First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron resonance heating (f = 53.2 GHz, P ECRH = 250 kW). Initial experiments have explored the TJ-II flexibility in a wide range of plasma volumes, different rotational transform and magnetic well values. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma wall interaction phenomena on TJ-II operation is discussed briefly.

Transitions to the regime with better confinement in the L-2M stellarator are presented. Transitions are indicated only at sufficiently high plasma densities, and for a given value of average density they appear only at higher heating powers. Each transition is easily identified by a sudden fast (<200 µs) small drop of total plasma energy fixed by diamagnetic measurements. After that plasma energy steeply regains its value and then slowly monotonically increases up to the end of the active heating phase (just as the line average plasma density n e ). In the bulk of the plasma parameters evolve slowly. Drastic changes are observed in the region close to the plasma boundary where two moderate order rational magnetic surfaces are located with the rotational transform µ taking the values 2/3 and 3/4. Relative values of plasma parameters' fluctuations and their spectrum widths decrease significantly in this region. The region has a definite sandwich structure being subdivided by the above-named moderate order rational magnetic surfaces into three smaller zones with different plasma parameter dynamics. Transition is triggered by local disturbances of plasma parameters that are caused by instabilities in the vicinity of magnetic surfaces where µ is equal to 2/3 or 3/4. Different hypotheses on the nature of the phenomenon are discussed.

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The results of experiments on plasma confinement at ECRH power density up to 1.5 MW/m 3 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.

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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.

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