Teacher s Assistant Laboratory Guide Joseph B. Hakanson 1, Warner Meeks 2, and Dr. Joshua L. Rovey 3 Missouri University of Science and Technology, Rolla, Missouri, 65409 These instructions will discuss the assembly of several experiments that can be used for undergraduate education in DC plasma. Experimental breakdown curves and Langmuir Probe curves will be studied with the use of a DC glow test article. p*d = pressure-distance parameter Nomenclature I. Introduction This document has been written to help the Teacher s Assistant (TA) in the preparation and execution of the laboratory experiment. This document includes instructions on how to assemble each part of the experiment, but does not address how to acquire or manufacture the parts themselves. Additionally, a separate document titled Plasma Theory for Undergraduate Education provides instructions on how to perform the experiments, which should also be given to the students. II. Experimentation Apparatus The AP Lab has assembled a DC glow discharge similar to that used by the Princeton Plasma Physics Laboratory [1]. The test article is constructed from a 3 ¾ diameter Pyrex tube T-joint, an Edwards Series 3 rough pump, and a Varian V70 turbo pump (see Figure 1). Two electrodes are suspended in a parallel configuration with a variable gap distance. Argon gas or air is used to backfill the chamber to the desired pressure from high vacuum. To control the backfill system, an Alicat Scientific digital flow regulator is used. To power the electric discharge a Glassman High Voltage Inc. 1000V 100mA variable DC power supply is connected to one electrode while grounding the second and is monitored by a Tenima 72-1020 Benchtop Digital Multimeter. For the Langmuir study, a BK Precision 1739 DC Power Supply was used to bias the probe. 1 Undergraduate Student, Aerospace and Mechanical Engineering, 2009 Maxwell St. Apt. A 2 Graduate Student, Mechanical and Aerospace Engineering, wcm994@mst.edu 3 Professor, Mechanical and Aerospace Engineering, 292D Toomey Hall 1
Figure 1. Apparatus Setup III. Experiment Assembly A. Lab Equipment Checklist To begin this lab, first verify that all necessary equipment has been identified and located: 1) DC Glow Apparatus a) Pair of rounded, polished electrodes for observation b) High-voltage cable Don't Touch 2) Argon Backfill Supply (optional) a) Filled tank of argon b) Bottle and Gauge valves c) Flow regulator 3) Power Supply a) 1000V power Supply b) 30V power supply 4) Pumps a) Rough pump b) Turbo pump (optional) 5) Sensors and Probes a) Digital pressure gauge b) Digital multimeters One to monitor discharge current and probe current and one to monitor discharge voltage c) Langmuir Probe d) Oscilloscope 2
B. General Assembly This section shall contain details of assembling parts of the experiment that shall be consistent between both the Paschen Study and the Langmuir Probe experiments. 1) DC Glow Apparatus a) As indicated in Figure 1., the two concentric openings of the T-joint shall be capped with stainless steel plates which will serve as the fixture point for each electrode. Just below the bottom plate is a 6-way cross piece that will allow the turbo pump to be connected and the electrode to slide directly through. The hole which the electrodes slide through is hermetically sealed, meaning that the electrode gap may be adjusted even when the chamber is evacuated. b) The 1000V power supply should be grounded on the negative side, and the positive side should be connected to the top electrode. The high-voltage line needs to have sufficient insulation to prevent accidental electrocution. 2) Electrical Sensors a) If the 1000V power supply electrical display lacks the precision needed for the experiments, then separate multimeters may be used to monitor the system. In this case, a multimeter should be set to measure DC voltage, and placed in parallel across the power supply. To monitor current, a multimeter can be set to measure DC current and arranged in series with the circuit. Because not much current is drawn by the plasma, the maximum current rating of the multimeter is not exceeded, and a clamp on induction meter is not needed. b) A digital pressure sensor should be placed in the chamber and can simply be powered by the cart s power strip. Therefore the sensor will turn on whenever the cart is turned on. c) The volumetric flow regulator should be connected to the argon supply line as close to the chamber as possible to eliminate as much line capacitance as possible. If there is much line between the regulator and the chamber, then there will be a substantial lag whenever the flow rate is changed because the gas requires much time to disperse through the long, skinny tube. The flow regulator should be turned on and off by connecting/disconnecting the power line directly to the flow meter. 3) Gas Pumps a) The turbo pump should be connected directly to the 6-way cross piece below the chamber and powered by a nearby power by the cart power strip. Note that the turbo pump is only necessary when purging the chamber for non-air gas studies. b) The rough pump should be connected to the exhaust from the turbo pump. The exhaust from the rough pump should be vented to a filtration system. The rough pump should be powered by a nearby wall outlet. When evacuating the chamber, the rough pump should be used initially to bring the pressure down to 20 mtorr, and then the turbo pump may be used to reduce the pressure to 0.05 mtorr. C. Paschen Study Assembly This section will specify all necessary modifications to the General Assembly that are needed to perform the Paschen Study. 1) Gap Distance a) The independent variable of the Paschen Curve, p*d, depends on d. Therefore, several iterations of the experiment can be performed at different gap distances to capture more of the p*d spectrum. b) To adjust the gap distance, loosen the nut on the bottom electrode by hand when the power supply is turned off. Then slide the electrode up or down. The top electrode need not be adjusted. D. Langmuir Probe Assembly This section will specify all necessary modifications to the General Assembly that are needed to perform the Langmuir Probe Study. 1) Gap Distance a) To adjust the gap distance, loosen the nut on the bottom electrode when the power supply is turned off. Then slide the electrode up or down. In doing so, make sure that the probe will still capture the desired region of the plasma. If needed, the top electrode may be adjusted by using 9/16 wrenches to loosen the two locked nuts. Be sure that the electrode is fully discharged before touching it because there may be some small capacitance associated with the electrode. 2) Langmuir Probe 3
a) The Langmuir Probe will be inserted into the vacuum chamber through the non-collinear leg of the T-joint. Position the probe so that the tungsten leads are along the perimeter of the plasma glow. This is done to insure that the probe isn t in the direct path of the electrons as they flow between probes, but is rather capturing the stray electrons of the glow. Make sure that the probe is positioned appropriately to capture the positive column of the glow. 3) Electrical equipment a) The Langmuir Probe circuit should follow the diagram in Figure 2. Note that the voltmeter in the diagram can be pulled from the initial apparatus setup after the 1000V supply has been tuned to a specific desired voltage. b) The 30V supply should initially be grounded on the negative terminal with the grounding strap and power should be drawn from the positive terminal. When the students desire to sweep the negative spectrum, the grounding strap should be flipped to the positive terminal, and the banana cable should be moved to the negative terminal. c) Because the plasma is drawing such little current, the voltage across a large resister is used to calculate the precise current. If a resistor matching the one used in the APL cannot be found (167 kohms), a resistor or resistor circuit of similar magnitude can be substituted. Figure 2. Langmuir Probe Circuit IV. Conclusion Through these experiments, undergraduate students should gain a richer understanding of the properties and characterization of DC plasma. With a solid grounding in these basics, more complicated applications of plasma technology may be explored. 4
References [1] Stephanie A. Wissel, A. Z. (2012). The Use of DC Glow Discharges as Undergraduate Education Tools. Princeton Plasma Physics Laboratory, 1-20. 5