TJIIRRS: Number 22

(21 May, 2011, ff)

Some time ago we were able to acquire, on eBay, an item that is clearly the head of a very small excimer laser. It has room for two preionizers; but when we got it, only one was present, and the other edge of the cathode had a white deposit on it that I was uneasy about.

           

(The deposit is not visible in these photos.)

For quite a while I didn’t do anything with this structure, but it has been on my mind a lot, and a few days ago I dragged it out and started thinking about it in earnest. I usually avoid commercial parts, but here was this very fine piece of equipment, lacking only a few capacitors, a switch, and some gas; and I do have a project or two in mind that could use it.

I have started to build a box for the head:

I have also made a new preionizer for it:

(I got some 5-mm borosilicate tubing on eBay. The vendor specified it as having 3.4-mm ID, but a 1/8" rod failed to fit inside it, and in fact the square-cross-section 3/32" brass tubing that I ended up using only fits in some of the pieces. Go figure.)

At least to begin with, I am expecting to build a Charge-Transfer circuit with a 35nf capacitor as dumper and a 6nf capacitor as peaker. These are commercial caps, with moderately low ESL, and although they may not produce much output if I run the head as a nitrogen laser, they should provide proof of principle. The switch is likely to be an old EG&G GP-70 spark gap, which should run well at about 20 kV.

Here’s a mockup or two:

           

I will also be trying to run this head as a nitrogen ion laser, btw (output at 428 nm), but there is no guarantee that it will work. We’ll see. First I must obtain a clean discharge in plain helium, and in order to do that I have to have a box that will hold the helium, and the usual electrics.

I have also bought some mirrors on eBay; they are listed as being for 326 nm, probably an Argon or Krypton ion line, and I am hoping that they will have significant reflectance at 337.1 and also at wavelengths a little longer than that. The channel of the laser is smaller than half an inch, so I should be able to use 12mm and 12.5mm optics if I’m careful about how I mount them.

(2011.0522, late evening)

I have nearly completed the box with the head inside it, and I should soon be able to hook up the electrics, tape temporary windows on, and fill the device with helium.

Here’s a photo, taken after I put pieces of microscope slide on as windows:

(2011.0523, early morning)

My initial attempt, late last night, produced bright sparks, but I am fairly certain that there was still plenty of air inside the box. The spacing between the electrodes is large enough that it would require considerable voltage to get a good discharge in air, so the sparks are not too surprising. I am hoping that 20 kV on the dumper cap will be enough to get a discharge in helium with just a little nitrogen, but I will have to move the laser closer to the helium tank for that. (I have already fixed some minor leaks.)

(2011.0523, late morning)

A retest provides a bit more information. This time I got cloudy sparks along the channel, a good indication (I think) of insufficient voltage but otherwise appropriate performance. This is fairly easy to ameliorate: I change from a single cap in the dumper to a Marx bank with 2 capacitors. I have already built this structure for the “minimalist” dye laser that I built a while back, and I have all of the requisite parts. They may even almost fit on the base...

(2011.0523, afternoon)

...Almost. I did manage to get everything crammed on, as you can see in this photo --

-- and ran a brief test. This did not provide the expected result; the sparking is just as bad with the “Marx dumper”, if not worse. Need to think about this and maybe test again before I proceed further.

(2011.0523, early evening)

I have now tested this setup with helium direct from the tank, and the results were identical. Something is not right here, and I am not yet sure what it could be.

[More as it transpires.]

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My email address is a@b.com, where a is my first name (just jon, only 3 letters, no “h”), and b is joss.

My phone number is +1 240 604 4495.

Last modified: Thu Jun 23 16:07:32 CDT 2016