Plasma disruptions affect plasma-facing and structural components of tokamaks due to electromechanical forces, thermal loads and generation of high energy runaway electrons (REs). Asymmetries in poloidal halo and toroidal plasma current can now be routinely measured in four positions 90 • apart. Their assessment is used to validate the design of the ITER vessel support system and its in-vessel components. The challenge of disruption thermal loads comes from both the short duration over which a large energy has to be lost and the potential for asymmetries. The focus of this paper will be on localized heat loads. Resonant magnetic perturbations failed to reduce the generation of REs in JET. An explanation of the limitations applying to these attempts is offered together with a minimum guideline. The REs generated by a moderate, but fast, Ar injection in limiter plasmas show evidence of milder and more efficient losses due to the high Ar background density.

The COMPASS tokamak, recently transferred from UKAEA Culham to IPP Prague, is equipped with a set of saddle coils for producing controlled resonant magnetic perturbations (RMPs). In the future experimental programme of COMPASS we plan to focus on studies of RMPs, especially in view of their application as an ELM control mechanism and their considered use in ITER. In the present contribution we describe the preparatory calculations for the planned experiments. We computed the spectra of perturbations for several different equilibria predicted by MHD simulations and determined the positions and sizes of the resulting islands. It is shown how the saddle coils of COMPASS can be adapted to our equilibria to obtain good island overlap at the edge, which is believed to be a key component in the ELM mitigation effect. The techniques used for adapting the coils to achieve this result are described. Those are fairly general and could be used in the design of RMP coils on other machines.

This paper discusses the stochastic (E × B) motion of ions in the homogeneous magnetic field and in the spatially periodical electrostatic potential. This approach is used for the simulation of ion diffusion in potential structures formed at the boundary plasma in tokamaks. We show that the ion motion in such structures scales with a single dimensionless parameter R, which is a combination of amplitude and dimension of potential structures, magnetic field intensity and ion charge/mass ratio. The presented results show that for sufficiently large values of R, the intrinsic stochasticity regime of ion motion appears, which results in the spatial diffusion of ions. The diffusion coefficient is sufficiently high for heavy impurities. Possible further applications are mentioned.

The diffusion of electrons in velocity space induced by a spectrum of electrostatic waves is numerically investigated. From the scatter of particles, the diffusion coefficient is calculated and compared with the quasilinear value. A strong disagreement is found at high electric field amplitudes, narrow spectra and short (spatial) lengths of the interaction region, which are however usual in Lower Hybrid Heating and Current Drive experiments in tokamaks. In particular, the (velocity) interaction region becomes significantly broader than the Landau resonance region for electric field amplitudes above ∼10 kV m −1 .

The results of computer simulations of lower hybrid current drive (LHCD) for ITER parameters are presented. The damping of LH waves by alpha particles due to the interaction at high cyclotron resonances is obtained by solving the 2-D Fokker-Planck equation with the corresponding quasi-linear term (FPA code). This code is incorporated into a spatially 1-D LHCD code so as to obtain radial profiles of the radiofrequency (RF) power absorbed by electrons (1-D Fokker-Planck code) and alpha particles as well as the radial profile of the driven current. One-pass absorption is assumed and verified by a detailed analysis of LH rays for an ITER-like magnetic configuration. Noticeable absorption by alpha paraticles is observed only below ∼5 GHz of the generator frequency and is still acceptable down to ∼3 GHz (19% of the input power Pin, with a 16% reduction of the total driven current for Pin = 50 MW). Two competing quasi-linear effects influence the damping rates: the RF field reduces the slopes of the alpha particle distribution function but, since the absorption is accompanied by acceleration of groups of alpha particles above their initial velocity, the RF field also increases the number of resonant particles. Both the global results and the damping rates are compared with those obtained for unmagnetized alpha particles and with the linear approximation. For frequencies above ∼3 GHz, the latter approach was found to significantly overestimate the absorption by alpha particles. Some further mechanisms not included in the code but having possible effects on the interaction are also discussed.