The Science Committee’s nitrogen laser currently is assembled in a configuration that we can test. The current configuration includes a pair of doorknob capacitors attached to it as the primary bank, but @Tron is making a fibroglass-and-epoxy capacitor, too. One of my hopes is that we can test several alternative configurations, to see how they function (or not).
In the current nitrogen laser configuration, I get arcing (stringers) across the active medium. This is not desirable. Although I do see occasional stringers half-way down the tube, I would like to have a more uniform discharge across the electrodes. The laser needs discharge along the entire length of the active medium. From my reading, there are several things likely causing this situation, chief among them the ratio of voltage to chamber pressure and spark gap spacing. Our tube has a fixed, 1-cm spark gap, and Andrew has lent us a 15 kV neon sign transformer. That implies that we need to drop the pressure inside the laser cavity to less-than 75 torr (on the engine vacuum gauge we are using, that’s more than 27 inches of Hg, or more than 66 cm of Hg). I tried to reach that last night, but then the tube apparently sprang an air leak.
An alternative to improving the vacuum sealing is to add helium, so the nitrogen remains at a small partial pressure. I’ve never favored that method, as it adds to the expense and complexity of the design, but if someone wants to experiment with it, that’s fine. It can be made to work. However, I think we ought to correct the deficiencies in our design, too.
We are using a square tube made from four straight pieces of plastic, which is pretty normal, but I’ve been advised that using a circular tube cut in half is much easier to seal. The trouble is, it is difficult finding a plastic tube of sufficient wall thickness in the size we are using. Our options seem to be either to make our own circular tube or use a smaller circular tube or figure out how to seal our square tube for the vacuum we need.
Our tube is on the large side of the range of nitrogen laser vacuum cavity typically used, meaning that we have more volume to evacuate than usual. It may take our current vacuum pump–a refrigeration compressor run in reverse–a long time to pump down the chamber, if it is able to do it at all. It should be able to do it, as these allegedly can get down to 10 torr or lower, but we aren’t getting anywhere near that. The main problem with the vacuum, then, is sealing the tube sufficiently for our current system to take the vacuum down where we need it.
Another issue is the composition of the windows used on the laser cavity. The original plans specify using microscope slides, which are thin pieces of glass. The trouble with glass is, it absorbs most UV radiation, and the thicker the glass, the more it absorbs. That’s pretty challenging when your laser is supposed to produce a beam of UV radiation! Specifically, we are trying to generate a laser beam of wavelength 337.1 nm. I estimate that it would be somewhere around 750 kW to a megawatt pulse. The windows currently on the laser tube are thick pieces of plate glass. That may not allow much of the beam to pass through it, especially as they currently have smudges of an unknown substance directly in front of the beam path. We could be generating a laser beam and never know it, because it cannot penetrate our output couplers.
If you would like to read a technical analysis of the electromagnetic absorption of silica glass, Harvard University has a paper available free online:
“Optical constants of silica glass from extreme ultraviolet to far infrared at near room temperature”
http://adsabs.harvard.edu/abs/1965JChPh..42.2623G
Another source on the topic is Sam’s Laser FAQ:
“Glass microscope slides at 20 to 30 degree angle (not Brewster windows) to minimize reflections back into the laser cavity. Note: The material used must have high optical quality and be fairly transparent to the 337.1 nm UV wavelength. Microscope slides are decent quality and thin enough not to absorb too much UV (just over 5 percent for a 1 mm thick slide). A higher cost alternative would be a flat quartz plate which will absorb even less UV (less than 1 percent for 1 mm thickness). DO NOT use ordinary window glass (low quality and absorbs too much UV) or plastic (totally opaque to UV). (From the Melles Griot 1997-98 Optics catalog: Approximate transmission at 337 nm for 1 cm thick Crown glass (probably similar to microscope slide glass): 55 percent; fused quartz: 93 percent. For 1 mm thickness, the transmission will be the 10th root of these values.)”
“Home-Built Nitrogen (N2) Laser”
http://www.repairfaq.org/sam/lasercn2.htm
My reference concerning the appropriate vacuum pressure is
“The TEA Nitrogen Gas Laser”
http://technology.niagarac.on.ca/people/mcsele/lasers/LasersTEA.htm
He states on another page that, “For a low-pressure design with a 1cm gap and an operating voltage of 15kV, the optimal gas pressure should be around 100 torr.” Our current system reaches 100 torr, but struggles to reach 75, so we are right on the edge of operation.
http://technology.niagarac.on.ca/people/mcsele/lasers/LasersN2.htm