A. Ya. Kislov

Electron cyclotron current drive experiments on T-10

The results of electron current drive experiments on the T-10 tokamak are presented. The total RF power was up to 2.5 MW, the electron temperature was up to 7 keV and the maximum driven current was 110 kA. The current drive efficiency ηCD was approximately 0.1 A/W. The value of ηCD and its dependence on the plasma parameters agree satisfactorily with the linear theory, corrected for the finite confinement time of resonant electrons. In discharges with large beta poloidal, βp ≈ 3, complete replacement of the inductive current by noninductive electron cyclotron current drive and bootstrap current was obtained

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

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption. (Some figures in this article are in colour only in the electronic version)

MHD activity and formation of the electron internal transport barrier in the T-10 tokamak

The plasma stability and confinement have been investigated through control of the safety factor profile q(r) by the electron cyclotron current drive in the T-10 tokamak. The regimes with dq/dr0 and dq/dr<0 in the plasma core were obtained. Various types of MHD activity were observed: ordinary sawtooth, saturated sawtooth, humpbacks, hills etc. It was shown that when the minimal value qmin increases from qmin <1 to qmin = 2 the plasma becomes strongly unstable due to the corresponding MHD activity or passes to the steady-state improved confinement mode. The latter is realized when the electron internal transport barrier (EITB) is formed. The condition for the appearance of the EITB is dq/dr0, where q = m/n lies near a rational value for low m and n.

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

Reduced core transport in T-10 and TEXTOR discharges at rational surfaces with low magnetic shear

It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τE), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ~ 1 × τE.

STUDY OF TWO TYPES OF T-10 REGIMES WITH ECRH AND OHMIC HEATING

The study of two types of T-10 regime-a regime with low impurity confinement (S regime) and a regime with enhanced impurity confinement (B regime) has been continued in deuterium discharges with ne approximately necrit. In the present experiments the transitions from S to B regimes is obtained by the method of gas puff cut-off. Detailed investigations of the profile evolution of Te(r), Ti(r), ne(r) and carbon density nc showed, that the ne(r) profile greatly changes after gas puff cut-off, while Te(r) and Ti(r) remain practically unchanged at the central zone of the plasma. This indicates the possibility of a relation between the limiting ne(r) which is established and either Te(r) or Ti(r). It is shown, that the main cause which is responsible for this transition between B and S regime, is a variation of the neutral atom influx to the plasma. The transition itself takes place at a given plasma radius when ne reaches some limiting value at this radius.

The main features of self-consistent pressure profile formation

The self-organization of a tokamak plasma is a fundamental turbulent plasma phenomenon, which leads to the formation of a self-consistent pressure profile. This phenomenon has been investigated in the T-10 tokamak in different experiments, excluding profiles with pronounced transport barriers. It will be shown that the normalized pressure profile can be expressed by the equation pN(r) = p(r, t)/p(0, t), over a wide range of plasma densities. It will also be shown that pN(r) is independent of the heating power and the deposition profile of electron cyclotron resonance heating. Experiments show that pN(r) depends only on the value of q at the plasma edge. During rapid current ramp-ups it has been demonstrated that the conservation of pN(r) is established during a time tc < 0.1τE, with τE the energy confinement time. It can be concluded that the self-consistent pressure profile pN(r) in tokamaks is linked to the equilibrium of a turbulent plasma.

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and qL<4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/aL≈0.6, where aL is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr∼0 with q≈1 in the barrier region.

Tokamak plasma self-organization—synergetics of magnetic trap plasmas

Analysis of a wide range of experimental results in plasma magnetic confinement investigations shows that in most cases, plasmas are self-organized. In the tokamak case, it is realized in the self-consistent pressure profile, which permits the tokamak plasma to be macroscopically MHD stable. Existing experimental data permit suggesting a hypothesis about the mechanism of pressure profile regulation and to give an explanation of such unusual phenomena as a nonlocal character of transport coefficients, enhanced speed of heat/cold pulse propagation and many modes of tokamak operation.