D. Diebold

The virtual cathode as a transient double sheath

The two‐dimensional plasma potential measurements are given of a space‐charge dominated double sheath near a hot cathode. Laboratory data show that a virtual cathode is a self‐consistent solution only for a transient cathode‐plasma system. Slow charge exchange ions get trapped in the potential dip that forms the virtual cathode and eventually destroy it.

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.

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.

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.

Optical fluctuation measurements of turbulence using a diagnostic beam on Phaedrus‐T

Plasma density turbulence has been measured with the beam emission spectroscopy (BES) diagnostic system, using a low‐power neutral beam with He0 and H0 as beam species. In general, He0 (588 nm) provided the best signal‐to‐noise ratio due to its lower edge plasma background interference. Simultaneous measurements of edge density fluctuations have been made with BES and Langmuir probes; the spectra are seen to be essentially identical, and the fluctuation amplitudes from both diagnostics are in close agreement. A poloidal coherence length of about 2–4 cm was observed. Radial propagation of modes was not seen, but a lab‐frame poloidal phase velocity at r/a=0.77 of about 7×105 cm/s in the electron diamagnetic direction was observed, corresponding to m=8–75 kHz.

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

Edge power deposition reduction in a tokamak by replacing the Faraday screen on an RF antenna with an insulator

Calculations show that the power coupled to the regions around the RF antenna (side, front and interior) due to the edge plasma interaction with voltages induced on the antenna structure of the Phaedrus-T tokamak was reduced from nearly 11 kW to a few hundred watts for 40 kW of RF input power by replacing the Faraday screen (FS) surrounding the antenna with insulating boron nitride (BN) side limiters. When an FS was on the antenna, calculations show that 7.7 kW of the power coupled to the edge was transferred to the electrons that passed through a potential gradient while crossing field lines and 3 kW was dissipated by ions falling through an increased sheath at the carbon limiter and FS. Experimental data of edge plasma parameters, loading resistance and intensity of VUV impurity light support the conclusion that less power was deposited in the edge plasma when the antenna had only BN side limiters than when it was covered with an FS

Comparison of the effects of an ICRF antenna with insulating side limiters with and without a Faraday screen on the edge parameters of a tokamak plasma

The PHAEDRUS-T tokamak was operated with radiofrequency power near the ion cyclotron frequency at 90 degrees phasing between two current straps with and without a stainless steel Faraday screen covering the antenna. In both cases, the sides of the antenna were protected by insulating limiters. The plasma parameters in the scrape-off layer were measured and were shown to be essentially the same when radiofrequency power was applied from the Faraday screen covered antenna as compared with the antenna without a Faraday screen. The intensity of Fe(XVI) light dropped an order of magnitude alter the screen was removed