The role of guard electrodes in probe diagnostics
Instruments and measurement techniques according to measurement types
Аuthors*, *, **
Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia
Flat remote and near-wall probes are widely used in probe diagnostics of rarefied plasma (static and dynamic). They are installed on hypersonic aircraft and space satellites in order to study the radiophysical parameters near their surface. Flat probes can be installed both parallel and perpendicular to the flow velocity in the rarefied plasma streams. Nonlinear border and edge effects occur at the edges of a flat probe, which have a significant influence on the probe current. The boundary effect is associated with the thickening of the lines of force of the electric field at the boundary of the active surface of the probe. The final effect arises under the influence of a directed flow velocity if the probe is oriented parallel to the flow velocity. Charged particles enter the space charge layer of the probe. They can fly through the probe line and not contribute to the probe current, depending on the potential of the probe and the velocity of the plasma flow.
The present article is devoted to an analysis of the effect of these nonlinear effects on the probe current. In the literature, there is insufficient information on this issue, the technique for conducting a probe experiment, and processing probe characteristics, taking into account the border and edge effects.
The physical and mathematical models of the problem are formulated. Extensive computational experiments were carried out. Conclusions and recommendations useful for the practice of probe measurements are made.
Consider a probe in the form of an elongated strip of width 2rр and potential φρ. The directional plasma flow rate U∞ is parallel to the probe surface and perpendicular to its short side (fig. 1).
a) the probe is oriented along the flow,
b) the probe is oriented towards the flow;
1 - probe surface,
2, 3 - the surface of the guard electrodes.
If the characteristic size of the probe is 2rр≥103rd, (rd is the Debye radius), then the effect of the border and edge effects can be neglected and the Bohm formula can be used when processing probe characteristics. If 2rр<103rd, then the arising nonlinear effects can be removed by constructing the guard electrodes in the form of strips along the elongated sides of the probe, separating them from the probe by thin insulating layers. In this case, the Bohm formula is also used. The width of the guard electrodes is calculated in numerical experiments and is presented in the work in the form of graphical dependences U∞, rp and the potential φρ (fig. 2, 3).
If the probe in the form of a strip is directed by its active surface towards the flow, the border effects is absent, and the edge effect is analogous to the case of parallel flow around the probe. If 2rр≥103rd, then the Langmuir formula, which describes the ion current with concentration, flow velocity and probe potential, is valid. If 2rр<103rd, then guard electrodes are placed, which remove the effect of edge effects. Their size is much smaller than with parallel flow around the probe. Recommendations for choosing the width of guard electrodes are given in the work as a function of the characteristic parameters of the problem.
If flat probes of another geometry (square, disk, etc.) are used, it is recommended to take their characteristic size 2rр>103rd. In this case, the role of border and edge effects is small and one can use the recommendations stated above for probes in the form of an elongated rectangle.
Keywords:probe diagnostics, border effect, edge effect, rarefied plasma, dense plasma
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