Uragan-3M Torsatron Research Articles

INFLUENCE OF THE PHOTODIODE SPECTRAL SENSITIVITY ON THE TEMPERATURE ESTIMATION BY SXR USING THIN FILTERING FOILS IN URAGAN-3M

Soft X-ray (SXR) diagnostics is routinely used in Uragan-3M (U-3M) torsatron. One of the SXR diagnostic applications is based on the plasma temperature estimation. The ratio of SXR signals passed through two different foils depends only on the plasma temperature and can be used for its estimation. An influence of spectral sensitivity of the photodiode on the two foils temperature estimations technique is considered in present work numerically. The influence is negligible in the case of the flat sensitivity in the energy range 5…500 eV in the case of thin 1.5/2.25 μm Al foils. In the opposite case of very low spectral sensitivity in 5…500 eV range and its increase at the 1 keV range the signals ratio can be very high. This result can explain high signals ratio experimentally observed in Uragan-3M stellarator, where photodiodes with unknown spectral sensitivity were used.

Escaping Electrons and Parametric Instability in the Uragan-3M Torsatron

Electromagnetic fluctuation spectra in the peripheral area of the Uragan-3M trap and plasma density fluctuation spectra in the plasma confinement zone have been compared with the modulation spectra of escaping electron flux. The dependence of the form of fluctuation spectra on the escaping electron flux intensity has been studied. Plasma confinement and heating conditions in the Uragan-3M torsatron have been discussed.

Structure of Magnetic Plasma Fluctuations in the Uragan-3M Torsatron at Rare Collision Frequencies

Fluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Characteristics of regular discharges in Uragan-3M torsatron

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.

MODELING OF TIME BEHAVIOR OF H, H2 NEUTRAL DENSITIES, AND Hα LINE INTENSITY IN THE RF PLASMA OF THE URAGAN-3M TORSATRON

In this paper, the results of computer simulation of time behavior of hydrogen atoms and molecules in the plasma produced by RF power in the Uragan-3M torsatron are presented. The evolution of the density of fast charge-exchange H atoms, slow H atoms, H atom population on the chamber walls, the intensity of the Hα line, and the H2 gas density and pressure has been studied using the system of differential equations.

Effect of Low-Frequency Magnetic Field Fluctuations on the Confinement of Plasma in the Uragan-3M Torsatron at Rare Collision Frequencies

The plasma energy content in the Uragan-3M torsatron was determined by means of diamagnetic measurements under the conditions of its radiofrequency (RF) generation and heating (at near ion-cyclotron frequencies) in the presence of bootstrap current. The balance of power under the fast heating of plasma was considered and used as a basis to estimate the radiofrequency power absorbed by the plasma in the confinement volume. The behavior of energy losses in the discharge period was calculated, and the effect of magnetic field fluctuations on the energy losses within a frequency range of 0.5–70 kHz was discussed.

Влияние низкочастотных флуктуаций магнитного поля на удержание плазмы в торсатроне Ураган-3М при редких частотах столкновений

AbstractThe plasma energy content in the Uragan-3M torsatron was determined by means of diamagnetic measurements under the conditions of its radiofrequency (RF) generation and heating (at near ion-cyclotron frequencies) in the presence of bootstrap current. The balance of power under the fast heating of plasma was considered and used as a basis to estimate the radiofrequency power absorbed by the plasma in the confinement volume. The behavior of energy losses in the discharge period was calculated, and the effect of magnetic field fluctuations on the energy losses within a frequency range of 0.5–70 kHz was discussed.

Структура магнитных флуктуаций плазмы в торсатроне Ураган-3М при редких частотах соударений

AbstractFluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Suppression of Runaway Electron Flows and Specific Features of Working Gas Breakdown in the Uragan-2M Torsatron

The results of experimental studies of the influence of runaway electrons on the parameters of the neutral gas breakdown and suppression of the runaway electron flow in the Uragan-2M torsatron are presented. The influence of the presence or absence of the runaway electron flow on the time characteristics of the breakdown is experimentally studied via comparative analysis. It is demonstrated that in the presence of the runaway electron flow the breakdown takes place earlier by an average of 1.5 ms, as compared to the breakdown in the absence of the runaway electron flow. This is also proved by the earlier results obtained at the Uragan-3M torsatron, which allows one to continue the description of some aspects of the runaway electron dynamics and its regularities and formulate a number of recommendations on controlling such electron flows in fusion installations.

Characterization of the 20 kHz transient MHD burst at the fast U-3M confinement modification stage

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 Alfvén 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 (ρ = 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.

Stimulation of high-frequency breakdown of gas in Uragan-3M torsatron by runaway electrons

In experiments on confinement and heating of plasma in the Uragan-3M torsatron, the method of high-frequency breakdown of the working gas is used. In these experiments, in conditions of a relatively stable magnetic field, the rf power supplied to the setup chamber has a frequency close to the ion-cyclotron frequency. Such a method of gas breakdown is not always sufficiently reliable. In our experiments, preliminary ionization of the working gas by the run-away electron beam is used for stabilizing the breakdown. This work contains the results of experiments on enhancement of the runaway electron beam and on the interaction of the runaway electron beam in the Uragan-3M torsatron with the HF electromagnetic pump field. This enables us to formulate a number of recommendations for using spontaneously formed beams of accelerated particles for stimulating the rf breakdown. Our results confirm the possibility of gas breakdown by runaway electrons.

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 Alfven 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 Alfven 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.

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.

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 Alfvén 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.

U-3M ion energy distribution measurements during frame antenna plasma production and heating in the ICRF range

An energy sweeping technique is developed and applied for charge exchange (CX) neutral particle diagnostics in the URAGAN-3M (U-3M) torsatron. Measurements are performed during low-density (ne = (0.5–1) × 1012 cm−3) frame antenna radio frequency (RF) plasma discharges in the RF frequency range close to the ion cyclotron frequency (ICRF). The presence of ions with energies up to 4 keV is established experimentally. According to the experimental data the ion energy distribution is close to Maxwellian in the energy range 0.4–2.5 keV and can be described by the ion temperature Ti = 300–600 eV. This is an indication of direct RF energy deposition onto the ions due to the negligible ion–electron energy exchange. The radial dependences of CX neutral flux and ion temperature under consideration are investigated using a set of similar discharges. The radial distribution of the studied temperature has a flat profile in the range ρ = 0.5–1. This is an indication of the ion energy deposition localization close to the plasma edge. The possible mechanisms of ion heating are briefly discussed.

Spectral and statistical analysis of fluctuations in the SOL and diverted plasmas of the Uragan-3M torsatron

In the l = 3/m = 9 Uragan-3M (U3-M) torsatron (R0 = 1 m, ā ≈ 0.12 m, ι(ā)/2π ∼ 0.3) with an open helical divertor and a plasma produced and heated by RF fields (ω ≲ ωci), studies of low frequency (5–100 kHz) density and potential fluctuations in the SOL plasma and in the diverted plasma flows (DPFs), have been carried out. It is shown, that in the SOL to more (less) distantly located points relative to the last closed magnetic surface, higher (lower) frequency fluctuations are inherent. Such a spectral splitting in two sub-ranges occurs in the DPFs too. A peculiarity of the spatial distribution of DPF fluctuation spectra is that lower frequency fluctuations dominate on the ion toroidal B × ∇B drift side. During L-H-like transition in U-3M simultaneously with strong Er shear formation, a suppression of lower frequency fluctuations and a decrease of local radial turbulent particle flux take place. Results are presented of investigation of plasma density fluctuations in SOL with the use of probability distribution function (PDF) analysis. Evaluations of skewness and kurtosis of fluctuations have been made. The analysis of Is fluctuations in DPF have been carried out in a similar way.

Fast ion loss-driven H-mode transition in RF discharge plasmas of the Uragan-3M torsatron

In the l = 3 Uragan-3M (U-3M) torsatron with a helical divertor the plasma is produced and heated by RF field in the ω ≤ ωci range of frequencies. A two-temperature ion perpendicular energy distribution with a suprathermal tail sets in with heating. If the heating power is high enough, a spontaneous transition to an H-like confinement mode is observed. Recently, it has been supposed that the transition is connected with hotter and suprathermal ions (common name “fast ions“, FI) loss. The objective of this work is an experimental elucidation of the real link between the H-transition and FI loss. To do this, a transient regime of the RF discharge with two H-mode states is chosen, and the evolution is followed of electron density, FI content in the confinement volume, FI outflow to the divertor and edge potential. On the basis of juxtaposing of these processes, a conclusion is made that the H-mode transition in U-3M is really driven by FI loss. Possible mechanisms resulting in the transition are discussed, among them the ion orbit loss and the radial drift of helically-trapped ion orbits seem most probable.

Studies of fast ion outflow to the helical divertor of the U-3M torsatron

In the l=3 Uragan–3M torsatron with RF-produced and heated plasmas (ω ≲ ωci), a two-temperature ion distribution with a suprathermal tail is formed. Faster ions (FIs) can be the characteristic of reactor-size stellarators in the long mean free path regime. The presence of the helical divertor offers new opportunities for studying FI loss by measuring ion fluxes and energies in the divertor plasma. Ion energy distributions were measured in divertor flows in two symmetric poloidal cross-sections in several field periods. It is shown that FI flows out to the divertor mainly on the ion B×∇B drift side in accordance with the assumptions of a determinative FI contribution to the plasma divertor flow vertical asymmetry inherent to torsatrons/heliotrons. Strong toroidal non-uniformities in flows and energies of ions outflowing into the divertor are observed. The island structure of the U-3M magnetic configuration and locality of RF power injection are considered as possible reasons for these non-uniformities.

Processes arising in the edge and diverted plasmas during ITB formation in the U-3M torsatron

Spontaneous changes in confined plasma parameters have been observed recently in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated plasma, these being interpreted as transition to an improved confinement mode due to ITB formation near the ι = 1/4 rational magnetic surface. In the work presented joint studies are carried out of changes in some edge and diverted plasma characteristics that accompany ITB formation. It is shown that ITB formation induces a hard Er bifurcation at the boundary presumably driven by the ion orbit loss. As a result, Er becomes more negative, and an Er shear layer occurs, where the low-frequency microturbulence and the turbulence-induced anomalous transport are suppressed, i.e. an ETB is formed. At the pre-bifurcation phase of transition a reduction of fast ion loss takes place. The bifurcation results in an improvement of electron confinement, while the ion loss increases.