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基于 PLT、EAST、WEST、ASDEX-Upgrade、JET等托卡马克装置开展的研究表明重杂质易产生聚芯现象，这会导致等离子体约束性能降低甚至引发等离子体大破裂事件。大破裂期间等离子体热能损失主要发生在快速热猝灭（thermal quench，TQ）阶段，但目前对于这一阶段的时间尺度定标关系并没有较为全面的物理解释。国际热核聚变实验堆（International Thermonuclear Experimental Reactor，ITER）将采用全钨壁材料，而钨作为高Z杂质，其较强的辐射能力将会对TQ过程的热能损失产生影响。为研究钨杂质对快速TQ时间尺度的影响，本工作通过同时考虑随机磁场导致的热扩散以及钨杂质辐射引起的热损失机制，建立了托卡马克等离子体电子温度演化的一维模型，并在典型类ITER参数下对这一阶段的电子温度演化进行了数值计算和分析，主要结论为：（1）快速TQ时间尺度的量级由热扩散水平决定，但钨杂质辐射可以定量上影响TQ时间尺度和TQ后期电子温度，钨浓度越高TQ时间尺度越短、后期电子温度越低，且数值与解析结果分析都表明该时间尺度与钨杂质浓度近似呈线性关系；（2）快速TQ阶段前期，通过钨杂质辐射损失的能量远小于通过随机磁场引起热扩散损失的能量，但在TQ后期，钨杂质辐射功率量级可以接近甚至超过热扩散功率，这也是导致TQ后期电子温度随钨浓度增大而减小的原因。因此，钨杂质辐射在TQ后期对热能损失的贡献不可忽略。Recent studies based on the PLT, EAST, WEST, ASDEX-upgrade, JET and other tokamaks have shown that the accumulation of heavy impurities in the core regime is unavoidable, which may lead to the degradation of the plasma confinement and even trigger the major disruptions. The plasma thermal energy loss during the major disruptions mainly occurs during the fast thermal quench (TQ) stage. However, there is no comprehensive physical explanation for the scaling of the timescale of this stage. Tungsten as high Z impurity, which will be used as the wall material in International Thermonuclear Experimental Reactor (ITER), has strong radiation power, and may affect the thermal energy loss during the fast TQ. This work considers both the thermal diffusion induced by the stochastic magnetic fields and the radiation from tungsten impurities as the dominant thermal loss mechanisms in this stage, and construct a one-dimensional model of electron temperature evolution in tokamak plasmas. We numerically calculate and analyze the evolution of the electron temperature in this stage with the typical ITER-like parameters, and here are our main conclusions: (1) The order of magnitude of the fast TQ timescale is mainly determined by the level of thermal diffusion. However, the radiation from tungsten impurities can quantitively influence on the timescale of fast TQ and the electron temperature in the late phase of fast TQ. The higher the tungsten concentration, the shorter the TQ timescale and the lower the electron temperature it will lead to in the late phase. Both the numerical and analytical results show that the timescale is approximately linear with the tungsten impurity concentration, as shown in Fig. 1. (2) Fig. 2 demonstrates the evolution of the global energy loss and the global power loss during the fast TQ. From Fig. 2 (a), it can be found that the global thermal energy loss via the radiation from tungsten impurities is much smaller than that via the thermal diffusion induced by the stochastic magnetic fields during the early phase of fast TQ stage. However, in the late phase of fast TQ stage, the global radiation power can be comparable to or even greater than that of the global thermal diffusion power as shown in Fig. 2 (b). This is also the reason why the electron temperature in the late phase of fast TQ decreases as the concentration of tungsten impurities increases. Therefore, the contribution of the radiation from tungsten impurities to the thermal loss cannot be ignored in the late phase of fast TQ.
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Keywords:
- Tokamak /
- major disruption /
- thermal quench /
- tungsten impurity radiation /
- evolution of the electron temperature /
- thermal energy loss
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