Demonstrations And Experiments In Plasma Physics

Portland Community College, through a grant from the Advanced Technological Education Program at the National Science Foundation, has implemented a suite of demonstrations and experiments in plasma physics. These activities, which focus on the optical and electrical characteristics of gas plasmas, have been classroom-tested at Portland Community College in PCC’s associate of applied science degree program in Microelectronics Technology. The demonstrations and experiment range from low-cost experiments based on NE-2 neon bulbs to more sophisticated studies using fiberoptic spectrometers and Langmuir probes. This paper will describe experimental activities in plasma physics and describe how these activities are integrated into a technician-level course in RF Plasma Systems. Introduction For the purpose of this paper, “plasma” refers to an ionized gas. It is often referred to as the “fourth state of matter.” In this state of matter, plasmas exist when enough energy is supplied to a gas to sustain the continuous creation of positively charged and free electrons. It is the creation of charged particles that makes the plasma useful in manufacturing processes, e.g. etching, sputtering, and deposition. Plasma technology is one of several enabling technologies that makes manufacturing at the nanoscale possible today. It is absolutely essential in the manufacture of integrated circuits as well as a variety of surface coating applications. We benefit from gas plasmas everyday. Gas plasmas produce the visible light in our universe, including our sun. In our offices, fluorescent lighting is based on producing a gas plasma within a coated glass tube. We seldom think of the variety of materials coated by a plasma deposition process, e.g. our eyeglasses with anti-reflective coatings. Gas plasmas are briefly mentioned in chemistry courses, but students enrolling in engineering technology programs lack an understanding of gas plasmas. The laboratory activities described in this paper are designed to provide a basic understanding of the electrical and optical properties of gas plasmas. They range from inexpensive demonstrations and experiments to more sophisticated studies using a Langmuir Probe. P ge 10397.1 “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright 2005, American Society for Engineering Education” They have been tested in the laboratory at Portland Community College in PCC’s MT 240 RF Plasma Systems course. Demonstrations Structure of a DC Glow Discharge in a Long Tube The equipment for this demonstration can be purchased from scientific supply companies, e.g. Fisher Scientific. The experimental set up is shown in Figure 1. Figure 1. Experimental apparatus to study the structure of a DC glow discharge in a long tube. A DC voltage in the range of 250 volts is applied to the terminals of a fluorescent tube. The tube is half-coated to allow viewing of the plasma inside the tube. The uncoated end should be connected to the negative terminal of the power supply and the terminal at this end will serve as the cathode. A Tesla coil is used to initiate, or strike, the plasma. The plasma produced by excitation of the argon-mercury gas mixture in the tube produces a visible structure. At the cathode, the cathode glow is visible followed by the Crooke’s dark space, negative glow, Faraday dark space, and positive column. The Aston dark space is covered by the cathode glow and the anode glow and anode dark space are behind the coating at the other end of the tube. The demonstration is static in that no parameters can be varied, e.g. excitation voltage. A modification that was made replaced the fixed output DC voltage source with a variable high voltage source, e.g. PS-310 High Voltage Power Supply P ge 10397.2 “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright 2005, American Society for Engineering Education” manufactured by Stanford Research Systems. Use of PS-310 requires a 50 kΩ limiting resistor in series with the power supply. The PS-310 produces output voltages with a one-volt resolution. Spectral Studies Using Gas Discharge Tubes Gas plasmas emit light as a result of the excitation-relaxation processes in the gas plasma. Specific wavelengths of light emitted by an excited gas provide a characteristic fingerprint for a given gas. Spectrum tubes for common gases, e.g. nitrogen, helium, argon, and neon, are relatively inexpensive, but require a spectrum tube power supply to produce an excitation voltage of 5000 volts @ 10 mA. Traditionally, the spectra for a given gas plasma is viewed by students using a hand-held spectroscope, having an accuracy of + 50 Angstroms. However, some students and instructors have difficulty viewing the colored lines produced by the diffraction grating. A suitable alternative is to use a fiberoptic spectrometer, e.g. Ocean Optics USB-2000 Spectrometer, to produce a graph of “Intensity versus Wavelength” on a computer monitor. Experiments NE-2 Neon Lamp Experiment The NE-2 neon lamp is a low-cost plasma chamber that can be used to study the breakdown, conduction, and turn-off characteristics of a simple gas plasma. The experiment requires a variable DC power supply with an output voltage range of 0 volts to 120 volts DC. A suitable DC power supply is the Agilent E3612A. Using the E2612A, a 200 k Ω series limiting resistor, NE-2 neon lamp, and two standard multimeters, data can be taken to construct an I-V characteristic curve for the NE-2 neon lamp. The experimental set-up is shown in Figure 2. The breakdown voltage is in the range of 65 – 75 volts. Conduction ceases when the voltage across the lamp drops below 55 volts.