J-TEXT上外加磁扰动抑制等离子体破裂下逃逸电子产生的实验研究

The major disruptions usually result in rapid increase of loop voltage, which is favor for the generation of runaway electrons. The runaway energy can reach several tens MeV during disruptions. Up to 70% of plasma current could be converted into runaway current during disruptions. These high energy electrons will be stopped by the first wall due to the lost of displacement control at last. A severe consequence of a disruption on ITER could be the generation of multi-MeV electron beams that could damage the vacuum vessel and the structures of the machine if they hit the wall unmitigated. This is a serious threat to the reliable operation of machine. Therefore the issue of how to avoid or mitigate the runaway generation is of prime importance. The mitigation of runaway electron beams is also a key requirement for reliable operation of ITER. Some experiments have shown that the magnetic perturbation can enhance the loss rate of runaway population and weaken the runaway generation process. This proposal is aimed to use external magnetic perturbation to deconfine runaway electrons before they have time to accelerate to high energies, and to investigate the runaway suppression efficiency and the runaway suppression mechanism in the J-TEXT tokamak. It is expected that it may provide a physical base for the runaway suppression on ITER.

等离子体大破裂通常引起等离子体环电压的急剧增加，从而容易产生大量的逃逸电子。等离子体破裂产生的逃逸电子的能量可达几十MeV，并携带高达70%的等离子体电流，这些高能电子束最终会由于位移失控而损失到装置的第一壁材料上。对于下一代装置ITER，仅一次等离子体大破裂产生的高能逃逸电子束就会严重损伤装置的第一壁材料，从而影响装置的安全运行。如何避免等离子体破裂引发强的逃逸电子束是目前破裂迁移研究的重点之一，也是今后ITER运行中必须解决的问题。实验发现磁扰动可以增强逃逸电子的损失，削弱逃逸电子的产生过程，这为逃逸电子控制提供了新的途径。本项目拟在J-TEXT托卡马克上研究外加磁扰动对等离子体破裂产生的逃逸电子的抑制效率以及抑制机制，从而为今后ITER上的逃逸抑制提供一定的物理基础。

托卡马克等离子体破裂后形成的逃逸电子会威胁装置的安全运行，是今后ITER这类大装置必须要解决的问题。在本课题项目中，我们对外加扰动磁场抑制破裂后逃逸的形成进行了理论计算和实验研究。研究发现破裂缓解阀门（MGI）注入氩气触发等离子体破裂会形成逃逸电流，并获得了J-TEXT装置上逃逸电流形成的参数区间。我们分别利用外加扰动场线圈（RMP）和超声分子束注入（SMBI）两种方式产生扰动磁场，研究这两种外加磁扰动对破裂时逃逸形成的影响。实验发现破裂时利用RMP线圈施加中等强度的磁扰动可以抑制逃逸电流大小，但更强的磁扰动反而增强了逃逸电流。破裂前通过施加RMP穿透形成磁岛，或者锁模可以有效抑制逃逸电流，抑制效率与边界安全因子、RMP形成的扰动磁场结构有关。破裂前用SMBI注入氢气激发磁扰动可以抑制逃逸电流。SMBI较早注入时，激发的扰动扩散到等离子体芯部，逃逸电流可以被完全抑制。这些研究结果表明外加磁扰动对逃逸电子的抑制不仅与扰动磁场的强度有关，还和磁扰动在等离子体中的分布结构密切相关。外加的磁扰动能否影响等离子芯部的磁面是抑制逃逸形成的关键，这为ITER上如何利用外加扰动磁场抑制逃逸提供了重要研究基础。