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.

Conceptual development activities on a stellarator-mirror-based fission-fusion hybrid system (SM hybrid) are reviewed.Intended for transmutation of spent nuclear fuel and safe fission energy production, SM hybrid consists of a fusion neutron source and a powerful subcritical fast fission reactor core.Its fusion component is a stellarator with an embedded magnetic mirror.The stellarator allows for the confinement of a moderately hot (1-2 keV) deuterium plasma.In the magnetic mirror, the hot sloshing tritium ions are trapped and fusion neutrons are generated.The magnetic mirror is surrounded by a fission mantle, where transmutation of minor actinides and energy generation take place.One candidate magnetic confinement device for the SM hybrid is the advanced DRACON magnetic trap system, which, unlike the «classical» DRACON version, has one short, rather than two longer mirrors with a relatively short size of 3-6 m.A comparative numerical analysis of collisionless losses occurring in the magnetic trap part of the single-mirror DRACON leads to a conclusion about the possibility for high-energy tritium ions to be fairly well confined in the magnetic trap area.The Uragan-2M (U-2M) stellarator is used to test the SM hybrid concept with experiment.To fit a magnetic trap into U-2M system, one of the toroidal coils had to be switched off.A radial escape of charged particles may spontaneously give rise to a weak radial electric field, which may result in closing the particles' drift trajectories and thereby substantially improve their confinement.Background plasma confinement without destructive instabilities is demonstrated in the stellarator-mirror regime of U-2M) operation.The sloshing ions driven by radio-frequency heating are detected in the mirror part of the device with NPA diagnostics.A novel fission mantle design for the SM hybrid is proposed.

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.

A unshielded two-strap antenna had been installed in Uragan-2M. A vacuum chamber inner walls conditioning regime with the two-strap antenna is studied in a weak magnetic field. Plasma with the density n(e)similar to(0.2...0.95).10(12) cm(-3) and sustained. The RF frequency was f(0)similar to 5 MHz, RF plasma was sustained in stationery magnetic field B-0 approximate to 0.01 T, at hydrogen pressure range 3.10(-3)...3.10(-2) Pa.

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

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.

For the past decades the microwave radiometry is a routinely used as diagnostic tool to obtain the information on temporal evolution and radial profile of the electrons temperature at Uragan-3M torsatron plasma experiments. However, in the case of low plasma density operation we observe the high level of emission at the frequencies that match the second and third harmonics of the extraordinary mode of electron cyclotron emission (ECE), after RF heating pulse off. This effect could be explained with the production of the suprathermal electrons. The present work describes the suprathermal electrons (SE) dynamics after turning off the RF heat pulse at the Uragan-3M torsatron (n(e)=2.10(12) cm(-3), Te=300 eV, P-RF=115 kW, B-0=0.69 T). In the absence of the well-known suppressive techniques (resonant magnetic perturbations and massive gas injection) an attempt was made to describe the factors, which contribute to the generation of the suprathermal electrons for the Uragan-3M plasmas. The temporal evolution of the ECE emission intensity and it dependence on the working gas pressure in the torsatron vacuum chamber is presented. The level of the ECE emission shows strong correlation with other diagnostics (plasma density, plasma current, HXR and H-alpha emission intensity). The gradual increase of the pressure (after RF off) could be is one of the reasons that temporarily sustain the process of electron acceleration. The dependence of the time ECE emission on the rate in magnetic field change (delta B) is also given. PACS: 42.25Bs, 42.30Rx, 42.68Ay, 42.82Et, 55.25Os, 52.40Db, 52.55.Hc, 52.70 -m, 52.70.Gw, 92.60Ta

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.

The operation of the 2 mm microwave superheterodyne interferometer, which allows to measure the linear electron density (average over the line of sight through the plasma) at the Uragan-2M stellarator is presented. Compared with the previously used 8 mm interferometer, this diagnostic does significantly expand the limits of the measured density of electrons which is now increased up to the value of 2.43.10(20) m(-3). Optimized receiving and transmitting waveguide line provided a significant reduction in attenuation of the microwave radiation introduced into the plasma. It ensures that the presented experiments the value of the minimum measured density does not exceed 1.5.10(16) m(-3). It is shown that for the different plasma discharges this system which have a high signal-to-noise ratio and high time resolution of the detector allowed to measure the "global" quasi-coherent fluctuations of the plasma density in the frequency range of 3 ... 20 kHz.

Commissioned a microwave 2 mm (140 GHz) superheterodyne interferometer, which allows to start the measurement of linear (average over the length of the chord passing through the plasma) of the electron density at the Uragan-2M torsatron. Compared with the previously used 8 mm interferometer, this diagnosis will significantly expand the limits of measurement. It is now possible to measure the plasma density up to the 2.43.10(20) m(-3). New receiving and transmitting waveguide line provided a significant reduction in attenuation of the microwave radiation introduced into the plasma. It ensures that the value of the minimum measured density does not exceed 1.5.10(16) m(-3). It is shown that a high signal-to-noise ratio and the temporal efficiency of the detection system allowed to measure the quasi-coherent plasma fluctuations in the range of 3...20 kHz.

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.

For the past decades the microwave radiometry is a routinely used diagnostics to obtain the information on temporal evolution and radial profile of the bulk electron temperature at Uragan-3M torsatron plasma experiments. However, in the case of low plasma density operation we observe the high level of electron cyclotron emission at the frequencies that match the second and third harmonics of the extraordinary mode after RF heating pulse off. This effect could be explained with the production of the suprathermal electrons. Study of the behavior suprathermal electrons is of great importance because: (a) suprathermal electrons significantly distort or make it impossible to measure the thermal electron temperature; (b) such electrons influence the ionization process, the excitation of the plasma ions and may lead to the occurrence of several plasma instabilities. The present work describes the results of experimental studies of the behavior of the emission after turning off the RF heat pulse.

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 observations of quasi-coherent fluctuations in the frequency range of 20–400 kHz in Alfvén-wave-heated plasmas of the U-3M torsatron are presented. The excitation conditions of these modes depend on the radio frequency antenna type and the plasma density, the appearance of the modes correlating with the presence of both suprathermal electrons and high-energy ions in the plasma, which supports our opinion that the modes are excited by energetic particles. Complicated evolution of the mode frequencies with abrupt changes at the instants of plasma confinement transitions is observed at the initial stage of each discharge. The frequencies become stable at the stage of the plasma current flattop. Raw estimates show that toroidicity-induced Alfvén eigenmodes could be responsible for the 150–400 kHz fluctuations. Low-frequency 20–70 kHz bursts are observed during plasma confinement transitions. The poloidal mode number of one of these bursts with the frequency of 20 kHz burst was determined to be m = 2. This mode rotated in the electron diamagnetic rotation direction with a frequency lower than the geodesic acoustic mode frequency and can be identified as a drift-sound-type mode.

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.