A DC discharge plasma experiment for undergraduate laboratories

TL;DR

This work presents the design, construction, and characterization of a DC glow-discharge plasma chamber intended for undergraduate laboratories. It integrates a movable-electrode quartz chamber, a versatile HV drive, GUI-based diagnostics, and homemade Langmuir probes to measure electron temperature and density, complemented by Boltzmann-plot spectroscopy for excitation temperatures. The study demonstrates Paschen breakdown behavior, V–I characteristics across pressures, radial temperature/density profiles, and, with custom Helmholtz coils, magnetic lensing of electrons supported by Runge–Kutta trajectory simulations. Together, these elements provide a flexible, student-driven platform to explore ionization, excitation, and charged-particle dynamics with potential expansions into microwave-plasma interactions. The approach emphasizes hands-on learning, quantitative diagnostics, and numerical modeling to connect fundamental plasma physics with undergraduate experimentation.

Abstract

Plasma physics offers a wide range of fundamental phenomena, making it an excellent subject for undergraduate laboratory instruction. In this work, we present the design, construction, and characterization of a DC glow-discharge plasma chamber developed for the junior-level curriculum, a project carried out by two undergraduate students. The apparatus consists of a 1-meter-long quartz tube with a movable electrode, enabling systematic exploration of plasma behavior under varying pressure, voltage, and geometry. Using this platform, we characterized the Paschen breakdown relation and the voltage-current characteristics of the plasma. We then developed Langmuir probes to map spatial distributions of electron temperature and density, and used Boltzmann plot spectroscopy to measure excitation temperatures across different plasma regions. Finally, with custom Helmholtz coils, we demonstrated magnetic focusing of electrons. We performed Runge-Kutta simulations of particle trajectories and analyzed the electron drift velocity by comparing the focal lengths. Overall, this plasma chamber provides a versatile platform for investigating fundamental plasma phenomena and offers potential for future studies, including microwave-plasma interactions and other student-driven investigations.

Figures (27)

• Figure 1: Voltage-current curve of different stages of DC discharge. This graph is from IV_ref. The Normal Glow discharge is studied in this thesis. This regime exhibits Ohm's law relation between current and voltage. The regime of DC discharge can be driven by a low power high voltage DC power supply that is common in undergraduate laboratories.
• Figure 2: Position dependent measurements in a DC glow discharge from fundamentals_of_dcThis figure shows the plasma potential, plasma density, space charge, and current across the various regimes of a DC glow discharge.
• Figure 3: DC discharge chamber design. The main structure of the chamber is a 1.1-meter-long T-shape quartz glass cell. There are two sides of vacuum components connected to the chamber by O-rings, the left side is the cathode, the right anode. There is a side tube in the middle. 1. Cathode section: we have an position-adjustable copper cathode that's sealed by compression fitting, a ball valve connected to the mechanical pump, a vacuum gauge meter, and a home-made needle valve in series with a ball valve to control the input flow of gas. This side is completely grounded. 2. Anode section: A brass plate is screwed onto a oxygen-free copper HV feed through. The high voltage power supply is connected to this side of the chamber. During experiments, this side is shielded by a big 3D-printed cylinder to avoid electric shocks. 3. The side-tube section: A KF50 quartz window installed for laser access and spectroscopy.
• Figure 4: Images of the chamber. (a) The chamber without foam boards covering. The entire setup is supported by aluminum extrusions. The anode is covered with a plastic cylinder at the right, the cathode is at the left. In this image, a CCD camera is set pointing at the anode. (b) During the experiment, the chamber is covered up with black foam boards.
• Figure 5: Schematic diagram of the electronic layout of our plasma experiment. The red wires correspond to high voltage (kV) cables, black correspond to ground. The high-voltage power supply creates kilovolts of potential difference relative to the ground. The current passes through a high voltage ceramic resistor, than connected to the anode. On the other side, the cathode is grounded. An amp meter is connected in series to monitor the current passing through the chamber. An individual 5A power supply provides current for the magnetic coils.