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Plasma Physics:Electrical Breakdown
Plasmas are the most common form of matter in the visible universe but remain one of the least studied topics in the undergraduate physics curriculum. While a fascinating multi-variable system on its own, the study of plasmas also has connections to a significant number of topics in an undergraduate physics curriculum, from classical mechanics to atomic physics.
In this Immersion, participants will first build a DC glow discharge, a simple plasma source that is understandable to undergraduate students yet flexible enough to be used in a variety of experiments (see our recent article in the Sept. 2013 edition of AJP for more details). This type of source is relevant to fluorescent lighting, plasma televisions, and surface treatment of materials. The dc glow discharge is composed of two conductors separated by some distance and electrical potential inside an evacuated vessel at moderately low pressures (between a few and a hundred Pascals). The electrical potential needs to be high enough (hundreds to thousands of Volts) to breakdown the gas into a plasma. The characteristics of the plasma are controlled by four variables: potential difference and distance between the electrodes as well as the type of gas used and its pressure. In our device, we evacuate glass vessels that house stainless steel electrodes. Each has a two-stage direct-drive vacuum pump, an inlet valve system, and a pressure sensor. The vessels can be filled with any gas (including air) through one fine and one coarse valve. These valves also enable fine control of the gas pressure from 3 to 300 Pa. We use power supplies that provide a potential difference of up to 2000 V and 20 mA.
Then, participants will investigate the transition from a gas to a plasma and correlate the breakdown voltage to the length of the tube and the gas pressure (Paschen’s Law). We compare our experimental measurements to the ideal curve and use that to discuss sources of error in this multivariable system.
To start the dc glow, air, argon, helium, or another gas filling the gap between the electrodes, must become conductive. The voltage applied by the high voltage power supply and hence the electric field in the gap, must be high enough to initiate electrical breakdown in the gap. In such a breakdown experiment, participants measure the starting potential needed to initiate a continuous, self-sustained flow of current through the dc glow tube. Participants will experimentally investigate the dependence of the breakdown or starting voltage as the balance between electron production and loss changes. Increasing d decreases the electric field strength, and decreasing p increases the electron mean free path.
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