Laser-direct-drive fusion target design with a high-Z gradient-density pusher shell

Laser-direct-drive fusion target designs with solid deuterium-tritium (DT) fuel, a high-Z gradient-density pusher shell (GDPS), and a Au-coated foam layer have been investigated through both 1D and 2D radiationhydrodynamic simulations. Compared with conventional low-Z ablators and DT-push-on-DT targets, these GDPS targets possess certain advantages of being instability-resistant implosions that can be high adiabat (α 8) and low hot-spot and pusher-shell convergence (CRhs ≈ 22 and CRPS ≈ 17), and have a low implosion velocity (vimp < 3 × 107 cm/s). Using symmetric drive with laser energies of 1.9 to 2.5 MJ, 1D LILAC simulations of these GDPS implosions can result in neutron yields corresponding to 50−MJ energy, even with reduced laser absorption due to the cross-beam energy transfer (CBET) effect. Two-dimensional DRACO simulations show that these GDPS targets can still ignite and deliver neutron yields from 4 to ∼10 MJ even if CBET is present, while traditional DT-push-on-DT targets normally fail due to the CBET-induced reduction of ablation pressure. If CBET is mitigated, these GDPS targets are expected to produce neutron yields of >20 MJ at a driven laser energy of ∼2 MJ. The key factors behind the robust ignition and moderate energy gain of such GDPS implosions are as follows: (1) The high initial density of the high-Z pusher shell can be placed at a very high adiabat while the DT fuel is maintained at a relatively low-entropy state; therefore, such implosions can still provide enough compression ρR >1 g/cm2 for sufficient confinement; (2) the high-Z layer significantly reduces heat-conduction loss from the hot spot since thermal conductivity scales as ∼1/Z; and (3) possible radiation trapping may offer an additional advantage for reducing energy loss from such high-Z targets.本研究针对采用固态氘氚（DT）燃料、高Z梯度密度推层壳（GDPS）与镀金泡沫层的激光直接驱动聚变靶设计，通过一维与二维辐射流体力学模拟开展了系统性研究。相较于传统低Z烧蚀层靶与DT推覆DT靶，此类GDPS靶具备抗不稳定性内爆的显著优势：可在高绝热因子（α=8）工况下运行，同时维持较低的热点与推层壳收敛比（CRhs≈22、CRPS≈17），且内爆速度更低（vimp < 3×10^7 cm/s）。当采用1.9至2.5 MJ的激光能量实现对称驱动时，即便因交叉束能量转移（CBET）效应出现激光吸收效率降低的情况，一维LILAC模拟结果显示，此类GDPS内爆的中子产额可对应50 MJ量级的聚变能量。二维DRACO模拟结果表明，即便存在CBET效应，此类GDPS靶仍可实现点火，中子产额可达4至约10 MJ；而传统DT推覆DT靶通常会因CBET导致烧蚀压力降低而无法实现点火。若对CBET效应进行抑制，在驱动激光能量约为2 MJ的工况下，此类GDPS靶的中子产额预计可超过20 MJ。此类GDPS内爆能够实现稳健点火与适度能量增益的核心机制如下：（1）高Z推层壳初始密度较高，可在较高绝热因子下运行，同时维持DT燃料处于相对低熵状态，因此此类内爆仍可提供足够的压缩积（ρR >1 g/cm²）以实现充分约束；（2）由于热导率与~1/Z正相关，高Z层可显著降低热点的热传导损失；（3）潜在的辐射俘获效应可进一步为降低此类高Z靶的能量损失提供额外增益。