In magnetic fusion reactors relying on the burning of deuterium and tritium, sufficient confinement of the alpha particles produced in the nuclear reactions is crucial to sustaining the burning plasma. In this article the interactions of these energetic particles with linear and nonlinear Alfve'n waves generated in the magnetized plasma are reviewed.

https://link.aps.org/accepted/10.1103/RevModPhys.88.015008

Physics of Alfvén waves and energetic particles in burning plasmas

A model for sawtooth oscillations in tokamak experiments is outlined. A threshold criterion for the onset of internal kink modes and a prescription for the relaxed profiles immediately after the sawtooth crash have been implemented in a transport code that evolves the relevant plasma parameters. In this paper, applications of this model to the prediction of the sawtooth period and amplitude in projected ITER discharges are discussed. It is found that sawteeth can be stabilized transiently by the fusion alpha particles in ITER for periods that are long on the energy confinement timescale (). The sawtooth period depends on the amount of reconnected flux at the preceding sawtooth crash. When Kadomtsev's full reconnection model is used, the period can exceed 100 s. The sawtooth mixing radius following long duration sawtooth ramps can easily exceed half the plasma minor radius, raising questions about the desirability of transient sawtooth suppression.

Model for the sawtooth period and amplitude

The heating neutral beam injectors (HNBs) of ITER are designed to deliver 16.7 MW of 1 MeV D0 or 0.87 MeV H0 to the ITER plasma for up to 3600 s. They will be the most powerful neutral beam (NB) injectors ever, delivering higher energy NBs to the plasma in a tokamak for longer than any previous systems have done. The design of the HNBs is based on the acceleration and neutralisation of negative ions as the efficiency of conversion of accelerated positive ions is so low at the required energy that a realistic design is not possible, whereas the neutralisation of H− and D− remains acceptable (≈56%). The design of a long pulse negative ion based injector is inherently more complicated than that of short pulse positive ion based injectors because: • negative ions are harder to create so that they can be extracted and accelerated from the ion source; • electrons can be co-extracted from the ion source along with the negative ions, and their acceleration must be minimised to maintain an acceptable overall accelerator efficiency; • negative ions are easily lost by collisions with the background gas in the accelerator; • electrons created in the extractor and accelerator can impinge on the extraction and acceleration grids, leading to high power loads on the grids; • positive ions are created in the accelerator by ionisation of the background gas by the accelerated negative ions and the positive ions are back-accelerated into the ion source creating a massive power load to the ion source; • electrons that are co-accelerated with the negative ions can exit the accelerator and deposit power on various downstream beamline components. The design of the ITER HNBs is further complicated because ITER is a nuclear installation which will generate very large fluxes of neutrons and gamma rays. Consequently all the injector components have to survive in that harsh environment. Additionally the beamline components and the NB cell, where the beams are housed, will be activated and all maintenance will have to be performed remotely. This paper describes the design of the HNB injectors, but not the associated power supplies, cooling system, cryogenic system etc, or the high voltage bushing which separates the vacuum of the beamline from the high pressure SF6 of the high voltage (1 MV) transmission line, through which the power, gas and cooling water are supplied to the beam source. Also the magnetic field reduction system is not described.

https://iopscience.iop.org/article/10.1088/1367-2630/19/2/025005/pdf

Overview of the design of the ITER heating neutral beam injectors

The interaction of externally applied small non-axisymmetric magnetic perturba16 tions (MP) with tokamak high-confinement mode (H-mode) plasmas is reviewed and illus17 trated by recent experiments in ASDEX Upgrade. The plasma response to the vacuum MP 18 field is amplified by stable ideal kink modes with low toroidal mode number n driven by the 19 H-mode edge pressure gradient (and associated bootstrap current) which is experimentally 20 evidenced by an observable shift of the poloidal mode number m away from field alignment 21 (m = qn, with q being the safety factor) at the response maximum. A torque scan experiment 22 demonstrates the importance of the perpendicular electron flow for shielding of the resonant 23 magnetic perturbation, as expected from a two-fluid MHD picture. Two significant effects of 24 MP occur in H-mode plasmas at low pedestal collisionality, νped ≤ 0.4: (a) a reduction of the 25 global plasma density by up to 50% and (b) a reduction of the energy loss associated with edge 26 localised modes (ELMs) by a factor of up to 10. A comprehensive database of ELM mitigation 27 pulses at low ν∗ in ASDEX Upgrade shows that the degree of ELM mitigation correlates with 28 the reduction of pedestal pressure which in turn is limited and defined by the onset of ELMs, 29 i. e. a modification of the ELM stability limit by the magnetic perturbation. 30 PACS numbers: 52.55.Fa, 52.55.Tn, 52.65.Kj 31 2

https://iopscience.iop.org/article/10.1088/0741-3335/59/1/014049/pdf

Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade

On the road to a fusion reactor, a thorough control of the fast-ion distribution plays a crucial role. Fusion-born $$\alpha$$
 -particles are, indeed, a necessary ingredient of self-sustained burning plasmas. Recent developments in the diagnostic of fast-ion distributions have significantly improved our predictive capabilities towards future devices. Here, we review key diagnostic techniques for confined and lost fast ions in tokamak and stellarator plasmas. We discuss neutron and gamma-ray spectroscopy, fast-ion D-
 $$\alpha$$
 spectroscopy, collective Thomson scattering, neutral particle analyzers, and fast-ion loss detectors. The review covers physical principles of each diagnostic, sensitivities, basic setups, and operational parameters. The review is largely (but not exclusively) based on the contributions from ASDEX Upgrade and JET. Finally, we discuss integrated data analysis of fast-ion diagnostics by velocity-space tomography which allows measurements of 2D velocity distribution functions of confined fast ions.

/pdf/recent-progress-in-fast-ion-diagnostics-for-magnetically-56gqrh8v70.pdf

Recent progress in fast-ion diagnostics for magnetically confined plasmas

This paper discusses the behaviour and consequences of the expected populations of energetic ions in ITER plasmas. It begins with a careful analytic and numerical consideration of the stability of Alfven Eigenmodes in the ITER 15 MA baseline scenario. The stability threshold is determined by balancing the energetic ion drive against the dominant damping mechanisms and it is found that only in the outer half of the plasma (r/a>0.5) can the fast ions overcome the thermal ion Landau damping. This is in spite of the reduced numbers of alpha-particles and beam ions in this region but means that any Alfven Eigenmode-induced redistribution is not expected to influence the fusion burn process. The influence of energetic ions upon the main global MHD phenomena expected in ITER's primary operating scenarios, including sawteeth, neoclassical tearing modes and Resistive Wall Modes, is also reviewed. Fast ion losses due to the non-axisymmetric fields arising from the finite number of toroidal field coils, the inclusio...

K. Tani

Papers

Energetic ions in ITER plasmas