Physics and Technological Issues for Steady-State Tokamak Operation on TRIAM-1M

Abstract

Subjects on TRIAM-1M are presented from the viewpoint of plasma physics and reactor technology associated with 'steady state operation' (SSO). For a future fusion power plant, for burning plasma, complete steady state operation is required. Control of the density under the complicated plasma-wall interaction, non-inductive current start-up, and sustainment of high performance are key areas in the present and future investigations.The following experimental results are reviewed. First, it is shown that the co-deposition of the metallic impurity and oxygen plays an important role in the temporary change in the wall pumping rate, and a model of the co-deposition probability agreed with the observation. It was also noticed that the thermal release of the hydrogen from the plasma-facing components affects the steady state density operation in the ultra long discharge. It was found that enhanced influx of metal impurities from the hot spot affect the steady state operation of the high performance plasma. Second, helium effects on microscopic damage on metals were studied in helium/hydrogen mixture discharges. A large quantity of dislocation loops and dense fine bubbles were observed by means of TEM even for exposure only 125 seconds in duration. From TDS for the specimens, the amount of retained helium was evaluated to be 3.9 × 10 20 He/m 2 . Third, the physics understanding for the enhanced current drive (ECD) mode around the threshold power level progressed from the viewpoint of transition probability. The forward transition frequency from a non-ECD plasma state to the ECD state was precisely determined under fixed LHCD power. Thus, a statistical probability for ECD transition was determined; that is, the transition behavior around the threshold power could be described in a statistical manner. Transition frequency showed a strong power dependence. Fourth, the current ramp-up scenario without using centre solenoid coils was reinvestigated at higher density, and controllability of the current ramp-up rate was studied. The plasma was initiated by ECH (fundamental O-mode at 170 GHz with 200 kW) at B = 6.7 T, and a ramp-up rate below the technical limit of 150 kA/s for ITER could be achieved by choosing LH power. A model to describe the ramp-up is proposed.

Study of lower hybrid current drive efficiency over a wide range of FTU plasma parameters

“…the EC waves are injected perpendicularly to B T . This is the so-called downshifted regime (EC resonance frequency pushed down at B = cost) where the e − relativistic mass increment is balanced by an increase of B. Synergetic effects have been already reported in FTU in previous works [4,5] and subsequently confirmed on Tore Supra [33] and TRIAM-1M [34]. Both devices, however, operate in up-shifted regimes, where the Doppler shift (n EC > 0) of the co-injected EC waves prevails on the relativistic mass increase, the cold EC resonance is retained inside the plasma and also some current is driven directly by the EC waves.…”

Steady-state tokamak operation, ITB transition and sustainment and ECCD experiments in TRIAM-1M

“…As shown in figure 9, PWI driven relaxation oscillations (ULF events) are observed for heat load, recycling flux and impurity behaviour [34]. These oscillations are semi-regular and should be distinguished from the random off-normal events.…”

Status report on fusion research

“…JT-60U has achieved quasi-steady sustainment of reversed shear configuration with a Q DT equivalent of 0.5 for 0.8 s [56] and maintenance of internal transport barrier with lower hybrid current and bootstrap alone for 6 s [57]; such experiments are critical for advanced tokamak reactors. Truly long pulse discharges with lower hybrid current drive have been achieved in TRIAM-1M in which a current of 15 kA was driven for 5 h 16 min [58]. TORE SUPRA has also demonstrated 6 min long 1 GJ, 0.5 MA discharges sustained by about 3 MW of lower hybrid power at an average plasma density of about 1.5 × 10 19 m −3 [59].…”