V.E. Moiseenko

RF plasma production and heating below ion-cyclotron frequencies in Uragan torsatrons

In the IPP-Kharkiv there are two torsatrons (stellarators) in operation, and in both of them Alfven resonance heating under high-k∥ conditions is used. This method of heating is advantageous for small-size devices, since in contrast to the minority and second-harmonic heating it can be realized at lower plasma densities. A series of experiments has been performed at the Uragan-3M torsatron with an aim to investigate the features of the discharge with a three-half-turn antenna. Electron temperatures in the range are achieved at plasma densities . The plasma energy content has increased by a factor of 2 with respect to the plasma produced with the frame antenna. A new four-strap shielded antenna has been manufactured and installed in the Uragan-2M. A high-frequency discharge for wall conditioning is introduced in the Uragan-2M torsatron. The discharge is sustained by a specially designed small frame antenna, and efficient hydrogen dissociation is achieved. A self-consistent model has been developed for simulation of plasma production in ICRF. The model includes a set of particle and energy-balance equations for the electrons, and the boundary problem for the Maxwell equations. The first calculation results on RF plasma production in the Uragan-2M stellarator with the frame-type antenna are presented.

A study of three-half-turn and frame antennae for ion cyclotron range of frequency plasma heating in the URAGAN-3M torsatron

Abstract Numerical and experimental results of Alfven wave heating of plasmas in the frequency range below the ion cyclotron frequency ( ω ω ci ) are presented. Two different types of antenna were used for plasma production and heating: a frame type antenna (FTA) conventionally used in the URAGAN-3M device and a three-half-turn antenna (THTA) proposed recently to avoid the deleterious effects of conversion of fast wave to slow wave in the plasma periphery and to perform plasma core heating more effectively. Numerical modeling of electromagnetic field excitation in the URAGAN-3M plasma by the FTA and THTA was performed using a one-dimensional code. The results of calculations showed better performance of the compact THTA compared with the FTA for the case of a high density plasma (approximately 10 13 cm −3 ). When using the THTA, the experiments performed showed the possibility of dense plasma production (more than 2 × 10 13 cm −3 ) and heating, which had not been obtained earlier in the URAGAN-3M. Shifting the power deposition profile deeper inside the plasma body with the THTA resulted in modification of the plasma density profile and an improvement in plasma confinement.

Progress in stellarator research in Kharkov IPP

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.

Cleaning of inner vacuum surfaces in the Uragan-3M facility by radio-frequency discharges

A method for cleaning vacuum surfaces by a low-temperature (Te ∼ 10 eV) relatively dense (ne ≈ 1012 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 ne ≈ 1012 cm−3, and an electron temperature of up to Te ∼ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.

VHF discharges for wall conditioning at the Uragan-2M torsatron

The very high frequency (VHF) discharge for wall conditioning with hydrogen atoms is studied. It is driven by the RF power at frequencies ∼140MHz, higher than usually used in ICRF. For wall conditioning a special small size antenna is designed. The antenna is aimed to excite the slow wave that is damped via electron collisions with neutral gas. The wave excitation is modelled using a 1D numerical code. In the experiment, the discharge parameters are studied as functions of confining magnetic field and gas pressure. The Langmuir probe measurements give the radial profiles of plasma density and electron temperature. The discharge is volumetric: plasma occupies whole confinement volume and even steps out at the edge. The characteristic value of plasma density is 10 10 cm −3 , electron temperature varies in the range 3‐10eV. The temperature values of probe measurements are compatible with the results of optical diagnostics. Such parameters of discharge are favourable for wall conditioning in hydrogen. The discharge parameters did not reveal any sensitive dependence on neutral gas pressure and the toroidal magnetic field. The mass spectrometry of the residual gas is used for monitoring the wall conditioning effect of the VHF discharge.

Dynamics of r.f. production of Stellarator plasmas in the ion cyclotron range of frequency

Abstract The present study investigated numerically the process of r.f. production of plasma in the URAGAN-3M torsatron in the frequency range below the ion cyclotron frequency (ω

Characteristics of the three-half-turn-antenna-driven RF discharge in the Uragan-3M torsatron

In the ℓ = 3 Uragan-3M torsatron hydrogen plasma is produced by RF fields in the Alfvén range of frequencies (ω ≤ ωci). 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.

Characteristic properties of the frame-antenna-produced RF discharge evolution in the Uragan-3M torsatron

In the ℓ = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ≲ ωci). 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.

RF Plasma Production in Uragan‐2M Torsatron

A single frame antenna is used for plasma production below the cyclotron frequency at the medium‐size Uragan‐2M torsatron which has started operation at the end of year 2006. Its sizes and the regime of plasma production are chosen using the numerical modeling. In the numerical analysis, time‐harmonic Maxwells equations are solved and the power deposition to the electrons is calculated. In the calculations the plasma density is varied in the range np∼108–1013u2009cm−3. The single frame antenna provides the core power deposition at the low densities. With increase of the plasma density the power deposition per particle decreases and the power deposition profile worsens since more power is delivered to the plasma periphery. The first experimental results on plasma production in the Uragan‐2M torsatron are presented and discussed.