Electron Beam Formation in a Glow Discharge Tube

   
				
The original way of producing beams of electrons (or cathode rays, prior to the formal discovery of the electron) was from a glow discharge. Braun's original oscilloscope tube used plasma produced electrons as the beam source. A bit later on Edison discovered thermionic emission (electron production from a hot filament) which led to the era of the high vacuum electron tube. The electron component of plasmas is, of course, still vital to plasma production.

This video shows the production of electrons from a plasma in a simple glow discharge tube. As shown in the figure below, a cathode electrode is at the top. In the main section of the tube is a grounded support rod that has affixed to it a piece of phosphor screen. The Zn-CdS screen is very sensitive to electrons (and x-rays) and will glow bright green when excited. Between the screen and the top of the rod is a little loop of wire arranged so that it will cast a shadow on the screen. The top of the rod serves as the anode and the region below is essentially devoid of electric field (i.e. it's a drift zone). The camera is viewing the apparatus from the left.

When a negative voltage is applied to the cathode (about -4 kV in this case) a glow discharge will form when the pressure is reduced to a few Torr. As the pressure decreases further the discharge becomes less bright (lower gas density). Eventually the screen starts to glow as electrons, escaping the plasma, are accelerated to the screen. Initially the screen is uniformly illuminated but as the pressure continues to decline the image of the wire loop can be seen as a shadow on the screen. At this point the glow discharge itself is very dim while the screen is glowing brightly. The relative crispness of the shadow image is due to the increasing mean free path for the electrons. Eventually the screen dims indicating that the pressure is too low to sustain a glow discharge. Toward the end of the video clip I raised the pressure slightly to bring back the beam and image.

The beam can be steered and the image distorted if a magnet is brought into the viscinity of the beam in the area below the wire loop and above the screen. A "donut" magnet placed around the tube can be used to enlarge the image of the loop thereby illustrating the basic principle of the transmission electron microscope.

The clip lasts about 60 seconds. The first 20 seconds or so are pretty boring as the pumpdown is in progress and the tube has not yet lit. Be patient!


					 
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