The Chodura sheath for angles of a few degrees between the magnetic field and the surface of divertor targets and limiters
The Chodura sheath for angles of a few degrees between the magnetic field and the surface of divertor targets and limiters
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DOI：
10.1088/0029-5515/52/8/083012
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发表时间：
2012-08
期刊：
影响因子：
3.3
通讯作者：
P. Stangeby
中科院分区：
文献类型：
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作者：
P. Stangeby
为了在磁聚变装置中实现低的沉积到固体表面的功率通量密度，需要非常小的α值，其中α是B与表面切线之间的角度。对于斜磁场，在固体表面前存在厚度为几个ρi的Chodura鞘（CS）（也称为“磁前鞘”），离子拉莫尔半径。标准假设是CS是德拜鞘（DS）的附加物，德拜鞘的厚度为几λD，德拜长度。无碰撞CS条件下的简单流体模型给出了CS上归一化静电势的下降，公式为eΔφCS/kTe = ln（sin α）。对于电浮壁，存在双极流对壁面的单独约束eΔφfloating/kTe = 0.5 ln[（2πme/mi）（1 + Ti/Te）]，其中Δφfloating = ΔφCS + ΔφDS。对于氘等离子体和Ti = Te的情况，|eΔφ浮动/kTe| = 2.84。对于α < 3.35°，|eΔφCS/kTe|超过2.84，这显然意味着对于这样的α值，DS不再存在，整个电位降将在CS上发生。CS的新分析提供了一些具有实际重要性的量的解决方案，这些解决方案改进了目前在模型和边缘杂质代码中使用的解决方案。与后者相比，本分析的结果表明，（i）指向固体表面的电场更强，（ii）等离子体密度下降更迅速地接近固体表面。（i）的效果是增加溅射粒子的迅速局部沉积的概率，而（ii）具有相反的效果。
To achieve low deposited power flux density to solid surfaces in magnetic fusion devices, very small values of α are required, where α is the angle between B and the surface tangent. For an oblique magnetic field, there exists in front of the solid surface a Chodura sheath (CS) (also known as the ‘magnetic pre-sheath’) of thickness several ρi, the ion Larmor radius. The standard assumption is that the CS is additional to the Debye sheath (DS) of thickness several λD, the Debye length. Simple fluid modelling for collisionless CS conditions gives the drop in normalized electrostatic potential across the CS as eΔφCS/kTe = ln(sin α). For an electrically floating wall there is the separate constraint of ambipolar flow to the wall eΔφfloating/kTe = 0.5 ln[(2πme/mi)(1 + Ti/Te)], where Δφfloating = ΔφCS + ΔφDS. For the case of a deuterium plasma and Ti = Te, |eΔφfloating/kTe| = 2.84. For α < 3.35°, |eΔφCS/kTe| exceeds 2.84 which evidently implies that the DS ceases to exist for such values of α and the entire potential drop would then occur across the CS. New analysis of the CS provides solutions for a number of quantities of practical importance, which improve on the solutions presently in use in models and edge impurity codes. Compared with the latter, the results of the present analysis indicate that (i) the E-field directed towards the solid surface is stronger and (ii) the plasma density drops more rapidly approaching the solid surface. The effect of (i) is to increase the probability of prompt local deposition of sputtered particles, while (ii) has the opposite effect.