The local stability analysis of plasma waves driven by runaway electrons (RE) is performed considering hot plasma Maxwellian background. This allows including hot plasma waves, such as the electron plasma waves (EPW) and the ion Bernstein waves (IBW), which are excluded by cold background plasma models. In addition, a new analytic model of RE distribution is proposed, based on the skew normal distribution. It seems appropriate to describe RE distribution with electrons that tend to accumulate around a peak in the momentum space. It is like distribution functions obtained by numerical solutions of the RE kinetic equation. Based on the perturbation theory, the wave equation and, for normal plasma modes, the growth rates or damping rates are derived. To this end, the contribution to the anti-Hermitian dielectric tensor due to the RE has been calculated for different RE distribution functions by the numerical code REDHPW (Runaway Electron Driven Hot Plasma Waves). It has been developed to analyse the local stability of the normal plasma modes. This analysis has been performed for typical plasma and RE parameters during the current ramp-up phase of FTU discharges. Lower hybrid waves (LH), already identified in FTU by experimental analysis, are shown as the leading unstable waves, with much larger growth rates than the whistler waves (WW). EPW and IBW are also found unstable. At the EPW-IBW confluence RF emissions near the integer multiples n of the ion cyclotron frequency fci, are expected. Nonlinear interactions, due to the large wave electric field expected at the confluence of the modes, might excite sub-harmonic oscillations at nfci/3 frequencies. Experimental data are discussed, suggesting the excitation of such hot waves during the ramp-up phase of plasma discharges in FTU.

The COMPASS tokamak provided unique conditions to study Runaway Electrons (RE) physics in the ITER-like geometry. Twelve experimental campaigns were dedicated to identify limiting thresholds for safe termination or efficient suppression of RE beam [1] and to thoroughly explore the possible advantages of alternative mitigation strategies. High flexibility and reproducibility of COMPASS RE scenarios allowed the execution of extensive scans devoted, for example, to decoupling effects of individual factors acting on RE dynamics. The studied factors include resonant magnetic field perturbations (RMP), radiation drag, instabilities, and the newly developed unique RE radial position feedback [2]. The main principle of the RE radial feedback was successfully used as a fast and robust method to obtain information about the RE energy in parallel to HXR radiation and unique calorimetry measurements. These methods are used to show the impact of different mitigation materials, externally triggered RMPs and the feedback itself on the RE beam energy and its current decay rate. The experimentally observed effects of RMPs are supported by a full orbit RE tracer developed at COMPASS, MARS-F [3] and ORBIT [4] codes. First tests with an active launch of low power 0.5 GHz waves into a RE populated low density plasma were performed. Preliminary results from measurements of electromagnetic waves in the 0.1-1.5 GHz range during this scenario are presented.

The study of Runaway Electron (RE) physics and their response to mitigation strategies is crucial to safeguard ITER structural integrity. During their motion REs collide with background ions before hitting the inner vessel of the machine and thus they emit Bremsstrahlung photons in the gamma range of the spectrum. It is possible to detect such radiation using a LaBr3(Ce) spectrometer with counting rate capability in the MHz range and high energy resolution [1][5]. The measured spectra contain information about the RE energy distribution, which can be reconstructed using specific inversion (or econvolution) algorithms. The deconvolution operation is computationally faster than first principles simulations and its use in RE studies might be many fold: it can be used to improve synthetic diagnostic calculations or as a preliminary method for RE spectra analysis.

Asymmetric vertical displacement event (AVDE) disruptions in ITER should produce a relatively small electromechanical force on the conducting structures surrounding the plasma, in contrast to previous predictions based on JET data. This is shown in simulations [1, 2] with theM3D 3DMHD code [3] and confirmed in JET experiments [4] in which the current was quenched with massive gas injection (MGI). In ITER the current quench (CQ) time tCQ is less than or equal to the resistive wall penetration time twall . JET is in a different param- eter regime, with tCQ/twall > 1. JET simulations were validated by comparison [1] to JET shot 71985 data and were in good agreement. The wall time twall was then artificially increased, keeping tCQ fixed, and it was found that the wall force decreased. The reduction of the asymmetric wall force was also found in analysis of experimental data of JET MGI mitigated disruption shots, although the published data only concerned the symmetric wall force [4]. Further simulations [2] were carried out of ITER AVDEs. For tCQ/twall 1, the force was 4MN, comparable to the force in JET. A fast CQ may cause production of runaway electrons (REs). Simulations using a modified version of M3D with a fluid RE model [5] will be presented.