1. 1 Technical Arguments in Favor of using the Cryomech PT-415 Cooler for Cooling the LH 2 Experiment Michael A. Green Lawrence Berkeley Laboratory Berkeley CA 94720

2. 2 My Understanding of the Purposes of the RAL Hydrogen System Test One must test the hydride bed hydrogen gas supply system. This test must include taking hydrogen gas back into the hydride bed as well as delivering it. One must demonstrate that hydrogen gas can be liquefied into the absorber. This should include using the cooler to do the liquefaction. One must demonstrate that the thermal siphon system will keep the hydrogen absorber cold over a range of T and P. In my view one must demonstrate that the system works when helium is used to keep the pressure in the absorber above 1.1 bar. Liquid helium operation should be demonstrated.

3. 3 The H 2 and He Liquefaction Problem The goal is to liquefy hydrogen into the absorber using the cooler in 16 to 24 hours. One wants to fill the absorber with helium in under 24 hours using the cooler. This means that the H 2 or He gas must be pre-cooled by the cooler before it can be liquefied. The pre-cooling must be applied at the rate of 100 W for H 2 and about 50 W for He. The pre-cooling is applied through a heat exchanger attached to the 1st stage or the tube between the 1st and 2nd stages and or a liquid nitrogen pre-cooler. The heat exchangers will be a laminar flow heat exchangers with enough area to ensure that the  T between the gas and the source of the cooling be low. The heat exchanger can not be a part of the vent circuit.

4. 4 Why is the cooler used important? The only way that one can pre-cool the gas being liquefied using a GM cooler is to attach a laminar flow heat exchanger to the first stage. LN 2 pre-cooling can be used as well. With a pulse tube cooler, one can pre-cool with the first stage and one can pre-cool off of the regenerator tube. Cryomech has a helium liquefier that does just this. Liquefaction using the pulse tube cooler should be more efficient. There is experimental data that suggests that this is true. MICE has decided to use pulse tube coolers on all of the magnets including the AFC module.

5. 5 The Cryomech PT-410 used as a Liquefier A PT-410 cooler (1 W @ 4.2 K) was used. The actual cooler capacity is not known Input power = 8 kW @ 60 Hz Liquefaction into a 60 liter storage dewar. The heat leak into the dewar is unknown The liquefaction rate for helium gas was 15.2 l/d (0.022 g/s). Dewar cool down took ~20 hrs until liquid accumulation starts. The start temperature is not known. The data was published in cryogenics late last year

6. 6 The Cryomech Pulse Tube Helium Liquefier

7. 7 Helium Liquefaction into the Absorber Using a Pulse Tube Cooler Liquefaction time H 2 = ~16 hr Liquefaction time He = ~23 hr

8. 8 Concluding Comments From the standpoint of being able to liquefy hydrogen or helium into a dewar using a cooler, the pulse tube cooler appears to be the better choice. The collaboration has made the decision to use PT-415 coolers on all of the magnets including the AFC module. Unless cost or schedule arguments dictate using a GM cooler the PT-415 cooler should be used for the MICE hydrogen test at RAL. There may be other unknown considerations for going either way.