A Joss Research Institute Report:
The Questek Impulse 4530 Excimer Laser

(04 June, 2004)

Today we took delivery of a Questek excimer laser, which was actually manufactured by Visx. (If I’ve capitalized that incorrectly, please forgive me; if I find out that it’s wrong, I’ll try to correct it.) I believe that this box was made in 1993; it is much heftier and more solidly made than the room-pressure nitrogen laser that I’ve been working on lately. In fact, this thing must weigh a good 500 lbs. It is rated for the usual excimers (F2, ArF, KrF, XeF, Br2 perhaps, XeCl, probably XeI, and maybe one or two others that are far less commonly used), and for both N2 and CO2, with pulsewidths from 5 ns (N2) up to 1 microsecond (CO2??) and pulse energy up to 4 Joules (I’ll believe it when I see it). I don’t know what kind of optics it has in it, but I’m sure I’ll find out.

(Note, added in proof: Milan Karakas suggests that 98% He / 2% N2 would be an appropriate mix for running this as a nitrogen laser. That sounds about right; but of course the design is not at all optimized for nitrogen, and the output would probably be only a millijoule or two. See "BotEC" estimate of electrical pulsewidth, below.)

I have only just begun to look inside the laser, warily and with mask and gloves on: it was almost certainly used with either F2 or NF3 as one of its feed gases, and it has a peculiar smell, somehow throat-catching. I got a tiny whiff or two of that smell, soon after the laser arrived, and I seem to be slightly hoarse now. I don’t think I want to get any more of it, thankyew.

There are several gas inlets on the box: He (the main carrier gas); Ne (better, if you can afford it, but I expect that most people mix it into the He); "Rare" (usually Kr for KrF, or Xe for XeF or XeCl or even XeI, though I’d guess that XeI is uncommon); and "Halogen", which has ucky-looking stains around it. There isn’t a high-pressure N2 or CO2 inlet, because there isn’t any spark gap: lasers of this sort are switched with thyratrons.

The laser came with an external control box, but no manuals. This is to be expected, but with a device this arcane [not to say fiendish] it is somewhat disturbing. I will be checking with the actual maker to see whether they archive any manuals. Unlikely, but worth checking.

Here are some early photos. First row begins with the device, looking at its output side. Then the front panel of the controller, and then the ID tag. Second row is the left rear external view, followed by two views of what’s inside it. Third row is the right rear external view, and the HV power supply controller that is just above and inside it. Fourth row shows the central HV section; first the left side, then the middle, then the trigger board, and finally a shot from the right that shows the number of the thyratron. I did not succeed in getting any of the numbers printed on the brown caps -- they are cleverly oriented to hide their specs -- but I have a strong suspicion that they are 2.2 nf @ 40 kV. I will try to get a better shot, and perhaps we’ll see whether I’m right. (Note, added in proof: I forgot to mention that there are 24 of these caps; they are 2 nF [Doctor Memory -- I conflated "202" with "2.2"], and in fact they are identical to a large number of caps we’ve acquired on eBay over the last few years. Either this was a very popular cap for excimer lasers of various sorts, or quite a few of these particular lasers have been scrapped out.) I’m not really certain about the peaker caps, but the choke on the HV line uses two that appear, at least superficially, identical, and they are 780 pf @ 30 kV.

(Back to material I wrote earlier...)

Given that we know what the caps are, and given the fact that what we appear to have here is a straightforward dumper-peaker circuit (with some sort of preionization, which I haven’t been able to see yet), it should be fairly easy to figure out the electrical characteristics. At 35 kV, 48 nF holds 29.4 joules, unless I’m miscalculating badly. If we ignore any real complexities like impedance, and just pretend that everything is overly simple, we can guess that the overall inductance of this circuit is on the order of 100 nh, and the electrical pulsewidth should be on the order of 200 nsec. It may actually be faster; about the only way to tell is to put a photodetector on the output and fill the laser with helium. That should give us a nice flash that doesn’t stay lit very long after we stop exciting it. I won’t be doing that, however, until we are fairly sure that it’s safe to do so.

I’m guessing, also, that this laser was scrapped out nonfunctional, though we have no easy way to determine that unless we can find someone from the lab that used to own it. It’s distant, but not impossible. I may have a contact. (We Shall See What We Shall See.) If it’s nonfunc, that could either be because a cap blew out or because the thyratron reached end of life. I don’t know which would be worse...

(2004 June 13/14)

The laser now has power, and I’ve connected it to the helium tank. The temptation to power it up was almost unbearable, but I was alone, it was after midnight, and I concluded that safety should rule the night.

Here are some newish photos. The first three show the back of the box, with the cover off. The computer is inside the black box on the right. The next three show the computer, its battery, and what I hope is the place where the little controller box plugs in. I have it plugged in there, and when I power up the laser, we stand at least a chance of finding out whether that is, indeed, the correct location. Sorry about the blurriness of that last shot -- it was a slightly long exposure, and I was holding the camera in one hand & the white LED flashlight in the other. This is an unresized crop from the middle, and is probably better viewed as it is here on the page than in 800x600.

That’s about where things stand now. More after I try to power up the machine, though if that’s as anticlimactic as I fear, it may just be a single unhappy sentence.

(2004 June 15)

Well, it’s not all good news -- the laser has not shown any sign of lasing yet -- but it’s not all bad news either. I powered it up, and the controller continued to say "Laser Not Communicating", so I looked around carefully. Inside the optics compartment at the output end, there’s a set of connectors with a legend, and one connector was labelled "LCD". That, of course, turns out to be the right one:

We’re not out of the woods yet, but I have succeeded in flushing the gas (three times, as it happens), and even in replacing the gas fill with Helium and Nitrogen. Unfortunately, I was unable to get the Nitrogen in through the "Halogen" port: either the line is blocked, or a solenoid valve isn’t working. The laser doesn’t really want to believe in Nitrogen, btw, despite the notice on the side that the pulsewidth can be as short as 5 nsec. Here are the allowable choices:

Perhaps worse, the laser doesn’t really show any sign of firing, despite the fact that the shot counter increments, and I’m fairly certain that there’s a problem in the high voltage section. (These next two are from different runs, which is why there’s a difference in the voltage setting.)

For the moment, that’s where it stands; I’m going to tear down the dumper cap soon, and see whether perhaps some of the doorknobs in it are visibly damaged. In the meanwhile, I leave you with this tiny wonder:

(23 June, 2004)

Last week I tore down the dumpers, and tested them... each cap, individually. There’s nothing wrong with them.

When I was putting them back in, I took a hard look and realized that some idiot had connected the HV to the grid of the thyratron instead of the anode (!). This may have damaged the trigger board; I’m not fully certain yet.

I moved the HV line to the anode of the thyratron, and succeeded in getting something vaguely resembling a pulse, but the machine didn’t lase, and there was a nasty "bang!" noise. The second pulse made an even louder bang. I shut down the power and thought about it for a while.

Then I checked some things. The reservoir heater voltage is adjustable, so I set it to 6.3V. The cathode heater voltage, on the other hand, is not adjustable, and was 6.93V, outside the permissible limits. (The absolute max is 6.8V.)

To make a long story short, I then found our HY-3025 tube, checked the specs, and used 6V batteries to run the heaters so I could test it. (The cathode heater takes 12.5A, more than I could provide with what we have on hand.) It turned out to be in good shape. This also obliged me to test one of our EG&G trigger units, which worked just fine.

When I swapped in the HY-3025, however, the cathode heater voltage was still 6.93, so I powered down the laser and went looking for power resistors. I figured that at 12.5A, 0.05 ohm should drop a little over half a volt, taking me down to 6.3V, which is where I wanted to be. Couldn’t find any 0.1-ohm resistors, however, so I was obliged to execute Plan B -- it occurred to me that a nice hefty diode would have about 1/3 to 1/2V forward drop, so I took apart an old Mac II power supply ...and found a heatsink with some very nice paired Schottky diodes on it, rated 60V, 30A.

(Rude ASCII diagram follows.)

   o---->|---+---|<---o
             |
             |
             o

I connected each of these so that the two diodes were in parallel, and then I connected the resulting dual diodes across each other, to pass AC. I figure that as long as each diode is rated for 30 amps, two in parallel shouldn’t have any trouble at all with 12.5 amps, and the heatsink is superfluous. I left it in place anyway, because it holds the devices nicely.

With this thing in the line, I measure about 6.4V on the cathode heater, which is bearable.

Having resolved that issue, I had the laser evacuate itself. Then I refilled it with about 80 mBar of Nitrogen, and added Helium to a final pressure of 1020 mBar. Then I crossed my fingers and pressed the Start button on the controller box.

It lased. Twice. I wasn’t absolutely certain, though, because the target was only about a foot from the end of the box -- it could conceivably have been just random light from the discharge. Unlikely, but certainly not impossible.

It made a "bang" noise at the second pulse, so I turned it off. Tried again, and got two fairly loud "bang" noises, but no lase. Hmmm... Had it pull down the pressure to about 880 mBar, moved the target back to about 10 feet, and tried again. This time it lased once, went "bang" once (and lased, but weakly), and then displayed its "Too Many Energy Dropouts" error. I got a few more pulses out of it, but it was clear that something was amiss.

So much for yesterday.

This evening, I made the obvious test: opened the cover, took the inner cover off the HV section, and watched the action (with my fingers in my ears). Most of the time, it arcs from the grid of the thyratron to the cathode; sometimes it arcs across one of the dumper assemblies. I take this to be an indication that either we aren’t getting a trigger pulse, or the trigger pulse is happening much too late in the game; the caps are overcharging, which is bad for them, for the thyratron, and probably even for the trigger board.

My next move, I think, is to check the trigger circuitry, and to see whether I can figure out what’s wrong. This probably won’t be easy, but at least I’m fairly sure that there’s a laser underneath all the problems, if I can only clear away the rubbish and get to it.

Contact Information:

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

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Last modified: Tue May 9 11:59:35 EDT 2017