2018/07/24 (Tuesday)
| overnight: stopped circulation yesterday due to suspected IVC leak, tower holding at 11.7 K overnight while pumping on IVC | |
| 0950 | Tower = 11.7 K. Close turbo pump valve. IVC = 22 nbar. |
| 0951 | Fill LN2 cold trap. |
| 1005 | Tower = 11.7 K, IVC = 25 nbar |
| 1125 | Tower = 11.1 K, IVC = 30 nbar. Decrease in temperature since we stopped pumping is quite clear. |
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| 1154 | Tower = 10.6 K, IVC = 30 nbar. |
| 1336 | Tower = 8.9 K, IVC = 30 nbar. |
| 1343 | During initial helium blowoff during the transfer, IVC pressure increased dramatically, to 460 nbar. (This is with the cryostat transfer stick just inserted into top port, with a lot of helium still escaping out the top.) As soon as transfer stick was lowered IVC pressure fell quickly, settling back to 30 nbar. Also a spike in Sorb temperature from 9.3→10.3 K then back down |
| 1343 | Start LHe transfer (MB = 33.9%) |
| 1350 | 1K Pot falls below 7K |
| 1400 | Sorb has now dropped to lower than before the transfer, to 6.6 K. About 10 minutes after starting transfer, Tower temperature stopped falling, flattened out at 8.67 K |
| 1405 | Watched IVC pressure while we ended the LHe transfer. It had settled to 19 nbar. While we disconnected the two pieces it fell a little, to 16 nbar. No increase when we pulled out the cryostat vacuum stick, although plenty of gas blew out as usual. |
| 1405 | End LHe transfer (MB = 52.3%) |
| 1405 | 1K Pot/Sorb reach minimum of 5.1/6.5 K then rise again. Then 5 min later they peak at 5.5/7.0 K then fall again - I guess that's after the cryostat stick was removed, releasing a lot of pressure. The rising part corresponds with a small rise in Tower too, from 8.7→8.8 K, then the Tower stays flat. |
| 1420 | IVC has risen to 22 nbar. Tower temperature is still basically flat, 8.81 K |
| 1425 | Did some tests squirting helium: 1. Squirt helium around stripline tube seals at top. No change in IVC pressure. 2. Squirt helium into bath port. Wait long enough until recovery gauge is spinning audibly. No change in IVC pressure. 3. Release built-up pressure from 2. IVC pressure drops a little, from 22 nbar to 19 nbar. Then rises back. Repeated this twice, happened both times. (Also it looks like Sorb reacted a little, falling from 6.7 to 6.6 K and back) |
| Conclusion: there's no clear indication that the rate of helium entering the IVC increases during a transfer. The rise and fall in IVC pressure during the transfer seems to correspond to changes in the bath temperature, i.e. the temperature of the gas above the LHe surface. Currently the Tower temperature is flat, rather then falling. It probably would have flattened naturally anyway, but this was a sudden flattening when the transfer began. Presumably it's because the bath is now colder, so that the helium in the IVC is not as efficient as an exchange gas. |
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| 1445 | Tower/Sorb/1K Pot = 8.86/7.1/5.6 K, IVC = 25 nbar. MC/Still = 3/3 mW. |
| 1505 | Tower/Sorb/1K Pot = 8.76/7.3/5.7 K, IVC = 30 nbar. MC/Still = 3/3 mW. So Tower is starting to cool again, but slowly. Still not as low as the minimum it achieved today after 4 hours of not pumping on the IVC. |
| 1610 | Fill LN2 cold trap full. |
| 1630 | Tower/Sorb/1K Pot = 8.36/7.8/6.1 K, IVC = 30 nbar. MC/Still = 3/3 mW. Tower is falling steadily but at a significantly lower rate than before the transfer. |
| 1935 | Tower/Sorb/1K Pot = 7.87/8.7/6.17 K, MC/Still = 3/3 mW. |
| 1935 | Increase MC/Still heaters → 20/20 mW |
| 1955 | Tower/Sorb/1K Pot = 9.23/9.6/>7 K, MC/Still = 20/20 mW. Looks like Tower is starting to level out. Compare to when IVC was being pumped out, Tower rose above 12 K before we lowered the power. |
| 2020 | Tower peaks around 9.46 K, then falls slowly |
| 2145 | Tower/Sorb/1K Pot = 9.23/9.6/>7 K, MC/Still = 20/20 mW. |