The runaway electron event is the fundamental physical phenomenon and tokamak is the most advanced conception of the plasma magnetic confinement. The energy of disruption generated runaway electrons can reach as high as tens of mega-electron-volt and they can cause a catastrophic damage of plasma-facing-component surfaces in large tokamaks and International Thermonuclear Experimental Reactor (ITER). Due to its importance, this phenomenon is being actively studied both theoretically and experimentally in leading thermonuclear fusion centers. Thus, effective monitoring of the runaway electrons is an important task. The synchrotron radiation diagnostic allows direct observation of such runaway electrons and an analysis of their parameters and promotes the safety operation of present-day large tokamaks and future ITER. In 1990 such diagnostic had demonstrated its effectiveness on the TEXTOR (Tokamak Experiment for Technology Oriented Research, Germany) tokamak for investigation of runaway electrons beam size, position, number, and maximum energy. Now this diagnostic is installed practically on all the present-day’s tokamaks. The parameter v┴/|v||| strongly influences on the runaway electron synchrotron radiation behavior (v|| is the longitudinal velocity, v┴ is the transverse velocity with respect to the magnetic field B). The paper is devoted to the theoretical investigation of runaway electron synchrotron radiation spot shape when this parameter is not small that corresponds to present-day tokamak experiments. The features of the relativistic electron motion in a tokamak are taken into account. The influence of the detector position on runaway electron synchrotron radiation data is discussed. Analysis carried out in the frame of the nonlinear cone model. In this model, the ultrarelativistic electrons emit radiation in the direction of their velocity v→ and the velocity vector runs along the surface of a cone whose axis is parallel to the magnetic field B. The case of the small parameter v┴/|v||| (v┴/|v|||<<1, linear cone model) was considered in the paper: Plasma Phys. Rep. 22, 535 (1996) and these theoretical results are used for experimental data analysis.

The secondary runaway electrons generation is the process in which already existing high energy runaway electrons knock out thermal plasma electrons directly into the runaway region by close Coulomb collisions. Such knocked-on electrons are immediately accelerated to ultrarelativistic velocities, since in the runaway region the toroidal electric field force overcomes the collisional friction force with thermal plasma particles. The avalanche of runaway electrons with mega-electron-volt energy emerges, hit of which with the construction elements of large-scale tokamaks and future international tokamak ITER can lead to catastrophic consequences. Due to its importance, this phenomenon is being actively studied both theoretically and experimentally in leading thermonuclear fusion centers. It is known that during secondary generation, the value of the transversal component of knocked-on electrons momentum with respect to the confining magnetic field may be significantly higher than the longitudinal one: p⊥ >> p∥. Thus, conditions for knocked-on electron trapping in a non-uniform tokamak magnetic field occur (banana orbits). Such electrons can no longer be accelerated by the inducted toroidal electric field to high energies, avalanche formation is partially suppressed. The question is how long this population of knocked-on and trapped electrons exists. In the presented paper, it is shown the additional possibility of formation and existence of such long-lived banana orbits of suprathermal electrons under conditions of plasma MHD activity when MHD instability spikes induced the strong burst of the toroidal electric field that results in the abrupt growth in these knocked-on and trapped electrons. This phenomenon is considered for the recent low-density EAST (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China) tokamak quasistationary runaway discharges. Long-lived trapped electrons (p⊥ >> p∥) also have an influence on the intensity of ECE emission. The considered phenomenon is important for correct interpretation of the runaway experiments on present-day tokamaks.

The energy of disruption generated runaway electrons can reach as high as tens of megaelectron volt energy and they can cause a serious damage of plasma-facing-component surfaces in large tokamaks like International Thermonuclear Experimental Reactor [1]. At the same time, the quiescent runaway electron generation during the flat-top of DIII-D low density Ohmic discharges allows accurate measurement of all key important parameters to runaway electron excitation [2]. Using a test particle description (like [3]) that includes acceleration in the toroidal electric field and collisions with the plasma particles the generation of suprathermal electrons is analyzed under conditions of gas puffing. In presented modeling, the plasma parameter behavior close to the DIII-D quiescent runaway shot #152895 parameters is used. For this puffed discharge the growth and decay of high-frequency ECE signal was in disagreement with the HXR and synchrotron emission signals. Possibility of formation of the suprathermal

The strong suprathermal electron generation is modeled in conditions of bursts of the strong MHD plasma activity in the Experimental Advanced Superconducting Tokamak (EAST) when magnetic field line reconnections took place. Because of the fast changes in the magnetic flux during these magnetic field line reconnections the instant bursts of the induced electric field occur. The instant changes in suprathermal electron density during these bursts of the induced electric field have been analyzed.

The energy of disruption generated runaway electrons can reach as high as tens of megaelectronvolt and they can cause a serious damage of plasma-facing-component surfaces in large tokamaks like International Thermonuclear Experimental Reactor (ITER).The synchrotron radiation diagnostic allows a direct observation of such runaway electrons and an analysis of their parameters and promotes the safety operation of present day large tokamaks and future ITER.Only this diagnostic will be applied in ITER.In the paper detail analysis of the synchrotron radiation spectra of runaway electrons for the recent Experimental Advanced Superconducting Tokamak (EAST, Institute of Plasma Physics of Chinese Academy of Sciences) experiment parameters has been presented.The calculations are carried out on the base of precise expression for synchrotron radiation spectral density.They make more precise spectra analysis of the previous paper by Zhou R.J., Pankratov I.M., Hu L.Q., et al. (Physics of Plasmas, 2014, Vol.21, No. 6, 063302).Obtained results are important for correct interpretation of runaway EAST experiments and runaway experiments in other tokamaks.

Peculiar phenomena were observed during experiments with runaway electrons: rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the electron cyclotron emission (ECE) signal (cyclotron radiation of suprathermal electrons). These phenomena were initially observed in TEXTOR (Tokamak Experiment for Technology Oriented Research), where these events only occurred in the current decay phase or in discharges with thin stable runaway beams at a q = 1 drift surface. These rapid changes in the synchrotron spot were interpreted by the TEXTOR team as a fast pitch angle scattering event. Recently, similar rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the non-thermal ECE signal were observed in the EAST (Experimental Advanced Superconducting Tokamak) runaway discharge. Runaway electrons were located around the q = 2 rational magnetic surface (ring-like runaway electron beam). During the EAST runaway discharge, stepwise ECE signal increases coincided with enhanced magnetohydrodynamic (MHD) activity. This behavior was peculiar to this shot. In this paper, we show that these non-thermal ECE step-like jumps were related to the abrupt growth of suprathermal electrons induced by bursting electric fields at reconnection events during this MHD plasma activity. Enhancement of the secondary runaway electron generation also occurred simultaneously. Local changes in the current-density gradient appeared because of local enhancement of the runaway electron generation process. These current-density gradient changes are considered to be a possible trigger for enhancement of the MHD plasma activity and the rapid changes in runaway beam behavior.

A detailed analysis of the spectra of synchrotron radiation emitted by runaway electrons, and an analysis of synchrotron radiation spot shapes are presented for EAST runaway cases. Conditions required for the asymptotic expressions of synchrotron radiation spectra to be valid are studied for these EAST parameters. We provide the correct synchrotron radiation spectra in typical EAST discharges, and we show results of calculations of the shape of the synchrotron radiation spots emitted by runaway electrons. These shapes are detected by a visible light camera in EAST. Safety factor q(r), the horizontal displacement of electron drift surfaces with respect to the magnetic surfaces δe, pitch angle θp, and the position of the camera were taken into account. Our results indicate that the θp and q profiles can significantly affect the synchrotron radiation spot shape; it is simpler to record all synchrotron radiation if the camera is placed far from the plasma. An asymmetrical synchrotron radiation spot shape can be deduced when the effect of the drift orbit shift is taken into account. Our results can explain the asymmetrical ring-like synchrotron radiation spot shape from runaway electron beams in EAST experiments.

In the ℓ = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ≲ ω ci ). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna.

Recent results of the experimental program on the stellarator-type device Uragan-3M at the IPP in Kharkov are presented. Efforts were focused mainly on optimization of the operation of the frame-type radiofrequency antenna to produce a target plasma for the three-half-turn antenna. Different regimes of the Uragan-3M operation, which are characterized by different temporal behavior of the average plasma density, electron cyclotron emission radiation intensity and particle confinement time, are considered. Elementary atomic processes responsible for plasma creation are studied. The particle confinement time for the Uragan-3M plasmas is estimated. Measurements of energy spectra of charge exchange atoms are carried out. The principal possibility of realizing a ‘stellarator–magnetic mirror’ scheme as a prototype of a stellarator-mirror fusion–fission hybrid is shown for Uragan-2M. Future plans are discussed.

Recently the possibility of resonant excitation of pressure perturbation by external helical magnetic perturbations near the rotating plasma edge was shown taking into consideration the finite plasma conductivity. In present paper the influence of the small safety factor variation on the pressure perturbation and on the plasma current response is studied. This phenomenon may explain the existence of the small window of safety factor values where ELMs were completely eliminated. The possibility to control the plasma current response to penetration of external helical resonant magnetic perturbations into the edge plasmas is shown. The investigation is carried out in the frame of one-fluid MHD.

In the frame of one-fluid MHD the pressure perturbation resonant excitation by external low frequency helical magnetic perturbations near the plasma edge is investigated. The plasma rotation plays a key role in this phenomenon. The plasma response has been taken into account. These pressure perturbations may affect stability of the ballooning and peeling modes.

ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (approximate to 10(16)-10(18) m(-3)) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters (B-T = 3.3 T, f = 25 MHz) resulted in a reliable operation of the JET A2 antennas and a high conditioning efficiency in a scenario imitating closely ITER full-field operation (B-T = 5.3 T, f = 40 MHz) with the fundamental ion-cyclotron resonance for deuterium (omega = Omega(D+)) located on-axis. Numerical modelling with the 3D electromagnetic code Micro Wave Studio, a 1D RF full wave code and a 0D plasma code allows extrapolating the results obtained on JET and other present-day tokamaks to ITER and provides good prospects for the use of the ITER ICRF antennas for ICWC purposes.

In the l=3 Uragan-3M torsatron a hydrogen plasma with the density (n) over bar (e) similar to 2x10(12) cm(-3) is produced and heated by RF fields in the omega less than or similar to omega(ci) range of frequencies with using a frame-like antenna. Time variations are considered of (1) density (n) over bar (e) and electron cyclotron emission at different values of the RF power fed to the antenna; (2) fast ion generation and loss; (3) edge electric field E-r and edge turbulent transport. Obtained results are of importance for (1) subsequent production and heating of denser plasmas; (2) understanding of processes resulting in the observed transition to the H-like confinement mode.

This paper focuses on further study of the Radio-Frequency (RF) power absorption mechanisms responsible for Ion Cyclotron Wall Conditioning (ICWC) discharge ignition and sustainment in fusion machines in the presence of high toroidal magnetic field. The dominant electron collisional, ion collisional and cyclotron absorption mechanisms are analyzed during local (antenna-near) gas breakdown ( ω ω pe ) phases of RF discharge. Optimization of the absorbed RF power in terms of (i) z E ~ field generation (electric field along BT-field lines), (ii) antenna phasing and (iii) waves excitation in plasmas with multi-ion species resulted in a successful performance of the JET ICWC experiments (BT=3.3 T, f=25 MHz) using the standard ICRF A2 antennas in a scenario envisaged at ITER full field (BT=5.3 T, f=40 MHz) – i.e. with the fundamental ion cyclotron resonance (ICR) of the deuterons, + = cD ω ω , on-axis.

Generation at Major Disruptions in Tokamaks V.V. Plyusnin and I.M. Pankratov 1 Association Euratom/IST, Instituto de Plasmas e Fusao Nuclear – Laboratorio Associado, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais, 1049 – 001 Lisboa, Portugal 2 Institute of Plasma Physics, National Science Center ‘Kharkov Institute of Physics and Technology’, Akademicheskaya str., 1, 61108 Kharkov, UKRAINE

Spatial and temporal behavior of the edge fluctuations and their correlation with plasma density behavior inside the confinement region of the Uragan-3M torsatron are investigated. The key role of the radial electric field in turbulent transport suppression is shown.

In the frame of one-fluid MHD a possibility of the pressure perturbation resonant excitation by external low frequency helical magnetic perturbations near the plasma edge is shown. The plasma rotation plays the key role in this phenomenon. The plasma response has being taken into account. These pressure perturbations may affect on the ballooning and peeling modes stability.

In the Uragan-3M (U-3M) and Uragan-2M (U-2M) torsatrons possibilities and prospects of Alfven method utilization for wall conditioning, plasma production and heating are studied. In U-3M the effect of fast ion loss on H-like mode formation is investigated. In U-2M the wall conditioning associated with the chemical reactivity of the atomic hydrogen to create volatile substances is used. A compact four-strap antenna is proposed for Alfven resonance heating in U-2M. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim