After building a flyback high voltage supply, which I will describe in a later post, I was experimenting with putting the electrodes into thin glass tubes. This produced much longer arcs due to the concentration of the ionized air.
To get an even better result, I tried evacuating the tubes with a syringe. To seal one end of the tube I placed a thick wire in one end and put a drop of hot glue around it. Over the other end I put a piece of PVC hose and pierced the wire through it into the tube. The entryhole was also sealed with hot glue. To ensure a tight seal some air is sucked out of the tube while the hot glue is still liquid. This way the glue fills the holes.
As the glass tubes only have a volume of about 5ml and the syringe 65ml, one pull reduces the pressure inside the tube from 1bar to about 0.05bar. (p*V = const.)
When the voltage (between 20kV and 30kV) is applied to the electrodes, a continuous channel of violet (the photo does not reproduce the color correctly) nitrogen plasma is formed inside the tube. The plasma quickly heats the glass and the hot glue, so don’t apply the voltage too long or the seal will break. Magnetic fields will deflect the plasma channel.
These small tubes quickly got boring, so I decided to make a bigger version with a Ø30mm L=100mm plexiglas tube. To seal of the ends, I turned caps on the lathe. The caps were then fitted with a hole for the electrodes and a hose connection. The holes around the electrodes are again sealed with hot glue. To get the caps airtight, o-rings were used.
To get a low enough pressure inside the tube, the syringe has to be pulled several times. Between each pull, the tube has to be closed. This is achieved with an electric valve.
When the pressure inside the tube is low enough and the voltage high enough, a glowing channel of plasma will form. Due to the heat it generates in the residual air, it will bulge upwards. This will heat the tube and slowly melt the plexiglas. The plasma stream can be deflected with strong magnets or by influencing the electrostatic field around the tube, i.e. by placing your fingers near or on the tube.
With one of the smaller tubes I put a drop of ethanol (C2H5OH) inside the syringe before pulling the air out. This produced a thin ethanol vapor inside the tube and caused the plasma to glow blueish-white instead of violet.