T. Kuwabara

Heat flows through plasma sheaths

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Hot spot formation on electron-emissive target plate with plasma potential variation across magnetic field

Nonlinear dynamic behavior of hot spot formation on tungsten (W) material surfaces is numerically investigated by introducing a transient heat pulse into the local area of the hot W sheet immersed in high heat flux plasma. It is found that the formation and development of hot spot depend not only on plasma parameters (plasma heat flow, plasma potential variation) and the energy and time scale of heat pulse but also sensitively on electron-emission characteristics of the tungsten surface as well as the size of hot spot in a nonlinear way.

Effect of Damage Introduction and He Existence on D Retention in Tungsten by High Flux D Plasma Exposure

Both of radiation-induced damages and helium (He) existence effects on deuterium (D) retention in tungsten (W) by D plasma exposure were evaluated using high flux divertor plasma exposure device, called Compact Divertor Plasma Simulator (CDPS). The results were compared with 3 keV D2+ implanted W with low flux and fluence. The thermal desorption spectra were consisted of three desorption stages at 400, 600, 780 K. Comparing to the undamaged W, the D desorption stages were shifted towards higher temperature side and the values of D retention increased. It can be said that the formation of stable trapping sites by damage introduction enhances the D trapping in the damaged W. For He+ irradiation, D desorption at lower temperature was enhanced, due to the formation of dense dislocation loops. In case of sequential Fe2+ and He+ implantation, D desorption at higher temperature was reduced, comparing to that for only Fe2+ damaged W. These facts show that the accumulation of He near surface region reduces D diffusion toward bulk, leading to the reduction of D trapping by voids.

Monte Carlo simulation on active helium-ash exhaust by divertor biasing

Abstract An effective helium-ash exhaust from the tokamak edge region using reduced helium recycling by divertor plate biasing is numerically studied. The amount of the passing neutral particles is calculated taking into account the reflection on the divertor plate and the recycling process of the reflected neutrals between the divertor and the SOL plasma. The ion incident energy is determined by the sheath voltage, which is controlled by the divertor biasing. Numerical studies show that the optimum structure of the divertor plate and the pump ducts, and the control of the ion energy imply a possibility of selective helium-ash exhaust. In this calculation we obtain an enhancement factor of about 3 for the ratio of helium to deuterium incident on the divertor plate.