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 3He 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 4He plasmas, modest heating efficiencies (¿ = Pabsorbed/Plaunched <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 3He 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 3He ions reached about 260 keV in the second harmonic 3He 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 3He 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 4He) were confirmed in the H majority case, the 3He concentration was the main handle on the heating efficiency in the second harmonic 3He heating scenario

Comparison between dominant NB and dominant IC heated ELMy H-mode discharges in JET

Abstract The experiment described in this paper is aimed at characterization of ELMy H-mode discharges with varying momentum input, rotation, power deposition profiles and ion to electron heating ratio obtained by varying the proportion between ion cyclotron (IC) and neutral beam (NB) heating. The motivation for the experiment was to verify if the basic confinement and transport properties of the baseline ITER H-mode are robust to these changes, and similar to those derived mostly from dominant NB heated H-modes. No significant difference in the density and temperature profiles or in the global confinement were found. Although ion temperature profiles were seen to be globally stiff, some variation of stiffness was obtained in the experiment by varying the deposition profiles, but not one that could significantly affect the profiles in terms of global confinement. This analysis shows the thermal plasma energy confinement enhancement factor to be independent of the heating mix, for the range of conditions explored. Moreover, the response of the global confinement to changes in density and power were also independent of heating mix, reflecting the changes in the pedestal, which is in agreement with globally stiff profiles. Consistently, the pedestal characteristics (pressure and width) and their dependences on global parameters such as density and power were the same during NB only or with predominant IC heating

Identification of the ubiquitous Coriolis momentum pinch in JET tokamak plasmas

A broad survey of the experimental database of neutral beam heated plasmas in the JET tokamak has established the theoretically expected ubiquity, in rotating plasmas, of a convective transport mechanism which has its origin in the vertical particle drift resulting from the Coriolis force. This inward convection, or pinch, leads to inward transport of toroidal angular momentum and is characterized by pinch numbers RV/¿, which rise from near unity at r/a ˜ 0.25 to around 5 at r/a ˜ 0.85. Linear gyrokinetic calculations of the Coriolis pinch number and the Prandtl number ¿/¿i are in good agreement with the experimental observations, with similar dependences on plasma parameters. The data, however, do not rule out contributions from different processes, such as residual stresses