Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas

Summary

The main goal of this work is to make it easier for research groups unfamiliar with Langmuir probes and emissive probes to use them as plasma diagnostics, especially near plasma boundaries. We do this by demonstrating how to build the probes from readily available materials and supplies.

Abstract

Langmuir probes have long been used in experimental plasma physics research as the primary diagnostic for particle fluxes (i.e., electron and ion fluxes) and their local spatial concentrations, for electron temperatures, and for electrostatic plasma potential measurements, since its invention by Langmuir in the early 1920s. Emissive probes are used for measuring plasma potentials. The protocols exhibited in this work serve to demonstrate how these probes may be built for use in a vacuum chamber in which a plasma discharge may be confined and sustained. This involves vacuum techniques for building what is essentially an electrical feedthrough, one that is rotatable and translatable. Certainly, complete Langmuir probe systems may be purchased, but they can also be built by the user at considerable cost savings, and at the same time be more directly adapted to their use in a particular experiment. We describe the use of Langmuir probes and emissive probes in mapping the electrostatic plasma potential from the body of the plasma up to the sheath region of a plasma boundary, which in these experiments is created by a negatively biased electrode immersed within the plasma, in order to compare the two diagnostic techniques and assess their relative advantages and weaknesses. Although Langmuir probes have the advantage of measuring the plasma density and electron temperature most accurately, emissive probes can measure electrostatic plasma potentials more accurately throughout the plasma, up to and including the sheath region.

Introduction

During this first century of plasma physics research, dating from Langmuir’s discoveries in the 1920s of the medium like behavior of a new state of matter, plasma, the Langmuir probe has proved to have been the single most important diagnostic of plasma parameters. This is true in part, because of its extraordinary range of applicability¹. In plasma encountered by satellites^2,3,4, in semiconductor processing experiments,^5,6,7,8

Protocol

1. Building Langmuir probes and Emissive probes to fit into a vacuum chamber

1. Planar Langmuir probe (see Figure 5 for more details)
   1. Take a ¼” diameter stainless steel tube as the probe shaft and bend one end to the desired 90° angle.
   2. Cut the unbent side to a length so that the probe can axially cover more than half of the chamber length.
   3. Fit the unbent side of the shaft through the brass tube by a SS-4-UT-A-8 adapter in combination with a B-810-6 union tube fitting.
   4. Use a ½” brass tube extending out of the customized flanges through a B-810-1-OR s....

Results

Langmuir probes, known to be sensitive to flows and to the kinetic energy of the particles they collect, have up till now have been considered to yield valid measurement of the plasma potential, except in sheaths. But direct comparisons of plasma potentials measured by Langmuir probes and emissive probes have demonstrated that in the quasineutral presheath region of the plasma immediately in contact with the sheath on the plasma side, Langmuir probes do not provide accurate measurements of the plasma potential

Discussion

Langmuir probes are used for particle flux measurements in an extraordinarily wide range of plasma densities and temperatures, from space plasmas in which the electron density is just a few particles 10⁶ m^-3 to the edge region of fusion plasmas where the electron density is more like a few times 10²⁰ m^-3. Moreover, electron temperatures between 0.1 and a few hundred eV’s have been diagnosed with Langmuir probes. Langmuir probes are often used to measure plasma density and.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.001" thick tungsten wire	Midwest Tungsten Service	0.001"	Emissive probe filament
0.005" thick tantalum sheet	Midwest Tungsten Service	0.005"	Heating filament to generate plasma
1/2" Brass supprting tube
1/4" Brass Ferrule Set	Swagelok	B-400-SET	Interface between stainless probe shaft and swagelok tube fitting
1/4" OD 304 or 315 stainless steel tube	Swagelok	SS-T4-S-035-20	Used to make the probe shaft, order seamless, sold in 20' lengths
Alumina tubes	COORSTEK	65655, single bore 0.156" OD 0.094 ID	single bore, double bore, quadruple bore, use for support structure for both emissive and Langmuir probes between the probe tip and shaft
Baratron gauge	MKS	Type 127	Display the pressure when there's gas flowing in the chamber
Brass Swagelok Tube Fitting	Swagelok	B-400-1-OR	Tube fittings used on the probe
Brass Swagelok Tube Fitting	Swagelok	B-810-6	Tube fittings used on the probe
Brass Swagelok Tube Fitting	Swagelok	B-810-1-OR	Tube fittings used on the probe
Ceramic liquid	Sauereisen	No. 31 Ceramic Encapsulant Liquid	Mix with No.31 cement power to make the ceramic paste
Ceramic powder	Sauereisen	Cement Powder No. 31 Off-White	There are Saureisen cements that cure with water, e.g. No.10 Powder
Gold plated nickel wire	SYLVANIA ELECTRIC PRODUCT		spod-welded to the probe tip to provide supports
Ion gauge controller	Granville-Phillips	270 Gauge controller	Heat up the ion gauge and display pressure inside the chamber
Mechanical pump	Leybold D60 D60AC	D60 D60AC	Bring the pressure down to ~10 mTorr then serve as the backing pump for the turbo pump
needle valve	Whitey	SS-22RS4	Metering Micro-Needle Micrometer Valve 1/4" Tube Swagelok fittings
Power supply	Kepco	ATE 100-10M	Voltage Bias supply of heating filament
Power supply	Sorensen	DCR 20-115B	Heating supply of heating filament
shutoff valve	Kurt J. Lesker	Nupro SS-4BK	Knob handle, for 1/4" tubing, swagelok fittings
Stainless Steel Ultra-Torr Vacuum Fitting	Swagelok	SS-4-UT-A-8	Tube fittings used on the probe
Teflon coated wire	Geyer Systems	P31546	Connect the gold-coated wire to BNC pin
Turbo pump	PFEIFFER	TPH 240 C	Bring the pressure down to 1E-6 Torr
Vacuum grease	APIEZON	L Ultra High Vacuum Grade Grease	Vacuum grease used to lubricate the oring
Viton Orings	Grainger	#031	Round #031 Medium Hard Viton O-Ring, 1.739" I.D., 1.879" O.D
Viton Orings	Grainger	#010	Round #010 Medium Hard Viton O-Ring, 0.239" I.D., 0.379"O.D