of the plasma column. Plasma parameter scans produced variations in the X-ray emission profiles that are consistent with changes in the launched Fourier power spectrum and the conditions imposed by lower hybrid wave accessibility. In addition, the velocity space distribution function of the energetic tail electrons has been determined using the angular variation in the X-ray emission.

Plasma current ramp-up and ohmic-heating transformer recharging experiments using lower-hybrid waves on a tokamak

Plasma current ramp‐up by lower‐hybrid waves in a high‐density regime (ne≳1×1019 m−3) is investigated on the Alcator C tokamak [Nucl. Fusion 25, 1127 (1985)]. A time‐averaged conversion efficiency (W−Pext)/Prf of over 15% has been obtained at a density of ne ≂1×1019 m−3, where W is the poloidal field energy and Pext is the external inductive power. The maximum conversion efficiency obtainable is lower at higher densities. Ramp‐up rates of over 1 MA/sec were obtained when rf current ramp‐up is applied to an Ohmically maintained plasma. Lower‐hybrid current drive can also be used to maintain the plasma while recharging the Ohmic‐heating transformer. The conversion efficiency of 10% obtained during recharging is found to be comparable to that obtained during ramp‐up under similar conditions.

Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

Sawteeth have been suppressed by lower hybrid current drive in high density Alcator C plasmas. At a density of 1.1 × 1014 cm−3 and q(a) = 4.7, the sawteeth were eliminated by application of PRF = 600 kW at 4.6 GHz, which represents an estimated 30% of the radiofrequency (RF) power required to fully sustain the discharge in the absence of Ohmic power. Strong electron heating is observed with both current drive phasing and anti-current drive phasing of the waveguide arrays, but sawtooth stabilization occurs only in the former case. With current drive phasing the sawtooth period increases with RF power until the sawteeth are stabilized, whereas with anti-current drive phasing the period remains unchanged. The internal inductance of the plasma increases in the case of current drive relative to the case of anti-current drive. The disappearance of sawteeth and the increase in internal inductance suggest that the RF current is driven near the q = 1 surface. At high power (PRF > 850 kW), the sawteeth return 40–60 ms into the RF pulse, indicating that an optimal power range, or RF deposition profile, exists for successful sawtooth suppression.

Impurity sources during lower hybrid heating on Alcator

Impurity source mechanisms which appear during the heating of the Alcator C tokamak by injection of lower hybrid frequency waves are described. Silicon is a dominant impurity in these experiments where SiC was the limiter surface material. At low values of injected power, the silicon source rate is dominated by physical sputtering. As the amount of injected power is raised, evaporation caused by the tail electron heat flux to the limiter becomes the primary source for impurities entering the plasma. Measurements of silicon line emission from the plasma as well as other central and edge parameters are presented as functions of injected radiofrequency power.

Energy confinement of the high energy tail electrons during lower hybrid current drive experiments on the Alcator C tokamak

Abstract The distribution function of the high energy electron tail generated during lower hybrid current drive experiments at a density of n e =3×10 19 m −3 in the Alcator C tokamak has been determined. By performing a convolution integral with the collision operator, the power flow into the bulk plasma was calculated. With the assistance of code modeling, the energy confinement time of the fast electrons, and its dependence upon energy, was determined. It is shown that the energy confinement of tail electrons is comparable with that of the bulk particles.

Energy confinement studies of lower hybrid current driven plasmas in the Alcator C tokamak

Energy confinement is studied in lower hybrid current driven (LHCD) plasmas in Alcator C in the density range e = (1?8) ? 1013 cm?3. In LHCD plasmas, the stored energy in the electron tail, Wtail, can be a significant fraction of the total stored energy, Wtot, especially at lower densities. At sufficiently low densities, the confinement time of the high energy electrons is expected to become shorter than their collisional slowing down time, and direct energy losses from the electron tail can become important in the overall power balance. The global energy confinement time, defined as is found to be comparable to or exceed that in ohmically heated (OH) plasmas at low densities e 3 ? 1013 cm?3, where a steady state current can be maintained with relatively low RF power. However, at higher densities where substantially more RF power is needed (relative to the Ohmic power required to maintain a similar plasma), a deterioration of relative to Ohmic confinement, similar in magnitude to that predicted by the neutral beam heated L-mode scaling, is observed. Theoretical modelling with the aid of a ray tracing Fokker-Planck transport code suggests that the deteriorated confinement in this high density, high power regime may be attributed to an enhanced bulk electron thermal diffusivity. In a combined OH-LHCD plasma, a value of greater than the Ohmic value is obtained as long as the applied RF power does not significantly exceed the Ohmic power.

LOWER HYBRID HEATING EXPERIMENTS ON THE ALCATOR-C AND THE VERSATOR-II TOKAMAKS

ABSTRACT We report on initial results from lower hybrid wave heating experiments carried out on the MIT Alcator C and Versator II tokamaks. In the Alcator C experiments a 4 waveguide array, with internally brazed ceramic windows has been used to inject 160 kW of microwave power at 4.6 GHz into the plasma with n O ≤ 1 × 10 15 cm −3 , and B O ≤ 12 T. An RF power density of 8 kW/cm 2 has been transmitted into the plasma without RF breakdown. RF coupling studies show optimal coupling (R ≤ 10%) when the local density at the waveguide mouth is 25–50 times overdense. Initial heating experiments show an ion tail formation in hydrogen discharge peaking at a density of at B = 8.9 T, and bulk ion heating at a density of at B ≃ 11 T. Evidence of RF current enhancement has been observed at a density of n ≃ 3 × 10 13 cm −3 .

Plasma X‐ray emission in the 20‐500 keV range during lower hybrid current drive on Alcator

An array of eight 1’’×3’’ NaI scintillators has been used to collect plasma hard x‐ray spectra (Eγ≳20 keV) emitted perpendicular to the magnetic axis during lower hybrid current drive on Alcator. The spectra exhibit a tail extending out to at least 300 keV and the profiles are generally peaked. These results show that the slope of the x‐ray spectra increases with increasing plasma radius. Equivalently, the emission profiles tend to broaden with increasing photon energy. Also, the x‐ray spectra slope increases at each radial location as the relative phasing of adjacent waveguides in the grill antenna is decreased. Preliminary results also suggest that the x‐ray spectra tend to flatten and that the emission profiles tend to peak up with decreasing plasma density or increasing magnetic field. In addition, the initial results of an array for measuring the high energy x‐ray emission from Alcator as a function of the emission angle relative to the magnetic axis are presented.

Lower hybrid current ramp‐up experiments and density limit on Alcator C

Lower hybrid plasma current ramp‐up efficiencies at densities ne≥1×1013 cm−3 are investigated. The ramp‐up efficiency Pel/Prf of almost 100% is obtained shortly after the rf turn on, but the efficiency drops to typically 5−10% after this initial transient which dies away in about 50ms. The cut‐off density for electron tail formation was also investigated. It correlated with the onset of parametric decay and ion tail formation near the plasma edge. However, at high magnetic fields and high plasma currents (B=10T, lp=0.5MA, H plasma) an ion tail was observed even below the parametric threshold density.

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.