Experimental studies of instabilities and confinement of energetic particles on JET and MAST

Abstract

In preparation for next step burning plasma devices such as ITER, experimental studies of instabilities and confinement of energetic ions were performed on Joint European Torus (JET) and on Mega-Amper Spherical Tokamak (MAST) with innovative diagnostic techniques, in conventional and shear-reversed plasmas, exploring a wide range of effects for energetic ions. A compendium of recent results testing capabilities of the present-day facilities for burning plasma relevant study is presented in this paper. ‘Alpha tail’ production using 3rd harmonic ion-cyclotron resonance heating (ICRH) of 4He beam ions has been employed on JET for studying 4He of the megaelectronvolt energy range in a ‘neutron-free’ environment. The evolution of ICRH-accelerated ions of 4He with E ⩾ 1.7 MeV and D with E ⩾ 500 keV was assessed from nuclear gamma-ray emission born by the fast ions colliding with Be and C impurities. A simultaneous measurement of spatial profiles of fast 4He and fast D ions relevant to ITER was performed for the first time in positive and strongly reversed magnetic shear discharges. Time-resolved gamma-ray diagnostics for ICRH-accelerated 3He and H minority ions allowed changes in the fast ion distribution function to be assessed in the presence of unstable toroidal Alfvén eigenmodes (TAEs) and sawteeth. A significant decrease of gamma-ray intensity from protons with E ⩾ 5 MeV was detected during the ‘tornado’ modes. This was interpreted as ‘tornado’-induced loss of fast ions with the drift orbit width, Δf, comparable to the minor radius of tokamak a. Experiments performed in the opposite case, Δf/a ≪ 1, for ICRH-accelerated 3He ions with E ⩾ 500 keV, have shown excitation of numerous Alfvén eigenmodes without a significant degradation of the fast ion confinement. The stabilizing effect of fast particles on ‘monster’ sawteeth was experimentally found to fail in low-density plasmas with high power ion cyclotron resonance frequency (ICRF)-heating. The transition from the ‘monster’ to short-period ‘grassy’ sawteeth was investigated with different ICRF phasing, which controls the pinch-effect and radial distribution of ICRF-accelerated ions. Instabilities excited by super-Alfvénic beam ions were investigated on the spherical tokamak MAST. Due to higher values of β and a higher proportion of fast ions on MAST than on JET, a wider variety of modes and nonlinear regimes for the Alfvén instabilities were observed, including the explosive TAE-regimes leading to the formation of hole-clump pairs on the fast ion distribution function. The MAST and START data showed that TAE and chirping modes decrease both in their mode amplitudes and in the number of unstable modes with increasing β.