D. Hwang

Fast-ion orbit effects during ion cyclotron range of frequency experiments on the Princeton Large Torus

The angular dependence of the flux of fast neutrals was measured in conjunction with Ion Cyclotron Range of Frequency experiments on the Princeton Large Torus. A regime was found where the energetic tail of the charge-exchange neutral flux was strongly anisotropic. Unexpectedly, however, the flux from co-going ions was peaked at an angle intermediate between perpendicular and parallel. This could be the consequence of a preferential filling of that class of banana orbits whose tips are located within the ion cyclotron resonance layer.

Plasma-materials interactions during RF experiments in tokamaks

Plasma-materials interactions studied in recent ICRF heating and lower hybrid current drive experiments are reviewed. The microscopic processes responsible for impurity generation are discussed. In ICRF experiments, improvements in machine operation and in antenna and feedthrough design have allowed efficient plasma heating at RF powers up to 3 MW. No significant loss of energy from the plasma core due to impurity radiation occurs. Lower hybrid current drive results in the generation and maintenance of hundreds of kiloamperes of plasma current carried by suprathermal electrons. The loss of these electrons and their role in impurity generation are assessed. Methods to avoid this problem are evaluated.

Radiation losses in PLT during neutral-beam and ICRF heating experiments

Radiation and charge-exchange losses in the PLT tokamak are compared for discharges with Ohmic heating only (OH), and with additional heating by neutral beams (NB) or RF in the ion cyclotron frequency range (ICRF). Spectroscopic, bolometric and soft-X-ray diagnostics were used. The effects of discharge cleaning, vacuum wall gettering, and rate of gas inlet on radiation losses from OH plasmas and the correlation between radiation from plasma core and edge temperatures are discussed. – For discharges with neutral-beam injection the radiation dependence on type of injection (e.g. co-injection versus counter- and co- plus counter-injection) was investigated. Radial profiles of radiation loss were compared with profiles of power deposition. Although total radiation was in the range of 30–60% of total input power into relatively clean plasma, nevertheless only 10–20% of the total central input power to ions and electrons was radiated from the plasma core. The radiated power was increased mainly by increased influx of impurities, however, a fraction of this radiation was due to the change in charge-state distribution associated with charge-exchange recombination. – During ICRF heating radiation losses were higher than or comparable to those experienced during co- plus counter-injection at similar power levels. At these low power levels of ICRF heating the total radiated power was ~ 80% of auxiliary-heating power. Radiation losses changed somewhat less rapidly than linearly with ICRF power input up to the maximum available at the time of these measurements (0.65 MW).

Changes in impurity radiation during ICRF heating of PLT tokamak plasmas

This paper presents the results of a study of the increase in impurity line radiation in the 15-360 A region during ICRF heating of PLT plasmas, with emphasis on metallic impurity (Ti and Fe) behaviour. Central titanium and iron densities are given for a variety of ICRF heating experiments; total central metallic impurity concentrations of up to about 0.3% of ne(0) are observed at the 2.0 MW RF power level. This study shows that the power radiated by these elements is a small (about 10%) fraction of the total input power to the plasma for the present heating efficiency at 2.0 MW RF power. The impurity line brightnesses scale approximately linearly with RF power up to 2.8 MW. The antenna Faraday shields are shown to be the primary source of metallic impurities during both ICRF heating and Ohmic heating only. The impurity content of discharges heated using a single half-turn antenna and a pair of centre-fed antennas (having the same total surface area but half the poloidal extent of the half-turn antenna) is the same at a relatively low RF power of 350 kW, indicating that the impurity influx does not depend on the poloidal length of the antennas (or that the plasma interacts only with a localized area on the Faraday shields).

Impurity ion transport studies on the PLT tokamak during neutral-beam injection

Radial transport of medium- and high-Z ions during co- and counter-neutral-beam heating in the PLT tokamak is studied, using molybdenum and scandium ions as tracer elements. The time evolution of the radial profiles of several ionization stages of both elements, injected by laser blowoff during the neutral-beam heating, is measured under three significantly different beam-plasma combinations. No noticeable differences in the radial profiles attributable to the beam direction are observed. However, a given injected amount resulted in considerably larger interior concentrations of the tracer element in the counter-beam heating cases, suggesting larger penetration of the plasma periphery. Computer simulation with the MIST code suggests a net inward drift of the order 103 cms−1 superposed to a diffusion coefficient of the order 104 cm2s−1 for both scandium and molybdenum ions. Injection of larger amounts of the tracer element, sufficient to cause measurable central electron temperature changes, resulted in dramatic changes in ion-state distributions, making some appear peaked in the centre while others disappeared. This effect could be produced with both co- and counter-beam heating, but with lesser amounts in the latter case. It is interpreted as rearrangement of the ionization balance, rather than any preferential accumulation of the injected element.

Measurements of low-energy neutral hydrogen efflux during icrf heating

Using the Low Energy Neutral Atom Spectrometer, measurements were made of the H/sup 0/ and D/sup 0/ efflux from PLT during ion cyclotron heating experiments. The application of rf power at frequencies appropriate to fundamental and 2nd-harmonic heating results in a rapid, toroidally uniform rise in the charge-exchange efflux at a rate of about 10/sup 15/ cm/sup -2/ s/sup -1/ MW/sup -1/. This flux increase is larger at lower plasma currents. The cause of this flux and its impact on plasma behavior are discussed.

Observation of lines above 2000 Å in O VIII and C VI in the Princeton Large Torus due to charge‐exchange processes: Diagnostic applications

Hydrogenlike oxygen and carbon lines from transitions (Δn=1) between levels of high principal quantum number n, with wavelengths above 2000 A have been observed. Observations of such transitions were possible due to charge‐exchange processes during neutral beam injection of hydrogen atoms into the Princeton Large Torus tokamak. The lines are O VIII 2976 A (8–7 transitions), C VI 3434 A (7–6), and C VI 5291 A (8–7). Application of these lines for ion temperature measurements and initial observations of neutral beam vertical distributions in the plasma are presented.

HIGH POWER ICRF HEATING ON PLT AND EXTRAPOLATION TO FUTURE DEVICES

ABSTRACT Presently, ICRF heating experiments are being conducted on PLT at rf powers up to ~ 2 MW for rf pulse durations of up to 0.30 sec. Efficient deuteron and electron heating are observed in the minority 3He and H heating regimes. In addition, efficient ion heating in the second harmonic II heating regime has been demonstrated while no significant deleterious effects on plasma confinement or stability have been observed in either case. These results provide a credible physics base for use of combined minority-second harmonic heating regimes in future devices in which the minority regime will transition to the majority second harmonic regime as the β increases. Extentions of the heating studies to higher power, higher harmonic regimes, and especially to higher β plasmas with greater correspondence to future devices are underway. Technological development in support of these experiments and of possible reactor applications is now being incorporated into the PLT program.

Modeling of ICRF heating in PLT

Significant heating with the fast magnetosonic wave near the ion cyclotron frequency has been demonstrated in the present generation of tokamaks. Effective wave absorption and heating can be achieved either by using the second harmonic or by heating at the fundamental of a minority ion component. Recent experiments in PLT have facilitated the refinement of a heating model which both shows good agreement with experiment and predicts favorable scaling to hotter, denser plasmas. Details of the model, including full wave theory, power deposition, Fokker-Planck theory, and scaling are discussed.

Comparison of bounce‐averaged quasilinear theory with charge exchange measurements during minority fundamental and majority second harmonic ICRF heating in PLT

Previous studies in PLT using charge‐exchange, edge probe, and fusion product diagnostics all indicate that ICRF tends to produce energetic trapped particles whose banana tips are near the resonance layer. A bounce‐averaged quasilinear operator which predicts this ‘‘resonance localization’’ has been implemented in a Fokker‐Planck code in order to make detailed comparisons with measurements. Good agreement is found with data from the horizontally‐scanning, mass‐resolving, charge‐exchange analyzer, although the RF power profile seems to be broader than expected. We have recently observed a deuterium tail during hydrogen minority heating. The shape of this tail and its scaling with RF power agree well with the quasilinear theory. These measurements indicate that as much as 30% of the central RF power goes into direct second harmonic deuterium heating.