Pulsed-laser ablation of hydrogen-dposited graphite target

碳材料因其高热通量和低原子序数的优越性，成为ITER和核聚变反应堆等离子体面材料的候选材料之一。然而，当碳被含氚的氢等离子体侵蚀时，会与氚共沉积，出于安全考虑需要定期去除。为了去除核聚变装置等离子面材料碳共沉积中的氚，研究了JT-60开导流瓦上脉冲激光对共沉积氢的解吸。利用Nd:YAG激光器基频（L= 1064nm）和四次谐波（L= 266nm）发射（1064nm时激光能量< 35mj /脉冲，266nm时激光能量< 3mj /脉冲，脉冲持续时间~ 20ps， I_L<6 × 10^<12> W/cm^2, 1064nm时I_L<6 × 10^<12> W/cm^2, 266nm时I_L< 5 × 10^<11> W/cm^2，重复频率= 10hz）。单次激光烧蚀深度与I_L的关系表明，烧蚀阈值激光强度I_<烧蚀> ~ 10^<10> W/cm^2与L无关，但L=266 nm时线性吸收系数较大。在L=1064和266 nm之间，碳I_<电离>的强电离阈值激光强度有较大差异；当L=266和1064 nm时，I_<电离>= 9 × 10^<10>和（3.4 ~ 7.4）× 10^<11>W/cm^2。L= 266nm时较低的I_<电离>可以用碳的三光子电离来解释。解吸氢效率（即激光照射解吸氢的数量与烧蚀体积中保留氢的数量之比）在C^+发生强电离的区域最大，并且在L=266 nm激光光子可以切割C- h键的区域比L=1064 nm的区域更大。对于烧蚀除氢，需要短波长和高功率的激光器。

Carbon materials are one of the candidates for plasma facing materials in ITER and nuclear-fusion reactors because of its superior nature at high-heat flux and low atomic number. However, when carbon is eroded by hydrogenic plasmas containing tritium, it co-deposits with tritium, and periodical removal is required from the safety reason. For the purpose of tritium removal from carbon co-deposits on plasma-facing material of fusion device, pulsed-laser induced desorption of hydrogen from co-deposits on JT-60 open-divertor tile has been investigated.The fundamental (L=1064 nm) and fourth harmonic (L=266 nm) emission of Nd:YAG laser (laser energy <35 mJ/pulse for 1064 nm and <3 mJ/pulse for 266 nm, pulse duration 〜20 ps, I_L<6 x 10^<12> W/cm^2 for 1064 nm and 5 x 10^<11> W/cm^2 for 266 nm, repetition rate=10 Hz) were used.Ablation depth per laser shot vs I_L showed that whereas the threshold laser intensity for ablation I_<ablation> 〜10^<10> W/cm^2 was nearly independent on L, the linear absorption-coefficient was larger for L=266 nm. A large difference in the threshold laser intensities of the strong ionization of carbon I_<ionization> between L=1064 and 266 nm was observed ; I_<ionization>= 9 x 10^<10> and (3.4〜7.4) x 10^<11>W/cm^2 for L=266 and 1064 nm, respectively. The lower I_<ionization> for L=266 nm was explained by three-photon ionization of carbon. Hydrogen-desorption efficiency, defined as the ratio between the number of desorbed hydrogen by laser irradiation and that of hydrogen retained in the ablated volume, was largest in the region, where strong ionization of C^+ occurred, and was larger for L=266 nm, in which a laser photon can cut C-H bond, compared with that for L=1064 nm. For the ablative removal of hydrogen, a short-wavelength and high-power laser is desirable.