Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak
Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak
复制标题
兆安球形托卡马克合并压缩等离子体启动过程中的粒子加速
DOI：
10.1088/1361-6587/aa98fa
复制
发表时间：
2018
影响因子：
2.2
通讯作者：
McClements K
中科院分区：
文献类型：
--
作者：
McClements K
磁重联发生在合并压缩等离子体启动在巨型磁球托卡马克（MAST），导致大量的离子和电子迅速加速到高超热能量。加速场对准离子（氘和质子）检测使用中性粒子分析仪在早期MAST脉冲合并期间的能量高达约20千电子伏，而非热电子已被间接检测到最近的脉冲通过微波爆发。然而，直到合并阶段后期才观察到软X射线发射的增加，此时通过汤姆逊散射测量检测到强电子加热。一个测试粒子代码CUEBIT是用来模拟离子加速的存在下，一个规定的空间分布和时间演变的基础上霍尔MHD模拟的合并过程的感应环形电场。模拟产生的粒子分布与实验观察到的性质相似，包括快速离子的强场对准和质子加速到比氘更高的能量。对含有稀场取向超热电子成分的等离子体的粒子模拟表明，至少有一些微波爆发可以归因于各向异性快电子驱动的异常多普勒不稳定性，这并不产生可测量的软X增强，射线发射，或者因为它们能量不足，或者因为在脉冲的这一阶段期间非热韧致辐射发射率低于检测阈值没有证据表明逃逸电子加速合并过程中，可能是由于三维场扰动的存在。
Magnetic reconnection occurred during merging-compression plasma start-up in the Mega Amp Spherical Tokamak (MAST), resulting in the prompt acceleration of substantial numbers of ions and electrons to highly suprathermal energies. Accelerated field-aligned ions (deuterons and protons) were detected using a neutral particle analyser at energies up to about 20 keV during merging in early MAST pulses, while nonthermal electrons have been detected indirectly in more recent pulses through microwave bursts. However no increase in soft x-ray emission was observed until later in the merging phase, by which time strong electron heating had been detected through Thomson scattering measurements. A test-particle code CUEBIT is used to model ion acceleration in the presence of an inductive toroidal electric field with a prescribed spatial profile and temporal evolution based on Hall-MHD simulations of the merging process. The simulations yield particle distributions with properties similar to those observed experimentally, including strong field alignment of the fast ions and the acceleration of protons to higher energies than deuterons. Particle-in-cell modelling of a plasma containing a dilute field-aligned suprathermal electron component suggests that at least some of the microwave bursts can be attributed to the anomalous Doppler instability driven by anisotropic fast electrons, which do not produce measurable enhancements in soft x-ray emission either because they are insufficiently energetic or because the nonthermal bremsstrahlung emissivity during this phase of the pulse is below the detection threshold. There is no evidence of runaway electron acceleration during merging, possibly due to the presence of three-dimensional field perturbations.
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