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.

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.

Plasmas heated by ICRF only in the JET tokamak show distinct structures in the toroidal rotation profile, with regions where dω/dr>0 when the minority cyclotron resonance layer is far off-axis. The rotation is dominantly co-current with a clear off-axis maximum. There is only a slight difference between a high-field side (HFS) or a low-field side position of this resonance layer: the off-axis maximum in the rotation profile is modestly higher for the HFS position. This is in contrast to the predictions of theories that rely mainly on the effects arising from ICRF-driven fast ions to account for ICRF-induced plasma rotation. The differences due to the direction of the antenna spectrum (co- or counter-) are small. A more central deposition of the ICRF power in L-mode and operation in H-mode both lead to more centrally peaked profiles, both in the co-direction. Strong MHD modes brake the rotation and lead to overall flat rotation profiles.

Using standard guiding centre (gc) variables, we have obtained general expressions for the contributions of individual gc orbits to the linear radiofrequency response of a tokamak plasma. The theory is therefore valid for general equilibrium distribution functions (namely, arbitrary functions of the constants of the motion). Particle motion is described to first order inclusive in the drift approximation.Particular emphasis is put on the importance of wave-particle phase decorrelation; a simplified model of decorrelation due to the effect of Coulomb collisions, based on other authors' work, is incorporated. This allows, for instance, the description of the transition from correlated to uncorrelated resonance crossings occurring near tangent resonance.Two successive asymptotic expansions, based on the important inhomogeneity along the trajectory induced by toroidicity and rotational transform, allow drastic analytical simplifications for a broad class of orbits and interactions; these situations generally coincide with a rapid phase decorrelation. Special attention is paid therein to a realistic description of tangent resonance phenomena.The opposite regime, where the inhomogeneity is weak along the gc orbit, lends itself to direct numerical investigation, and is generally associated with stronger non-local effects such as resonances between the gyromotion, the gc bounce motion and the Doppler-shifted wave frequency.Our results allow the self-consistent (full-wave) study of many radiofrequency wave propagation and absorption scenarios, as well as instabilities; we have indeed applied a common mathematical treatment to these various mechanisms. The present work has been carried out with a marked orientation toward practical applications, which will follow in future publications. † We use the terms 'poloidal bounce' and 'bounce frequency ω b ' indistinctly for passing and trapped orbits. They always relate to a full poloidal revolution of the projection of the gc trajectory in a reference poloidal plane. † This property is absent from several existing models. It is lost as soon as the poloidal magnetic field is taken into account and different geometries are used to solve the Vlasov equation and Maxwell's equations. 'Non-resonant' global wave absorption (i.e. linked with the real part of the plasma dispersion function in a Maxwellian plasma) results and may dramatically alter the numerical solutions.

Ion cyclotron resonance frequencies (ICRF) mode conversion has been developed for localized on-axis and off-axis bulk electron heating on the JET tokamak. The fast magnetosonic waves launched from the low-field side ICRF antennas are mode-converted to short-wavelength waves on the high-field side of the 3 He ion cyclotron resonance layer in D and 4 He plasmas and subsequently damped on the bulk electrons. The resulting electron power deposition, measured using ICRF power modulation, is narrow with a typical full-width at half-maximum of ≈30 cm (i.e. about 30% of the minor radius) and the total deposited power to electrons comprises at least up to 80% of the applied ICRF power. The ICRF mode conversion power deposition has been kept constant using 3 He bleed throughout the ICRF phase with a typical duration of 4-6 s, i.e. 15-40 energy confinement times. Using waves propagating in the counter-current direction minimizes competing ion damping in the presence of co-injected deuterium beam ions.

The ITER Ion Cyclotron Heating and Current Drive system will deliver 20MW of radio frequency power to the plasma in quasi continuous operation during the different phases of the experimental programme. The system also has to perform conditioning of the tokamak first wall at low power between main plasma discharges. This broad range of reqiurements imposes a high flexibility and a high availabiUty. The paper highlights the physics and design reqiurements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedide.

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This paper summarizes the physical principles behind the novel three-ion scenarios using radio frequency waves in the ion cyclotron range of frequencies (ICRF). We discuss how to transform mode conversion electron heating into a new flexible ICRF technique for ion cyclotron heating and fast-ion generation in multi-ion species plasmas. The theoretical section provides practical recipes for selecting the plasma composition to realize three-ion ICRF scenarios, including two equivalent possibilities for the choice of resonant absorbers that have been identified. The theoretical findings have been convincingly confirmed by the proof-of-principle experiments in mixed H–D plasmas on the Alcator C-Mod and JET tokamaks, using thermal 3He and fast D ions from neutral beam injection as resonant absorbers. Since 2018, significant progress has been made on the ASDEX Upgrade and JET tokamaks in H–4He and H–D plasmas, guided by the ITER needs. Furthermore, the scenario was also successfully applied in JET D–3He plasmas as a technique to generate fusion-born alpha particles and study effects of fast ions on plasma confinement under ITER-relevant plasma heating conditions. Tuned for the central deposition of ICRF power in a small region in the plasma core of large devices such as JET, three-ion ICRF scenarios are efficient in generating large populations of passing fast ions and modifying the q-profile. Recent experimental and modeling developments have expanded the use of three-ion scenarios from dedicated ICRF studies to a flexible tool with a broad range of different applications in fusion research.