I need a multiline Ar laser to ionise the pressurised Argon gas in the LIBS target chamber for RELIPS experimentation [L16]. http://www.dzlaser.com/Laser_Physics

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To avoid over-pressure, I run all gas lasers once a month for at least 1 hour. I had been doing this at minimum intensity (to preserve their life) for several years when one day in 2022 this 500M laser wouldn't start. Eventually it did start and I measured its output at over 500mW. I then ran it for about 4 hours at minimum power, thinking the extra time would de-pressurise it.
 

However when I tried to start it a month later, it again would not ignite.

Removing its cover I found unlike the 50S, this one has a pressurised Ar canister in it that 'drip-feeds' the resonator, and it is this that leads to over-pressurisation if the laser is left unused. It occurred to me it had been a mistake to run it at minimum power.

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I tried to help it by ionising its gas using the 50S but this didn't work. Reading up about Ar lasers, the most common way of ignition is by a high voltage spike on the anode. I also read this is often derived from the mains supply. It occurred to me if I increased the mains supply voltage I would also increase this striking voltage. This is not something I would want to do often as it is overstressing the tube, but it was now my last resort. I then hooked it up to my 13 Amp lab variac which has an output from 0-270Vac.

In [Repairs: Variac Repair & Add Meters] I discovered my UK mains supply is typically ~250Vac instead of the nominal 230Vac.  I set the variac to its maximum which I measured at 287Vac and to my relief it started. I reduced the variac to 240Vac and measured max power was now 485mW way above the rated 300mW.

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I then ran it for 12 hours at what seemed a reasonable 120mW but it again failed to start the next month.

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I had also checked it has a BS1352 compliant 13A fuse fitted, which is capable of carrying 20.8A and should only blow at 24.7A. [U10]: A BS1362 fuse must carry 1.6x its rated current; the fuse must blow at 1.9x its rated current within 30 minutes. This is just as well because when I momentarily put the laser up to max power it drew 20A. It would be catastrophic if the fuse blew and the fan stopped as normally the fan runs for a good couple of minutes after the beam is turned off. I fitted a 15A fuse: carry 24A and blow at 28.5A [U10] and decided to run the laser off the 32A oven socket in my kitchen

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I left it for another month before trying to start it normally off UK mains (250Vac) but once again it failed to ignite. I also tried it on the variac and it again failed to start and I wondered if it might be a good idea to leave it for a few days. Up until this point I had been running it in a centrally heated room, although the temperature was only around 20°C.

A fortnight later I tried again with the variac at 287Vac on a cold Spring morning with all windows open when I imagined pressure would be lower [Q10a]: Gay-Lassac's Law, and this time it fired up. I reduced the variac voltage and set power to the maximum 13 amp current my mains variac could carry (higher voltage would be lower current but again I wanted to minimise stress on the tube, so I lowered the voltage).

I measured 270mW at 13A. I then left it running at this level for 12 hours. For the first 30 minutes there were fluctuations and I had to manually adjust the power to stop thermal runaways but eventually it settled into a pattern of slowly decreasing current over an hour or more from ~13A to ~11A, at which point I increased power, noted power was now greater than before, then the current began to slowly increase and I lowered the power, until once again I had equilibrium.

Finally after 12 hours I measured maximum power at 436mW. Maybe it's over-pressure as it's is only rated to 300mW, but this is about the power I got when it first arrived, although it dropped to ~270mW when I ran it for more than a couple of minutes. At the time I was unfamiliar with Ar lasers and was worried it would keep going down the longer I ran it at high power, much the same as some high power LDs I bought from eBay China (which of course don't have a pressurised gas reserve inside them).
I assumed it had been slowly rising over the several years I've owned it even though I'd run it every
month, albeit at the minimum power setting. However the next month it again didn't start.

Having tried everything else, I left it for 6 months and in November 2022 tried it again and it fired up first time on UK Mains. I forgot to measure it at the start but at the 20 minute point once I'd taken it up to max power then reduced it to minimum, the minimum output increased from 65mW to 74mW.

20mins: Min power measured: 65mw, max power measured: 463mW

60mins: Min power measured: 78mW, max power measured: 474mW  
 

I left it for another 6 months but it didn't fire up and it is now officially dead.

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Below, the Laser Physics Reliant Ar lasers range and their basic specifications: