High pressure HGM5 - I'm screwed, aren't I?

[buffo]

Well, crap. I had a spare HGM5 that I pulled out of storage to play with over the weekend. The PSU had some sort of general fault, so I hooked it up to my other HGM5 supply, which until that point had been happily powering my (smaller) 903 tube.

Fired it up, and it started with the first tick of the ignitor... Hooray! But then it winked out after about 20 seconds. Managed to light it again, but it winked out again after just 10 seconds. Hmmm... Then it wouldn't re-light at all.

I shut it down and called Bruce Rodgers (medical laser guru) to ask him what might be the problem. He suggested checking the tube voltage right after it started. Now, bear in mind that an HGM5 tube normally runs around 185V or so...

I ended up waiting a day before I tried it, and to my utter astonishment, the tube voltage was 248 volts when it started! (This was at around 7 amps of tube current; basically it was at low idle.) And as it ran, I watched the tube voltage climb rather quickly to 310 volts, at which point the tube winked out again. (I was ready for this, so I was able to disconnect the meter before the starter could fire again, which would have otherwise destroyed my meter.) It took maybe 15 seconds to wink out, and it would not re-start. (The ignitor just kept ticking away.)

The thing was lit down the bore, and I had a visible beam, so everything else is fine. But evidently it's gone high pressure on me while it's been sitting these past 9 months. Crap!

Bruce suggested that I dial the current way back and see if I can get it to run continuously so I can bury some gas. Unfortunately, even at 5 amps of tube current (the lowest I can get it to run), it still winks out after about 20 seconds, and it won't re-start until I shut it down and let it cool off. (Not that it's very warm to begin with, mind you. The blower I'm using is oversize, and the exhaust air is more or less ambient temperature.)

So unless someone here has another idea, I think my only option is to have this sucker pumped down. Which sucks, because it's going to be expensive. I don't want to toss it though, because a year ago this tube was making 3.5 watts, and it doesn't have a lot of hours on it. If I can solve the pressure problem, it will be a great laser to keep.

Adam

[mixedgas]

248 seems way high... I'm betting your getting some oscillation on top of the pressure. Is the discharge a nice pure argon color or is there some orange (nitrogen..) in it ?

Do what Bruce says, and see if the internal gettering cleans it up. Some ALC/HGM products have a getter strip that is activated by the cathode heat and plasma, but it has to be running, not just the cathode heat.

Steve

[buffo]

I can't get the tube to stay ionized for more than 20-30 seconds, so I don't think the getter will do me any good. (Unless it's hot even when just the cathode is lit.) Once it winks out, I have to wait several hours (if not a whole day) before it will light again. (I even tried "tickling" the cathode end bell with my BD-10A oudin coil - no dice. Once it winks out for good, there's no re-starting the tube.)

The plasma discharge looks normal to me. There is some orange visible before the tube lights, but I'm assuming that this is just the normal glow from the cathode. I can't say that I noticed any pronounced orange color in the plasma though.

Bruce suggested running it to bury some gas, but in order to do that I need to get it to run for longer than 20 seconds. I can't figure out how to do that, because at 310 volts across the tube, I'm really close to the theoretical maximum voltage for direct-rectified AC. Unless I put my PSU on a variac and jack up the incoming line voltage a bit... But Bruce warned me that doing so was a good way to blow the filter caps in the PSU, as they're only rated for 350 volts. So I'm screwed either way.

Crud...

Adam

[Xytrell]

Would another heating method work?

[Laserman532]

STEVE PLEASE CONFIRM THIS METHOD.

I know nothing of the HGM-5 or what ever it is...BUT...

With all sp glass plasma tubes if a tube was high pressure, and we were positive that the magnet was working properly. We would "flash the cathode" (usually due to the customer over filling it) (it also sometimes works with water vapor contamination from long term storage) It was also a last ditch effort to avoid the customer having to buy a new PT. (note: will not work on a PT that is up to air)

it would work like this...

We would disconnect the cathode wires from the terminal block and connect the cathode wires to a set of jumper cables. We would remove the battery from our rental car and drag it into the lab.

You connect the negative cable to the battery using the jaws (+ or - dosnt really matter) to one side of the cathode. Then with the water flowing you touch the positive cable to the terminal post. The cathode will start to glow then glow brighter then all of a sudden you will see the cathode FLASH, FLASH, FLASH, FLASH!!! (like a strobe light) Then you remove the battery connection and allow to cool.

Each time we did this we would burn off about 10 volts of PT pressure. We found it was a good idea to do it twice then hook back up the laser and run it normally to measure how much gas you burned off.

it works well with SP tubes...disclaimer...attempt at your own risk

[300EVIL]

Would another heating method work?

That's what I was thinking. Maybe drop the bare tube in the oven for a couple hours.

[Experimentonomen]

What happens if you try and keep the tube voltage constant ? Would current just drop until it winks out instead ?

Now i dunno if its actually possible to trick the psu into outputting a constant voltage rather than a constant current.

[MarioMaster]

The problem is that the psu is already outputting its maximum voltage and the current drops to nothing as the psu can no longer regulate, and increasing the voltage would most likely damage the filtering capacitors as already discussed.

Maybe he could make his own basic linear psu capable of outputting a higher voltage so the tube does not wink out, and run it for a while to bury some gas into the walls of the tube. AFAIK Pat's suggestion of flashing the cathode should only be done as an absolute last resort since it can destroy the cathode and tube.

[mixedgas]

You connect the negative cable to the battery using the jaws (+ or - dosnt really matter) to one side of the cathode. Then with the water flowing you touch the positive cable to the terminal post. The cathode will start to glow then glow brighter then all of a sudden you will see the cathode FLASH, FLASH, FLASH, FLASH!!! (like a strobe light) Then you remove the battery connection and allow to cool.

Each time we did this we would burn off about 10 volts of PT pressure. We found it was a good idea to do it twice then hook back up the laser and run it normally to measure how much gas you burned off.

it works well with SP tubes...disclaimer...attempt at your own risk [/QUOTE]

That is so glass SP its not even funny... And I understand it is something you only do for customers you really like or feel sorry for before you tell them they need a new tube. Or if your going to have to sit in a hotel for 3 days waiting for the new tube to come from the factory... He could try it as a last resort. Buffo, this method works by evaporating the barium emission layer from the cathode and using it as a getter. When the barium cloud gets hot enough, it conducts between the cathode leads like a arc lamp, hence the flash.. It is known not to work on Coherent, and I've never used it with any luck on anything other then SP Glass.. And even then its a last resort. If your cathode is depleted, sagged a little, or not barium based your hosed.
That is a end of the world I cant afford a repump and I have nothing to loose case...

It sounds like a residual pocket of hydrogen has worked its way loose from the brazing.. The Conundrum is hydrogen will only "burn off" by reacting with the cathode/other materials if the tube is lit.

leave the cathode on for 24 hours with no HV.

The reverse bias illusion:
Then try removing the tube from the laser wiring, and hook a hene supply with a 50K ballast resistor in series with the anode lead. Use the smallest ballast value that will not blow up the psu, ie higher current is better. Hook the anode lead to the CATHODE end of the argon tube, ie reverse bias it, and let it run a week. (crude ion sputter pump, will drive H2 but not the heavy argon) rumored to work sometimes when the moon is full, jupiter is in the seventh house and aligned with mars... At least the hene supply or rectified neon transformer will only cost you 10 cents a day to try.. Again, strictly rumored to work.. I had a hard to fire 60X clean up exactly ONCE from this trick..

I'm weary of telling you to bake the tube in a oven, as I do not know if you have contacted brewster windows or epoxed ones..

Steve

[Xytrell]

Can he heat just the tube? like wrapping around it one of those contraptions that prevent water from freezing in pipes? Or maybe even a propane torch with a deal of distance?