Divertor heat flux challenge and mitigation in SPARC

The SPARC experiment, because of its high magnetic field, high power, and compact size, will operate with divertor conditions at or above those expected in reactor-class tokamaks. Power exhaust at this scale remains one of the key challenges for practical fusion energy. Based on empirical scalings, the peak unmitigated divertor parallel heat flux is projected to be greater than 10 GW/m2. This is nearly an order of magnitude higher than has been demonstrated to date. Furthermore, the divertor parallel ELM energy fluence projections (~11-34 MJ/m2) are comparable to those for ITER. However, the relatively short pulse length (~25 second pulse, with a ~10 second flat top) provides the opportunity to consider mitigation schemes unsuited for long pulse devices including ITER and reactors. The baseline scenario for SPARC employs a ~1 Hz strike point sweep to spread the heat flux over a large divertor target surface area to keep tile surface temperatures within tolerable levels without the use of active divertor cooling systems. In addition, SPARC operation presents a unique opportunity to study divertor heat exhaust mitigation at reactor level plasma densities and power fluxes. Not only will SPARC test the limits of current experimental scalings and serve for benchmarking theoretical models in reactor regimes, it is also being designed to enable the assessment of long-legged and X-point target advanced divertor magnetic configurations. Experimental results from SPARC will be crucial towards retiring risk for a fusion pilot plant divertor design.

SPARC实验凭借高磁场、高功率与紧凑结构，将在达到甚至超过反应堆级托卡马克（tokamak）预期的偏滤器（divertor）工况下运行。在此规模下的热排气问题仍是实用聚变能源的核心挑战之一。基于经验定标律，未实施缓解措施时的峰值偏滤器平行热通量预计将超过10 GW/m²，该数值较当前已实现的水平高出近一个数量级。此外，偏滤器平行边缘局域模（Edge Localized Mode，ELM）能量通量的预测范围约为11~34 MJ/m²，与国际热核聚变实验堆（International Thermonuclear Experimental Reactor，ITER）的相关预测值相当。不过，SPARC相对较短的脉冲时长（总脉冲约25秒，平顶段约10秒）为研究不适用于长脉冲装置（含ITER与聚变反应堆）的热排气缓解方案提供了契机。SPARC的基准运行方案采用约1 Hz的打击点扫描策略，将热通量分散至更大面积的偏滤器靶面，从而在无需启用主动偏滤器冷却系统的前提下，将靶面瓦片的表面温度控制在可耐受范围内。此外，SPARC的运行为在反应堆级等离子体密度与热通量条件下开展偏滤器热排气缓解研究提供了独特机遇。它不仅将检验当前实验定标关系的极限，为反应堆工况下的理论模型提供基准验证，其设计还支持对长腿型与X点靶先进偏滤器磁构型开展评估。SPARC的实验结果对于消除聚变示范堆偏滤器设计的技术风险至关重要。