D.A. 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

Coupling of internal m=1 and m=2 modes at density limit disruptions in the T-10 tokamak

A soft X-ray imaging system consisting of three arrays of silicon surface barrier diodes is applied to the tomographic analysis of internal plasma perturbations in the T-l0 tokamak (R0=1.5 m, a=0.32 m). It is found that density limit disruptions in a plasma with a high safety factor at the edge (qa=3.5-4.5) are associated with joint rotation of the m=2, n=1 and m=1, n=1 modes, overlapping at the energy quench stage. Electron cyclotron resonance heating is used to prevent density limit disruptions or to recover stable operation of the discharge after the energy quench at the density limit

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)

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.

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

Objectives, physics requirements and conceptual design of an ECRH system for JET

A study has been conducted to evaluate the feasibility of installing an electron cyclotron resonance heating (ECRH) and current drive system on the JET tokamak. The main functions of this system would be electron heating, sawtooth control, neoclassical tearing mode control to access high beta regimes and current profile control to access and maintain advanced plasma scenarios. This paper presents an overview of the studies performed in this framework by an EU-Russia project team. The motivations for this major upgrade of the JET heating systems and the required functions are discussed. The main results of the study are summarized. The usefulness of a 10 MW level EC system for JET is definitely confirmed by the physics studies. Neither feasibility issues nor strong limitations for any of the functions envisaged have been found. This has led to a preliminary conceptual design of the system.

Tokamak T‐10 soft x‐ray imaging diagnostic

Three arrays of silicon surface‐barrier diodes were recently installed on T‐10 (R=1.5 m, a=0.3 m). The detectors view the plasma cross section along 58 chords spaced in the poloidal direction at one toroidal location. The tomographic reconstruction technique allows one to obtain the time evolution of the two‐dimensional soft x‐ray intensity profiles in the energy range of 2.5–15 keV. The field of view covered the main part of the plasma (r/a<0.7) with a spatial resolution as small as 2 cm, which is consistent with the scale of the processes under study. The signals are digitized at rates up to 100 kHz and stored in 464K (total) memory (8K per channel). The measured soft x‐ray emission was applicable for investigation of the magnetohydrodynamic instabilities, heat and particle transport, and plasma position control. Studies of the evolution of soft x‐ray perturbations were made in ohmically and ECRH heated plasmas. It was shown that the effect of ECRH on the plasma parameters (transport coefficients, sawto...

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.