Experimental investigation of ion cyclotron range of frequencies heating scenarios for ITER's half-field hydrogen phase performed in JET

Abstract

Two ion cyclotron range of frequencies (ICRF) heating schemes proposed for the half-field operation phase of ITER in hydrogen plasmas-fundamental H majority and second harmonic 3 He ICRF heating-were recently investigated in JET. Although the same magnetic field and RF frequencies (f ≈ 42 MHz and f ≈ 52 MHz, respectively) were used, the density and particularly the plasma temperature were lower than those expected in the initial phase of ITER. Unlike for the well-performing H minority heating scheme to be used in 4 He plasmas, modest heating efficiencies (η = P absorbed /P launched < 40%) with dominant electron heating were found in both H plasma scenarios studied, and enhanced plasma-wall interaction manifested by high radiation losses and relatively large impurity content in the plasma was observed. This effect was stronger in the 3 He ICRF heating case than in the H majority heating experiments and it was verified that concentrations as high as ∼20% are necessary to observe significant ion heating in this case. The RF acceleration of the heated ions was modest in both cases, although a small fraction of the 3 He ions reached about 260 keV in the second harmonic 3 He heating experiments when 11 Romanelli F et al 2010 Proc. 23rd IAEA Fusion Energy Conf. (Daejeon, Korea, 2010 5 MW of ICRF power was applied. Considerable RF acceleration of deuterium beam ions was also observed in some discharges of the 3 He heating experiments (where both the second and third harmonic ion cyclotron resonance layers of the D ions are inside the plasma) whilst it was practically absent in the majority hydrogen heating scenario. While hints of improved RF heating efficiency as a function of the plasma temperature and plasma dilution (with 4 He) were confirmed in the H majority case, the 3 He concentration was the main handle on the heating efficiency in the second harmonic 3 He heating scenario.

Observations and modelling of ion cyclotron emission observed in JET plasmas using a sub-harmonic arc detection system during ion cyclotron resonance heating

“…The minority concentration was determined by comparing the intensity of a 3 He emission line to that of Dα emission in the outer divertor region. This is a plasma edge measurement, but the concentration estimated using this method is consistent with a comparison between experimental and modelled radio-frequency heating efficiencies performed in these pulses [21], and therefore the divertor measurements are believed to provide a reasonable estimate of the 3 He concentration in the plasma core. The minority ions were accelerated by ICRF waves via the second harmonic resonance.…”

Benchmarking ICRF full-wave solvers for ITER

“…Cases 2 and 3 (accelerating He 3 at its second harmonic) are numerical explorations of a heating scenario under assessment but currently not considered a main scenario for ITER. Experiments with this scheme using low concentrations of He 3 have been conducted in JET [9] and Tore Supra [10]. The results have not been encouraging.…”

Impact of minority concentration on fundamental (H)D ICRF heating performance in JET-ILW

“…which for the case of ion cyclotron resonance frequency (ICRF) heating translates into total absorption of the waves in the bulk plasma (no edge losses). The ICRF heating scenarios available for the hydrogen plasma operation (fundamental majority H heating and 2nd harmonic minority 3 He heating) have been extensively modelled [2][3][4][5] and were experimentally studied a few years ago in dedicated JET experiments [6]. The results indicate poor RF wave absorptivity leading to relatively low heating efficiency (<50%) in both cases: fundamental H majority heating suffers from the well-known unfavourable polarization of the RF fields near the majority cyclotron layer (screening effect) while for the second harmonic 3 He heating scenario it is necessary to work at prohibitively high 3 He levels to increase the wave absorption.…”