Abstract
Study of behavior of accelerated electrons (AE) in the ohmically heated (OH) plasma and low-hybrid current drive (LHCD) in tokamak has the practical interest especially for estimations of AE radiation energy losses and solving of the non-inductive CD problem for the ITER. Studies based on measurements of non-thermal microwave radiation (MR) intensity and hard X-ray (HXR) spectra are conducted in the FT-2 tokamak having large local magnetic ripples under LHCD in the omically heated (OH) plasma with the fan instability excited. The paper presents experimental data first obtained in such conditions witnessing about some AE behavior features. Nature of MR abnormal intensity and short giant flashes together with the fast additional electron heating in the plasma core are discussed. The first high additional fast electron heating was registered together with the synchrotron radiation (SR) intensity increase. It was accompanied by short MR spikes observed in the narrow frequency range (53 ÷ 78) GHz. They arise owing to the AE transverse energy and pitch angle increase under each crossing the cyclotron auto-resonance region and SR maser gain. It is proposed the heating mechanism owing to absorption of SR and Bernstein waves arising at linear transformation the extraordinary component of intensive MR spikes in the black plasma layers. It was found that the non-thermal MR of abnormal intensity arising during OH in the frequency range (10 ÷ 40) GHz is due to the fan instability development and the substantial local magnetic ripples. MR is accompanied by short giant flashes having a narrow frequency spectrum. Together with the SR growth the less intensive MR flashes appear in the range (57 ÷ 75) GHz. In our case it becomes possible the maser amplification of both SR and collective radiation. Appearance of the giant flashes may be initiated under transition of the maser - amplifier into the self - excitation regime, when low-frequency quasi- coherent MR flashes are generated.
Highlights
On several tokamaks [1÷6], short flashes of intense microwave radiation (MR) arising from the interaction of accelerated electrons with the harmonics of local toroidal magnetic ripples on the plasma periphery in the auto-resonance cyclotron mode has been discovered [7÷9]
Similar MR flashes were recently registered on the FT-2 tokamak with a significantly greater depth of magnetic ripples, ΔBt/Bt ~ 10%, Nt = 24, in experiments on generation of the lower hybrid current drive (LHCD) in the initially ohmic heated plasma [4, 10, 11] with the excited fan instability
Observations of an abnormally high level and intense flashes of microwave radiation at electron plasma and cyclotron frequencies in the Globus-M spherical tokamak OH plasma [3] and in the FT-2 tokamak (OH+LHCD) experiment [4, 6] have directed our efforts toward exploring the origin of maser effects that may arise in ohmically heated plasma in particular, upon formation of fan instability in more broader frequency diapason, [5]
Summary
On several tokamaks [1÷6], short flashes of intense microwave radiation (MR) arising from the interaction of accelerated electrons with the harmonics of local toroidal magnetic ripples on the plasma periphery in the auto-resonance cyclotron mode has been discovered [7÷9]. Under such conditions, maser amplification of synchrotron radiation (SR) proves to be possible in the frequency range corresponding to the width of the cyclotron auto-resonance.
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