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.

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.

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.

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.

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.

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.

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.

The possibility of suppressing the flux of runaway electrons at the stage of their generation in an Uragan-3M stellarator has been experimentally studied and theoretically analyzed.

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.

Unshielded THT antenna is successfully used: (i) for heating of plasma prepared by the frame antenna pulse, (ii) for making an initial plasma with low density similar to 10(10) cm(-3) for further frame antenna operation, (iii) for independent generation and heating plasma at low magnetic fields B-0<0.7 T and (iv) for mutual operation with frame antenna. In the last scenario both antennas contribute to plasma heating.

На установке торсатрон “Ураган-3М” отработан и успешно применяется метод очистки вакуумных поверхностей установки плазмой, со следующими основными параметрами: а) низкая электронная температура (порядка 10 эВ); б) достаточно высокая плотность плазмы (ne 1012 см-3). Чистящий разряд удобен тем, что его можно проводить, используя те же антенные устройства и ВЧ-генераторы, которые используются для создания и нагрева плазмы в рабочих режимах, причем без какой-либо перестройки частоты антенных и генераторных ВЧ-контуров. Разряд имеет достаточно хорошую эффективность с точки зрения очистки вакуумных поверхностей. После проведения цикла очистки низкотемпературной плазмой ВЧ-разряда (примерно 20 000 импульсов): а) существенно понижается интенсивность свечения примесной линии СIII; б) в рабочих разрядах достигается квазистационарный режим продолжительностью до 50 мс с плотностью плазмы ne 1012 см-3 и радиационной электронной температурой до Те 1 кэВ; в) при масс-спектральном анализе остаточного газа в камере наблюдается значительное уменьшение примесей.

An procedure for the determination of electron temperature profiles from electron cyclotron emission measurements on URAGAN-3M is described. Both the nature of the plasma (low density, moderate temperature which is leads to low plasma optical depth) and the characteristics of the radiometry diagnostic (probing beam through plasma cross-section), require that additional effects be considered. This paper takes these into account when deducting electron temperature profile. Initial test-run data from the new radiometer system also described.

A method for cleaning vacuum surfaces by a low-temperature ( T e ∼ 10 eV) relatively dense ( n e ≈ 10 12 cm −3 ) plasma of an RF discharge was developed and successfully applied at the Uragan-3M torsatron. The convenience of the method is that it can be implemented with the same antenna system and RF generators that are used to produce and heat the plasma in the operating mode and does not require retuning the frequencies of the antennas and RF generators. The RF discharge has a high efficiency from the standpoint of cleaning vacuum surfaces. After performing a series of cleanings by the low-temperature RF discharge plasma (about 20000 pulses), (i) the intensity of the CIII impurity line was substantially reduced, (ii) a quasi-steady operating mode with a duration of up to 50 ms, a plasma density of n e ≈ 10 12 cm −3 , and an electron temperature of up to T e ∼ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.

Frequency spectrum (radial profile) of X-mode second harmonic electron cyclotron emission was observed for optically thin plasma produced by Alfven resonance heating in Uragan-3M torsatron. Radial electron temperature profile within frequency range 31.5-37.5GHz is covered a significant portion of the plasma column radius. Temperature profile derived from "radiation temperature" profile. This procedure neglects multiple reflections of ECE radiation from the torsatron inner structure (mainly from helical coils). We relate the mismatch effect of the ECE radiation data by the strong modification of emission level by plasma opacity (small plasma optical depth) and by the scrambling effect. This effect results from both O-X mode conversion. Electron temperature is calculated from radiation temperature using tokamak approximation for the optical thickness. The difference in ECE and other data is explained using some modification of electron density profile.