Absolute electron density fluctuation reconstruction for two-dimensional Hydrogen beam emission spectroscopy

Scrape-off layer (SOL) and edge plasma turbulence contribute significantly to the radial particle and heat transport lowering plasma confinement and increasing heat load on the plasma facing components. SOL turbulence is predominantly intermittent which manifest in the occurrence of isolated density filaments or blobs. Filaments propagate radially outwards towards plasma facing components limiting their lifetime by erosion and sputtering. To characterize this phenomenon in detail few diagnostic techniques are available. Beam emission spectroscopy is a diagnostic capable of measuring plasma turbulence in both SOL and edge plasmas. Due to the finite lifetime of the excitation states during the beam - plasma interaction, and the misalignment between the optics and the magnetic field, spatial smearing is introduced in the measurement. In this paper a novel method is introduced to overcome this hindering effect by inverting the fluctuation response matrix on an optimally smoothed signal. We show that this method is fast and provides significantly more accurate absolute density fluctuation reconstruction than the direct inversion technique. The presented method is usable for all types of beam emission diagnostics where the spatial resolution is higher than the combined smearing of the atomic physics and the observation.

刮削层（Scrape-off Layer, SOL）与边缘等离子体湍流对径向粒子及热输运贡献显著，会降低等离子体约束，并增大面向等离子体部件的热负荷。刮削层湍流以间歇性为主，表现为孤立密度丝状体或团块的出现。丝状体沿径向向外传播至面向等离子体部件，通过侵蚀与溅射缩短其服役寿命。目前可供详细表征该现象的诊断技术较为有限。束发射光谱法（Beam Emission Spectroscopy）是一种可同时对刮削层与边缘等离子体中的湍流进行测量的诊断手段。由于束流-等离子体相互作用过程中激发态的有限寿命，以及光学系统与磁场的不对准，测量过程中会引入空间展宽效应。本文提出一种新颖方法，通过对最优平滑后的信号求解涨落响应矩阵的逆问题，以克服该不利效应。研究表明，相较于直接反演方法，该方法运算速度更快，且能更精准地重建绝对密度涨落。所提出的方法适用于所有空间分辨率高于原子物理过程与观测过程共同带来的总展宽的束发射诊断系统。