Name: building langmuir probes and emissive probes for plasma potential measurements in low pressure, low temperature plasmas
Uploaded: 2021-05-25T12:30:00
Description: Watch this Scientific Journal Video about Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas at JoVE.com

This work was partially funded by the U.S. Department of Energy (DOE), through grantDE-SC00114226, and the National Science Foundation through grants PHY-1464741, PHY-1464838, PHY-1804654, and PHY-1804240
Tribute to Noah Hershkowitz: 
Noah Hershkowitz made groundbreaking contributions to plasma physics while earning the respect and admiration of his colleagues and students, both as a scientist and a human being.&#160; &#8220;Physics,&#8221; he once explained, &#8220;is like a jigsaw puzzle that&#8217;s really old. All the pieces are worn down. Their edges are messed up. Some of the pieces have been put together in the wrong way. They sort of fit, but they&#8217;re not actually in the right places. The game is to put them together the right way to find out how the world works.&#160; He died on November 13, 2020, at age 79.

1. Building Langmuir probes and Emissive probes to fit into a vacuum chamber
<ol>
	<li>Planar Langmuir probe (see Figure 5 for more details)
	<ol>
		<li>Take a &#188;&#8221;&#160;diameter stainless steel tube as the probe shaft and bend one end to the desired 90&#176; angle.</li>
		<li>Cut the unbent side to a length so that the probe&#160;can axially cover more than half of the chamber length.</li>
		<li>Fit the unbent side of the shaft through the brass tube by a SS-4-UT-A-8 adapter in c...

<ol>
	<li>Godyak, V. A., Alexandrovich, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+analyses+of+plasma+probe+diagnostics+techniques.">Comparative analyses of plasma probe diagnostics techniques.</a> Journal of Applied Physics. 118, 233302 (2015).</li><li>Gurnett, D. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Cassini+Radio+and+Plasma+wave+investigation.">The Cassini Radio and Plasma wave investigation.</a> Space Science Reviews. 114, 395-463 (2004).</li><li>Olson, J., Brenning, N., Wahlund, J. E., Gunell, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+interpretation+of+Langmuir+probe+data+inside+a+spacecraft+sheath.">On the interpretation of Langmuir probe data inside a spacecraft sheath.</a> Review of Scientific Instruments. 81, 105106 (2010).</li><li>Lebreton, J. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+ISL+Langmuir+probe+experiment+processing+onboard+DEMETER:+Scientific+objectives,+description+and+first+results.">The ISL Langmuir probe experiment processing onboard DEMETER: Scientific objectives, description and first results.</a> Planetary and Space Science. 54, 472-486 (2006).</li><li>Godyak, V. A., Piejak, R. B., Alexandrovich, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+electron+energy+distribution+in+low-pressure+RF+discharges.">Measurements of electron energy distribution in low-pressure RF discharges.</a> Plasma Sources Science and Technology. 1, 36-58 (1992).</li><li>You, K. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+and+computational+investigations+of+the+effect+of+the+electrode+gap+on+capacitively+coupled+radio+frequency+oxygen+discharges.">Experimental and computational investigations of the effect of the electrode gap on capacitively coupled radio frequency oxygen discharges.</a> Physics of Plasmas. 26, 013503 (2019).</li><li>Sobolewski, M. A., Kim, J. 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L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+Langmuir+Probes+Work.">How Langmuir Probes Work.</a> Plasma Diagnostics Volume 1 Discharge Parameters and Chemistry. , 114 (1989).</li><li>Lee, H. C., Lee, J. K., Chung, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evolution+of+the+electron+energy+distribution+and+E-H+mode+transition+in+inductively+coupled+nitrogen+plasma.">Evolution of the electron energy distribution and E-H mode transition in inductively coupled nitrogen plasma.</a> Physics of Plasmas. 17, 033506 (2010).</li><li>Amemiya, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmas+with+negative+ions-probe+measurements+and+charge+equilibrium.">Plasmas with negative ions-probe measurements and charge equilibrium.</a> Journal of Physics D: Applied Physics. 23, 999 (1990).</li><li>Andreu, J., Sardin, G., Esteve, J., Morenza, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Filament+discharge+plasma+of+argon+with+electrostatic+confinement.">Filament discharge plasma of argon with electrostatic confinement.</a> Journal of Physics D: Applied Physics. 18, 1339-1345 (1985).</li><li>Bohm, D., Guthrie, A., Wakering, R. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Minimum+Kinetic+Energy+Requirement+for+a+Stable+Sheath.">Minimum Kinetic Energy Requirement for a Stable Sheath.</a> The Characteristics of Electrical Discharges in Magnetic Fields. , (1949).</li><li>Chen, F. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Introduction to Plasma Physics and Controlled Fusion, 3rd Ed. , (2016).</li><li>Sheehan, J. P., Hershkowitz, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emissive+probes.">Emissive probes.</a> Plasma Sources Science and Technology. 20, 063001 (2011).</li><li>Barnat, E. V., Laity, G. R., Baalrud, S. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Response+of+the+plasma+to+the+size+of+an+anode+electrode+biased+near+the+plasma+potential.">Response of the plasma to the size of an anode electrode biased near the plasma potential.</a> Physics of Plasmas. 21, 103512 (2014).</li><li>Mausbach, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Parametrization+of+the+Laframboise+theory+for+cylindrical+Langmuir+probe+analysis.">Parametrization of the Laframboise theory for cylindrical Langmuir probe analysis.</a> Journal of Vacuum Science and Technology A. 15, 2923-2929 (1997).</li><li>Li, P., Hershkowitz, N., Wackerbarth, E., Severn, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+studies+of+the+difference+between+plasma+potentials+measured+by+Langmuir+probes+and+emissive+probes+in+presheaths.">Experimental studies of the difference between plasma potentials measured by Langmuir probes and emissive probes in presheaths.</a> Plasma Sources Science and Technology. 29, 025015 (2020).</li><li>Goeckner, M. J., Goree, J., Sheridan, T. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+ion+velocity+and+density+in+the+plasma+sheath.">Measurements of ion velocity and density in the plasma sheath.</a> Physics of Fluids B: Plasma Physics. 4, 1663 (1992).</li><li>Lee, D., Hershkowitz, N., Severn, G. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+Ar++and+Xe++velocities+near+the+sheath+boundary+of+Ar-Xe+plasma+using+two+diode+lasers.">Measurements of Ar+ and Xe+ velocities near the sheath boundary of Ar-Xe plasma using two diode lasers.</a> Applied Physics Letters. 91, 041505 (2007).</li><li>Yan, S., Kamal, H., Amundson, J., Hershkowitz, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Use+of+emissive+probes+in+high+pressure+plasma.">Use of emissive probes in high pressure plasma.</a> Review of Scientific Instruments. 67 (12), 4130-4137 (1996).</li><li>Smith, J. R., Hershkowitz, N., Coakley, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inflection-point+method+of+interpreting+emissive+probe+characteristics.">Inflection-point method of interpreting emissive probe characteristics.</a> Review of Scientific Instruments. 50, 210-218 (1979).</li><li>Campanell, M. D., Umansky, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strongly+Emitting+Surfaces+Unable+to+Float+below+Plasma+Potential.">Strongly Emitting Surfaces Unable to Float below Plasma Potential.</a> Physical Review Letters. 116, 085003 (2016).</li><li>Kraus, B. F., Raitses, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Floating+potential+of+emitting+surfaces+in+plasmas+with+respect+to+the+space+potential.">Floating potential of emitting surfaces in plasmas with respect to the space potential.</a> Physics of Plasmas. 25, 030701 (2018).</li><li>Yip, C. -. S., Jin, C., Zhang, W., Xu, G. S., Hershkowitz, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+investigation+of+sheath+effects+on+I-V+traces+of+strongly+electron+emitting+probes.">Experimental investigation of sheath effects on I-V traces of strongly electron emitting probes.</a> Plasma Sources Science and Technology. 29, 025025 (2020).</li></ol>

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&#8217;s have been diagnosed with Langmuir probes. Langmuir probes are often used to measure plasma density and...

During this first century of plasma physics research, dating from Langmuir&#8217;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 satellites2,3,4, in semiconductor processing experiments,5,6,7,8 at the edges of plasma confined in tokamaks,9,10,11 and in wide range of basic plasma physics experiments, Langmuir probes have been used to measure plasma densities and temperatures spanning the ranges 108&#8804;ne&#8804;1019m-3, and 10-3&#8804;Te&#8804;102eV&#160;, respectively. Simultaneously in the 1920s, he invented the probe now named after him and the emissive probe12. The emissive probe is now primarily used as a diagnostic of plasma potential. Although it cannot measure the breadth of plasma parameters that the Langmuir probe can, it too is a&#160;diagnostic of wide utility when it comes to the measurement of plasma potential, or, as it is sometimes called, the electrostatic space potential. For example, the emissive probe can accurately measure space potentials even in a vacuum, where Langmuir probes are incapable of measuring anything.
The basic setup of the Langmuir probe consists of putting an electrode into the plasma and measuring the collected current. The resulting current-voltage (I-V) characteristics can be used to interpret plasma parameters such as electron temperature Te, electron density ne, and plasma potential &#981;13. For a Maxwellian plasma, the relationship between collected electron current Ie (taken&#160;to be positive) and probe bias VB can be expressed as14:
<img alt="Equation 1" src="/files/ftp_upload/61804/61804equ1v2.jpg" />
where Ie0 is the electron saturation current,
<img alt="Equation 2" src="/files/ftp_upload/61804/61804equ2v2.jpg" />
and where S is the collecting area of the probe, <img alt="Equation 9" src="/files/ftp_upload/61804/61804equ09.jpg" /> is the bulk electron density, e is the electron charge, Te is the electron temperature, me is the electron mass. The theoretical relation of I-V characteristics for the electron current is illustrated in two ways in Figure 1A and Figure 1B. Note, Eq. (1a,b) only applies to bulk electrons. However, Langmuir probe currents can detect flows of charged particles, and adjustments must be made in the presence of primary electrons, electron beams, or ion beams etc. See Hershkowitz14 for more details.
The discussion here takes up the ideal case of Maxwellian electron energy distribution functions (EEDF). Of course, there are many circumstances in which non-idealities arise, but these are not the subject of this work. For example, in materials processing etching and deposition plasma systems, typically RF generated and sustained, there are molecular gas feed stocks that produce volatile chemical radicals in the plasma, and multiple ion species including negatively charged ions. The plasma becomes electronegative, that is, having a significant fraction of the negative charge in the quasineutral plasma in the form of negative ions. In plasma with molecular neutrals and ions, inelastic collisions between electrons and the molecular species can produce dips15 in the current-voltage characteristics, and the presence of cold negative ions, cold relative to the electrons, can produce significant distortions16 in the vicinity of the plasma potential, all of which of course are non-Maxwellian features. We prosecuted the experiments in the work discussed in this paper in a single ion species noble gas (argon) DC discharge plasma, free of these kinds of non-Maxwellian effects. However, a bi-Maxwellian EEDF is typically found in these discharges, caused by the presence of secondary electron emission17 from the chamber walls. This component of hotter electrons is typically a few multiples of the cold electron temperature, and less than 1% of the density, typically easily distinguished from the bulk electron density and temperature.
As VB becomes more negative than &#981;, electrons are partially repelled by the negative potential of the probe&#160;surface, and the slope of the ln(Ie) vs. VB is e/Te, ie. 1/TeV where TeV is the electron temperature in eV, as shown in Figure 1B. After TeV is determined, the plasma density can be derived as:
<img alt="Equation 3" src="/files/ftp_upload/61804/61804equ3v2.jpg" />
Ion current is derived differently than electron current. Ions are assumed to be &#8220;cold&#8221; due to their relatively large mass, Mi &#62;&#62; me, compared to that of the electron, thus, in a weakly ionized plasma, the ions are in fairly good thermal equilibrium with the neutral gas atoms, which are at the wall temperature. Ions are repelled by the probe sheath if VB &#8805;&#160;&#981; and collected if VB &#60; &#981;. The collected ion current is approximately constant for negatively biased probes, while the electron flux to the probe decreases for probe bias voltages more negative than the plasma potential. Since the electron saturation current is much larger than the ion saturation current, the total current collected by the probe decreases. As the probe bias becomes increasingly negative, the drop in current collected is great or small as the electron temperature is cold or hot, as described above in Eq. (1a). The equation for ion current in this approximation is:
<img alt="Equation 4" src="/files/ftp_upload/61804/61804equ4v2.jpg" />
where
<img alt="Equation 5" src="/files/ftp_upload/61804/61804equ5v2.jpg" />
and
<img alt="Equation 6" src="/files/ftp_upload/61804/61804equ6v2.jpg" />
We note that constant ion flux collected by the probe exceeds the random thermal ion flux due to acceleration along the presheath of the probe and thus ions reach the sheath edge of the probe at the Bohm speed18, uB, rather than the ion thermal speed19. And the ions have a density equal to the electrons since the presheath is quasineutral. Comparing the ion and electron saturation current in Eqn.5 and 2, we observe that the ion contribution to the probe current is smaller than that of electrons by a factor of <img alt="Equation 10" src="/files/ftp_upload/61804/61804equ10.jpg" />. This&#160;factor is about 108 in the case of argon plasma.
There is a sharp transition point where the electron current goes from exponential to a constant, known as the &#8221;knee&#8221;. The probe bias at the knee can be approximated as the plasma potential. In the real experiment, this knee is never sharp, but rounded due to the space-charge effect of the probe, that is, the expansion of the sheath surrounding the probe, and also to probe contamination, and plasma noise13.
The Langmuir probe technique is&#160;based on collection current, whereas the emissive probe technique is based on the emission of current. Emissive probes measure neither temperature nor density. Instead they provide precise plasma potential measurements and can operate under a variety of situations due to the fact that they are insensitive to plasma flows. The theories and usage of emissive probes are fully discussed in the topical review by Sheehan and Hershkowitz20, and references therein.
For plasma density 1011 &#8804; ne &#8804; 1018 m-3, the inflection point technique in the limit of zero emission is recommended, which means to take a of series of I-V traces,&#160;each with different filament heating currents, finding the inflection point bias voltage for each I-V trace, and extrapolate the inflection points to the limit of zero emission to get the plasma potential, as shown in Figure 2.
It is a common assumption that Langmuir and emissive probe&#160;techniques agree in quasineutral plasma, but disagree in the sheath, the region of the plasma in contact with the boundary in which space-charge appears. The study focuses on the plasma potential near plasma boundaries, in low temperature, low pressure plasma in an effort to test this common assumption. To compare potential measurements by both Langmuir probe and emissive probe, plasma potential is also determined by applying inflection point technique to Langmuir probe I-V, as shown in Figure 3. It is generally accepted1 that the plasma potential is found by finding the probe bias voltage at which the second derivative of the current collected differentiated with respect to the bias voltage, <img alt="Equation 11" src="/files/ftp_upload/61804/61804equ11.jpg" />, that is, the peak of the dI/dV curve, with respect to the probe bias voltage. Figure 3 demonstrates how this maximum in dI/dV, the inflection point of the current-voltage characteristic, is found.
Langmuir probes (collecting) and emissive probes (emitting) have different I-V characteristics, which also depend on the geometry of the probe tip, as shown in Figure 4. The space-charge effect of the probe must be considered before the probe fabrication. In the experiments, for the planar Langmuir probes, we used a &#188;&#34;&#160;planar Tantalum disk. We could collect more current and get bigger signals with a larger disk. However, in order for the analyses above to apply, the area of the probe, Ap must be kept smaller than the electron loss area of the chamber, Aw, satisfying21 the inequality <img alt="Equation 12" src="/files/ftp_upload/61804/61804equ12.jpg" />. For the cylindrical Langmuir probe, we used a 0.025 mm thick, 1 cm long Tungsten wire for the cylindrical Langmuir probe and a same thickness for the Tungsten wire for the emissive probe. It is important to note that for cylindrical Langmuir probes, for the plasma parameters of these experiments, the radius of the probe tip, rp, is much smaller than its length, Lp, and smaller than the Debye length, &#955;D; that is, <img alt="Equation 13" src="/files/ftp_upload/61804/61804equ13.jpg" />, and <img alt="Equation 14" src="/files/ftp_upload/61804/61804equ14.jpg" />. In this range of parameters, applying Orbital Motion Limited theory and Laframboise&#8217;s development of it22 for the case of thermal electrons and ions, we find that for probe bias voltages equal to or greater than the plasma potential, the electron current collected may be parameterized by a function of the form <img alt="Equation 15" src="/files/ftp_upload/61804/61804equ15.jpg" />, where the exponent <img alt="Equation 16" src="/files/ftp_upload/61804/61804equ16.jpg" />. The important point here is that for values of this exponent less than unity, the inflection point method for determining the plasma potential, as described in the paragraph above, applies to cylindrical Langmuir probes too.

<table>
	<tbody>
		<tr>
			<td>0.001&#34; thick tungsten wire</td>
			<td>Midwest Tungsten Service</td>
			<td>0.001&#34;</td>
			<td>Emissive probe filament</td>
		</tr>
		<tr>
			<td>0.005&#34; thick tantalum sheet</td>
			<td>Midwest Tungsten Service</td>
			<td>0.005&#34;</td>
			<td>Heating filament to generate plasma</td>
		</tr>
		<tr>
			<td>1/2&#34; Brass supprting tube</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>1/4&#34; Brass Ferrule Set</td>
			<td>Swagelok</td>
			<td>B-400-SET</td>
			<td>Interface between stainless probe shaft and swagelok tube fitting</td>
		</tr>
		<tr>
			<td>1/4&#34; OD 304 or 315 stainless steel tube</td>
			<td>Swagelok</td>
			<td>SS-T4-S-035-20</td>
			<td>Used to make the probe shaft, order seamless, sold in 20&#39; lengths</td>
		</tr>
		<tr>
			<td>Alumina tubes</td>
			<td>COORSTEK</td>
			<td>65655, single bore 0.156&#34; OD 0.094 ID</td>
			<td>single bore, double bore, quadruple bore, use for support structure for both emissive and Langmuir probes between the probe tip and shaft</td>
		</tr>
		<tr>
			<td>Baratron gauge</td>
			<td>MKS</td>
			<td>Type 127</td>
			<td>Display the pressure when there&#39;s gas flowing in the chamber</td>
		</tr>
		<tr>
			<td>Brass Swagelok Tube Fitting</td>
			<td>Swagelok</td>
			<td>B-400-1-OR</td>
			<td>Tube fittings used on the probe</td>
		</tr>
		<tr>
			<td>Brass Swagelok Tube Fitting</td>
			<td>Swagelok</td>
			<td>B-810-6</td>
			<td>Tube fittings used on the probe</td>
		</tr>
		<tr>
			<td>Brass Swagelok Tube Fitting</td>
			<td>Swagelok</td>
			<td>B-810-1-OR</td>
			<td>Tube fittings used on the probe</td>
		</tr>
		<tr>
			<td>Ceramic liquid</td>
			<td>Sauereisen</td>
			<td>No. 31 Ceramic Encapsulant Liquid</td>
			<td>Mix with No.31 cement power to make the ceramic paste</td>
		</tr>
		<tr>
			<td>Ceramic powder</td>
			<td>Sauereisen</td>
			<td>Cement Powder No. 31 Off-White</td>
			<td>There are Saureisen cements that cure with water, e.g. No.10 Powder</td>
		</tr>
		<tr>
			<td>Gold plated nickel wire</td>
			<td>SYLVANIA ELECTRIC PRODUCT</td>
			<td></td>
			<td>spod-welded to the probe tip to provide supports</td>
		</tr>
		<tr>
			<td>Ion gauge controller</td>
			<td>Granville-Phillips</td>
			<td>270 Gauge controller</td>
			<td>Heat up the ion gauge and display pressure inside the chamber</td>
		</tr>
		<tr>
			<td>Mechanical pump</td>
			<td>Leybold D60 D60AC</td>
			<td>D60 D60AC</td>
			<td>Bring the pressure down to ~10 mTorr then serve as the backing pump for the turbo pump</td>
		</tr>
		<tr>
			<td>needle valve</td>
			<td>Whitey</td>
			<td>SS-22RS4</td>
			<td>Metering Micro-Needle Micrometer Valve 1/4&#34; Tube Swagelok fittings</td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Kepco</td>
			<td>ATE 100-10M</td>
			<td>Voltage Bias supply of heating filament</td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Sorensen</td>
			<td>DCR 20-115B</td>
			<td>Heating supply of heating filament</td>
		</tr>
		<tr>
			<td>shutoff valve</td>
			<td>Kurt J. Lesker</td>
			<td>Nupro SS-4BK</td>
			<td>Knob handle, for 1/4&#34; tubing, swagelok fittings</td>
		</tr>
		<tr>
			<td>Stainless Steel Ultra-Torr Vacuum Fitting</td>
			<td>Swagelok</td>
			<td>SS-4-UT-A-8</td>
			<td>Tube fittings used on the probe</td>
		</tr>
		<tr>
			<td>Teflon coated wire</td>
			<td>Geyer Systems</td>
			<td>P31546</td>
			<td>Connect the gold-coated wire to BNC pin</td>
		</tr>
		<tr>
			<td>Turbo pump</td>
			<td>PFEIFFER</td>
			<td>TPH 240 C</td>
			<td>Bring the pressure down to 1E-6 Torr</td>
		</tr>
		<tr>
			<td>Vacuum grease</td>
			<td>APIEZON</td>
			<td>L Ultra High Vacuum Grade Grease</td>
			<td>Vacuum grease used to lubricate the oring</td>
		</tr>
		<tr>
			<td>Viton Orings</td>
			<td>Grainger</td>
			<td>#031</td>
			<td>Round #031 Medium Hard Viton O-Ring, 1.739&#34; I.D., 1.879&#34; O.D</td>
		</tr>
		<tr>
			<td>Viton Orings</td>
			<td>Grainger</td>
			<td>#010</td>
			<td>Round #010 Medium Hard Viton O-Ring, 0.239&#34; I.D., 0.379&#34;O.D</td>
		</tr>
	</tbody>
</table>

building langmuir probes and emissive probes for plasma potential measurements in low pressure, low temperature plasmas

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&#160;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.

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<sup class=...

Watch this Scientific Journal Video about Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas at JoVE.com

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

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.

Langmuir probes have long been used in experimental plasma physics research as the primary diagnostic for particle fluxes (i.e., electron and ion fluxes) ...

We provide a step-by-step protocol on how to make Langmuir probes and emissive probes using readily available lab material for researchers who want to build themselves. Using readily available material in the lab makes it cheaper and easier to build and maintain electrostatic probes for plasma diagnostics, and make the system more adaptable to different lab settings. The vacuum interface using in our building protocol can also be used as feed-through methods for other low-pressure devices.When performing this protocol vacuum techniques electric continuity check, curing the ceramic paste, and managing the fragility of the probe can seem challenging at first. Begin by constructing the Langmuire probes. Take a quarter inch diameter stainless steel tube as the probe shaft.Cut the tube so that the probe can axially cover more than half of the chamber length. Fit the unbent side of the shaft through a brass tube by a SS-4-UT-A-8 adapter in combination with a B-810-6 Union tube fitting. Use a half inch brass tube extending out of the customized flanges through a B-810-1 OR Swagelok interface to provide axial support for the probe shaft.Connect the unbent end of the probe shaft to the BNC housing through a B-400-1 OR Swagelok fitting. Fit the gold coated nickel wire through two single bore alumina tubes, fitting the thicker tube inside the probe shaft. Spot weld one end of gold coated nickel wire onto a piece of stripped wire, which is saudered onto the pen of the BNC feed through, at the end of the probe shaft.Then, cut the gold coated wire so that the joint with the stripped wire fits inside the alumina tube to prevent short circuit with the probe shaft. Punch through a tantalum sheet to make a quarter inch planer Langmuir probe tip, and spot weld to the other end of the gold coated nickel wire onto the edge of the tip, setting the probe tip normal to the axis of the boundary plate. Position the probe tip slightly forward so that the body of the probe does not touch the boundary plate while taking measurements inside the sheath.Seal all joints with ceramic paste to insulate the probe circuit components from the plasma, and use a heat guy to bake the ceramic joints for five to 10 minutes. Use a multimeter to measure the resistance between the probe tip and the BNC connector. If continuity is demonstrated, the probe is ready to be put into the vacuum chamber.To build a cylindrical admissive probe, follow the previously described procedure with the exception of using a one eighth inch two bore aluminum tube instead of a single bore one. Cut the 0.025 millimeter diameter tungsten wire to about one centimeter, and spot weld the tungsten filament onto gold coated wires. Seal alt joints with ceramic paste, making sure the paste does not get onto the tungsten filament.Check continuity between the two BNC ends. Turn on the ion gauge to check the base pressure before putting gas into the chamber. Otherwise, check for leaks in the system.Use the pen to calibrate the baratron display until the number floats within plus or minus 0.01 millitorr. Make sure that the needle valve is at a closed position. Then open the shutoff valve and check that there is no pressure change on the baratron reading.Slowly turn the knob of the needle valve to release the gas into the chamber until the required pressure is reached. Turn on the voltage power supply, and set the voltage to negative 60 volts to provide sufficient electron energy for the maximum ionization cross section of Argonne. Then, turn on the heating power supply for the filaments and slowly adjust the level until the discharge current reads the required value.Connect the voltage supply to the boundary plate, and adjust the bias to the desired level. Then proceed with measurement. Attach the probe to the data acquisition and control circuit, and proceed to sweep the voltage applied to the probe while simultaneously measuring the current drawn by the probe.Save the current voltage trace. Repeat data acquisition with the admissive probes, data acquisition and control circuit. Langmuir probes were built in four different configurations, and were labeled as LPJ, with J being an integer from one to four.The probe designs included the cylindrical Langmuir probe LP one, the double-sided Langmuir probe, LP two, the planer Langmuir probe, LP three, with the side facing the boundary plate sealed by ceramic paste, and the planar Langmuir probe, LP four, with the side facing away from the boundary plate covered by ceramic paste. The comparison between Langmuir probe and a missive probe potential measurements are shown here. All four Langmuir probe types were compared to a missive probe measurements to determine how well they measure plasma potentials near plasma boundaries, specifically in the pre sheath region.Different probe designs were tested to determine whether the way the Langmuir probes are used to measure plasma potentials, gives accurate results. All the Langmuir probe measurements were compared to admissive probe measurements of the plasma potential. In the pre sheath, all Langmuir probe measured plasma potentials differ from those measured by admissive probes.The difference widens with proximity to the sheath edge, growing to a value of many electron temperatures. Plasma parameters such as temperature, density, to buy lengths, and child Langmuir sheath lengths obtained from the measurements by LP two in the bulk of the plasma are shown here. These parameters help establish good estimates of the nominal sheath thickness for planar Langmuir probes.To successfully perform this protocol, bake all the air bubbles out when drying the ceramic paste. And before putting the probe into the vacuum chamber, don't forget to do the continuity check between the probe tape and the BNC head, and also the installation check between the probe tape and the probe shaft. Electrostatic probes are used to detect and launch the plasma waves to measure the presence of the plasma instabilities, and to map out the coherent structure found in plasma, such as double layers and solid tons.

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