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A special experiment performed at the TJ-II stellarator set-up has demonstrated how a relatively weak wave returned from an external mechanically modulated reflector influences upon the gyrotron radiation spectrum.

A heavy ion beam probe diagnostic was installed on T‐10 tokamak. Cs+ and Tl+ beams with the energy 100–280 keV and intensity about a few dozen μA were used. The diagnostic is able to make two‐dimensional measurements of plasma parameters along the detector line. The detector grid covered the first quadrant of the plasma cross section. The measurements are possible with r/a=0.3–1, and ω=0°–90° for B=1.5 T and Cs+ ions. High‐voltage stabilizers keep the analyzer and accelerator voltage drift on the level ΔU/U∼10−5. An absolute in situ calibration of the energy analyzer, using He target will allow to make a more accurate absolute referencing of the potential. The time dependence of the local plasma parameters was obtained. A scan along the detector line by variation of injection angle and obtaining the whole profile during one pulse is possible. The diagnostic is also sensitive to poloidal magnetic field.

D. K. Akulina, G. M. Batanov, M. S. Berezhetskii, G. S. Voronov, G. A. Gladkov, S. E. Grebenshchikov, I. S. Danilkin, N. P. Donskaya, L. V. Kolik, N. F. Larionova, A. I. Meshcheryakov, K. A. Sarksyan, 0. I. Fedyanin, N. K. Kharchev, Yu. V. Khol’nov, and S. V. Shchepetov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, I19991 Russia, e-mail: akulina@pl.gpi.ru Abstract Results are presented from experimental studies of the behavior of plasmas with moderate ( n , l 3 x m”) down to low (n, = 0.3 x I O l 9 m”) densities produced and heated by microwaves If= 75 GHz) with power PS 400 kW It is shown that, as the plasma density is reduced, the plasma emission spectrum is modified. In spite of the fact that the second harmonic emission from the plasma increases in this case, the plasma energy measured by the diamagnetic diagnostics does not increase. This casts doubts on the correctness of ECE measurements of the plasma temperature under these conditions. The formation of an internal transport barrier was not observed in the available magnetic-field structure of L-2M.

Experiments at the stellarator L-2M show that even a small reflection from the plasma may result in an essential broadening of the gyrotron output spectrum. The effect is explained as the resonant locking of the gyrotron by the reflected wave fluctuations.

Comparative studies of the structure of turbulence have been carried out in stellarator plasmas with low (TJ-IU) and medium (L2-M) magnetic shear. In both devices, the value of the radial coherence of fluctuations is bursty. In the plasma edge region, the radial coherence is dominated by low frequency fluctuations ({lt}200 KHZ) in TJ-IU, whereas in L2-M, the radial coherence is due to high frequency fluctuations ({gt} 100 kHz). Numerical calculations of resistive ballooning turbulence show that in the non-linear phase, the correlation length is of the order of the width of the linear mode poloidal components, whereas in the linear phase, the correlation length is related to the width of the envelope. Toroidal coupling effects appear to be relevant in the nonlinear phase in the proximity of instability thresholds. The electrostatic Reynolds stress shows a radial gradient close to the velocity shear location in the TJ-IU torsatron, thus suggesting that this mechanism can drive poloidal flows.

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 results of experiments on plasma confinement at ECRH power density up to 1.5 MW/m3 are reported. It is shown that the results of measurements of the diamagnetism and poloidal rotation velocity of the plasma in the peripheral region agree with numerical calculations using a neoclassical model with allowance for anomalous losses. The correlation has been found between the low-frequency modulation of the gyrotron power radiation and the modulation of the radiation scattered by the turbulent plasma density fluctuations.

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.

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.

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (f = 53.2 GHz, P-ECRH approximate to 250 kW) with a pulse length of Delta t approximate to (80-200) ms. The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P-ECRH = 300 kW, power density = 1-25 W cm(-3)). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.

The first results of ECH experiments in the L-2M stellarator are presented. The main goal of the experiments is to investigate the physics of ECH and plasma confinement at very high values of the v...

The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5−2)×10−, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (B v /B 0 =5×10−3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (B v/B 0∼3×10−3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.