R. Majeski

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.

Measurements of electromagnetic waves in Phaedrus-B: bench-mark test of antena wave field calculations

It is shown that the predictions of a numerical code (ANTENA) and the data of wave field measurements in the Phaedrus-B tandem mirror are consistent (±25%) for right-handed (−) wave fields and less so (±40%) for left-handed (+) wave fields in the plasma core, and that they disagree for + fields near the column edge. Shorting out or reduction of the wave azimuthal electric fields by limiters is the probable cause of this discrepancy. The ICRF fluctuating wave fields are shown as || contour maps in the r-z plane, where the + data peak at a smaller radius than predicted. The waves are characterized by different dominant axial wave numbers for the left- and right-handed circularly polarized fields.

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

Radio-frequency wave interchange stability experiments below the ion cyclotron frequency

It has been demonstrated in the Phaedrus‐B tandem mirror [Phys. Fluids 31, 714 (1988)] that radio‐frequency (rf) waves applied below the ion cyclotron frequency (ω/Ωi ≊0.75) can stabilize a mirror plasma against the interchange instability. Results presented here demonstrate stabilization over a range of frequencies (0.65≤ω/Ωi≤0.8) with the mMHD =−1 instability stabilized and higher‐order (‖mMHD‖≥2) low‐amplitude modes present. Radial profiles of the rf magnetic fields at a number of axial locations as well as parallel wavenumber measurements indicate mrf =−1 slow shear Alfven wave excitation over this frequency range. Measurements at frequencies below 0.75Ωi indicate attenuation of rf waves by gas baffles and a gas box.

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

Interchange stabilization of a mirror plasma using radio‐frequency waves below the ion cyclotron frequency

It is demonstrated that radio‐frequency (rf) waves applied below the ion cyclotron frequency (ω/Ωi≊0.75) can stabilize a mirror plasma against the interchange instability. A set of phased antennas in the Phaedrus‐B tandem mirror central cell [Phys. Rev. Lett. 59, 206 (1987)] is used to select the rf azimuthal mode number (m). When the m=−1 mode is selected, the ponderomotive force from the left‐hand polarized wave fields is sufficient to stabilize the plasma. Measurements of the rf mode number and of the wave polarization indicate strong excitation of m=−1 modes with much of the wave having a left‐hand polarization.

RF generated voltage on the Faraday screen of an ICRF antenna and its effect on the Phaedrus-T edge plasma

Work done on the Phaedrus-T tokamak is summarized regarding RF voltages induced on the sides of slotted side Faraday screens (FSs). Direct in-air measurements show voltages induced on slotted side FSs, while theory and computer simulations suggest that these voltages are induced by radial currents on antenna feeds. Triple probe measurements in the edge region of the Phaedrus-T tokamak suggest that the induced voltages modify edge plasma parameters and increase sheath potentials, which increase sputtering of the FS material and result in an increase of the impurity content of the plasma. These effects of the induced voltages can be eliminated by using insulating plates as FS side limiters

Measurements of the ponderomotive force including sideband mode coupling effects and damping rates

Measurements of the interactions of waves in the ion cyclotron range of frequencies (ICRF) with flute interchange modes are presented. Interactions between the applied l=1 radio frequency (rf) wave and an m=−1 flute mode give rise to sidebands above and below the rf frequency with mode numbers of l=0 and l=+2, respectively. The contribution of the sideband terms to stability are shown to cancel 40% of the direct ponderomotive contribution. This is less than the 90% predicted by theory [Phys. Fluids 30, 148 (1987)] for an applied l=+1 rf wave above the ion cyclotron frequency with a large separation between the plasma and the vacuum vessel. Measurements of the linear growth and damping rate of the flute instability in the presence of rf are in good agreement with that calculated by considering only the curvature‐driven instability and the ponderomotive force. The growth rate of the magnetohydrodynamic mode is consistent with the primary force acting on the plasma being the curvature force. This method allo...