Alfvén ion–ion hybrid wave heating in the Phaedrus-T tokamak

Abstract

In the Phaedrus-T tokamak ͓R. A. Breun et al., Fusion Technol. 19, 1327 ͑1991͔͒, Alfvén waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions are shown to have a large effect on the Alfvén spectrum, as measured at the edge. An ion-ion hybrid Alfvén mode has been identified by measuring dispersion properties. Landau damping is predicted to be large and spatially localized. These Alfvénic waves are experimentally shown to generate correlated electron heating and changes in density near the core of the tokamak plasma. Fast wave antenna fields can mode convert at a hybrid Alfvén resonance and provide a promising route to spatially localized tokamak heating and current drive, even for low effective ionic charge Z eff Ϸ1.3-2.

Discrete spectrum of Alfvén ion–ion hybrid waves

“…The split signal drove two broadband power amplifiers, which excited the two antenna straps through matched lengths of cable with ͓0, ͔ phasing. 7 The antennas were driven through a voltage measurement feedpoint, inserted so as to avoid the series matchbox capacitor, and drive the parallel combination of the strap inductance and parallel matching capacitor directly. The parallel capacitor was adjusted to the minimum value so that the resonance was at the highest possible frequency ͑Ͼ25 MHz͒.…”

“…In order to investigate the wave physics, we performed low RF power experiments on standard Ohmic tokamak discharges, where RFinduced core electron heating was observed. [7][8][9] The plasma current was I p Ϸ70 kA, edge q(a)Ϸ3.7, line average density ͗n e ͘Ϸ8ϫ10 18 m Ϫ3 . A two strap fast wave antenna array had two toroidally spaced poloidal current straps driving low phase velocity wave fields.…”

“…The plasma was nominally pure H ϩ with impurity minority species C VI . This electron heating was shown to be caused by Alfvén ion-ion hybrid waves 8 by comparing high-power heating shots with low-power frequency swept measurements of wave number k z and dispersion relations (k z ) at the plasma edge.…”

“…The presence of impurity ions such as fully stripped C VI , D ϩ create a gap in the dispersion curve near the ion minority gyrofrequency and add another mode above the ion-ion hybrid frequency, as well as changing the discrete Alfvén spectrum. This approach is a different way of looking at the well-known ion-ion hybrid wave spectrum, 8 and is not limited to the hydrogen majority plasmas that we used in Phaedrus-T.…”

“…16,17 A cylindrical model, not a toroidal one, appears to be adequate to explain the data shown in this paper, even though the Phaedrus-T tokamak has significant toroidicity at an aspect ratio AϷ3. 7.…”