Vacuum Service for a Gemini 200 NMR

The following photo shows Varian's Gemini 200 now relocated from room 130 to room 38. The instrument is being serviced to restore full vacuum.  When the seals have completely degrade then the magnet must be de-energized before the seals can be replaced.  In this case the vacuum for the magnet was pumped down while it was still at field. The vacuum seals for this magnet which were part of the original XL spectrometer. These seals are made of indium and were replaced, once. The vacuum is then reestablished and checked for leaks. A second photo. below at the right, shows the rear of the magnet and the left leg which contains the helium and nitrogen off gas flow meters. The vacuum port is located in the rear center of the magnet. The complete set up for bringing the vacuum back is also shown in the third photo. The third photo was taken in front side of the magnet and shows the set up used with the leak detector, the diffusion pump and controller along side the liquid helium Dewar and cylinder of helium gas.

On Indium Seals --

• Both seal surfaces and the indium should be kept as clean as possible.
• Acetone does an adequate job cleaning surfaces and indium.
• When making a seal out of a wire of indium, cut the wire to length, beveling the edges to make the joint. The freshly cut edges will stick together readily.
• Not all sealing schemes make reliable indium seals. An improperly designed seal geometry can often be sealed initially, and then after a couple of days, will open up. For example, a seal between a knife edge and a flat. It initially worked, but after a couple of days, the indium crept inwards, over the knife edge, and a leak opened up. The problem was solved by machining washers which prevented the indium from flowing inwards.
• A good rule of thumb for determining the number of fasteners needed to secure an indium seal is - as many as you can possibly fit on the surface!
• Indium seals can be reliable down to very low temperatures. The fill tube was reliably sealed to the L_He reservoir with 0.125 inch indium.
• Indium can be reformed into useful seals after use. Successfully extruded indium wire from scrap indium. First clean the indium with acetone. The extrusion mechanism consisted of a hollow cylinder with a bolt screwed into one end, a plunger and a stand. The bolt had a hole drilled lengthwise through which the indium was extruded. A 0.070 inch hole yielded indium with a 0.079 inch diameter. Use a hydraulic press to extrude the indium. Smaller diameter indium required larger forces (500 pounds for 0.125 inch wire, 2000 lbs for 0.080 inch wire).

AMMRL General Comments about Loss of Vacuum and Cold-pumping --  A quench start was reported for a Varian 200 MHz "R₂D₂" type magnet due to low L_He levels.

Symptoms noted were high He gas flows (increased from ~ 4 cc/hr to ~ 20 cc/hr) and extreme lock problems due to the decreasing field (²H lock frequency loss rate of ~ 300 Hz/hr).

They were able to arrange for same-day delivery of L_He.  At NMSU delivery of liquid helium takes a week. After filling the magnet, the field stabilized. Net result was 230 ppm loss of field. After adjusting NMR frequencies, the system was able to meet all specs, including line shape.

Cold-pumping magnet Dewars there clearly are different experiences: sometimes it helps, sometimes it doesn't. When somehow He diffuses into the vacuum space from the He can, cryopumping by the liquid He does not do the job but external cold pumping will remove the He gas. With larger leaks bringing in air from the room atmosphere this may not be so effective. When He gas is the reason for a poor vacuum, cold pumping will certainly help.

As an alternative to periodic pumping with a high throughput turbopump  A large diameter five foot copper tube connected the pump to the magnet Dewar via a high vacuum valve. The current in the vac-ion pump shows the vacuum at any point in time. The only disadvantage is that He is not too well absorbed by the electrodes of the Vac-ion pump. A diffusion pump had to be used about every three to four months to bake out the Vac-ion pump overnight to recondition it.

Consider -- the much higher partial He pressure in the NMR lab as compared to the Natural abundance of He in air (about 5 10^-4 torr) and how much faster will a leak in a Dewar lead to excessive He boil-off as a result?

When the L_He boil-off rate doubled and tripled on one Oxford magnet, there was moderate success in cold-pumping the magnet with the proviso that the turbo pump must be no closer than the 50-gauss line to avoid eddy current forces from overloading it. To reduce the flow resistance, if possible make the stainless line from magnet to pump much larger in diameter than the magnet vacuum valve. In the case of a moderately slow leak, this procedure can easily extend the life of the Dewar to a time more convenient and economical for you to bring down the field and warm it up for proper repair.   The suggestion to pump a failing magnet while cold, remember there are safety valves to protect against accidental loss of vacuum if the power fails during the cold pump-out. These are simply solenoid-activated gate valves which are normally closed. They are wired to the pump power mains so that if there is a loss of electric power, the gate will fall, closing the valve. These valves are available from manufacturers of high vacuum equipment like Alcatel, Leybold, and Varian. Cold-pumping a magnet is usually not very effective. A magnet filled with liquid helium is an excellent cryopump, exceeding the pumping speed of just about any commercially available turbopump system. Also, it is not possible to bring a turbopump very close to a large NMR magnet without having serious eddy current problems. Thus, there usually has to be at least a one meter metal hose between the magnet and the pump, and this can seriously degrade pumping efficiency. In particular, it is very hard to pump helium gas out of a cold magnet cryostat (or a warm one). Since the presence of traces of helium gas in the cryostat (due to an O-ring seal frozen during a helium fill) is often the cause for degraded magnet cryogenic performance, removing this helium will usually be necessary to restore the magnet to specifications. Provided that proper safety devices and practices are used, pumping a cold magnet does not present a great hazard and it is not a very expensive procedure, but there is not much benefit from cold pumping our magnets. In particular, pumping one magnet had no benefit at all in restoring its liquid helium hold time to specifications, and pumping   One magnet provided only a few days of reduced boil off. Ultimately, that magnet continued to degrade, and a complete overhaul was necessary.

 

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