Fast ignition realization experiment with high-contrast kilo-joule peta-watt LFEX laser and strong external magnetic field
Fast ignition realization experiment with high-contrast kilo-joule peta-watt LFEX laser and strong external magnetic field
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DOI：
10.1063/1.4948278
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发表时间：
2016-05-01
影响因子：
2.2
通讯作者：
Azechi, Hiroshi
中科院分区：
文献类型：
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作者：
Fujioka, Shinsuke;Arikawa, Yasunobu;Azechi, Hiroshi
用于快速点火实验的拍瓦激光器[N. Miyanaga等人，J. Phys. IV France 133，81（2006）]，其目前能够使用4个激光束在1.5ps脉冲中递送2kJ，已经在GEKKO-XII激光设施旁边建造，用于在快速点火实现实验（FIREX）项目[H. Azechi等人，Nucl. Fusion 49，104024（2009）]。在FIREX实验中，一个圆锥体被连接到一个含有燃料的球形靶上，以防止电晕等离子体进入强烈加热的LFEX激光束的路径。LFEX激光束聚焦在锥体的尖端以产生相对论电子束（REB），该相对论电子束加热由GEKKO-XII激光束诱导的球形氘化塑料靶压缩产生的致密燃料核心。最近的研究表明，当前的加热效率仅为0.4%，并且已经确定了利用当前GEKKO和LFEX系统实现快速点火（FI）方案的更高效率的三个要求：（i）减少REB的高能尾部;（ii）使用有限数量（十二个）的GEKKOXII激光束以及有限的能量形成具有高面密度的燃料芯（4千焦的0.53 μ m的光在1.3纳秒脉冲）;（iii）引导和聚焦的REB的燃料核心。长尺度等离子体中的激光-等离子体相互作用产生的电子能量太大，无法有效地加热燃料核心。为满足第一项要求采取了三项行动。首先，足脉冲与LFEX的主脉冲的强度对比度提高到> 10（9）。第二，5.5毫米长的圆锥体被引入，以减少由未转换的1.053 μ m内爆光束（GEKKO-XII）的光照射引起的内锥壁的预热。第三，锥壁的外部涂有40 μ m的塑料层，以保护其免受内爆等离子体造成的压力。在上述改进之后，实现了13%的LFEX激光能量到REB的低能量部分的转换，其斜率温度为0.7 MeV，这接近于有质动力标度值。为了满足第二个要求，通过激光驱动的球形会聚冲击波，压缩直径为200 μ m的实心球形球，形成面密度接近0.07 g/cm（2）的致密核。会聚激波压缩比壳体内爆更流体动力学稳定，而固体球靶不能产生热点。由于冲击压缩区域的相对小的磁雷诺数，实心球压缩对于压缩外部磁场以将REB准直到燃料芯也是优选的。为了满足第三个要求，我们已经产生了一个强大的千特斯拉磁场使用激光驱动的电容线圈目标。磁场的强度和时间历程的特点与质子偏转仪和B点探针。在LULI 2000激光设施中，已经证明了在平面几何形状中使用0.6 kT场的REB的指导。在一个现实的FI方案中，磁镜之间的REB生成点和燃料芯形成。利用数值模拟研究了强磁场对REB输运和等离子体压缩的影响。根据传输计算，通过满足上述三个要求，GEKKO和LFEX激光系统的加热效率可以从0.4%提高到4%。通过增加燃料芯的面密度，该效率可扩展到加热效率的10%。出版社：AIP Publishing
A petawatt laser for fast ignition experiments (LFEX) laser system [N. Miyanaga et al., J. Phys. IV France 133, 81 (2006)], which is currently capable of delivering 2 kJ in a 1.5 ps pulse using 4 laser beams, has been constructed beside the GEKKO-XII laser facility for demonstrating efficient fast heating of a dense plasma up to the ignition temperature under the auspices of the Fast Ignition Realization EXperiment (FIREX) project [H. Azechi et al., Nucl. Fusion 49, 104024 (2009)]. In the FIREX experiment, a cone is attached to a spherical target containing a fuel to prevent a corona plasma from entering the path of the intense heating LFEX laser beams. The LFEX laser beams are focused at the tip of the cone to generate a relativistic electron beam (REB), which heats a dense fuel core generated by compression of a spherical deuterized plastic target induced by the GEKKO-XII laser beams. Recent studies indicate that the current heating efficiency is only 0.4%, and three requirements to achieve higher efficiency of the fast ignition (FI) scheme with the current GEKKO and LFEX systems have been identified: (i) reduction of the high energy tail of the REB; (ii) formation of a fuel core with high areal density using a limited number (twelve) of GEKKOXII laser beams as well as a limited energy (4 kJ of 0.53-mu m light in a 1.3 ns pulse); (iii) guiding and focusing of the REB to the fuel core. Laser-plasma interactions in a long-scale plasma generate electrons that are too energetic to efficiently heat the fuel core. Three actions were taken to meet the first requirement. First, the intensity contrast of the foot pulses to the main pulses of the LFEX was improved to > 10(9). Second, a 5.5-mm-long cone was introduced to reduce pre-heating of the inner cone wall caused by illumination of the unconverted 1.053-mu m light of implosion beam (GEKKO-XII). Third, the outside of the cone wall was coated with a 40-mu m plastic layer to protect it from the pressure caused by imploding plasma. Following the above improvements, conversion of 13% of the LFEX laser energy to a low energy portion of the REB, whose slope temperature is 0.7 MeV, which is close to the ponderomotive scaling value, was achieved. To meet the second requirement, the compression of a solid spherical ball with a diameter of 200-mu m to form a dense core with an areal density of similar to 0.07 g/cm(2) was induced by a laser-driven spherically converging shock wave. Converging shock compression is more hydrodynamically stable compared to shell implosion, while a hot spot cannot be generated with a solid ball target. Solid ball compression is preferable also for compressing an external magnetic field to collimate the REB to the fuel core, due to the relatively small magnetic Reynolds number of the shock compressed region. To meet the third requirement, we have generated a strong kilo-tesla magnetic field using a laser-driven capacitor-coil target. The strength and time history of the magnetic field were characterized with proton deflectometry and a B-dot probe. Guidance of the REB using a 0.6-kT field in a planar geometry has been demonstrated at the LULI 2000 laser facility. In a realistic FI scenario, a magnetic mirror is formed between the REB generation point and the fuel core. The effects of the strong magnetic field on not only REB transport but also plasma compression were studied using numerical simulations. According to the transport calculations, the heating efficiency can be improved from 0.4% to 4% by the GEKKO and LFEX laser system by meeting the three requirements described above. This efficiency is scalable to 10% of the heating efficiency by increasing the areal density of the fuel core. Published by AIP Publishing.