2019/07/16 (Tuesday)

Previous Day

Setup	DIB1/3/4 → 3.2 MHz filters 2/3/1 → cable 3/1/2 → gender changer ?/3/2 → HVConn 1/3/2 → HV 1/2 → DCRCX(4)/DCRC55(1) DCRC33(2) → 1kV power supply → HV input Three walls of the lead/poly shield are up.
0835	End overnight activation. Remove PuBe source from fridge shield and return it to PAN 443 storage. 15.75 hours = 945 minutes PuBe activation.
0845	Check event rate. Count number of visible pulses in 50 traces, three trials: count 36, 32, 41 pulses. 222±21 Hz
0910	Tower/MC/Coldplate = 38.4/29.3/197 mK, Still = 1.34 K / 0.18 mbar / 6 mW, G1/G2 = 222/214 mbar, 1K Pot = 1.62 K / 19 mbar. MB = 37.2%
0915	Fill LN2 cold trap bath.
0921	Begin LHe transfer @ MB = 37.1%
	note: in order to get in sync with the Goldman lab, with whom we are sharing the transfer stick, we'll do an off-schedule transfer tomorrow. (Today's may not be a full transfer because the 250L Dewar is only at 43.9 L.) Tomorrow we won't have access to the transfer stick until after 3pm.
0948	Finish LHe transfer MB = 54.2%
1100	Check event rate. Count number of visible pulses in 50 traces, three trials: count 24, 32, 27 pulses. 169±19 Hz
1215	Check event rate. Count number of visible pulses in 50 traces, three trials: count 30, 21, 24 pulses. 159±18 Hz
1335	Check event rate. Count number of visible pulses in 50 traces, three trials: count 25, 30, 26 pulses. 165±19 Hz
1555	Check event rate. Count number of visible pulses in 50 traces, three trials: count 17, 22, 22 pulses. 124±16 Hz
1558	Start a sequence of “8” @ 50 min series at HV = 180→306V, rate 2.49V/min, with 10% prebias for 2 minutes, flash at end with 30 min wait. CDMS detector: Driver Gains = 1x, DCRC4/1 QET biases = (490/400/495)/(420/480/450) uA, Front-End Gains = (2/2/3)/(5/5/5) 1000 BOR, 5Hz InRun randoms, trigger on DCRC1pD threshold = 3, baseline/sample length = 20/13 (standard 16/5),phonon pre-pulse 500 (standard is 500). Use sequence HVdrift_8X180V_2p49VPM_loopversion_prebias_10_2_dur_50min_flashDCRC_1_4.msl
1610	Fill LN2 cold trap bath.
2150	Notice that Tower temperature is very high, started rising about 6 hours ago (why didn't we notice?). Currently 2.0 K - yikes.

14:45: temperature starts to wiggle by about 0.4 mK
15:58:05: 1K Pot temperature and pressure start rising. Still pressure starts rising at exactly the same time.
15:58:17: Circulation pressures start falling
16:00: Tower temperature starts rising very quickly
16:03: 1K Pot pressure so high that G3 reads an overpressure. Rises to about 110 mbar above 1atm
16:15: Sorb temperature starts rising noticeably
16:25: Circulation pressures reach min of 55 mbar then begin rising
17:00: 1K Pot reaches 4K and plateaus
19:58: 1K Pot starts rising above 4K!
19:59: Sorb reaches 5.3 K then starts rising much faster
20:30: 1K Pot reaches 7K!
21:10: Sorb reaches 10K and plateaus
22:20: Circulation pressures around 650 mbar and rising

So what caused the fridge failure? Best guess: helium buildup in the IVC. We haven't been pumping on it. Could be that superfluid built to the point of a thermal short somewhere - whether or not that happened, a boiloff occurred causing a sudden thermal short between IVC walls and all internal components. Naturally the dilution unit is immediately affected, with the still showing the first symptoms. Should the 1K pot react too? Makes sense - helium gas will also thermally short the pot to the IVC walls too. And eventually that is a short to parts of the IVC above the bath level, so the rise above 4K is plausible too. Could be that we are experiencing a rapid boiloff too. No remote access to the reco meter, but I can keep an eye on the bath level.

2223	Close V6 to stop circulation
	Looks like maybe circulation pressures stopped rising. G1/G2 = 670/640 mbar. But probably they'll rise again. Better relieve the pressure.
2230	Open V9 to let mixture out of fridge into dump. Let G1/G2 fall to 200 mbar, then close it again.
2245	Tower 2.5 K and rising, looks like it's on its way to 4K.
2340	G1/G2 have risen back to 637/586 mbar. I'll just open V9 and leave it open.
2340	Open V9
2340	Tower = 2.7 K and rising. 1K Pot still at overpressure but hanging at around 110 mbar above 1atm, so seems ok.
	Main bath hit 41.2% some time in the last hour. According to the main bath calculator this is not a fast boiloff, rather significantly slower than usual. strange, is the calculator wrong?
2350	Turn off still heater.

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