Yu. V. Esipchuk

The m=2, n=1 mode suppression by ECRH on the T-10 tokamak

Experiments on m=2, n=1 tearing mode suppression and on avoidance of density limit disruptions by electron cyclotron resonance heating (ECRH) were performed on the T-10 tokamak. Partial suppression of the m=2, n=1 mode by the high frequency (HF) power deposition in the vicinity of the q=2 surface was observed. Development of external kink modes with HF power injection can result in m=2, n=1 mode destabilization under specific operating conditions. ECRH suppresses m=2, n=1 mode activity at extremely high values of electron densities and prevents the density limit disruptions practically independently of EC resonance position. Complete compensation of the additional peripheral heat losses near the density limit by ECRH should be responsible for this result. No effect of electron cyclotron current drive (ECCD) on m=2, n=1 mode stability has been observed because of insufficient values of HF driven current in the vicinity of the q=2 surface under the operating conditions of the experiment

Beta limit due to resistive tearing modes in T-10

Soft β limiting phenomena have been observed in T-10 in ECRH heated plasmas. Neoclassical tearing modes are supposed to be responsible for the β limitation. MHD onset was observed at high βp values but low βN values. The critical β has been found to be almost independent of the collisionality parameter νe*. Sawtooth stabilization by ECCD does not result in an increase of critical beta. A dependence of the critical β on the q(r) profile (modified by ECCD) has been observed.

Second harmonic electron cyclotron current drive experiments on T-10

Results of the electron cyclotron current drive experiment at the second harmonic resonance on the T-10 tokamak are presented. High frequency (HF) power up to 1.2 MW was launched from the low field side. A maximum driven current of 35 kA and current drive efficiency ηCD = 0.05 A/W at an electron temperature Tc(O) = 4 keV and a density nc(0) = 1 × 1013 cm-3 were obtained. For low HF power, the current drive efficiency was less than predicted by the linear theory unless the effect of the elliptical polarization from non-perpendicular injection is considered, in which case the efficiency is close to the theoretical value. The experimental dependence of HF on the absorbed HF power indicated a strong increase of ηCD with power. Suppression of sawtooth oscillations and improvement of confinement during electron cyclotron heating has also been demonstrated

Power deposition profile effect on the ECH efficiency in T-10

The results of the electron cyclotron heating (ECH) experiments in T-10 are presented. An 11-tube gyrotron set-up with a total power of 4 MW was used in the experiments. The set-up consists of two types of gyrotrons with different wavelengths. The dependence of the energy confinement on the RF power deposition profile was investigated. An electron temperature of reactor level was obtained.

Increased understanding of the dynamics and transport in ITB plasmas from multi-machine comparisons

OAK A271 INCREASED UNDERSTANDING OF THE DYNAMICS AND TRANSPORT IN ITB PLASMAS FROM MULTI-MACHINE COMPARISONS.

Investigation of ECCD on the first and second harmonics ECR

Electron cyclotron current drive (ECCD) on the 1st and 2nd harmonics of the electron cyclotron resonance (ECR) is considered. Mainly the T-10 experimental results are described and comparison with the experimental data from various devices is performed. Features of ECCD are studied by comparison of Fokker-Planck (FP) calculations with measurements of X-ray spectra in discharges with ECCD. It is shown that ECCD efficiency is in a good agreement with FP calculations. Database obtained on various devices allows to assume that in ITER the ECCD efficiency can be achieved as high as eta CD=0.3*1020A/Wm2.

Internal transport barrier formation in experiments with reverse magnetic shear in T-10 tokamak

T-10 experiments with electron internal transport barrier (ITB) formation in discharges with reverse shear q(r) profile are described. Reverse magnetic shear was formed in the central region characterized by r/aL ≤ 0.3. It is shown that electron thermal conductivity decreases essentially in comparison with the value typical for the L-mode. It is found that degradation of the ITB correlates with development of MHD activity in the internal part of the plasma column.

Investigation of the second harmonic electron cyclotron current drive efficiency on the T‐10 tokamak*

Experiments on second harmonic electron cyclotron current drive were done on the T‐10 tokamak using four gyrotrons. Total powers up to 1.2 MW at a frequency of 140 GHz were injected. Current generation by electron cyclotron (EC) waves was demonstrated in the experiments. The efficiency η of current generation and its dependence on plasma parameters were measured and it was shown that the efficiency is a nonlinear function of input power, more closely predicted by Fokker–Planck calculations than by linear theory. The interaction of EC waves with the tail of the electron distribution was shown to be important. It was also found that current density profile redistribution played an important role in the plasma behavior.

High density experiments with gas puffing and ECRH in T-10

High density experiments were carried out in T-10 with gas puffing and electron cyclotron resonance heating (with absorbed power value up to 1.4 MW) with oblique and perpendicular power launch. Densities exceeding the Greenwald limit (nGw) by up to a factor of 1.8 were achieved in a regime with a high value of the edge safety factor at the current flat-top, q(a)8.2. The decrease of q(a) to a value of 3 led to the reduction of the ratio (e) lim /nGw to 1. Confinement degradation with density increase was not significant up to the density limit. However, the typical T-10 linear increase of energy confinement time with density saturates at e≥0.6nGw. This saturation is the result of the development of an additional transport in the electron heat channel. However, the saturated τE values exceeded the ITER L-mode scaling predictions by up to a factor of 1.2 and were close to the value predicted by the ITER H-mode scaling. Effect of the strong gas puffing on the plasma confinement and experiments with neon seeding are also discussed in this paper.

Reversed-shear experiments in the T-10 tokamak

Results from T-10 experiments in regimes with nonmonotonic plasma current profiles are presented. The possibility of controlling the current profile j(r) by electron-cyclotron current drive is demonstrated experimentally. Nonmonotonic q profiles with the reversed shear are obtained in which the qmin value varies in a wide range, qmin=1–2.3. It is shown that the current profiles with qmin∼2 (in this case, there are two resonant magnetic surfaces q=2 in the plasma) can cause the onset of MHD instabilities. The possibility of the formation of an internal transport barrier in reversed-shear discharges in the T-10 tokamak is analyzed. In T-10, electron transport is governed by short-wavelength electron turbulence. As a result, there is no clear evidence of the formation of an inner transport barrier in these experiments.