A self-balanced electron-emissive double Langmuir probe drawing no electron loss from its diagnosed plasma

Abstract

In this work, a new form of double Langmuir probe (DLP) system, an emissive double Langmuir probe (EDLP), which connects a collecting probe tip and an electron-emitting probe tip to form a DLP system, has been proposed as a replacement of the currently more common asymmetric double Langmuir probes (ADLPs). The EDLP was both computationally and experimentally investigated in this work. Using an emissive probe to provide an emission current I E to balance the electron collection current I C,e, the EDLP can obtain a full I–V trace when I E,TL > I C,es and be used in a similar manner to a single Langmuir probe with the exception that the EDLP, as with the ADLP, does not measure the local plasma potential. I E,TL ≫ I C,es can be realized on an EDLP without needing the much larger ion collection area required by the ADLP, and at I E,TL ∼ 2I C,es the relative error between the EDLP and a single Langmuir probe is ∼15% due to space-charge limited effects, which is better than that of the ADLP at ∼30% under similar conditions. The performance of an EDLP depends on whether its electron emission current sufficiently offsets its electron-collecting current, making it particularly fitting for scenarios where the plasma density is low but a large probe is difficult to employ due to the limited balancing ion current. This makes the EDLP potentially useful on satellites, which operate in very low temperature plasmas with a limited ion loss area to balance a Langmuir probe’s electron-collecting current. With the advances in highly emitting materials, EDLPs are expected to significantly remove the design barriers of Langmuir probes on satellites.