In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50% and 20%, respectively, at n _e = 8 10²⁰m⁻³. These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In low-density conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ion–neutral friction and electron–ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p–n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.

在面向等离子体的表面附近，入射等离子体粒子与表面重组的中性粒子相互作用。在高通量线性等离子体发生器 Magnum-PSI 中观察到显着高的近地表等离子体压力损失。结合非相干和相干 Thomson 散射诊断，我们直接测量了距离等离子体目标表面 3 毫米以内的粒子、动量和能量通量。在表面，还分别使用目标内朗缪尔探针和热成像方法测量了颗粒和总热通量。近地表动量和能量损失与密度成比例，在n _e = 8 10 ²⁰ m ^{-3 时}分别达到至少 50% 和 20%. 这些损失归因于入射等离子体和表面复合中性粒子之间电荷、动量和能量的有效交换。在具有足够密度的低温等离子体中，入射粒子在离开等离子体之前经历了几个相互作用和表面沉积的循环，从而为入射等离子体提供了一个有效的替代耗散通道。平行等离子体参数分布随着等离子体密度的增加而呈现转变​​。在低密度条件下，由于双极电场，等离子体温度恒定并且观察到近表面离子加速。相反，在密集条件下观察到减速和等离子冷却。这些结果可以通过所谓的热碰撞预鞘中的离子 - 中性摩擦和电子 - 离子热平衡的组合效应来解释。可用于双极加速的能量因此减少，以及上游流速。在 ITER 偏滤器中，考虑到等离子体条件的重叠，预计也会增强近表面 p-n 相互作用。在有限元刮削层模型中包含这些影响需要比中性平均自由程更小的近地表分辨率。这在 Magnum-PSI 中相当于 1mm，在 ITER 中可能小一个数量级。鉴于等离子体条件的重叠。在有限元刮削层模型中包含这些影响需要比中性平均自由程更小的近地表分辨率。这在 Magnum-PSI 中相当于 1mm，在 ITER 中可能小一个数量级。鉴于等离子体条件的重叠。在有限元刮削层模型中包含这些影响需要比中性平均自由程更小的近地表分辨率。这在 Magnum-PSI 中相当于 1mm，在 ITER 中可能小一个数量级。