2016/04/19 (Tuesday)

Previous Day

	Status: at 77K yesterday we discovered a blockage in the fridge circulation. Allowing the fridge to warm overnight (LN is blown out, exchange gas in IVC, flushed MB)
0900	MC(Cernox) = 150 K, IVC = 1.6 mbar
0910	All IGH and fridge valves closed, pumps off. Heaters off.
0915	Throughput test. Open Gate and Condenser valves. Open V9.
	Open V14 and V3, wait for G1&G2 to stabilize
	Close V3 and V14, open V1, monitor G1.
0920	G1 peaked at about 680 mbar then fell to 40 mbar. Then barely rose at all. 0.2 mbar / 5 minutes. 0.04 mbar/min.
0935	Pump fridge contents to dump through Condenser valve, Gate valve, V1, V3, V9, V6
0940	Close valves and turn off pump
1015	Fill LN2 cold trap full.
1020	Turn He3 pump back on and commence cleaning mixture through Cold Trap 2.
1225	Stop cryocleaning mixture and pump to dump.
1240	Isolate LN2 Cold Traps. Turn on He4 pump to pump out lines on right-side of IGH leading to vent valve in preparation for letting cold traps warm up into P2. P2 pumped down all the way reads 17ubar. Isolate He4 pump by closing manual valve on intake. Turn off He4 pump.
1245	V6 closed all the way. Isolate dump (787mbar of mixture) and turn of He3 pump. Open V7, V2A, and V5A, (pipes leading up to cold traps) and P2 rises to 0.5mbar. Open V11A/V11B to connect cold traps with P2. P2 immediately jumps to 7.5mbar. Pull out LN2 cold traps from LN2 bath and let warm up.
1310	Traps still warming, but P2 already reads 1000mbar. Turn on He4 pump, and open manual valve to begin pumping out traps.
1430	Perform magnetic field measurements with and without Mu-metal shield using the SLAC magnetometer. Mu metal shield was lowered into the pit, just below the pit doors. Probe was inserted 65 cm down into the shield, such that the X direction was pointing north/northeast, the Y direction was facing east/southeast (the lab's door), and the Z direction was downward. The probe is connected to a Mag-03PSU power supply, which has a port for each direction (X, Y, Z) that can be connected in turn to a voltmeter.The X,Y, and Z components of the magnetic field were recorded from the voltmeter for this orientation, in Volts. Then the Mu-metal shield was raised to its standard position, and the B field of the same location inside the pit was measured without the shield. Results included in table below. Test was performed twice for each position. This test was performed with a different Magnometer setup on 2016/03/09, with a more indepth look in the Run 56 summary. These results can also be compared to Tsuguo's SQUID modulation depth vs. MuMetal Shield on/off study.

	With Mu-metal shield			Without Mu-metal shield
	X	Y	Z	X	Y	Z
Test 1	-0.055 V	0.011 V	0.160 V	1.301 V	0.120 V	5.406 V
Test 2	-0.054 V	0.010 V	0.159 V	1.163 V	0.160 V	5.439 V

As mentioned on 2016 March 28, one volt corresponds to 10 μT. So the field is

	Magnetic field (μT)
	With shield	Without shield
North	-0.545	12.32
West	-0.105	-1.40
Up	-1.595	-54.225
uncertainty	~0.01	~1

The measurements without the shield seem to have significant error between the two tests. However, if you calculate the magnitudes of the field vector in Test 1 compared to Test 2, they only differ by 0.01 μT. So it's likely that the probe was not in quite the same orientation in the two tests.

The magnetic field attenuation for our shield is about a factor of 30, much smaller than Tsuguo's. However the geometry of the SLAC75 fridge's shield is very different - it's much narrower and has a bottom.

Our mu-metal shield dimensions: 1/16“ thick, 21” inner diameter, 49“ tall. Detector Position 2, indicated by blue tape, is about 23” from bottom of shield.

1503	With P2=22 ubar, stop pumping on cold traps. Return traps to LN2 bath. Fill LN2 cold trap bath full.
1505	Pump out main bath and flush with warm Nitrogen gas.
1530	Commence cryocleaning through cold trap 1.
1535	Pump IVC with scroll pump, add 3 mbar He exchange gas.
1710	Cease cryocleaning. Pump mixture to dump.
1718	Isolate dump with pressure 785mbar.

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