K. Makino

Recent progress on high performance steady state plasmas in the superconducting tokamak TRIAM-1M

An overview of TRIAM-1M experiments is presented. The current status of issues related to steady state operation is presented with reference to the achievement of super-ultra-long tokamak discharges sustained by LHCD for over 2 h. The importance of control of the initial phase of the plasma, the avoidance of high heat load concentration, wall conditioning and the avoidance of abrupt termination of long discharges are discussed as the crucial issues for the achievement of steady state operation of the tokamak. A high ion temperature (HIT) discharge fully sustained by 2.45 GHz LHCD with both high ion temperature and steep temperature gradient was successfully demonstrated for longer than 1 min in the limiter configuration. The HIT discharges can be obtained in a narrow window of density and position. The avoidance of heat load concentration on a limiter is the key point for the achievement and long sustainment of the HIT discharge. As the effective thermal insulation between the wall and the plasma is improved for the single null configuration, HIT discharges with peak ion temperature > 5 keV and a steeper temperature gradient of up to 85 keV/m can be achieved through the fine control of density and position. Plasmas with high κ ≈ 1.5 can also be demonstrated for longer than 1 min. The current profile is also well controlled for a time about 2 orders of magnitude longer than the current diffusion time using combined LHCD. The serious damage to the material of the first wall caused by energetic neutral particles produced by charge exchange is also described. As the neutral particles cannot be affected by a magnetic field, this damage by neutral particles must be prevented by a new technique.

Recent progress on TRIAM-1M

A steady state plasma with high performance and high current drive efficiency is reported. In 2.45 GHz LHCD plasmas Ti is studied as a function of ne at the edge of the high ion temperature (HIT) window. Different characteristic timescales are found for Ti and ne to enter the HIT regime and the observed hysteresis behaviour of Ti with respect to ne is attributed to this difference. The electromagnetic emission (<3.5 GHz) is studied in order to understand ion heating mechanisms in the HIT regime. The spectrum shows several sidebands whose peak frequencies correspond to the ion plasma frequency. The spectral narrowing of the width of the sideband shows a clear correlation with ion heating. In 8.2 GHz LHCD plasmas an enhanced current drive (ECD) regime where both current drive efficiency ηCD( = eICDR0/PLH ~1 × 1019 A m-2/W) and energy confinement time τE (~8-10 ms) are simultaneously improved is obtained at an e of 4.3 × 1013 cm-3 and B = 7 T under full current drive conditions. There exists a certain threshold power above which the ECD transition occurs. A hysteresis of ηCD is found around the threshold power, which is explained by the different characteristic time for the ECD transition in power rampup and rampdown schemes. Current profile control experiments are performed by using two opposite travelling LHWs. Current compensation (ΔICD/ICD < -10%) is clearly seen when the backward (BW) travelling LHW (8.2 GHz) is added to a target plasma whose current is driven by a forward travelling LHW (8.2 GHz). As the BW wave power is increased, however, the current tends to flow in the forward direction. The mechanisms of this non-linear behaviour of the driven current with respect to the BW wave power are discussed.

A rotating coil probe for the magnetic field measurement on a long pulsed tokamak

In order to determine the magnitude and the position of the plasma current in a long pulsed tokamak such as the International Thermonuclear Experimental Reactor (ITER), it is urged to establish a reliable method which is free from the zero-level drift of the integrator as well as the radiation damage for the steady-state magnetic field measurement. For this purpose, we have developed a hybrid system, a combination of a conventional magnetic probe for the measurement of fast varying magnetic field and a rotating coil magnetic probe for that of slowly varying field. The rotating coil is energized by an air turbine to avoid electromagnetic interference and the induce signal with a constant rotation frequency is picked up through a transformer to eliminate mechanical contacts. An automatic gain control circuit is also designed for the compensation of rotation speed fluctuation. The system is proved to achieve a flat frequency response with a proper choice of cross over frequency for high- and low-frequency systems. The rotating coil probe is tested for over 170 h without any trouble. The probe was applied to the poloidal magnetic field measurement on the TRIAM-1M long pulsed tokamak, and proved to work satisfactorily.

High ion temperature discharge and its long sustainment in both limiter and single null configurations on TRIAM-1M

A high ion temperature discharge in the lower hybrid current drive scheme has been obtained for the first time in the world in the TRIAM-1M superconducting tokamak. The high ion temperature mode is triggered by a transition which occurs within an operation window for the density, horizontal plasma position and antenna phasing. A steep ion temperature gradient (>80 keV/m) is formed near the core region at the transition. Long duration operation of this discharge has been successfully demonstrated in both the limiter and single null configurations by multiple control techniques.

Current profile control and improvement of current drive efficiency by combined lower hybrid waves in TRIAM-1M

Current profile control experiments were carried out in TRIAM-1M by the combination of two lower hybrid waves (LHWs) at 8.2 and 2.45 GHz with different spectra of parallel refractive index N||. In the combined lower hybrid current drive (LHCD) discharge, the current drive efficiency was improved by a factor of >1.5 compared with 8.2 GHz LHCD only and 2.45 GHz LHCD only. Measurements of the internal inductance and the radial profile of hard X ray emission indicated that the current profile can be controlled by changing the additional power of the 2.45 GHz LHW to the 8.2 GHz LHCD plasma. The improvement of the current drive efficiency and the modification of the current profile are considered to be attributable to the enhancement of the absorption of lower N|| waves by higher N|| waves in the two kinds of LHWs. Moreover, the controlled current profile can be successfully sustained for about two orders of magnitude longer than the current diffusion time.