I’m afraid that it’s dredging the bottom of my memory to recall the details of operating the CU10, even though I built it myself on the Oxford Lasers production line in the first year of my DPhil. I’m afraid they’re temperamental beasts, though the control system is fairly straightforward in principle. Most of the control system was built from relays (partly because they’re more noise immune), so decoding the circuit boards shouldn’t be too difficult, though buzzing through the wiring looms could take a while.
I forget the exact start sequence, but you probably have to wait a while until all the interlocks and monitors are happy: these may include pumping down to an appropriate pressure, warming up the thyratron and so on. Some of the monitors/interlocks can be overridden by toggle switches on the circuit boards inside the power supply, but it’s probably not wise to do so unless you know what’s up and why.
The laser runs on pulsed discharge, and starts in the buffer gas (various noble gases work) that you need to supply at an appropriate pressure. This is continuously pumped through the laser tube, because at the temperatures involved the system is far from clean. I think there’s a pressure gauge built into the control unit to indicate the tube pressure. If you can strike a discharge, then the tube should warm up over a period of 10-20 minutes or so until it’s hot enough for a substantial vapour pressure of copper; this has a lower ionization energy, so the discharge switches to the copper, and the discharge will turn green.
The high voltage power supply and switching were, I remember, always a bit prone to failure. High voltage capacitors can break down, and the thyratrons can give up. You can probably replace the capacitors, though they’re likely to be tricky to find (do remember that they can store a lethal charge, so should always be kept shorted by a piece of wire when not installed in the power supply, where the job should be done by a ‘dump switch’ – but be wary of them at all times). I suspect that the thyratrons are no longer made, and they were always rather expensive, so if the thyratron has failed that’s probably it for that laser. The thyratron gets hot, so sits in a tank of insulating oil; the tank usually leaked a bit around the top, so if there are signs of significant leakage you might want to check the level – a messy job, I’m afraid. It wasn’t unknown for the high voltage transformers to fail either.
If the interlocks are happy, the transformer and capacitors still intact, the thyratron okay and the oil level sufficient, then the supply of an appropriate frequency from the built-in clock generator (a roughly 4” square box in the sloping front panel of the power supply) should trigger the discharge to run. With the big current spikes involved, the thing can be sensitive to self-generated noise, though I think the production models were generally fine in this respect; if the clock box doesn’t work, though, any suitably grounded pulse generator should do the job. The pulse repetition frequency shouldn’t matter too much – say to within a factor of two – and I think 8-10kHz was a typical range, though it was possible to change the internal circuitry at manufacture to optimize for other frequencies, so you might have to play with this.
If everything works, then you still have to watch some of the adjustable parameters:
pressure: too high and the discharge won’t strike; too low and you may get the same problem, or (if memory serves) just a rather low power discharge that won’t heat the tube. I *think* that 20-40mbar was about right. A higher pressure also limits the migration of copper from the discharge region. If you run at too low a pressure, you might also find that the discharge runs from the tube back down the gas lines to the power supply, which isn’t generally damaging for short periods at least, but can be a bit disconcerting.
repetition frequency: I forget the real limitations here, but I think it was that, since each pulse delivers roughly the same energy, too high a frequency overloads the dc hv power supply, and too low a frequency means insufficient heating of the laser tube. (If you can get the thing lasing, I seem to remember that the tube temperature is, under normal running conditions, reflected in the green:yellow ratio, green being cold.)
If you have a bright green/yellow discharge, and the tube windows are reasonably clean, then getting the thing to lase is straightforward because the gain is very high. If you twiddle the back (100% reflecting) mirror, then you should see a bright region in the output where the amplified spontaneous emission from the first pass down the tube (from output to back mirror) is aligned with the second pass from back mirror to output. You can then align the output mirror (uncoated glass/silica) to reflect this back down the tube. When the two are parallel, and normal to the tube axis, it’s as good as it gets.
The beam is, of course, rather bright, so we used to let it fall onto a plate of black-anodised aluminium, and view through dark smoked Perspex. If you get the full beam, or specular reflection of it, into your eyes, it’ll blind you; diffuse scatter from a distant black object, suitably attenuated, should be okay – though you should do your own calculations and remember that you’re dealing with ~10ns pulses, so there’s an acoustic/percussive damage mechanism as well as simple heating. If you’re used to Q-switched doubled YAGs, though, you should be okay: the CVL pulses are longer and less coherent.
Two things that are rather routine problems are the copper and the windows. The laser needs copper to operate, and it tends to have a finite life because of the gas flow and migration within the tube. Ordinary electrical copper wire, cut into a dozen inch-long bits and placed at regular intervals along the tube, should be fine: we had a loading tool made by cutting a piece of plumbing pipe in half lengthways, so that you could place the copper in it, slide it into the laser tube, and then rotate the pipe to leave the copper in roughly the right place. If the existing copper’s been in there a while it could well have oxidised and be reluctant to vaporize, so new copper is probably required. The old stuff can build up on the bottom of the tube and partially obscure the beam; you may be able to chip it off gently, or just ignore it. (The other thing that can obscure the beam is if the tube has been hot enough for the tube liner to sag.)
To load copper, you need to let the tube up to atmospheric pressure, isolate the power supply, ground the big air-cooled electrode connections at the ends of the laser tube, and undo one or both of the rings that hold the windows onto them: do this evenly, as you would the head gasket of a car. Beneath the window is an o-ring, which might need to be replaced by now (it should be a standard size). The windows should be standard 6mm silica, and you can probably clean the current ones somehow if you’re not too fussed about the beam quality. I forget what we used to use, I’m afraid. If you need new ones, the company Comar can probably still supply them.
Generally, the electrode blocks were lined with some ‘electrodes’ made by punching holes to leave spikes in a piece of thick metal foil / thin plate of some sort that had been bent round to form a rough tube that slid into the blocks. I forget what material we used, though titanium springs to mind. Without these – or if they’re there but too far eroded – the discharge should still start by might need a higher voltage or lower pressure to compensate for the lower electric fields without the spikes; the discharge will also erode the aluminium of the electrode block faster than the harder material of the electrode foil.
That’s about all I can remember, I’m afraid. There are enough difficult parts in the lasers (electrodes, thyratron, transformer, capacitors) that there’s a good chance you won’t be able to coax the thing into life at all – but good luck trying, and do send me a photo if you get it running! In terms of hazards, the most serious is definitely the high voltage power supply; then laser beams, high temperatures and mains voltages; and you should be careful with the bottled gas and tube pressure too.
Creaky Old Award Winning Bastard Technologist
Originally Posted by mixedgas
Posting Permissions
Reply With Quote