Pulsed-laser ablation of hydrogen-dposited graphite target

碳材料因其在高热通量和低原子序数下的上级性能而成为ITER和核聚变反应堆中面向等离子体材料的候选材料之一。然而，当碳被含氚的类氢等离子体侵蚀时，它与氚共沉积，并且出于安全原因需要定期去除。为了从聚变装置面向等离子体材料上的碳共沉积层中去除氚，研究了JT-60开放偏滤器瓦上碳共沉积层中氢的脉冲激光诱导脱附，获得了Nd：YAG激光（1064 nm激光能量&lt;35 mJ/pulse，266 nm激光能量&lt;3 mJ/pulse，脉宽≤ 20 ps，1064 nm激光I_L&lt;6 × 10 ~ 4 W/cm ~ 2，266 nm激光I_L&lt;5 × 10 ~ 4 W/cm ~ 2）的基波（L=1064 nm）和四次谐波（L=266 nm）<12>发射<11>，每次激光照射的烧蚀深度与I_L的关系表明，烧蚀阈值激光强度I_<ablation>（10 μ <10>W/cm ^2）几乎与L无关，而L=266 nm时的线性吸收系数较大。在L=1064和266 nm之间，观察到了碳的强电离的阈值激光强度有很大的差异<ionization>; L=266和1064 nm时，I_分别<ionization>为9 × 10 <10>^ W/cm ^2和（3.4 × 7.4）× 10^<11>W/cm ^2。L=266 nm时的较低I_<ionization>2可由碳的三光子电离解释。氢脱附效率，定义为激光照射脱附的氢数与烧蚀体积中保留的氢数之比，在C^+发生强电离的区域最大，并且在激光光子可以切断C-H键的L=266 nm时比L=1064 nm时更大。对于氢的烧蚀去除，短波长和高功率的激光是可取的。

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.