The PITA voltages for positive and negative helicity have been gradually diverging (see for example this Log Book Entry). The effect of this will be, eventually, to introduce some linear polarization into the laser light, which will generate helicity correlated position differences due to the analyzing power of the crystal. At the next convenient oportunity (possibly NOT the IHWP change on Saturday), Kaz will attempt to reduce the PITA voltage difference.
There has been much controversy about the possibility of a current dependence of the CFD thresholds. A check was carried out with data from the BCM calibration runs (Log Entry Link). The results are consistent with a small increase in the threshold for the NA CFDs and an even smaller decrease for the French detectors. For the moment, CFD thresholds will NOT be readjusted to compensate for this effect.
It was reported (Log Entry Link) that both the NA and French electronics experienced an unexplained rate loss for a few runs this morning. This is NOT explained by an erroneous HV file. The cause of this rate loss remains mysterious, although the time of its appearance coincides with a sharp rise in the temperature of the electronics room (Log Entry Link) due to a failure of the air conditioning. That was repaired and (as far as I know) the problem has also disappeared. We need to remain vigilent.
The French electronics has been plagued by timing spikes (Log Entry Link and followup) and alerts (Log Entry Link). The source of the former is being investigated. The latter may be caused by bad beam (see next concern).
The beam position on the target has changed a number of times during this run by amounts of order 0.8 mm. The table below shows some of the "nominal" values (for G0 and G0B only) that we have used (note values that have changed in red). These changes have been necessitated by the inability of MCC to position the beam without causing high halo. Over the last two days, we have experienced another change, this time in G0Bx.
| Date |
G0x |
G0y |
G0Bx |
G0By |
Source |
|---|---|---|---|---|---|
| Pre-Jan 25, 2004 |
-1.3 |
-2.6 |
-1.8 |
1.1 |
|
|
Jan 25, 2004 |
-1.0 |
-1.9 |
-1.5 |
1.1 |
|
|
February 15, 2004 |
-1.0 |
-1.9 |
-1.7 |
1.8 |
Log Entry Link (Posted under Xterm |
|
Now |
-1.0 |
-1.9 |
-0.8 |
1.8 |
BPM positions of last 2 days |
Normally we would repetedly do harp scans at the 4 places on our beam line which preced G0 with no other magnetic elements, steering the beam until the harps show the beam to be on our surveyed "zero" line. Unfortunately, these harp scans seem to be inconsistent (for a given beam position, the scanned positions do not lie on a line). This could be due to beam movement during the scans, but there is a pattern to the "non-linearity" which is not random and which suggests a misalignment or faulty readout of at least one harp. Without reliable harps, we have no way to define our beam line and therefore no way to find the nominal BPM readings. The right thing to do is to re-survey the harps and verify their operation. This, however is very invasive and time-consuming (it would take a shift to check the outside of the harps and probably we would need more time to check the read-out and certainly much more time if the internal alignment was to be checked). One more piece of information: a mispositioning of the beam on target of 2-3 mm causes an increased and probably time-dependent inelastic proton background. We will continue to run for now at the latest nominal position, and a meeting with experts will be scheduled for early next week to hash out what to do. Suggestions are welcome.
Thursday
Swing and ABUs
Friday
Owl and ABUs
Friday
Day and ABUs