S. Texter

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.

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.

Impurity generation during intense lower hybrid heating experiments on the Alcator C tokamak

Abstract Experiments are underway on the Alcator C tokamak with over 1 MW of RF power injected into the plasma at a frequency of 4.6 GHz to study both heating and current drive effects. During these studies, impurity generation from limiter structures has been observed. The RF induced impurity influx is a strongly nonlinear function of net injected power. For PRF P RF = 1.0 MW , n e = 1.3 × 10 14 cm −3 , and the SiC coated limiters, Zeff is seen to increase from 1.5 before the RF pulse to about 4 during the heating. At the same time, central Te increases from 2000 to 3000 eV and central Ti from 1200 to 1800 eV. Similar effects are seen in both H2 and D2 working gas discharges. The contribution to impurity generation of nonthermal electrons, which are produced by the RF, is under investigation. Changes in edge plasma temperature and density, as well as the possibility that the particle transport is affected by the RF, are also being examined. Results of the experiments with the three different limiter materials are compared, and contributions of impurity radiation to the overall power balance are estimated.

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.