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

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 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.

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