Bathing The HeNe

[pschlosser]

We can bathe our older HeNe tubes in Helium, and rejuvenate older tubes with weak output.

I sometimes read through Sam’s Repair FAQ when I want a break at work. I reread the same sections, over and over, and my understanding (and retention) is improving. But a little knowledge can be a dangerous thing. I read something over the weekend that has me excited.

I have some HeNe tubes that are near end of life. They have low output, one at 20uW, yup microwatts. I have a red and a yellow HeNe dating back to 1987. These are the common 14-inch soft-seal tubes mounted within 1.75-inch back aluminum tubes. I concede these tubes show some tiny contamination. One emits a slight magenta glow. The magenta, in my mind, implies blue in the spectra, and blue, sadly, can mean N2 and O2 contamination. It is a very faint magenta, and I am optimistic. But I suspect there is also a lack of Helium. Of all the atoms we are discussing, Helium may be the tiniest, and as such, may leak from the tube at a higher rate than the Neon leaks out, or the O2 and N2 leak in.

According to Sam's FAQ, the concentration of Helium and Neon in these tubes, even when new, was pretty low. Sam’s FAQ suggests that a foot-long tube with moderate vacuum has only about a cubic centimeter of gas in it. In the section on rejuvenating, the FAQ suggests soaking the HeNe tube in a garbage bag full of balloon Helium for a couple days at a time can diffuse Helium from the high concentration in the garbage bag back into the HeNe laser’s low concentration. Thereby, putting lost Helium BACK into the tube.

When I read that, I thought “Could I rejuvenate these old tubes?”

But there is a caveat, a warning. There are two types of Helium gas. He-3 and He-4. He-4 is common, and found in most balloons. He-3 is kind of a rare isotope, and I may need to hunt far and wide before locating some cubic feet of He-3.

The question in my mind is, what type of Helium is found in these tubes? Does anyone have data on the history of HeNe gas tubes, and the mixtures used therein? A few decades ago, were makers using more He-4, and only began using He-3 a decade ago? Or vice-verse? How might I begin to identify the type of Helium that is in these tubes.

The 632.8 Red: 1987 Manufacture Date, Spectra Physics.
The 594.1 Yellow: unmarked tube head, I suspect 1980s, it’s the same relative age as the SP.

[Meatball]

I think that since the electronic structures of two isotopes of the same atom are nearly identical, that any difference in emission characteristics would be negligible. E.G. The difference is the emission lines between U 238 and 235 is on the order of thousandths of a nano-meter. That's why detection of isotopes via spectroscopy can be tricky business.

Perhaps I am missing something though.

[mixedgas]

Isotopic gas is used to reduce Doppler Broadening which, if left unchecked, reduces overall gain bandwith of a given lasing transition. The idea is to avoid slight adsorption of a lasing line at the band edge overlap of the two isotopes. There are other reasons for using isotopic gas, but they are minor. Meatball is right., For most materials except Cadmium and Deuterium, the effect is not that bad.

A typical extreme example of this is Helium Cadmium, where using a natural mix of the eight Cadmium isotopes reduces power of the laser some 30%. This is one reason why HeCads are so expensive, isotopic cadmium is not cheap.

HeNe is not that sensitive. If you have soft seal tubes, natural helium is all you can get anyways, short of paying ~1700 to 2200$ a flask for the good stuff. (recent quote) He-3 needs a permit to purchase anyways. So I suggest gentle soaking in a bag of ordinary balloon or welding helium.
He diffuses so much faster then the other gasses, that purity of the gas in the plastic bag is a non-issue, if you flush it once or twice.

If you need to read more, get a copy of Silfvast's book, "Lasers" at the library via interlibrary loan. Silfvast's book is quite readable even if you have not had physics. He takes you through designing a red tube as a example in the 2nd edition. He spends time on the isotope issue, as he's the inventor of the HeCAD. The math is not that bad, if you had Algebra One, its overkill.

A caveat, leave the HENE power supply off when the tube is submerged in the helium. The helium outside the tube has a tendancy to conduct better then the ballast resistors.The resulting arc across the resistors may damage the power supply.

There have been papers on calculating the diffusion rate of the gasses, I just dont have them around right now. A rule of thumb mentioned in the HENE regeneration papers is 12 to 24 hours in the bag of gas, if the bag is just above atmospheric pressure. I'm paranoid, so I'd check the tube out every six hours.

If we were actually repumping the tube commercially, I'd consider isotopic gas for the "special" wavelengths where the 3-5% difference in gain matters. If you actually have a badly leaking tube, some time in the next few months I'll have the pumping station back up for other reasons. I'd rather you do the "soak", it is much less risky.

Most of the leakage occurs at the epoxy, however the vacuum epoxy used by the tube makers is loaded with about 80% ceramic or oxide dust. That slows the diffusion.

By 1988, most tubes were already hard seal.

The yellows from that day and age were almost all made by REO. REO tended to use hard seals, except for external Brewster tubes. Epoxy was placed around the hard seal to reduce minor leaks that may occur during hard sealing. Epoxy is cheap, so the idea was better safe then sorry on a expensive tube.

I've never seem mix data published for yellow or green. Orange has been mentioned in early research and leans toward more neon.



Does it matter, somewhat. Can you do something about it,.... Well, not much. In other words, JUST DO IT!

PS, one more thought... Check the alignment by applying gentle stress with a piece of stiff plastic tubing, might save you some on gas bills.


Steve (that other author of the Gas Lasers chapter)

[pschlosser]

Good stuff, thank you. I think you're saying isotopic helium is hard to find and expensive. Thus, I have little to lose by using toystore helium?

[mixedgas]

Mr. Schlosser,

I have a gas laser "Guru" friend 1 hour from you downstate, over the river, up the mountains, across the bay, and through the woods.

I don't know if he does factory tours, but I sent you a PM.

He's president of the company, a hands on guy, and while not patient, loves to teach about gas lasers.

I imagine he can answer some of your questions, as big HENE tubes are part of their product line.

Steve

[macona]

$2k a liter for He3 sounds a bit low. I was hearing higher prices. And even if you have the authorization to buy it you wont get any since all of it is going to Homeland Security for neutron detectors. You can find He3 tubes on ebay every now and then, they are snapped up pretty quickly by the guys who build the little Farnsworth fusors.

The helium diffusion problem is also apparent in the chinese CO2 laser tubes. I have a 40W one that is about 2 years old. Never used for more than a test to verify it work. After the two years sitting on a shelf it is down to 29 watts. The mirrors are just epoxied on with some sort of yellowish translucent epoxy, not Torr-seal/Hysol. I have one of the 'better" Reci CO2 tubes I am going to attempt to analyze for a guy, these have metal ends on them but the ends are just glued on as well. Mu guess is they just last longer because of the larger reservoir of gas.

Toy store helium gas is only partially helium, there is a lot of air in there as well, you dont need a high percentage of helium to make a balloon float. It just means you need to let it sit longer in the gas. Helium works its way through anything, just about. Thats why I and the rest of the vacuum world use it for leak checking.