Bulk ion heating with ICRH in JET DT plasmas

Abstract

Reactor relevant ICRH scenarios have been assessed during DT experiments on the JETtokamak using H mode divertor discharges with ITER-like shapes and safety factors. Deuteriumminority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, anICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermaldeuterons and thermal tritons. The Q value of the steady state discharge reached 0.22 for thelength of the RF flat-top (2.7 s), corresponding to three plasma energy replacement times. TheDoppler broadened neutron spectrum showed a deuteron energy of 125 keV, which was optimumfor fusion and close to the critical energy. Thus, strong bulk ion heating was obtained at the sametime as high fusion efficiency. Deuterium fractions around 20% produced the strongest ionheating together with a strong reduction of the suprathermal deuteron tail. The ELMs had low amplitude and high frequency and each ELM transported less plasmaenergy content than the 1% required by ITER. The energy confinement time, on the ITERH97-Pscale, was 0.90, which is sufficient for ignition in ITER. 3He minority heating, in approximately50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating. Central iontemperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV. The normalized H mode confinement time was 0.95. Second harmonic tritium heatingproduced energetic tritons above the critical energy. This scheme heats the electrons in JET,unlike in ITER where the lower power density will allow mainly ion heating. The invertedscenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successfulheating method producing both suprathermal neutrons and bulk ion heating. Theoreticalcalculations of the DT reactivity mostly give excellent agreement with the measured reactionrates.