Plasma production experiments in helium at Uragan-2M have been performed to investigate the role of the hydrogen minority in helium. The experiments presented here were carried on with a controlled minority hydrogen concentration. The hydrogen minority allowed one to increase plasma density more than three times as compared with pure helium. The obtained plasma density is highest for whole time of Uragan-2M operation. The developed scenario allowed to decrease the neutral gas pressure at which the plasma production is possible. This is a requirement for achieving regimes of plasma production with full ionization. Although the initial gas mixture 14%H 2 + 86%He can be treated as optimum, there is no sensitive dependence on hydrogen minority concentration, which makes the scenario robust. This study, together with initial LHD experiments, confirm the prospects of target plasma production by ICRF waves for stellarator type machines.

We consider a scenario of the initial stage of the RF breakdown of a working gas in torsatrons Uragan-3M and Uragan-2M and the roles of runaway electrons in this process. In our previous works, we studied only the acceleration factor of the breakdown process which occurs, when the intensity of the flow of runaway electrons increases due to the stimulation by an additional ultrahigh-frequency discharge at the front edge of a magnetic field pulse. This work attempts to describe the individual phenomena that accompany the initial stage of plasma formation in the confinement areas of torsatrons Uragan-3M and Uragan-2M in the presence of the flow of runaway electrons.

In support of the ICRF experiments planned on the Wendelstein 7-X (W7-X) stellarator, i.e. fast ion generation, wall conditioning, target plasma production and heating, a first experimental study on plasma production has been made in the Uragan-2M (U-2M) stellarator using W7-X-like two-strap antenna. In all the experiments, antenna monopole phasing was used. The W7-X-like antenna operation with launched radiofrequency power of ~100 kW have been performed in helium (p = (4–14) × 10−2 Pa) with the vacuum vessel walls pre-loaded with hydrogen. Production of plasma with a density higher than 1012 cm−3 was observed near the first harmonic of the hydrogen cyclotron frequency. Operation at first hydrogen harmonic is feasible in W7-X future ICRF experiments.

The Uragan-3M device is equipped with two antennas which are fed by RF power with the frequency below ion cyclotron. The frame antenna was used for pre-ionization and the three-half turn antenna makes plasma heating. In this experimental series, the radial profiles of C III, O V and C V optical line intensity and the second cyclotron harmonic emission are measured using a pulse-by-pulse technique. The results of these measurements and Biot-Savart calculations of the Uragan-3M magnetic configuration could be explained by the existence of a small central area with relatively high electron temperature and good plasma confinement surrounded by a zone where the electron temperature and confinement are worse. The relatively low average electron temperature and high RF power needed to sustain plasma are the consequences.

The start-up experiments were carried out at Uragan-2M stellarator with the Three-Half-Turn antenna (THT) without any pre-ionization. Conditions for optimal gas breakdown were found out through the variation of the neutral gas pressure, magnetic field strength and anode voltage of RF generator. The plasma parameters were measured with three Langmuir probes, optical spectroscopy and mutichord optical diagnostics. PACS: 52.55.Hc: 52.50.Qt

A variant of the B4C movable biased pumped limiter for the Uragan-2M torsatron is described. Some results of experiments with the limiter moved from the vacuum chamber wall to the central region of the plasma column in the work mode of Uragan-2M operation are reported. The effect of limiter-plasma interaction on parameters of the RF plasma discharges is discussed.

Generation of runaway electrons in fusion experiments can drive to serious damage of plasma devices components. Injection of gas with a large mass number decreases the generation processes. Also, magnetic perturbations decrease generation of runaway electrons by increasing the loss rate. We investigated the influence of working gas pulse injection and natural fluctuations of the magnetic confining field on runaway electrons dynamics. The interaction of runaway electrons with an Alfven wave in plasma is noted.

In the URAGAN-3M (U-3M) torsatron the low-frequency transient 20-30 kHz mode is observed during the plasma confinement transition that occurs at a plasma current value of about 1 kA. The burst of this mode is always accompanied by the fast jump of the Alfven eigenmode frequency. The transient 20-30 kHz mode contains two parts. The non-rotating part of the mode has higher amplitude and is localized in the stochastic region of the plasma. It is observed only in the vicinity of the radio-frequency antenna used for plasma production and does not propagate along the torus because of fast losses. Its high amplitude indicates that the major part of the 20-30 kHz mode is excited in the stochastic region near the antenna. In contrast, the second rotating part of the mode is localized everywhere along the torus near the plasma edge (p = 0.8-1). This is the n/m = 1/2 mode that rotates in the electron diamagnetic direction. It is observed in different toroidal cross-sections by various diagnostics (magnetic probe array, optics, Langmuir probe). Appearance of the 1/2 rational surface at the stochastic magnetic field line region near the plasma edge at 1 kA plasma current stage can be responsible for the mode generation. Modification of electron component gradients in the mode generation region near the antenna and the drop of the fast ion concentration (above 1 keV) in this region are observed simultaneously with the mode generation. The mode can be exited by the strong transient plasma gradients generated in the vicinity of the rational surface by the antenna.

In this work the results of amplification of the runaway electrons flow and interaction runaway electrons with RF-heating wave on the Uragan-3M torsatron are presented. Results described in the article confirm using runaway electrons for gas breakdown. The results allow making some recommendations for using of self-created flows of accelerated particles for stimulation of gas breakdown.

A new antenna of the crankshaft type is successfully employed in experiments on Uragan2M. It creates and heats plasma at frequencies below the ion-cyclotron frequency. The discharge created with the antenna is "hot" during few milliseconds and then fades under strong impurity influx into plasma. The radial profiles of OV and CV optical emissions are investigated. Both are concave, especially the OV profile. This can be explained by some burnout of O4+ and C4+ ions and its transition to O5+ and C5+ ionization states near the magnetic axis. The magnetic field optimisation and the Alfven heating allowed burning-out the light impurities prior to the discharge degradation. The experiments in support of the fusion-fission stellarator reactor concept are also carried out. The stellarator-mirror machine is modeled by switching off a toroidal field coil at Uragan-2M. The embedded mirror with lower magnetic field is created in this way. Plasma is successfully produced and heated in such a combined magnetic trap.

A double frame antenna with a broad spectrum of parallel wavenumbers (with respect to the magnetic field) is used for radio-frequency (RF) plasma production in Uragan-3M stellarator type device. The delay between the start of RF pulse and the discharge development (breakdown (delay) time) is analyzed as functions of the magnetic field strength, neutral gas pressure and anode voltage of the RF generator. The reproducibility of the RF discharges is improved by the pre-ionization by the pulse of the three-half-turn antenna preceding the main RF pulse. The preionization also results in shortening of the breakdown time for the frame antenna. The Langmuir probe measurements are made with two probes located at the plasma edge near and far from the double frame antenna. The measurements give rather high edge electron temperature, about 100 eV, at the initial stage of the frame antenna discharge both near and far from the antenna. The information on the plasma build-up is also given by the Ha chord measurements.

First observations of plasma fluctuations in the frequency range of 1...20 kHz in various plasma conditions of the URAGAN-2M (U-2M) torsatron are presented. Observed fluctuations of the Ha emissivity are coherent with the line-averaged density fluctuations measured by the 140 GHz heterodyne interferometer. Bursts of 1. 2 kHz, m= 1 oscillations, localized in the central region of the plasma column, are always observed at the heating degradation stage of the crankshaft antenna operation in "standard" U-2M discharges (B-0= 0.35 T). For this " standard" discharges with low RF power 10. 15 kHz plasma density fluctuations are strongly localized at the region of substantial Ha emissivity gradients. Transient burst of 6 kHz central fluctuations are also observed in the medium magnetic field (B-0= 0.067 T) discharges. Strong m = 1 fluctuations in the frequency range of 10. 20 kHz are observed in the plasma conditioning discharges with low temperature and magnetic field (B-0= 0.01 T). These fluctuations are localized in the central part of the plasma. Direction of poloidal rotation of the mode which causes fluctuations is reversed during the discharge simultaneously with its substantial amplitude increase and frequency decrease from 20 to 10 kHz. The fluctuations observed in standard U-2M discharges have frequency below the geodesic acoustic mode (GAM) frequency and can be caused by drift plasma modes. The frequency of fluctuations in the plasma conditioning discharges is observed in the range near the GAM frequency or higher.

First estimations of plasma electric potential and density in URAGAN -2M (U-2M) torsatron were done by the Heavy Ion Beam Probing Diagnostic (HIBP). The estimated plasma potential has the negative potential value of -(80. 195) V. The experimental values of the secondary ion beam current are well correlated with average plasma density measured by radio-interferometer. -(1.25...2.5) x10(12) cm(-3). Notable oscillations of the secondary beam current were observed, which were caused by fluctuations of the torsatron magnetic field.

A new antenna of ‘crankshaft’ type has been installed in the Uragan-2M device in order to increase the plasma density and heating below the ion cyclotron frequency. Antenna operation is modelled by 1D code, which solves boundary problem for time-harmonic Maxwell’s equations in radially non-uniform plasma cylinder. In recent experiments with this antenna, the SXR, CV, OV and OII emission measurements indicate that the light impurity radiation barrier is overcame at this device. Plasma with a temperature of ~50 eV exists during a short period of a few milliseconds. Then the radiation collapse comes owing to strong influx of impurities to the plasma column. A new magnetic diagnostics has been installed at Uragan-3M. Using it the poloidal magnetic field is measured and the shift of toroidal current in major radius is registered. A miniature pinhole camera array for spatially and temporally resolved measurements of soft X-ray (SXR) plasma emission has been recently installed on the U-3M. Different shapes of the SXR emission profile has been observed in different discharge conditions.

Deficient shot-to-shot stability of Uragan-3M discharges makes difficult to reproduce experimental results over the period of experimental session. An efficient way of reducing difference between shots is creation of initial low density plasma before the main discharge to start up. A RF pre-ionization in the same frequency range as that of the main discharge is used in the Uragan-3M torsatron. The pre-ionization provides stable discharges during the whole experimental campaign. The main parameters of the pre-ionization plasma are measured and discussed.

This paper describes a renewed system of pulse injection of working gas into the vacuum vessel of the stellarator Uragan-3M. The system was designed and fabricated in the Institute of Plasma Physics of NSC "Kharkov Institute of Physics and Technology" and is intended for providing of the fast-acting dosed injection of hydrogen by the given algorithm. A distinctive feature of the developed system is the use of designed microprocessor-based controller to control the operation of the piezoelectric valve.

In the l=3/m=9 Uragan-3M (U-3M) torsatron a hydrogen plasma is produced and heated by RF fields in the Alfven range of frequencies (omega <=omega(ci)). Peripheral plasma is investigated using two moveable Langmuir probes. Spatial distributions of plasma parameters, V-f, T-e and n(e) in two operating regimes and in three cross-sections are measured. Link between confinement volume and transition layer is shown. RF electric antenna field influence on the probes is discussed.

In the ℓ = 3 Uragan-3M torsatron hydrogen plasma is produced by RF fields in the Alfvén range of frequencies (ω ≤ ω ci ). The initial (target) plasma with the line-averaged density of units 1012 cm−3 is produced by a frame antenna with a broad spectrum of generated parallel wavenumbers. After this, to heat the plasma and bring its density to ~1013 cm–3, another, shorter wavelength three-half-turn antenna with large transverse currents is used. The behavior of the density, electron temperature, and loss of the plasma supported by the three-half-turn antenna is studied depending on the RF power fed to the antenna and initial values of the density and electron temperature supplied by the frame antenna.

The microwave radiometry is a well-known diagnostics to obtain the information on temporal evolution and radial profile of the electron temperature at U-3M torsatron plasma experiments. However, under low plasma density with this diagnostics we report on the large production of runaway electrons after RF heating pulse off. We notice a gradually increasing of the radiometer signal at the frequencies that match the second and third harmonics of electron cyclotron emission of the extraordinary mode. This effect could be explained with the existence of the "runaway" electrons in U-3M discharge. A phenomenological description of this process is presented, where the time evolution of the ECE radiation signal is compared to the electron density evolution.