Removal of the deposition on JT-60 tile by nano-sec pulsed-laser irradiation

Abstract

To clarify the property of tritium removal from carbon codeposition by using pulsed-laser induced desorption, hydrogen removal from co-deposits on JT-60 divertor tile using a fourth-harmonic emission (266 nm) of a nano-sec Nd:YAG laser was demonstrated. The threshold laser fluence for ablation was ~ 0.3 J/cm 2 , which was slightly higher compared to that of the pico-sec laser irradiation. The energy absorption coefficient for the nano-sec laser, which was obtained by fitting the removal rate by so-called Beer's law, was larger than the pico-sec laser. Since the time constant of thermal wave propagation into the target is of the order of nanosecond, such differences between nano-and pico-sec lasers could be attributed to thermal effects. The ablation threshold of the deposited layer was lower than that of a pure graphite, which could be attributed to the difference of thermal conductivity between deposited layer and pure graphite. The removal rate of the nano-sec laser was higher than that of pico-sec laser in the fluence range of < 0.5 J/cm 2 . On the other hand, the production ratio [hydrocarbon species] / [H 2 ] continuously increased with the laser fluence, and no significant ionization of carbon was observed in this fluence range. This indicated that the fluence range in this study was "weak"-ablation range, which was still not sufficient to minimize the hydrocarbon production.

A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

“…Carbon deposits can be removed with different techniques, e.g. with pulsedlaser [5] or with oxidation, which can be divided into three categories: plasma driven, activated oxygen and thermooxidation [6][7][8]. Applications of mild plasma of oxygen, hydrogen, or nitrogen (ammonia) were found suitable [9][10][11][12][13][14][15] but this technique cannot be applied in ITER because of the strong permanent magnetic fields produced by superconducting magnets.…”