E. Jotaki

Recycling and wall pumping in long duration discharges on TRIAM-1M

Recycling and wall pumping have been studied comparing low (?1018?m-3) and high (?1019?m-3) density long duration plasmas in TRIAM-1M. The recycling coefficient of each plasma increases with time. There exist two time constants in the temporal evolution of the recycling coefficient. One is a few seconds and the other is about 30?s. The wall pumping rates of low and high density plasmas are evaluated to be ?1.5?1016?atoms m-2?s-1 and ?4?1017?atoms m-2?s-1, respectively. The difference may be caused by the total number of diffused ions and the charge exchange neutral flux with an energy of less than 0.7?keV. In an ultra-long discharge (?70?min), the recycling coefficient becomes 1 or more before again decreasing below 1, i.e. the wall repeats the process of being saturated and refreshed. In high power and high density experiments, wall saturation phenomena have been observed. The discharge duration limited by wall saturation decreases with an increase in density.

Overview of steady state tokamak plasma experiments in TRIAM-1M

An overview of steady state tokamak studies in TRIAM-1M (R0 = 0.8 m, a × b = 0.12 m × 0.18 m and B = 8 T) is presented. The current ramp-up scenario without using centre solenoid coils is reinvestigated with respect to controllability of the current ramp-up rate at the medium density region of (1–2) × 1019 m−3. The plasma is initiated by ECH (fundamental o-mode at 170 GHz with 200 kW) at B = 6.7 T, and the ramp-up rate below the technical limit of 150 kA s−1 for ITER can be achieved by keeping the LH power less than 100 kW during the current ramp-up phase. The physics understanding of the enhanced current drive (ECD) mode around the threshold power level has progressed from a viewpoint of transition probability. A transition frequency, ftrans, for the ECD transition is determined as a function of PCD. At ~70 kW no transition occurs for an ftrans value of ~0.017 Hz, meaning almost zero transition probability. With increasing PCD > Pth, ftrans increases up to 10 Hz, and the transition tends to occur with high probability. The record value of the discharge duration is updated to 3 h 10 min in a low and low power (<10 kW) discharge. The global particle balance in long duration discharges is investigated, and the temporal change in wall pumping rate is determined. Although the density was low, the gas supply had to be stopped at 30 min after the plasma initiation to maintain the density constant. After that the density was sustained by the recycling flux alone until the end of the discharges. In addition to the recycling problem, in the high power and high density experiments, the localized PWI affects the SSO of the tokamak plasma. The effects of enhanced influx of metal impurities (Fe, Cr, Ni, Mo) on sustainment of the high performance ECD plasma are investigated. In order to evaluate the helium bombarding effects on the plasma facing component and hydrogen recycling in the future burning plasma, microscopic damage of metals exposed to long duration helium discharges was studied. The total exposure time was 128 s. From thermal desorption experiments for the specimens the amount of retained helium was evaluated as 3.9 × 1020 He m−2 and the scale length to be ~1 mm in the SOL.

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.

Global particle balance and wall recycling properties of long duration discharges on TRIAM-1M

The longest tokamak discharge, with a duration of 11 406 s (3 h 10 min), has been achieved. The global particle balance has been investigated. In the longest discharge, the global balance between the particle absorption and release of the wall was achieved at t ~ 30 min. After that, the plasma density was maintained by the recycling flux alone until the end of the discharge. The maximum wall inventory is about 3.6 × 1020 H at t ~ 30 min, but it is finally released from the wall at the end of the discharge. The hydrogen release seems to be caused by the temperature increase in the whole toroidal area of the main chamber. Moreover, it has been observed that there is a large difference between the properties of wall recycling in the continuous gas feed case (i.e. static condition) and in the additional gas puff case (i.e. dynamic condition). In the static condition, the effective particle confinement time increases to ~10 s during the 1 min discharge and it increases to ~100 s before the global balance in the longest discharge. In the dynamic condition, the decay time of the electron density just after the gas puff, i.e. the effective particle confinement time, is constant at 0.2–0.3 s during the discharge. The large difference in the effective particle confinement time between the static and dynamic conditions seems to be caused by the reduction in the recycling coefficient due to the enhanced wall pumping resulting from the additional gas puff.

Ultra-long pulse operation using lower hybrid waves on the superconducting high field tokamak TRIAM-1M

Ultra-long pulse operation (> 3 min) was achieved on the superconducting high field tokamak TRIAM-1M. In this operation, the plasma current was maintained with a relatively peaked current distribution by the 2.45 GHz radiofrequency power (PRF ≤ 35 kW) alone. A stationary plasma with a driven current of up to 35 kA and a line averaged electron density of up to 3 × 1012 cm−3 was produced by precise plasma position and gas feed control. The extremely long discharge showed the interesting characteristics that the high temperatures of about 1 keV for the electrons and about 0.5 keV for the ions were kept almost constant during steady state current drive and that there was no impurity accumulation which could have a fatally adverse effect on steady state tokamak operation.

Continuous Monitoring and Data Acquisition System for Steady-State Tokamak Operation

Long discharges have been demonstrated by lower hybrid current-drive experiments on some tokamak devices. Discharges of longer than 1000 s are also planned for the International Thermonuclear Experimental Reactor (ITER) and Tokamak Physics Experiment (TPX) projects. In the case of long-time or steady-state operation, it is important to monitor the plasma parameters continuously and change the operational conditions during the discharge to maintain the plasma current. However, a conventional data acquisition and analysis system cannot follow these operations because it must show the results after each pulse. A new system that can continuously monitor and support steady-state operation is necessary. A new system is developed in which the signal flow is divided into branches, and one series of processing is made to switch alternately among the groups in every regular desired interval. An application of this system has been demonstrated on a 1-h discharge by TRIAM-1M. 9 refs., 6 figs.

Development of completely solenoidless tokamak operation in JT-60U

Plasma current start-up to 100 kA was achieved successfully in the JT-60U tokamak without the use of the centre solenoid (completely solenoidless tokamak operation). Only poloidal field coils located on the outboard side of the torus were used, in combination with strong ionization by electron cyclotron (EC) power. The presence of a field null was not necessary for plasma current start-up, but the flux conversion efficiency was low in such a case. In a nearly solenoidless start-up, low neutral pressures were favoured, and the optimum location of the EC resonance was slightly to the high field side of the vacuum vessel centre. The required EC power for efficient utilization of flux swing in JT-60U was about 1 MW. A plasma current of 260 kA was maintained for 1 s by NB only, and plasma current ramp-up from 215 to 310 kA was achieved by EC and neutral beam (NB) only (without lower hybrid current drive (LHCD)). However, the ramp-up efficiency was much lower compared with LHCD. Recharging of the centre solenoid was observed with only counter and perpendicular NB injection, indicating bootstrap overdrive. Integration of these elements can lead to the achievement of a completely solenoidless tokamak operation.

ECR—discharge cleaning for TRIAM - 1M

Abstract Electron cyclotron resonance-discharge cleaning (ECR-DC) was carried out in tokamak experiments to reduce the impurities on the surface of plasma chamber, yielding good results. However, quantitative analysis for residual gases by long time ECR-DC has not yet been reported. In the TRIAM-1M device, the surface of the plasma chamber was cleaned using long time ECR-DC without baking the chamber, and clean plasmas were produced. The effects of ECR-DC were analyzed by the measuring of 17 (OH), 18 (H2O), 28 (CO) and 44 (CO2) with quadrupole mass analyzer (QMA).

Research activities on tokamaks in Japan: JT-60U, JFT-2M, and TRIAM-1M

Research activities of the Japanese tokamaks JT-60U, JFT-2M, and TRIAM-1M are described. The recent JT-60 program is focused on the establishment of a scientific basis of advanced steady-state operation. Plasma performance in transient and quasi steady states has been significantly improved, utilizing reversed shear and weak shear (high-βp) ELMy H-modes characterized by both internal and edge transport barriers and high bootstrap current fractions. Development of each key issue for advanced steady-state operation has also been advanced. Advanced and basic research of JFT-2M has been performed to develop high-performance tokamak plasma as well as the structural material for a fusion reactor. Toroidal field ripple reduction with ferritic steel plates outside the vacuum vessel is successfully demonstrated. No adverse effects to the plasma were observed with poloidal fields inside the vacuum vessel (partial covering). Preparation is in progress for full-scale testing of the compatibility of the ferritic steel wall (full covering) with plasma. A heavy ion beam probe has been installed to study H-mode plasmas. Compact toroid (CT) injection experiments are performed, showing deep CT penetration into the core region of the H-mode. The TRIAM project has investigated steady-state operation and high-performance plasma of a tokamak with the high toroidal magnetic field superconducting tokamak. Four important contributions in the fields of fusion technology of superconducting tokamaks, steady-state operation, high-performance plasma, and startup of plasma current without the assistance of center solenoid coils have been achieved on TRIAM-1M, especially regarding steady-state operation by realization of a discharge for >3 h.

Steady state experiments on current profile control and long sustainment of high performance LHCD plasmas on the superconducting tokamak TRIAM-1M

The main purpose of TRIAM-1M (R0 = 0.8?m, a ? b = 0.12?m ? 0.18?m, B = 8?T) is to study the route towards a high field compact steady state fusion reactor. In the advanced steady state operation programme, a heating mechanism for the high ion temperature mode with an internal transport barrier has been studied, an enhanced current drive mode in an extended (higher power and higher density) operation regime has been obtained, current density profile control experiments using multicurrent drive systems have been performed and the effects of wall recycling, wall pumping and wall saturation on particle control have been investigated.