2011/02/24

1300	Fill N2 cold trap full.
1330	Electrical check at 77 K.
1400	IVC pressure 8.6E-7 mbar. Isolate vacuum pump.
1415	Background He leak rate 2.4E-9 mbar l/s.
	Blow out LN2.
1630	LN2 is out of main dewar. He leak rate 3.5E-9 mbar l/s: no leak.
1740	Pump mixture into dump. Evacuate main bath and overpressurise with He-4 gas.
	Fill IVC with 7.4E-3 mbar of He-4 exchange gas.
1800	Begin LHe transfer.
1920	Mixing chamber temperature dropping very slowly. Increase pressure of exchange gas in IVC: dropping like a stone on the moon.
1935	Stop LHe transfer. Liquid level meter at 88.0%; mixing chamber temperature 62 K.
1950	Liquid helium level 72.9%: burned through a lot of helium in a very short space of time. Might have to perform a second helium transfer earlier on Saturday morning. Our poor cooling power might be due to an insufficient exchange gas pressure in the IVC.
2115	Mixing chamber temperature decreases at a slower rate at around 11 K. Perhaps pumping out at 10 K would be a little premature. Wait for a while.
2125	Fill N2 cold trap full.
	Mixing chamber temperature 9.6 K.
2135	Start cryocleaning mixture. Valve 6 open 13.9%.
	Mixing chamber temperature 8.9 K.
2140	Open valve 6 by 14.0%. Mixing chamber temperature 8.5 K.
2145	Mixing chamber temperature 8.1 K. Pump out He-4 exchange gas.

<note> The MC temperature dropped very slowly during the initial He transfer. The temperature was still high (~50K?) when the He transfer was done. The rest of the cool down from 50K to 4K had to be done by evaporating liquid helium, which was a waste of liquid helium due to its low latent heat. It's best to use the cold helium gas after evaporation to cool the load to close to 4K. Effective heat exchange by exchange gas is needed to make use the cooling power from the cold gas. From previous runs, 0.1mbar He4 exchange gas might be a little too much; but 7.4E-3 mbar in this run was definitely too little. If there is no problem to pump out the He4 exchange gas, it's best to go higher rather than lower. </note>