The Gustavsson Macroscopic Plasma Experiment

How do you demonstrate fundamental plasma physical effects to students? What is a Langmuir wave? Prof. Björn Gustavsson came up with a brilliant way to visualize motions of charged particles on a macroscopic scale with the help of 2 mm bearing balls that are trapped in between two plastic plates. Look at the video? It shows charged ball bearings trapped between two plastic plates. When perturbed, they oscillate.

What are Langmuir waves? They are quasi-harmonic oscillations of electrons in plasma. If you look at individual bearing balls in the video, you can see them oscillating back and forth. This is because each ball bearing is charged with approximately the same charge. Due to Coulumb forces between particles, any small perturbation to equilibrium will result in oscillations of the particles. These oscillations occur at a certain frequency, which is called the plasma frequency. From the video, it is possible to estimate that the plasma frequency for this specific plasma is a few Hz. A very small plasma frequency indeed. Typical plasma frequencies in the ionosphere are between 1 MHz and 15 MHz. Even in in space, at one AU, the plasma frequency of solar wind is measured in tens of kHz.

Plasma is neutral, so where are the oppositely charged "ions" in this plasma? The plastic plate contains a charge opposite to the bearing balls.

You can do a few calculations with the demonstration. If you know the plasma frequency (few Hz), the density of charged particles (about 100 per cubic cm), and the mass of the particles (0.1 g), it is possible to estimate the unit charge on each one of the spheres. We estimated this to be about 0.1 micro Coulombs -- which seems plausible.

It might also be possible to determine other plasma physics phenomena too. The charge to mass ratio is about 1 in this "macroscopic" plasma, whereas in reality this ratio is about 11 orders of magnitude higher.

I'm sure this is not the first time someone has played with charged bearing balls between two pieces of plexi glass, but I am not sure anyone has thought of the analogy to plasma physics.