General schematic of a glow discharge incorporating a movable cathode, a permanent magnet, a Helmholtz coil, and Langmuir probe. The dc glow has an anode (a) and a cathode (c) housed in a glass vessel. The electrodes are powered by a high-voltage power supply in series with a ballast resistor. The vacuum system controls the gas pressure via a coarse (ball) and fine (needle) valve that leaks gas into an inlet port while the vacuum pump removes gas through the outlet port.
(Color online) Three educational plasma devices based on dc glow discharges: (a) a basic, introductory laboratory device, (b) and (c) an advanced dc glow discharge with magnets, (d) a discharge with Langmuir probes. See text for more details.
The electrical breakdown voltage depends on the pressure of the gas and the distance between the high-voltage electrodes (circles, experimental data; curve, Eq. (1) with constants for air).
The relationship between resistance and current for an air plasma at 3.2 Pa (circles, experimental data; curve, fit to Ohm's Law).
Survey spectrum (a) and high-resolution spectrum (b) of argon taken in the 780–920 nm range yields spectral lines used to construct a Boltzmann plot (c) in the positive column. The linear fit yields an electron temperature of ∼0.5 eV.
The ratio of visible plasma area inside and outside of the Helmholtz coil as a function of increasing magnetic field. The radius of the plasma in the vertical direction was estimated through a moment analysis that fits the visible plasma to an ellipse. The radius was then used to estimate for different field strengths (circles). A linear fit (curve) demonstrates that magnetic flux is conserved.
Data set from the Langmuir probe. Below the floating potential , ion current is dominant; above, only electron current is collected. The inset shows the data in the electron collection region plotted on a semi-logarithmic scale.
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