Understanding the impact of the synergistic effects on fast ion distribution function (DF) is essential for maximising the fusion rates in magnetic confinement fusion plasma. Because fusion reactions such as D-D, D-3He, D-T etc. have different energy dependencies, tailoring the energies of the fast ions to predetermined values optimal for fusion generation is of an asset. Recently, a new technique allowing to heat the plasma and generate energetic ions and modify their DF has been proposed [1]. The idea relies on using the three ion ICRH scheme, in which fast ions from neutral beam injection (NBI) resonate and absorb RF power in the vicinity of the mode conversion (MC) layer in mixed plasmas.
In this contribution, we extend the studies of recent JET experiments in H-D ~ 85%-15% plasma (2.9T/2MA) in which neutron rate was enhanced by a factor of 10-15 by applying 2.5MW of ICRH using D-(D NBI)-H three-ion scheme. An extensive analysis of this novel heating scenario has been carried out by means of integrated TRANSP/TORIC [2] modelling, and a comprehensive validation of the computed fast ion DF with a range of fast ion diagnostics available at JET is presented. The predicted acceleration of D NBI ions beyond their injection energies and the associated changes in fast ions DF by RF wave are found to be in good agreement with measured neutron yield and TOFOR neutron spectrometer measurements, as well as with multi-channel neutron camera observations and neutral particle analyser diagnostic. The results of our analysis confirm strong synergetic absorption of RF power by fast D NBI ions in the vicinity of the MC layer. In addition, this ICRH+NBI heating scheme was found to generate a large number of energetic passing ions since the condition for fast ions to have their Doppler-shifted resonance matching the MC layer is fulfilled for ions with parallel velocities being far from the trapped/passing boundary.
The impact of the modified fast ion DF on the Beam-Target (BT) fusion rates is analysed in details. The significant enhancement in the observed neutron rates is shown to be largely due to the strong tail in fast ions DF driven by ICRH. Finally, we give an outlook of the possible applications of the developed technique for future DTE2 studies on JET [3]. In particular, controlled acceleration of T-NBI ions in D-rich and D-NBI ions in T-rich plasmas to optimal energies can be applied to maximise BT fusion rates and contribute to the success of future D-T experiments at JET and ITER as illustrated in this study.
Acknowledgment: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
[1] J. Ongena, Ye. O. Kazakov, Yu. Baranov et al 2017 EPJ Web of Conferences 157 02006
[3] E. Joffrin and JET contributors 2018 27th IAEA FEC, Ahmedabad, India