Name: building langmuir probes and emissive probes for plasma potential measurements in low pressure, low temperature plasmas
Uploaded: 2021-05-25T12:30:00.000+00:00
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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 combination with a B-810-6 union tube fitting.</li>
		<li>Use a &#189;&#8221; brass tube extending out of the customized flanges through a B-810-1-OR swagelok interface to provide axial support for the probe shaft.</li>
		<li>Connect the unbent end of the probe shaft to the BNC housing through a B-400-1-OR swagelok fitting, as shown in Figure 6.</li>
		<li>Fit the gold-coated nickel wire through two single-bore alumina tubes (1/8&#8221; and 3/16&#8221; in diameter) with the thicker one fits inside the probe shaft, as shown in Figure 7.</li>
		<li>Spot weld one end of gold-coated nickel wire onto a piece of stripped wire, which is soldered onto the pin of the BNC feedthrough at the end of the probe shaft.</li>
		<li>Cut the gold-coated wire to length such that the joint with the stripped wire fits inside the alumina tube to prevent short circuit with the probe shaft.</li>
		<li>Punch through a tantalum sheet to make a planar Langmuir probe tip (&#188;&#8221; in diameter)</li>
		<li>Spot weld the other end of the gold-coated nickel wire onto the edge of the probe tip and set the probe tip to be normal to the axis of the boundary plate.</li>
		<li>Position the probe tip a little forward so that the body of the probe does not touch the boundary plate while taking measurements inside the sheath.</li>
		<li>Seal all joints with ceramic paste (e.g., Sauereisen Cement No. 31) to insulate the probe circuit components from plasma. Use a heat gun&#160;to bake the ceramic joints for 5-10 min.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Building a cylindrical emissive probe (see Figure 8 for more details)
	<ol>
		<li>Follow step 1.1.1-1.1.4 and repeat step 1.1.5-1.1.7 on the same probe shaft twice with the exception of using a 1/8&#8221;, two-bore alumina tube instead of a single-bore one.</li>
		<li>Cut the 0.025 mm diameter tungsten wire to about 1 cm.</li>
		<li>Spot weld the tungsten filament onto gold-coated wires.</li>
		<li>Seal all the joints with ceramic paste and make sure the ceramic paste does not get onto the tungsten filament.</li>
		<li>Check continuity between two BNC ends.</li>
	</ol>
	</li>
</ol>
2. Generate plasma
<ol>
	<li>Turn on the ion gauge to check the base pressure before putting gas into the chamber. Proceed with zeroing of the baratron gauge if the pressure is in the low 10-6 Torr range. Otherwise, check the leak in the system.&#160;The positions of needle valve and the shut off value are open and closed, respectively.&#160;</li>
	<li>Use a plastic screwdriver&#160;to calibrate the baratron display until the number floats between &#177;0.01 mTorr.</li>
	<li>Close the needle valve so that it is gently seated in a closed position.</li>
	<li>Open the shutoff valve. Check that there is no pressure change on the baratron reading.</li>
	<li>Slowly turn the knob of the needle valve to release the gas into the chamber until the pressure reaches the requirement for the experiment. The typical working pressure stem from 10-5 ~ 2 x 10-3 Torr. Working gases have included argon, xenon, krypton, oxygen etc.</li>
	<li>Turn on the KEPCO voltage power supply and set the voltage to -60 Volts to provide sufficient electron energy for the maximum ionization cross section of argon. Turn on the heating power supply for the filaments and slowly adjust the level until the discharge current reads the required value. The discharge current tends to drop quickly in the first few minutes. Keep adjusting the current level for about 30 minutes until the discharge stabilizes</li>
	<li>Connect the voltage supply to the boundary plate and adjust the bias to desired level.</li>
</ol>
3. Take measurements
NOTE: I-V traces for Langmuir probes and emissive probes are acquired by a 16-bit DAQ board controlled by a Labview program. The details are not presented here since different users have different preferences for taking the data. However, there is a protocol for how to use the probes.
<ol>
	<li>Take the load line: obtain an I-V trace without any plasma discharge in the chamber with all connections made between the probe and its measuring circuit (see Figure 9, Figure 10 &#38; Figure 11 for the UW-Madison and the USD setup).</li>
	<li>Langmuir probes
	<ol>
		<li>Clean the probe tip (this step is critical, as a clean probe exhibits a sharper &#8216;knee&#8217; than a dirty probe) by biasing the probe positively to collect a large electron current.
		<ol>
			<li>Draw a current through the probe with a variable power supply and 50 Ohms to the machine ground to heat the tip so as to evaporate the layer of impurities that immediately attaches to the probe surface in the plasma and increase the surface resistivity of the probe.</li>
			<li>Slowly increase the bias positively to surpass the plasma potential, permitting the probe to begin to draw the electron saturation current.</li>
			<li>Continue to raise the potential; once one sees the probe tip glowing cherry red, the probe is clean. It is necessary to have a view of the probe tip in the plasma through a vacuum viewport.</li>
			<li>Be careful and vigilant while varying the bias on the probe. If the probe is allowed to get too hot, the probe tip itself could become warped, and worse things can happen, such as the tip could have holes in it, it could evaporate, it could fall off; wires could melt and lose their insulation, and so forth.</li>
			<li>Attach the probe to the data acquisition and control circuit (this is the part that will vary from lab to lab), and proceed to sweep the voltage applied to the probe while simultaneously measuring the current drawn by the probe. Save the I-V trace.</li>
		</ol>
		</li>
		<li>Attach the probe to the data acquisition and control circuit (this is the part that will vary from lab to lab), and proceed to sweep the voltage applied to the probe while simultaneously measuring the current drawn by the probe. Save the I-V trace.</li>
	</ol>
	</li>
	<li>Emissive probes
	<ol>
		<li>Repeat step 3.2.2 with the emissive probe&#8217;s data acquisition and control circuit.</li>
	</ol>
	</li>
</ol>
4. Data analysis
<ol>
	<li>Langmuir Probes (See Figure 12, Figure 13 for more details).
	<ol>
		<li>Subtract the load line from the total I-V characteristic.</li>
		<li>Fit the ion saturation current and subtract from the remaining I-V characteristics.</li>
		<li>Take the natural log of current and plot it against the probe voltage.</li>
		<li>Take linear fits of transitional region and saturation current separately.</li>
		<li>Take the inverse of the slope of the transitional region and obtain the electron temperature value.</li>
		<li>Obtain the plasma density by plugging the current at the crossing where the two fitted lines cross each other into Eq.3.</li>
		<li>Apply the inflection point technique to the Langmuir probe trace and determine the plasma potential.</li>
	</ol>
	</li>
	<li>Emissive Probe (refer to Figure 2).
	<ol>
		<li>Repeat step 4.1.1-4.1.2 for individual I-V characteristics, then smooth each trace.</li>
		<li>Differentiate each I-V trace and apply appropriate smoothing.</li>
		<li>Locate the peak of each smoothed dI/dV (inflection point).</li>
		<li>Apply a linear fit to the inflection points.</li>
		<li>Obtain the plasma potential by locating the zero crossing of the fitted line.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

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&quot; thick tungsten wire</td><td>Midwest Tungsten Service</td><td>0.001&quot;</td><td>Emissive probe filament</td></tr><tr><td>0.005&quot; thick tantalum sheet</td><td>Midwest Tungsten Service</td><td>0.005&quot;</td><td>Heating filament to generate plasma</td></tr><tr><td>1/2&quot; Brass supprting tube</td><td></td><td></td><td></td></tr><tr><td>1/4&quot; 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&quot; 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&#039; lengths</td></tr><tr><td>Alumina tubes</td><td>COORSTEK</td><td>65655, single bore 0.156&quot; 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&#039;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&quot; 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&quot; 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&quot; I.D., 1.879&quot; 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&quot; I.D., 0.379&quot;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) ...

building-langmuir-probes-emissive-probes-for-plasma-potential

Research

JoVE Journal

Engineering

3.9K Views.  University of Wisconsin. 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.

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

Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses

Early plasma is generated owing to high intensity laser irradiation of target and the subsequent target material ionization. Its dynamics plays a significant role in laser-material interaction, especially in the air environment1-11.
Early plasma evolution has been captured through pump-probe shadowgraphy1-3 and interferometry1,4-7. However, the studied time frames and applied laser parameter ranges are limited. For example, direct examinations of plasma front locations and electron number densities within a delay time of 100 picosecond (ps) with respect to the laser pulse peak are still very few, especially for the ultrashort pulse of a duration around 100 femtosecond (fs) and a low power density around 1014 W/cm2. Early plasma generated under these conditions has only been captured recently with high temporal and spatial resolutions12. The detailed setup strategy and procedures of this high precision measurement will be illustrated in this paper. The rationale of the measurement is optical pump-probe shadowgraphy: one ultrashort laser pulse is split to a pump pulse and a probe pulse, while the delay time between them can be adjusted by changing their beam path lengths. The pump pulse ablates the target and generates the early plasma, and the probe pulse propagates through the plasma region and detects the non-uniformity of electron number density. In addition, animations are generated using the calculated results from the simulation model of Ref. 12 to illustrate the plasma formation and evolution with a very high resolution (0.04 ~ 1 ps).
Both the experimental method and the simulation method can be applied to a broad range of time frames and laser parameters. These methods can be used to examine the early plasma generated not only from metals, but also from semiconductors and insulators.

An experimental method to examine the early plasma evolution induced by ultrashort laser pulses is described. Using this method, high quality images of early plasma are obtained with high temporal and spatial resolutions. A novel integrated atomistic model is used to simulate and explain the mechanisms of early plasma.

Early plasma is generated owing to high intensity laser irradiation of target and the subsequent target material ionization. Its dynamics plays a ...

How to Ignite an Atmospheric Pressure Microwave Plasma Torch without Any Additional Igniters

This movie shows how an atmospheric pressure plasma torch can be ignited by microwave power with no additional igniters. After ignition of the plasma, a stable and continuous operation of the plasma is possible and the plasma torch can be used for many different applications. On one hand, the hot (3,600 K gas temperature) plasma can be used for chemical processes and on the other hand the cold afterglow (temperatures down to almost RT) can be applied for surface processes. For example chemical syntheses are interesting volume processes. Here the microwave plasma torch can be used for the decomposition of waste gases which are harmful and contribute to the global warming but are needed as etching gases in growing industry sectors like the semiconductor branch. Another application is the dissociation of CO2. Surplus electrical energy from renewable energy sources can be used to dissociate CO2 to CO and O2. The CO can be further processed to gaseous or liquid higher hydrocarbons thereby providing chemical storage of the energy, synthetic fuels or platform chemicals for the chemical industry. Applications of the afterglow of the plasma torch are the treatment of surfaces to increase the adhesion of lacquer, glue or paint, and the sterilization or decontamination of different kind of surfaces. The movie will explain how to ignite the plasma solely by microwave power without any additional igniters, e.g., electric sparks. The microwave plasma torch is based on a combination of two resonators &#8212; a coaxial one which provides the ignition of the plasma and a cylindrical one which guarantees a continuous and stable operation of the plasma after ignition. The plasma can be operated in a long microwave transparent tube for volume processes or shaped by orifices for surface treatment purposes.

This movie shows how an atmospheric plasma torch can be ignited by microwaves with no additional igniters and provides a stable and continuous plasma operation suitable for plenty of applications.

This movie shows how an atmospheric pressure plasma torch can be ignited by microwave power with no additional igniters. After ignition of the plasma, a ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Experimental Methods of Dust Charging and Mobilization on Surfaces with Exposure to Ultraviolet Radiation or Plasmas

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using three recently developed experiments in which dust particles are exposed to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. The UV light source has a narrow bandwidth in wavelength centered at 172 nm. The beam electrons with the energy of 120 eV are created with a negatively biased hot filament. When the vacuum chamber is filled with the argon gas, a thermal plasma is created in addition to the electron beam. Insulating dust particles of a few tens of microns in diameter are used in the experiments. Dust particles are recorded to be lofted to a height up to a few centimeters with a launch speed up to 1 m/s. These experiments demonstrate that photo and/or secondary electron emission from a dusty surface changes the charging mechanism of dust particles. According to the recently developed &#34;patched charge model&#34;, the emitted electrons can be re-absorbed inside microcavities between neighboring dust particles below the surface, causing the accumulation of enhanced negative charges on the surrounding dust particles. The repulsive forces between these negatively charged particles may be large enough to mobilize and lift them off the surface. These experiments present the advanced understanding of dust charging and transport on dusty surfaces, and laid a foundation for future investigations of its role in the surface evolution of airless planetary bodies.

Dust charging and mobilization is demonstrated in three experiments with exposure to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. These experiments present the advanced understanding of electrostatic dust transport and its role in shaping the surfaces of airless planetary bodies.

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using ...

Environment

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Applying X-ray Imaging Crystal Spectroscopy for Use as a High Temperature Plasma Diagnostic

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line intensity ratios. By using a Johann spectrometer viewing the plasma, it is possible to construct profiles of plasma parameters such as density, temperature, and velocity with good spatial and time resolution. However, benchmarking atomic code modeling of X-ray spectra obtained from well-diagnosed laboratory plasmas is important to justify use of such spectra to determine plasma parameters when other independent diagnostics are not available. This manuscript presents the operation of the High Resolution X-ray Crystal Imaging Spectrometer with Spatial Resolution (HIREXSR), a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma. In addition, this manuscript covers a laser blow-off system that can introduce such ions to the plasma with precise timing to allow for perturbative studies of transport in the plasma.

X-ray spectra provide a wealth of information on high temperature plasmas. This manuscript presents the operation of a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma.

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line ...

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.

This article describes a flowing microwave reactor that is used to drive efficient non-equilibrium chemistry for the application of conversion/activation of stable molecules such as CO2, N2 and CH4. The goal of the procedure described here is to measure the in situ gas temperature and gas conversion.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose ...

Chemistry

An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. The reactions of cold plasma with the surrounding atmosphere yield a variety of reactive species, which can define its effectiveness. While efficient development of cold plasma therapy requires kinetic models, model benchmarking needs empirical data. Experimental studies of the source of reactive species detected in aqueous solutions exposed to plasma are still scarce. Biomedical plasma is often operated with He or Ar feed gas, and a specific interest lies in investigation of the reactive species generated by plasma with various gas admixtures (O2, N2, air, H2O vapor, etc.) Such investigations are very complex due to difficulties in controlling the ambient atmosphere in contact with the plasma effluent. In this work, we addressed common issues of &#39;high&#39; voltage kHz frequency driven plasma jet experimental studies. A reactor was developed allowing the exclusion of ambient atmosphere from the plasma-liquid system. The system thus comprised the feed gas with admixtures and the components of the liquid sample. This controlled atmosphere allowed the investigation of the source of the reactive oxygen species induced in aqueous solutions by He-water vapor plasma. The use of isotopically labelled water allowed distinguishing between the species originating in the gas phase and those formed in the liquid. The plasma equipment was contained inside a Faraday cage to eliminate possible influence of any external field. The setup is versatile and can aid in further understanding the cold plasma-liquid interactions chemistry.

An experimental setup was created for the helium-operated kHz frequency plasma jet. The setup includes a cage for the plasma power supply and jet and an in-house built reactor to monitor plasma-induced reactive species without the interference of the ambient atmosphere.

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. ...

Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 &#181;s), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons&#39; absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2-(aq)), nitrate (NO3-(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface

This article presents a total internal reflection absorption spectroscopy (TIRAS) method for measuring short-lived free radicals at a plasma-liquid interface. In particular, TIRAS is used to identify solvated electrons based on their optical absorbance of red light near 700 nm.

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated ...

Treating Surfaces with a Cold Atmospheric Pressure Plasma using the COST-Jet

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in biological applications. However, the scientific results often suffer from reproducibility problems due to unreliable plasma conditions as well as complex treatment procedures. To address this issue and provide a stable and reproducible plasma source, the COST-Jet reference source was developed.
In this work, we propose a detailed protocol to perform reliable and reproducible surface treatments using the COST reference microplasma jet (COST-Jet). Common issues and pitfalls are discussed, as well as the peculiarities of the COST-Jet compared to other devices and its advantageous remote character. A detailed description of both solid and liquid surface treatment is provided. The described methods are versatile and can be adapted for other types of atmospheric pressure plasma devices.

This protocol is presented to characterize the setup, handling, and application of the COST-Jet for the treatment of diverse surfaces such as solids and liquids.

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in ...