Pulsed-laser ablation of hydrogen-dposited graphite target

碳材料具有高热流密度和低原子序数的优异性能，是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，当碳被含氚氢等离子体腐蚀时，它会与氚共沉积，出于安全原因，需要定期清除。为了从聚变装置面向等离子体材料的碳共沉积中脱除氚，研究了脉冲激光诱导从JT-60开放偏滤器瓷砖上共沉积的氢的脱附。脉冲激光的基频(L=1064 nm)和四倍频(L=266 nm)发射(激光能量35mJ/脉冲1064 nm和3mJ/脉冲266 nm，脉冲宽度~20ps，i_L&lt；6×10^&lt；12&gt；W/cm^2,1064 nm和5×10^&lt；11&gt；激光单次烧蚀深度与I_L的关系表明，尽管烧蚀的阈值激光强度几乎与L无关，但当L=266 nm时，线性吸收系数较大。结果表明，在L=1 0 6 4 nm和2 66 nm之间，碳原子的强电离阈值激光强度有很大差异，对于L=2 6 6 nm和2 0 6 4 nm，碳原子的强电离阈值分别为9×10^lt；10&gt；和(3.4~7.4)x10^&lt；11&gt；W/cm2。用碳的三光子电离解释了L=266 nm较低的I&lt；电离和Gt；。氢的脱附效率，定义为激光照射解吸的氢的数量与烧蚀体积中保留的氢的数量之比，在C^发生强烈电离的区域最大，当L=266 nm时，激光光子可以切断C-H键，而L=1064 nm时，解吸效率较大。对于烧蚀除氢，短波长和高功率的激光器是可取的。

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.