MeV-range fast ion losses induced by fishbones on JET

Abstract

Abstract. Energy and pitch angle resolved measurements of highly energetic (megaelectronvolt range) suprathermal ions ejected from the plasma through interaction with fishbone oscillations are presented. The measurements are obtained with a 2-D scintillator probe diagnostic installed on JET, which is designed to detect lost ions only above a certain energy threshold (E min,D ∼ 200 keV). In the case reported here the lost ions are identified as fast protons which had been accelerated to high energies by ICRF minority heating. The energy of the lost protons (∼ 0.5-4 MeV) is approximately one order of magnitude higher than the energy of the injected beam ions (max. 130 keV) driving the fishbone. Losses arriving at the probe are enhanced by about a factor 10-20 with respect to MHD-quiescent levels, and are found to increase quadratically with the fishbone amplitude. Using a number of simplifying assumptions, numerical simulations have been performed which combine the HAGIS, MISHKA and SELFO codes (where the distribution function predicted by SELFO has been validated against NPA measurements). The losses are found to originate from orbit stochastic diffusion of trapped protons near the plasma boundary or from counter-passing protons deep in the plasma core which transit under the influence of the fishbone into an unconfined trapped orbit. The simulations show further that the losses are of nonresonant type. The simulated energy and pitch angle distribution of the losses, the ‡ See the Appendix of F. Romanelli et al., Proceedings of the 22nd IAEA Fusion Energy Conference, Geneva, Switzerland, 2008 MeV-range fast ion losses induced by fishbones on JET 2 temporal behaviour of the losses during a fishbone cycle and the scaling of the losses with the fishbone amplitude are compared with experiment. The simulation results are mostly in broad agreement with experiment, but some of the predictions could not be reconciled with experiment using this model.

Study of Interaction between Fast Ions and MHD Instabilities by Fusion Product Measurements in Joint European Torus

“…The experimental observation of the non-resonant losses of trapped energetic ions in the presence of NBIdriven low-frequency fishbones [29] was found to be in line with theory [28]. This effect could be important for fusion-born alpha-particles in scenarios with fishbones (e.g.…”

“…In order to validate the theory of the non-resonant losses, JET experiments were performed by measuring losses of highly energetic ions in the MeV energy range in the presence of fishbones driven by NBI ions with energy 80 -100 keV. Namely, the energy and pitch angle resolved SP measurements of MeV-ions ejected from the plasma during the non-resonant fishbone oscillations were studied [29]. The lost ions are identified as fast protons accelerated by ICRH (∼0.5 -4 MeV).…”

Fusion product losses due to fishbone instabilities in deuterium JET plasmas

“…However, a comparison with the MeV H-ion loss findings obtained by authors in [13], in particular the quadratic increase with the FB amplitude, is not possible in this case; an accurate dependence of the FP loss vs the FB amplitude cannot be determined because most of the magnetic signals were saturated.…”

“…The high energy particles ejected from the plasma during the FB oscillations were identified as H-ions accelerated during ion cyclotron range of frequencies (ICRF) heating. The losses were enhanced by about a factor 10 -20 with respect to the MHD-quiescent levels, and the magnitude of the losses was found to increase quadratically with the FB amplitude [13].…”

Study of fast-ion transport induced by fishbones on JET

“…This is a well diagnosed discharge that has been previously documented for the study of fishbonerelated fast ion losses with a scintillator probe detector [24,25]. The main discharge characteristics and experimental findings are repeated below for convenience, for further details the reader is referred to [24]. Discharge 69100 is an ELMy H-mode discharge with conventional (fully relaxed) qprofile.…”