Fast ions in mode conversion heating (³He)–H plasmas in JET

Abstract

Fast ions were analysed in experiments focusing on fundamental 3 He minority and mode conversion (MC) in the ion cyclotron resonance range of frequencies (ICRF) in H plasmas and on second harmonic heating of 3 He ions at 2.65 T mimicking D-T plasma heating in ITER at half its nominal toroidal magnetic field. Gamma-ray spectrometry, neutral particle analysers and fast-ion loss diagnostics provided information on the generation of fast-ion populations and on the distribution of ICRH power among the species in various heating scenarios and for a large range of 3 He concentrations. In the scenario with the fundamental 3 He minority and MC wave heating at B T (0) = 3.41 T and f ≈ 32 MHz, fast 3 He ions accelerated by ICRH in the MeV energy range were detected in discharges with low 3 He concentration. In the experiments with a 3 He concentration scan it was found that at a 3 He concentration of ≈2.2% the 3 He ion losses disappeared while a population of energetic D ions gradually built up due to a redistribution of the ICRH power between species on reaching the first MC regime. Under those conditions the ICRF-heated D beam ions effectively absorbed the wave power at their Doppler shifted resonance, which was close to the plasma centre. In discharges with second harmonic heating of 3 He ions at B T (0) = 2.65 T and f ≈ 52 MHz, the confined energetic 3 He ions were found in the MeV energy range. There is some evidence that the D ions were also accelerated by ICRF. This paper also demonstrates that the synergy of the various fast ion diagnostics allows making a broad picture of the physics of the redistribution of the absorbed ICRH power in complicated heating scenarios of JET.

Minority and mode conversion heating in (3He)–H JET plasmas

“…Rather, moderately fast RF heated D beam ions colliding with 3 He trigger the D( 3 He,p) 4 He reaction forming 3.6 MeV α-particles and 15 MeV protons; in the range of effective (D) temperatures of 200-400 keV, this reaction has a cross-section that is about 4 times higher than that of the D(D,n) 3 He reaction. The fact that reasonably fast D is present is also inferred from evidence that the D( 3 He,γ ) 5 Li branching reaction is taking place at high 3 He concentrations: this reaction gives rise to a broad spectrum of gamma rays with energies between 11 and 17 MeV (not shown); for more details both on the fast 3 He as on the fast D and 4 He subpopulations in the presently studied scenario, see [20].…”

“…JET has a number of diagnostics that allow monitoring the fast particles, either directly (as is the case for the fast lost ion collector [20] and neutral particle analyzer [21]) or indirectly (as is the case for the gamma ray spectrometers [20] or the time-of-flight neutron diagnostic [22]). …”

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

“…The detection of γ-ray emission and fast ion losses indicated the presence in these pulses of 3 He ions with energies in the MeV range [22]. Trace quantities of deuterium (D) and 4 He ions were also present, principally due to the use of deuterium neutral beam injection and wall recycling [22]. Table 1 lists some of the relevant parameters for ten shots in which ICE was observed, including the vacuum toroidal field at a reference major radius of 2.96 m (B T ), and the ICRF power (P RF ) and neutral beam power (P NBI ) at the time t ICE of ICE detection.…”

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

“…Gamma-ray measurements suggest, however, that the D ions are not accelerated to energies above 500 keV in any of the discharges. A more detailed discussion of the fast ions observed in these experiments is presented in another paper of this edition [27].…”