Since the target has been cold, we have puzzled over the mismatch between the cooling power we seem to be receiving from the ESR and the heat load we seem to have as evidenced by the extraordinarily small value of the heater power.
First, what's the cooling power we're getting? If we take the values from the ESR at face value (and one can argue whether we should do that or not), then we can estimate the cooling power from the 15K coolant temperature and pressure instrumentation at the ESR, plus the ESR flowmeter. Typical values from today are shown in the screenshots below. For a 15K, 12 atm supply the enthalpy is 67.2 J/g. For an 8 atm, 19K return it's 93.6 J/g. So we have a dH of 26.4 J/g of cooling power being sent to us from the ESR. At 20.5 g/s that means a whopping 540 W of cooling power, almost perfect for a 100 uA boiling study. 15 cm of LH2 will receive 530 W of heat from a 100 uA beam, and so we have 10 W of reserve heater power in principle. We need at least 50 W of reserve power, but it's close.
The ESR parameters are not 100% typical- usually the return pressure is a bit less than 3 atm, and the supply temp is usually 14K. But the ESR instrumentation says what it says. Note that the 15K supply temp on the ESR side will be warmer by the time it gets to us, and the 19K return temp I used is our loop temp, not the 19.5K return temp measured at the ESR. So the actual enthalpy available in Hall A will be a bit less than this.
BUT, what the hell, we only have about 220 W on our heater! Where is the missing 540-220=320 W of heat load? Some comes from the fan, we don't know for sure how much but 50-75 W is typical. Of course there is conductive and radiative heat loss to the outside world, maybe another 50 W. Is it possible we have ~200 W of heat loss in the transfer lines? That would be pretty awful, but not impossible. Unfortunately the coolant temperature information we have in Hall A does not work. You can see those temps in the cryostat secondary GUI- they read nonsensical values. Apparently those diodes are very old and no longer reliable. Too bad, if they worked we'd know for sure if it was the transfer line or not.
One last piece of info- the U-tubes for the target were pumped out before the run by the target group so they should be OK. The rest of the xfer line to the ESR is common with the HRS's so it can't be that bad.
In any case, whatever the reason for the mystery heat load, it means we can't go much past about 40 uA on LH2 with the present coolant mass flow of 20.5 g/s if we still want about 50 W of reserve heater power. We can probably go to 50 uA if we raise the operating temperature of the loop from 19K to 20 K. Then the dH becomes 32 J/g (instead of 26.4 J/g) and at 20.5 g/s of coolant flow that provides another 100 W or so of cooling power (about another 5 uA reach in beam current). They ran briefly at 24 g/s this morning but had to back off for some reason I have forgotten.
Dave Meekins talked to cryo and says that our usual coolant mass flow ceiling of 24 or 25 g/s is not the actual ceiling at the moment because the loads in Halls B & C are low, and they are supplying us with 15K coolant rather than the more usual 14K coolant. The Hall C 4K load is not zero, but this seems to mean we can squeeze more than the usual 25 g/s out of the ESR and who knows, maybe we can actually get enough coolant for a full current test. It remains to be seen, and after all, we still don't know how much current can be delivered from the accelerator anyway. With what we have right now, 50 uA seems doable for sure with only modest stretching. More means crashing through our usual coolant flow ceiling, which cryo apparently says should be doable at the moment.