This paper reports recent progress in the field of
γ-ray diagnosis of fast ions in the JET tokamak. The γ-rays, born in
nuclear reactions between fast ions and main plasma impurities and/or plasma
fuel ions, are analysed with a new modelling tool (the GAMMOD code) that has
been developed for a quantitative analysis of the measured γ-ray energy
spectra. The analysis of the γ-ray energy spectra identifies the
different fast ions giving rise to the γ-ray emission and assesses the
effective tail temperatures and relative concentrations of these fast ions.
This assessment is possible, since the excitation functions for the
different nuclear reactions are well established and exhibit a threshold
or/and a resonant nature. The capabilities of the γ-ray spectral
analysis are illustrated with the examples from the recent γ-ray
diagnostic measurements of 4He, 3He, deuterium and hydrogen ions
accelerated by ion-cyclotron resonance frequency heating in JET.
Simultaneous measurements of several fast ion species, including highly
energetic α-particles, are demonstrated. In addition to the
γ-spectroscopy, tomographic reconstructions of the radial profile of the
γ-ray emission are performed using the JET neutron profile monitor, thus
providing direct measurements of the radial profiles of fast ions in JET.
The onset of a neoclassical tearing mode (NTM) depends on the existence of a large enough seed island. It is shown in the Joint European Torus that NTMs can be readily destabilized by long-period sawteeth, such as obtained by sawtooth stabilization from ion-cyclotron heating or current drive. This has important implications for burning plasma scenarios, as alpha particles strongly stabilize the sawteeth. It is also shown that, by adding heating and current drive just outside the inversion radius, sawteeth are destabilized, resulting in shorter sawtooth periods and larger beta values being obtained without NTMs.
Gamma-ray images of fast D- and 4He-ions accelerated with third-harmonic ion-cyclotron-resonance heating of 4He-beam were simultaneously recorded for the first time in JET tokamak experiments dedicated to the investigation of burning plasmas with 3.5 MeV fusion alpha (α) particles. Gamma (γ) rays, born as a result of nuclear reactions, 9Be(4He, nγ)12C and 12C(D, pγ)13C, between the fast ions and the main plasma impurities, are measured using a two-dimensional multicollimator spectrometer array, which distinguishes the γ-rays from accelerated D- and 4He-ions. Tomographic reconstruction of the γ-ray emission profiles gives images of the fast-ion population in the poloidal cross-section. The potential of this technique to visualize several energetic ion species and to determine their behaviour in different plasma scenarios is demonstrated.
Abstract:During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimised by using hydrogen (H) plasmas where the reference ion cyclotron resonance frequency (ICRF) heating scenarios rely on minority species such as helium ( 3 He) or deuterium (D). This paper firstly describes experiments dedicated to the study of 3 He heating in H plasmas with a sequence of discharges in which 5 MW of ICRF power was reliably coupled and the 3 He concentration, controlled in real-time, was varied from below 1 % up to 10 %. The minority heating regime was observed at low concentrations (up to 2 %). Energetic tails in the 3 He ion distributions were observed with effective temperatures up to 300 keV and bulk electron temperatures up to 6 keV. At around 2 %, a sudden transition was reproducibly observed to the mode conversion regime, in which the ICRF fast wave couples to short wavelength modes, leading to efficient direct electron heating and bulk electron temperatures up to 8 keV. Secondly, experiments performed to study D minority ion heating in H plasmas are presented. This minority heating scheme proved much more difficult since modest quantities of carbon (C) impurity ions, which have the same charge to mass ratio as the D ions, led directly to the mode conversion regime.Finally, numerical simulations to interpret these two sets of experiments are under way and preliminary results are shown.
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