Lab Exercises with an Electron Beam

Various exercises with an electron beam are performed as part of the undergraduate plasma laboratory at UCLA. This course (look up PHYS 180E at the UCLA Physics and Astronomy Dept.) is intended to introduce upper division undergraduates to experimental plasma physics. While acting as the TA for the course during the Spring 2006 quarter I had the opportunity to take some pictures of the electron beam during testing of the setup before the students arrived.

The beam is produced by an oxide coated nickel cathode that is negatively biased with respect to the conducting aluminum vacuum chamber. Electrons are effectively boiled off of the cathode and then accelerated to the chamber. In this manner a steady state current may be driven between the cathode and chamber. For the pictures below, the chamber was filled with argon gas and that is what leads to the blue color of the beam. The beam also ionizes some of the background argon, hence producing a plasma for study.

electron beam in a uniform magnetic field

The electron beam is set at an angle with respect to a uniformly applied background magnetic field. This image has been post-processed to enhance the beam path.

Two of the images have been post-processed in order to more clearly illustrate the electron beam behavior. All of these pictures were taken with a simple, commercially available Sony digital camera.

In this picture the beam is being fired at an angle with respect to a uniformly applied background magnetic field. Cyclotron orbits are clearly seen. There is also a small dark region between the glowing red-hot cathode and the first blue light signifying the electron beam. This dark region is a physical effect and is not simply due to image processing. A sheath develops near the cathode because of the plasma generated by the beam. Collisions cause the beam to diffuse as it passes through the chamber and this leads to the blurred pattern seen further away from the cathode.

The beam appears to change color based on energy, but this effect is also not visible with the basic camera. More sensitive cameras (CCD’s) can be used in the lab and have proven useful in the past.

 

mirror effect of electron beam due to gradient in an applied magnetic field

The beam is fired into an increasing magnetic field. Magnetic mirroring occurs and the beam appears to stop and then move backward.

The lab uses a large dipole magnet to perform studies on electron behavior in the presence of a magnetic field gradient. This picture is taken from a window above the electron beam. The mirroring effect causes the beam to be reflected backward, which is only slightly visible as a light blue haze.

It is possible to position the dipole magnet in such a way that the cyclotron radius of the electrons decreases as they approach. The dipole is held next to a window on the chamber and the beam is directed approximately 30o from a geometrical line between the cathode and window. This produces a very nice tightening corkscrew. Unfortunately, I was unable to get a good picture of this effect.

beam reflected by a negatively biased grid

The beam is repelled by a negatively biased grid. A uniform magnetic field accounts for the apparent cyclotron motion. This image has been post-processed.

Biased grid studies are also performed in the lab. In this image the electron beam is directed toward a conducting mesh (not visible) that has been biased negatively with respect to the chamber. This produces a potential barrier that the electrons must overcome in order to reach the grid. The case shown features a grid with potential of -150 V and a beam of approximately 120 eV energy. This beam is not able to overcome the potential barrier of the grid and is fully deflected.

A lot of interesting physics can be observed with a very simple setup. Technically speaking, however, the setup is difficult in the beginning, such as when designing and building the vacuum chamber and gathering all of the accompanying equipment. The PHYS 180E lab at UCLA has been designed, built, and maintained by the Stenzel group. More information about their work may be found at the following location: UCLA Basic Plasma Physics Laboratory.

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