During two busy down days (the beam was off on Tues. and Wed.) we fixed a
large number of problems. Here's what we did as well as other assorted
thoughts.
The June 5-9 run period flushed out a number of problems in the chambers.
Since we had all 18 chambers operational, we saw some of our old problems
as well as some brand new ones.
HIGH VOLTAGE (shorts, open circuits, excessive current draw):
A number of high voltage shorts have been found which are likely due
to broken wires. The shorted wires were taken out of the system.
After data was taken, the absence of counts for wires corresponding
to a sense wire high voltage bus indicated an accidentally disconnected
HV baby cable. Most of these were re-connected. It is still an open
question whether we can detect a dis-connected field wire bus from the
data.
There still are a number of so-called "super-hot" wires in the
system. My current favorite theory is that they are caused by unbalanced
electrostatic forces on a (loose?) field wire, pulling it closer to
its neighbor sense wire and causing a higher gain and current draw.
We're investigating whether this might be caused by (as yet undetected)
nearby HV disconnects.
GAS (alarms on pressure, O2 and H2O levels; mix ratio mis-calibration):
Gas pressure control is not steady for Reg. 1 because of bubblers placed
in the output gas stream. This caused a number of spurious alarms.
The return lines for Reg. 2 have numerous leaks which pulls in oxygen
and water above our limits. Since we are not recirculating this has
had no adverse effects on the chambers. It was discovered that Regions
1 and 3 were not flowing 90:10 Ar/CO2 but more like 88:12. This has
been fixed and the mixture will slowly approach the 90:10 ratio.
This explains why the Region 2 efficiency plateaued at a lower than
expected voltage compared to Regions 1 and 3.
LOW VOLTAGE (blown fuses, supply trips, open circuits):
Several blown fuses were found and replaced. We have also had a few
occurences of low voltage supplies tripping on an over-current condition.
There are also several instances in which the power circuit is open
somewhere on the chambers, which can be recovered only by removing the
chambers.
ADB CRATES (trips, malfunctions):
One ADB crate (R2S6AX) tripped off because the TCI board detected inadequate
airflow. This was fixed by changing an air filter. Another crate
(R1S2) had a bad power connection which was fixed.
FASTBUS (TDC) CRATES (controller malfunctions):
Crate DC3 had a bad SFI and/or PowerPC card. They were replaced and
now the crate reads out properly. This affected R3S2(AX & ST) and R3S1AX.
SIGNAL DISCONNECTIONS (ADB, TDC):
The connections to our ADB and TDC boards do not have locks. We have
found a number of these connectors partially or entirely pulled out.
In our next long down period we will have to re-do the strain relief
on several of our crates.
NOISE (single signal cable groups, R3S6 noise, 60 Hz):
Much of the noise on single events comes in groups of 16 wires corresponding
to a single signal cable. On a 'scope (through a VPI post-amp) the noise
appears as a 2 MHz oscillation without observable subharmonics. The
channels which appear noisy in DAQ have noise amplitudes of about +- 10 mV;
since the ADB's have about 3 times the gain of the VPI post-amps this
corresponds to 30 mV which is the discriminator level currently set for
Regions 1 and 2 (see note on Reg. 3 following). This noise is NEW since
the February run and should be tracked down. However, we don't know why
some signal cables have noise at this level while most are a factor of 2
or so quieter. Several things are common to a group of sixteen signals
on a cable: the group of 16 sips which share the 17th pair calibration
trace and a voltage regulator as well as the ground braid in the cable
itself. In fact, we noticed that touching the ground braid to the
ADB crate caused a huge increase in the ADB output noise presumably
because we are closing a large ground loop. Some of the signal cables
did not have their ground braids taped back (primarily in the early
sectors - 5 and 6). We temporarily fixed the problem by pulling these
braids back away from the crate.
During an attempt to measure wire tensions Reinhard and Richard noticed
a large 60 Hz noise ripple which was proportional to the minitoroid
current. We need to try to fix this specific problem. I have also
started an effort to measure the 60 Hz phase for every event so that
we could look for 60 Hz effects in the data.
Finally, we see an anomalously large noise level in Reg.3, Sec. 6 (not
in the other Reg. 3 chambers!). It may be a grounding problem. We have
raised the threshold for all Reg. 3 chambers to 45 mV for now.
"once more into the breach, dear friends"
-Mac