[time-nuts] Is it possible to "recharge" hydrogen maser?

[John Ponsonby]

An H-maser works by continuously bleeding a beam of monatomic hydrogen into the system. This gas has to be continuously 'pumped' out. The ion pumps used don't have an outlet port. They work by burying the H2 gas in titanium plates. Thus as Ole Ronningen says the pumps have a finite working life which is reached when they are swollen with gas. If you need to replace old plates you must find someone who can manufacture them for you if the original supplier no longer exists. I think the new plates must be 'activated' by heating them as hot as possible in ultra-high vacuum until they no longer outgas and then of course allowing them to cool to room temeprature whilst still under vacuum. I guess it is possible, though I've never heard of it being done, to revive old plates by the same treatment.  
The hydrogen coming into the system may come from a standard 'lecture bottle' of ordinary molecular H2. Alternatively it may come from heating a can of H2 stored in solid solution in a metal hydride. It can also come from electrolysis of water. One can run an H-maser on water! These two latter options may be used on masers intended for air transportation as the airlines are understandably scared of cylinders of hydrogen gas. 
The hydrogen gas is reduced in pressure to perhaps twice atmospheric by an ordinary pressure indicating regulator, and is then further regulated and purified by passing it through a membrane of either palladium-silver alloy or perhaps nickle. I think most western masers use palladium-silver but the KVARZ masers use nickel. I only have experience of palladium-silver. At room temperature it is a complete barrier to hydrogen but when heated it allows hydrogen to diffuse through. I understand that it allows H2 and only H2 to diffuse through: actually as free protons. So it acts as a purifier keeping out all other gasses and is even as an isotopic filter blocking deuterium. Thus ordinary commercial grade H2 can be used. It doesn't have to be isotopically pure or especially free of contaminants. The flow rate is controlled smoothly and without hysteresis by varying the temperature of the palladium 'leak/valve'. One uses a miniature thermistor as a Pirani gauge on the low pressure side of the membrane to sense the pressure. This may be used as an arm in a wheatstone bridge run at constant voltage, in which case the out-of-balance voltage is a measure of the H2 pressure, or as I did, run the thermistor at constant temperature by varying the bridge voltage to keep it balanced so that the bridge voltage becomes the measure of the pressure: the lower the pressure the lower the bridge voltage.
The low pressure (~1torr) H2 then passes to the RF 'dissociator' which turns the H2 to monatomic H which is what actually powers the maser proper. 
Blind ended thin walled Palladium-silver tubes about 1.5mm OD and about 40mm long, described by Vanier and Audoin* as 'glove fingers', can be conveniently used. Something like 1A passed through them heats them to produces a suitable H2 flow rate (~10E^16 atoms per sec) .  The high pressure hydrogen is applied to the inside and the low pressure or vacuum side is the outside. 
Anders Wallin describes a procedure involving cooling the new hydrogen bottle in liquid nitrogen. I think this would be an extremely dangerous as well as a pointless thing to do with an ordinary cast-iron 'lecture bottle' of high pressure gas. I think what he describes may be appropriate to changing a can of metal hydride.  
Dana Whitlow is correct is saying that getting a new ion pump to start is a lengthy process. One needs to get the vacuum pressure down to about 10E^-6 torr by other means. Then on turning on the ion pump very soon the pressure will rise as the titanium plates heat up and outgas. Once the pressure has fallen one tries again. This time it takes a bit longer before the outgassing makes the pressure rise. One has to go on doing this for an hour or several hours before the pressure no longer rises but instead falls. The pressure should get to about 10E^-8 torr with the hydrogen beam turned off. 
I hope this helps.
Regards 
John P
* The Quantum Physics of Atomic Frequency Standards. Jacques Vanier and Claude Audoin.