Torsional Alfvén wave excitation by mode conversion in a tokamak plasma

Abstract

The paper presents the results of an experimental study of the wave fields excited in a tokamak plasma under Alfvén wave heating conditions, with a large number of Alfvén resonance surfaces present in the plasma. The wave fields excited by Faraday shielded antennas with current elements oriented in the poloidal direction were measured by an array of magnetic probes positioned along a vertical diameter of the plasma. Pronounced enhancement of the wave fields at the radii of Alfvén resonance surfaces was observed. The dependence of the wave field profiles on the plasma parameters and on the toroidal and poloidal positions is analysed with the help of a simple theoretical model. Evidence of the excitation of the outward propagating surface electrostatic wave is presented. The profiles excited by a number of phasings of an array of three toroidally spaced antennas are compared. The results provide unambiguous evidence that the peaks in the profiles are due to mode conversion of the compressional Alfvén wave to the torsional Alfvén wave at Alfvén resonance surfaces. Measurements of the resistive loading of the antenna indicate that up to 50% of the total wave power was deposited at Alfvén resonance surfaces well inside the plasma and 40% at a single Alfvén resonance surface.

Discrete spectrum of Alfvén ion–ion hybrid waves

“…This is similar to the well-known discrete spectrum for Alfvén waves in nominally one species plasmas, as measured on many devices, including PRETEXT, 12 Tokamak de Chauffage Alfvén TCA, 13 Torus of the University of Sydney TORTUS, 14 Tokamak Experiment for Technically Oriented Research ͑TEXTOR͒. 15 The discrete Alfvén spectrum follows from many theoretical treatments of Alfvén waves, which include boundary conditions.…”

“…It is well known that near an Alfvén resonance zone, the wave group speed slows down and the B wave energy ''piles up'' and swells. Historically the experimental signature of Alfvén waves, even at the core of a largely inaccessible plasma, has been observed to be the edge B wave field, [12][13][14]29 so our use of edge B-dot probes to measure core wave properties has some tradition behind it.…”

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

“…A number of laboratory measurements, including Alfvén resonant heating 7,8 antenna loading, 9 and wave fields in tokamaks 10 that have been carried out with nominally ''pure'' hydrogen plasmas, may actually have been studies of the Alfvén ion-ion hybrid waves. These comments are not restricted to H-minority plasmas, because even plasmas with a nominal D fill gas and partially stripped impurities will still exhibit these Alfvén-hybrid properties, although probably only on the edge.…”

“…At low power we show that small impurity fractions, e.g., effective ionic charge Z eff Ϸ1.3-2 induce large changes from the single species Alfvén spectrum. This may help to explain a number of laboratory measurements, including Alfvén resonant heating, 7,8 antenna loading, 9 and wave fields in tokamaks 10 that have been carried out with nominally ''pure'' hydrogen plasmas. Impurity or minority ion effects may be very difficult to avoid when explaining the interactions of the Alfvén wave with plasma electrons.…”

Far-forward scattering observations of an Alfven wave experiment

“…In this paper we describes the use of far-fonvard scattering (Evans er a1 1982) at submillimetre wavelengths to observe and distinguish between both resonance conditions in the TORTUS tokamak. Magnetic probe measurements of the Alfvtn resonances in TORTUS, made using low-current, short-duration discharges so that probes could be inserted into the plasma, have been reported elsewhere (Murphy 1991).…”