Karen A. Sarksyan

Electron Bernstein Wave Heating Calculations for TJ-II Plasmas

Abstract The feasibility of heating TJ-II plasmas by electron Bernstein waves (EBWs) is studied. As a first approach, the Clemmov-Mullaly-Allis diagram is studied to explore the possible heating regimes, and the TRUBA ray- and beam-tracing code, which has been adapted to the complicated TJ-II geometry, is used to perform detailed calculations. The final result is that it is possible to heat plasmas by overcoming the cutoff density of electromagnetic modes by injecting the O mode and X mode at the first harmonic, exploiting the O-X-B1 and the X-B1 schemes. Transport simulations are performed to estimate the plasma parameters that are expected in those regimes and to study the transition from the X mode at the second harmonic to EBW heating at the first harmonic.

Response of a gyrotron to small-amplitude low-frequency-modulated microwaves reflected from a plasma

Experiments in the L-2M stellarator revealed the intense noise modulation of the gyrotron power and the change in its mean value under the action of the noise modulation of radiation reflected from the plasma column. The effect observed is explained in terms of the resonant locking of the gyrotron self-oscillations due to wave reflection from the fluctuating plasma load.

Effect of ECRH regime on characteristics of short-wave turbulence in plasma of the L-2M stellarator

This paper reports on studies of short-wave turbulence in the plasma of the L-2M stellarator under markedly different conditions: with doubling the ECR heating power (100 and 200 kW) and with restricting the plasma radius by a sector limiter. The role of such short-wave turbulence in anomalous transport can appear important for conditions of a thermonuclear reactor. Experiments were carried out in a basic magnetic configuration of the L-2M stellarator during ECRH at the second harmonic of the electron gyrofrequency (75.3 GHz) at average electron densities of (1.5–1.7) × 1013 cm−3. The energy confinement time was ~3.5 ms at P0 = 100 kW and was reduced to ~2 ms at P0 = 200 kW. When the limiter was introduced inside the plasma to a depth of 2 cm from the last closed flux surface, τE decreased by a factor of 1.3–1.4. Plasma density fluctuations were measured from the scattering of gyrotron radiation at the second harmonic of operating frequency (~150 GHz). A quasioptical receiving system allowed measurements of scattered radiation from plasma regions r/a ≤ 0.6 at scattering angles π/4 ≤ Θ ≤ π/2 (24 cm−1 ≤ k⊥ ≤ 44 cm−1). The short-wave turbulence was studied for two radial positions of the scattering region: r/a = 0.3–0.4 and r/a = 0.5–0.6. Short-wave turbulence exhibits features of strong plasma turbulence. It is experimentally established that a change in the energy confinement time in the L-2M stellarator correlates with the level of short-wave turbulence.

Optimization of operation of a three-electrode gyrotron with the use of a flow-type calorimeter

Results are presented for measurements of microwave power of the Borets-75/0.8 gyrotron with recovery of residual electron energy, which were performed by a flow-type calorimeter. This gyrotron is a part of the ECR plasma heating complex put into operation in 2010 at the L-2M stellarator. The new calorimeter is capable of measuring microwave power up to 0.5 MW. Monitoring of the microwave power makes it possible to control the parameters of the gyrotron power supply unit (its voltage and current) and the magnetic field of the cryomagnet in order to optimize the gyrotron operation and arrive at maximum efficiency.

The Design of an Electron Bernstein Wave Heating System for the TJ-II Stellarator

Abstract Electron Bernstein waves excited by either X-B or O-X-B conversion scheme can be used to create and heat a dense plasma in TJ-II in the first harmonic. Two gyrotrons operating in the regime of second-harmonic electron cyclotron resonance heating (53.2 GHz) create a target plasma, and then a 28-GHz gyrotron is switched on. The power of the gyrotron is 300 kW and the pulse length is 100 ms. A new high-voltage power supply was designed for this gyrotron. It supplies 70 kV and a maximum current of 25 A. Corrugated waveguides will be used to transmit the microwave radiation. The distance between the position of the gyrotron and the TJ-II window is ~7 m. The microwave beam is launched through the D6 port of TJ-II. A movable internal mirror is needed to focus the beam and to accomplish the restrictive launching angle conditions. The layout and the main features of the new system are presented.

Testing of the spectroscopic method for location of water microleakages in ITER at the L-2M stellarator

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator (P = 0.5 MW, V = 0.24 m3, and the specific heating power ∼2 MW/m3) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ∼10−6 Pa m3 s−1 and spatial resolution of ∼0.5 cm.

Spectra of low-frequency modulation of gyrotron radiation during electron-cyclotron resonance heating of plasma in the L-2M stellarator

Results from experimental studies of the modulation of the gyrotron power during electron cyclotron resonance heating of plasma L-2M stellarator are presented. It is shown that the modulation spectrum consists of separate spectral bands, among which a 20-kHz peak with a spectral density exceeding by one order of magnitude the spectral density of the other peaks is observed. This can be explained by the gyrotron operation being affected by the wave reflected from long-wavelength plasma fluctuations.

Study of plasma confinement in the L-2M stellarator during the formation of an edge transport barrier

A plasma confinement mode characterized by the formation of an edge transport barrier (ETB) was discovered in the L-2M stellarator after boronization of the vacuum vessel wall. The transition into this mode is accompanied by a jump in the electron temperature by 100–200 eV at the plasma edge and a sharp increase in the gradient of the electron temperature Te in this region. The threshold power for the transition into the ETB confinement mode with an increased electron temperature gradient is Pthr▿Te = (60 ± 15)ne [1019 m−3] kW. The formation of the ETB manifests itself also in a substantial change in the electron density profile. A density peak with a steep gradient at the outer side forms at the plasma edge. The threshold power for the transition into the ETB confinement mode corresponding to a substantial increase in the plasma density gradient near r = a is Pthr▿Te = (67 ± 9)ne [1019 m−3] kW, which agrees to within experimental error with the threshold power for the transition into the ETB confinement mode determined from the sharp increase in the gradient of the electron temperature Te. The value of Pthr for the L-2M stellarator agrees to within 25% with that obtained from the tokamak scaling. In the ETB confinement mode, the plasma energy W and the energy confinement time τE determined from diamagnetic measurements increase by 20–30% as compared to those obtained from the stellarator scaling for the confinement mode without an ETB. When the heating power increases by a factor of 2–3 above the threshold value, the effects related to improved energy confinement disappear.

Amplification of ion-acoustic turbulence upon electron-cyclotron heating of plasma

In this paper, we present the results of the study of ion-acoustic turbulence upon electron-cyclotron heating of low-temperature plasma in a TAU-1 model setup. Two MI-167 magnetrons were used as microwave sources. The peak power of each magnetron was 1 kW, the pulse duration was 7.5 µs. An increase in the temperature of most electrons and an increase in the energy of nonthermal electrons were observed in experiments. Microwave field turning-on caused an increase by an order of magnitude in the spectral density of ion-acoustic noise in the entire frequency range from 0.3 to 3 MHz under study.

Collective backscattering of gyrotron radiation by small-scale plasma density fluctuations in large helical device.

A version of the collective backscattering diagnostic using gyrotron radiation for small-scale turbulence is described. The diagnostic is used to measure small-scale (k(s) approximately 34 cm(-1)) plasma density fluctuations in large helical device experiments on the electron cyclotron heating of plasma with the use of 200 kW 82.7 GHz heating gyrotron. A good signal to noise ratio during plasma production phase was obtained, while contamination of stray light increased during plasma build-up phase. The effect of the stray radiation was investigated. The available quasioptical system of the heating system was utilized for this purpose.