2016/09/07 (Wednesday)
| Status: fridge at room temperature, cryostat Dewar is down, IVC is open. Pumping on still with leak detector, attached at top of fridge. Mixture is in dump and 3He pump, manual dump valves are closed. |
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| Yesterday a slight room temperature leak from IVC to fridge lines was detectable when the IVC was filled with helium to 1 atmosphere, and disappeared when the IVC was evacuated again. | |
| 0750 | Leak = 4.0 pbarL/s |
| 0820 | Attempting to repeat yesterday's test without a sealed IVC: currently the lower and middle sections of the IVC are removed. I've mostly covered the openings at the bottom. I used some kapton tape to mostly block the stripline hole, and used a piece of plastic sheet to cover the central hole, taped to the ceiling of the IVC can with packing tape. I cut a 1/4 inch hole in the middle of this sheet as an outlet. The idea is to now push helium in through the top of the IVC, which should mostly displace the air and fill the IVC. Look for increase in leak signal. If we see an increase we can then open a top port of the IVC and push nitrogen in from the bottom, which should displace the helium. |
| 0825 | Feed helium into IVC from the green valve on the turbo pump station. Can feel air coming out from the hole in the plastic sheet. Over 10 minutes, leak rate rises steadily from 4.0 to 5.5 pbarL/s, then levels off. |
| 0845 | Disconnect an IVC connection at the top of the fridge, at the end of the metal hose that attaches to the central IVC port. Then feed a plastic hose from the nitrogen supply to the hole in the plastic sheet and begin feeding nitrogen in from the bottom. Over the first 15 minutes, the leak actually increases more, from 5.5 to 6.0 pbarL/s. It stays at that level for another 5 minutes. |
| 0910 | Remove the nitrogen hose from the plastic sheet at the IVC opening and tape it to the opening at the top, to blow nitrogen the other direction. Maybe this will help push out the helium. After 15 minutes, no significant change, leak = 5.9 pbarL/s. |
| 0925 | Move nitrogen hose to plastic sheet on IVC opening again, continue to blow nitrogen. Very slow decrease in signal, from 5.9 to 5.7 pbarL/s over 15 minutes. |
| 0945 | Increase nitrogen flow by opening regulator 1/3 turn. Over 15 minutes leak falls from 5.7 to 5.5 pbarL/s. |
| Maybe helium leaked into the condenser side (at the primary impedance seal?) and is slowly leaking past the impedance to the still side. Would prefer to pump from both sides with the LD… | |
| 1005 | Disconnect leak detector plumbing to add an additional tee so we can pump on still and condenser simultaneously. Stop pushing nitrogen into IVC for now. |
| 1200 | Came up with complicated plumbing solution, but the background with the fridge valves closed was around 80 pbarL/s, so it must have been sort of leaky. Gave up on that… |
| 1220 | Attach leak detector to condenser port of fridge, start pumping it out |
| 1230 | Leak rate is down to 0.78 pbarL/s |
| 1247 | Repeat this mornings test with the bottom of the IVC taped off as before, this time attaching the leak checker to the condenser line. Feed helium into IVC from the green valve on the turbo pump station. Can feel air coming out from the hole in the plastic sheet. Over 15 minutes, leak rate rises steadily from 0.68 to 1.2 pbarL/s, then seems to levels off. |
| 1304 | Disconnect an IVC connection at the top of the fridge, at the end of the metal hose that attaches to the central IVC port. Then feed a plastic hose from the nitrogen supply to the hole in the plastic sheet and begin feeding nitrogen in from the bottom. Over the first 15 minutes, the leak actually increases more, from 1.2 to 1.4 pbarL/s. It stays at that level for another 5 minutes, before the N2 was shut off. |
| 1400 | Tighten MC seal screws to 10lb-in using torque wrench. They seems to already be at least this tight. Replace bag seal on bottom of IVC. |
| 1418 | Moved leak checker to fridge still line, leak rate = 3.3 pbarL/s. Fill IVC from top with He. |
| 1423 | After 5 mins, leak rate = 3.2 pbarL/s. |
| 1431 | After 13 mins, leak rate = 3.4 pbarL/s. |
| 1433 | After 15 mins, leak rate = 3.6 pbarL/s. |
| 1437 | Disconnect an IVC connection at the top of the fridge, attach N2 hose at bottom of IVC. |
| 1440 | leak rate = 3.9 pbarL/s. Begin flushing with N2 gas. |
| 1441 | leak rate = 4.1 pbarL/s. |
| 1445 | After 5 mins, leak rate = 4.1 pbarL/s. |
| 1458 | After 18 mins, leak rate = 4.1 pbarL/s. |
| 1500 | Oh man, we forgot to open the gate valve! It seems this ~0.2 pbarL/s increase was not due to internal fridge lines. |
| 1505 | Open gate valve and let leak checker stabilize. |
| 1527 | Leak checker has stabilized at 5.6 pbarL/s, Fill IVC from the top with He. |
| 1536 | After 9 mins, leak rate = 5.9 pbarL/s. |
| 1542 | After 15 mins, leak rate = 6.2 pbarL/s. |
| 1546 | Disconnect an IVC connection at the top of the fridge, attach N2 hose at bottom of IVC. Start with a leak rate of 6.2 pbarL/s. |
| 1551 | After 5 mins, leak rate = 6.4 pbarL/s. |
| 1556 | After 10 mins, leak rate = 6.5 pbarL/s. |
| 1602 | After 16 mins, leak rate = 6.4 pbarL/s. Stop flushing IVC with N2, and remove tape from bottom of IVC in preparation for removing the top of the IVC can. |
| 1725 | After dismantling upper 4K can and upper 100mK can, look at mixing chamber and impedance flanges for any signs of leaks. Begin spray-test leak checking. Background leak rate = 4.8 pbarL/s. |
| 1730 | No reproducible leaks found during initial spray test of Mixing Chamber flanges or impedance flanges. Will continue with more extensive spray test tomorrow. |