Transition to the detached divertor regime that allows lowering the peak power loads on the divertor targets to a tolerable level requires low plasma temperature Te ~ 1 eV and high plasma density ne ~ 1021 m−3 in front of the target. Under such conditions, radiation trapping of the Lyman lines of hydrogen isotopes becomes important. It can influence both energy balance and the ionization/recombination rates significantly. Nevertheless, opacity is typically neglected in the 2D edge transport codes used to study the divertor plasma detachment. We report on the first in-depth investigation of radiation opacity effects on transition to detachment. Simulations are performed with the SOLPS 4.3 code package using a DIII-D size tokamak as a particular example. It is found that in pure hydrogen plasma suppression of the neutral hydrogen radiation loss due to the photon absorption makes reaching the detached plasma regime more difficult. A significantly higher average separatrix plasma pressure is required to reach a similar degree of detachment. Adding plasma impurities compensates for the reduced neutral radiation and offsets the effect of opacity. In a carbon device, where the impurity source is due to erosion of the divertor components and is strongly connected to the edge plasma density, the absorbed fraction of the hydrogen radiation is easily compensated with the increasing carbon radiation loss. However, nowadays, in the more relevant case of the full-metal wall and seeded impurity with the feedback-controlled content, the increase of the edge plasma density alone is insufficient to compensate for trapping of the hydrogen radiation. In this case, achievement of the desired degree of detachment requires higher average separatrix plasma pressure or seeded impurity content than those obtained from the transparent plasma model.
偏滤器脱靶状态的实现，使得偏滤器靶板上的峰值热负荷能够降低至可承受水平，这要求靶板前等离子体温度Te ~ 1 eV且等离子体密度ne ~ 1021 m−3。在此条件下，氢同位素莱曼线的辐射俘获变得重要，它可能显著影响能量平衡以及电离和复合速率。然而，在用于研究偏滤器脱靶状态的二维边界等离子体输运代码中，不透明度效应通常被忽略。我们首次深入研究了辐射不透明度对脱靶状态转变的影响。模拟采用SOLPS 4.3代码包，以DIII-D尺寸的托卡马克为例。研究发现，在纯氢等离子体中，由于光子吸收导致的中性氢辐射损失抑制，使得达到脱靶等离子体状态更加困难。要达到相似的脱靶程度，需要显著更高的平均边界等离子体压力。添加等离子体杂质可以补偿中性氢辐射的减少，从而抵消不透明度效应。在碳壁装置中，杂质源来自偏滤器部件的腐蚀，且与边界等离子体密度紧密相关，氢辐射的吸收部分可以很容易地通过增加的碳辐射损失来补偿。然而，在更现实的金属壁和反馈控制杂质注入的情况下，仅增加边界等离子体密度不足以补偿氢辐射的俘获。在这种情况下，要达到所需的脱靶程度，需要比透明等离子体模型所预测的更高的平均边界等离子体压力或更多的注入杂质含量。