Plasma heating and generation of energetic D ions with the 3-ion ICRF + NBI scenario in mixed H-D plasmas at JET-ILW

Abstract

Dedicated experiments were conducted in mixed H-D plasmas in JET to demonstrate the efficiency of the 3-ion ICRF scheme for plasma heating, relying on injected fast NBI ions as resonant ion species. Strong core localization of the RF power deposition in the close vicinity of the ion-ion hybrid layer was achieved, resulting in an efficient plasma heating, generation of energetic D ions, strong enhancement of the neutron rate and observation of Alfvénic modes. A consistent physical picture that emerged from a range of fast-ion measurements at JET, including neutron and gamma-ray measurements, a high-energy neutral particle analyzer and MHD mode localization analysis, is presented. The possibility to moderate the fast-ion energies with the ratio PICRF/PNBI and the choice of the NBI injectors is demonstrated. An outlook of possible applications of the 3-ion schemes, including a recent example of its use in mixed D-3 He plasmas in JET and promising schemes for D-T plasmas, are presented.

Generation and observation of fast deuterium ions and fusion-born alpha particles in JET $\mathrm{D-^3He}$ plasmas with the 3-ion radio-frequency heating scenario

“…Note the dual function for 3 He ions in this fast-ion scenario. The concentration of 3 He ions not only defines the location of the IIH layer (similar to the effect of the isotopic H:D ratio in earlier experiments with the 3-ion D-(D NBI )-H scenario [5]), but the 3 He ions also act as the target in D − 3 He fusion reactions [7] with deuterons accelerated by ICRF: D (fast) + 3 He (target) → 4 He(3.6MeV) + p (14.7MeV)…”

“…This observation, in combination with the direct comparison of T e and T i profiles for D − 3 He plasmas heated with NBI-only at the same total heating power, plasma density and other operational conditions (see Fig. 12 in [5]), implies that turbulent transport may have been reduced when the 3-ion D − (D NBI ) − 3 He was applied for plasma heating. The rich and partly unexpected variety of phenomena that are present in the discharges we have studied raise a number of questions that require a detailed further investigation.…”

“…1), there is a weak electromagnetic branch of this reaction, i.e. d + 3 He → γ + 5 Li , with 5 Li as the fusion product and a probability ≈10 −5 . This leads to the generation of high-energy gammas, E γ ≈ 17 MeV [24,25].…”

“…The fast-ion distribution function of the RF-accelerated beam ions can be controlled by varying the ratio P ICRF /P NBI . In this way rather high neutron rates can be obtained with a moderate amount of total auxiliary heating power, as shown in proof-of-principle studies in H-D plasmas on JET [5,6]. In view of demonstrating the feasibility of this heating scenario for alpha physics studies in D-T, we have performed a series of dedicated experiments in mixed D − 3 He plasmas on JET (B 0 ≈ 3.7T, I p = 2.5MA, n e0 ≈ 6 × 10 19 m −3 , f = 32.2-33.0 MHz, dipole phasing; B 0 , I p , n e0 and f are here the core magnetic field, plasma current, core electron density and applied RF frequency, respectively).…”

“…To this end, the Joint European Torus (JET) has established an ambitious scientific program in preparation of its future deuterium-tritium (DTE2) campaign, aiming, among others, at developing dedicated scenarios where the effects of alpha particles from the fusion reactions can be studied [1,2]. One possibility is offered by the 3-ion radio-frequency (RF) heating scenario [3,4] in D-T plasmas, by accelerating tritons or deuterons from neutral beam injection (NBI) to high enough energies that maximize the D-T reactivity [5] at moderate input auxiliary heating power. This fast-ion scenario relies on the combined use of NBI and ICRF where NBI provides the source of resonant absorbers at the ion-ion hybrid (IIH) layer in mixed plasmas, where the left-hand RF polarization is strongly enhanced.…”