Space-charge limits of ion sensitive probes

Abstract

Ion sensitive probes (ISPs) are used to measure ion temperature and plasma potential in magnetized plasmas. Their operation relies on the difference in electron and ion Larmor radii to preferentially collect the ion species on a recessed electrode. Because of their simple two-electrode construction and optimal geometry for heat flux handling they are an attractive probe to use in the high heat flux boundary of magnetic confinement fusion experiments. However, the integrity of its measurements is rarely, if ever, checked under such conditions. Recent measurements with an ISP in the Alcator C-Mod tokamak have shown that its ion current is space-charge limited and thus its current-voltage (I -V ) response does not contain information on the ion temperature. We numerically solve a 1D Vlasov-Poisson model of ion collection to determine how much bias is needed to overcome space-charge effects and regain the classic I -V characteristic with an exponential decay. Prompted by the observations of space charge in C-Mod, we have performed a survey of ISP measurements reported in the literature. Evidence of space-charge limited current collection is found on many probes, with few authors noting its presence. Some probes are able to apparently exceed the classic 1D space-charge limit because electrons can E × B drift into the probe volume, partially reducing the net ion charge; it is argued that this does not, however, change the basic problem that space charge compromises the measurement of ion temperature. Guidance is given for design of ISPs to minimize the effects of space charge.

Determining the ion temperature and energy distribution in a lithium-plasma interaction test stand with a retarding field energy analyzer

“…(a) Currents relative to the ion discriminating potential for the primary feature. The collected current appears to never reach a saturation value, which may be caused by the minimal influence of the space charge effect, 19 the sheath expansion effect, 17,21 or a combination of both. (b) Currents relative to the ion discriminating potential for the secondary feature.…”

“…5(a) appears to never reach a saturation value may indicate a minor influence from the space charge effect. 19,20 If space charge does play a role, the effect seems to be time-dependent since the initial linear drop vanishes as the plasma pulse continues to interact with the analyzer. The degree to which space charge may or may not influence this analyzer is unclear; however, the empirically determined shift potential described in Fig.…”

Development of Ion Sensitive Probe and Its Application to RF Plasma Device DT-ALPHA

“…One possible reason of this deviation is the space-charge limit. According to Brunner et al [12], the shape of an I-V curve is affected when a probe is limited by the space-charge. In Ref.…”

Retarding field analyzer for the wendelstein 7-X boundary plasma

“…The alternative Ion Sensitive Probe (ISP) technique is in principle able to measure ion temperatures but suffers from space charge limitation problems in practical applications. 1 The Retarding Field Analyzer (RFA) remains a practical tool to measure the edge ion temperature. To date, RFAs have been used on JET 2,3 , Tore Supra 4 , Alcator C-mod 5 , ISTTOK 6 , STOR-M 7 , and EAST 8 tokamaks and other plasma devices to measure the edge ion temperature profile.…”