High Harmonic Fast Wave Coupling And Heating Experiments In The Cdx U Spherical Tokamak
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High-harmonic Fast Wave Heating Experiments in CDX-U.
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| Release | : 2001 |
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One of the primary objectives of the proposed National Spherical Tokamak Experiment (NSTX) is the investigation of very high[beta] regimes. Consequently, finding efficient methods of non-inductive heating and current drive required to heat and sustain such plasmas is of considerable importance. High-frequency fast waves are a promising candidate in this regard. However, in NSTX, the field-line pitch at the outer midplane will range from 0 up to 60 degrees from plasma start-up to current flattop. Thus, antenna strap orientation with respect to the edge magnetic field may have a serious impact on power coupling and absorption. To address this issue, the vacuum vessel of the Current Drive Experiment -- Upgrade (CDX-U) spherical tokamak has been upgraded to accommodate a rotatable two-strap antenna capable of handling several hundred kilowatts in short pulses. Details of the antenna design and results from loading measurements made as a function of power, strap angle, and strap phasing will be presented. Results from microwave scattering experiments will also be discussed.
Review of Tokamak Experiments on Direct Electron Heating and Current Drive with Fast Waves
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| Total Pages | : 13 |
| Release | : 1993 |
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Results from tokamak experiments on direct electron interaction with the compressional Alfven wave ({open_quote}fast wave{close_quote}) are reviewed. Experiments aimed at electron heating as well as those in which fast wave electron current drive was investigated are discussed. A distinction is drawn between experiments employing the lower hybrid range of frequencies, where both the lower hybrid wave ({open_quote}slow wave{close_quote}) and the fast wave can propagate in much of the plasma, and those experiments using the fast wave in the range of moderate to high ion cyclotron harmonics, where only the fast wave can penetrate to the plasma core. Most of the early tokamak experiments were in the lower hybrid frequency regime, and the observed electron interaction appeared to be very similar to that obtained with the slow wave at the same frequency. In particular, electron interaction with the fast wave was observed only below a density limit nearly the same as the well known slow wave density limit. In the more recent lower frequency fast wave experiments, electron interaction (heating and current drive) is observed at the center of the discharge, where slow waves are not present.
Applications of Fast Wave in Spherical Tokamaks
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| Total Pages | : 8 |
| Release | : 1997 |
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In spherical tokamaks (ST), the magnetic field strength varies over a wide range across the plasma, and at high betas it deviates significantly from the 1/R dependence of conventional tokamaks. This, together with the high density expected in ST, poses challenging problems for RF heating and current drive. In this paper, the authors investigate the various possible applications of fast waves (FW) in ST. The adjoint technique of calculating current drive is implemented in the raytracing code CURRAY. The applicability of high harmonic and subharmonic FW to steady state ST is considered. They find that high harmonic FW tends to be totally absorbed before reaching the core and may be considered a candidate for off axis current drive while the subharmonic FW tends to be absorbed mainly in the core region and may be considered for central current drive. A difficult problem is the maintenance of current at the startup stage. In the bootstrap ramp-up scenario, the current ramp-up is mainly provided by the bootstrap current. Under this condition, the role of rf becomes mainly the sustainment of plasma through electron heating. Using a slab full-wave code SEMAL, the authors find that the ion-ion-hybrid mode conversion scheme is a promising candidate. The effect of possible existence of edge Alfven resonance and high harmonic cyclotron resonance is investigated and regimes of minimization of edge heating identified.
