The Use of Emissive Probes in Laboratory and Tokamak Plasmas

Abstract

Emissive probes offer the possibility for a direct determination of the plasma potential since for sufficient electron emission its floating potential is ideally equal to Φ pl . By means of two such probes the true electric field can be derived. Emissive probes also deliver reliable results if there are electron drifts and beams in the plasma. A conventional emissive probe consists of an electrically heated loop of refractory wire. We have also developed emissive pin probes of graphite or LaB6, heated by a focused infrared laser beam. In this contribution we first discuss the question to what extent emissive probes perturb the plasma and thereby might falsify the measurement. Furthermore, we present recent measurements with an array of emissive probes in a small tokamak to detect plasma potential fluctuations and electric fields and the related radial plasma transport. Eventually, we present results of a laser-heated emissive probe in a helicon discharge device.

Strongly Emitting Surfaces Unable to Float below Plasma Potential

“…A positive floating potential of an emissive probe was actually reported in Ref. [30]. However we note that our study only directly applies to thin (planar geometry) sheaths.…”

How plasma parameters fluctuations influence emissive probe measurements

“…It is then usually accepted that the measurement of the floating potential of the emissive probe provides a direct measurement of the plasma potential. While this is probably true to measure the time-averaged plasma potential, as shown by many successful measurements performed in laboratory [9][10][11] and fusion devices 5 plasmas worldwide, the question whether it is possible to directly and reliably estimate the plasma potential fluctuations using the floating potential fluctuations of an emissive probe remains open. This statement can be understood if one recall that I es and I is oscillate as the product of the electron density times the square root of the electron temperature (n ffiffiffiffi ffi T e p ) and that I em does not follow necessary these oscillations.…”

Entire plasmas can be restructured when electrons are emitted from the boundaries

“…Important examples include fusion machines, 2,9 dusty plasmas, 10 the moon, 11 thrusters, 12,13 RF discharges, 14 probe diagnostics, 15 and heated cathodes. 16 Emission alters the charge balance at a wall, thereby altering the sheath potential.…”