R. Breun

Neutral pumping in a helicon discharge

At powers of 500-800 W of 13.56 MHz rf power in a 1 kG axial magnetic field, the neutral pressure in the bulk of a helicon discharge with electron densities of and electron temperatures of eV in argon decreased by a factor of ten compared to the pressure before the discharge. These experimental results are compared with a simple axial diffusion model of ionization and neutral transport and agree qualitatively with calculated axial profiles of neutral pressure. The effects of cusped fields on neutral density profiles and plasma confinement are also examined.

The Phaedrus-T antenna system

Abstract A two strap fast wave antenna has been developed which is capable of operating at arbitrary phasing for any level of plasma loading resistance. Recent advances in the understanding of the rf-edge plasma interaction have been incorporated in the design as well. The result is an antenna which operates without Faraday shielding while greatly reducing rf-induced scrape-off layer perturbations and impurity influx.

Diagnostics for plasma processing (etching plasmas) (invited)

Plasma processing diagnostics play two different roles—characterization and control. The goal of plasma characterization is to establish connections of data with external parameters and to verify models. The goal of control diagnostics is to make noninvasive in situ measurements of relevant processing parameters. Diagnostics used in semiconductor etching are considered. These include Langmuir probes, laser induced fluorescence, optical emission spectroscopy, infrared and Fourier transform infrared absorption spectroscopy, mass spectrometry, microwave interferometry, and radio frequency diagnostics. An example is given of the use of many diagnostics in characterizing SiO2 and Si etching by fluorocarbons.

Alfvén wave experiments in the Phaedrus‐T tokamak*

Heating in the Alfven resonant regime has been demonstrated in the Phaedrus‐T tokamak [Fusion Technol. 19, 1327 (1991)]. Electron heating during injection of radio‐frequency (rf) power is indicated by a 30%–40% drop in loop voltage and modifications in sawtooth activity. Heating was observed at a frequency ωrf≊0.7Ωi on axis, using a two‐strap fast wave antenna operated at 7 and 9.2 MHz with 180° phasing (N∥∼100). Numerical modeling with the fast wave code fastwa [Plasma Phys. Controlled Fusion 33, 417 (1991)] indicates that for Phaedrus‐T parameters the kinetic Alfven wave is excited via mode conversion from a surface fast wave at the Alfven resonance and is subsequently damped on electrons.

Fueling and heating of tandem mirror end cells using rf at the ion‐cyclotron frequency

Fueling and heating of tandem mirror end cells by rf at the ion‐cyclotron frequency have been studied experimentally in the Phaedrus Tandem Mirror. The end cell density is found to increase linerly with rf voltage. The total plasma energy is observed to increase with rf power with no evidence of saturation at high power. The plasma axial length decreases with increasing rf power down to a length of approximately the distance between the two resonance locations in the end cell. The highest density and average ion energy are achieved with the resonance closest to the midplane. It is necessary to assume that the rf electric field at the resonance decreases with increasing density and with distance from the antenna in order to model the ion particle and power balance. The particle and power balance model predicts that dense, hot end cells may be maintained with E+∼1 V/cm and a fueling efficiency of a few percent when the resonance is close to the midplane because of better ion confinement and small plasma vol...

Biased H mode experiments in Phaedrus-T

Inserting a positively biased electrode to just inside the Phaedrus-T tokamak limiter results in typical H mode behaviour (i.e. Hα or Dα drop, density rise, increase in stored energy, profile steepening, and reduction of edge turbulence and radial transport) in deuterium, hydrogen and helium discharges. Hα or Dα emission suggests that the improvement in particle confinement with H mode is poloidally asymmetric, with the greatest improvement occurring on the low field side. The radial conductivity is examined and measured values are compared with theory

Triple Langmuir probe measurements in the Phaedrus‐T tokamak

Triple Langmuir probe data have been obtained in the initial phase of Phaedrus‐T operation. The data were obtained during ohmic discharges of roughly 60 kA, 〈n〉=5×1018 m−3, Te(0)=0.5 keV, BT=0.7 T, q(a)=4 and 80 ms duration. Measurements of Te, φF, and n were made in a radial scan in the scrape‐off layer of the plasma. The toroidal and poloidal positions were fixed throughout the scan. The spectra of Te, φF, and n fluctuations up to 100 kHz were also measured. One of the interesting features of these data is that m=2 oscillations detected by Mirnov coils were very pronounced in the φF and Te fluctuation spectra, but are not as evident in the n fluctuations. These data are presented and the techniques employed to get these data are discussed. Future plans to extend the maximum frequency resolution of the fluctuation measurements up to and beyond the frequency range of the ICRF power (13–18 MHz) that will be injected into Phaedrus‐T are also discussed.

Alfvén wave current drive in the Phaedrus‐T tokamak

The first experimental evidence of Alfven Wave Current Drive (AWCD) in a tokamak is shown. In a low‐density experiment, an estimated 20–35 kA out of 65 kA total current, or 30%–55% of the total current has been driven. The estimated efficiency for current driven per unit RF input power is approximately ICD/PRF≊0.2 A/W, which is near the predicted efficiency, and corresponds to the commonly used figure of merit, neR0ICD/PRF≊0.4×1018 A m−2 W−1, where ne is plasma density and R0 is the major radius. The significant 30%–40% drop in loop voltage observed cannot be explained by any plausible increase in electron temperature Te, or decrease in inductive plasma energy, or changes in plasma resistivity. Independently measured loop voltage, Te, effective ionic charge Zeff, and plasma inductance and resistance are all consistent with this conclusion.

Poloidally asymmetric potential increases in tokamak scrape-off layer plasmas by radiofrequency power

Langmuir probe data are presented which show poloidally asymmetric increases in floating potential, electron temperature and, hence, plasma potential on magnetic field lines which map to the Faraday shield of an ICRF antenna in a medium size tokamak, Phaedrus-T, during radiofrequency power injection. These data are consistent with and suggestive of the existence of radiofrequency generated sheath voltages on those field lines

Beam emission spectroscopy diagnostic for the study of turbulence in Phaedrus-T tokamak plasmas

A flexible beam emission spectroscopy diagnostic system is being installed on the Phaedrus‐T tokamak. It consists of a low‐power diagnostic neutral beam (H0 or He0) coupled with visible and vacuum UV collection optics to study low‐amplitude, high‐frequency fluctuations in local plasma density in ohmically and rf‐heated plasmas. Neutral beam geometry and optical sightlines are chosen to optimize localization and radial resolution (<1 cm) over the whole plasma region. The ability to inject either a H or He neutral beam and observe in both the vacuum UV and the visible spectral ranges allows a wide choice of atomic transitions (e.g., Hα, Lα, He singlet or triplet lines, etc.) to be compared and optimized for a given experimental condition.