These are some TPC tests that need to be taken before Run 18 isobar operations end.

iTPC GridLeak wall voltage tests
For better understanding of the impact of the iTPC GridLeak wall on distortions and how best to operate it

• Roughly minbias trigger with a wide vertex z selection (bbc-mon? needs Jeff's assistance to set up a configuration)
• Necessary detectors: tpc+itpc+btof+emc+eemc+(trigger detectors)
• Non-critical but welcome detectors: gmt+etof+mtd
• 4 runs of 100k events each at each (~1 minute) level of the Vernier scan
  • The 4 runs are 4 different voltage settings on the GridLeak wall (115/115, 115/700, 700/115, 115,0)
• Vernier scan should include higher luminosity (e.g. 20 kHz) above normal 10 kHz operation, and lower luminosity (e.g. 2 kHz)
  
  • Going to even lower luminosity would be welcome, but not critical; going below 2 kHz the acquisition will become noticeably slower

Anode load tests
For confidence in sectors being able to return to charge exposure levels comparable to pp500 when BES II is finished. During pp500, we see currents as high as 1.5 μA, and that is with voltages reduced. We will not see levels like this during BES II. During the current isobar running, we are at 0.2 μA while at full voltage. 

• No DAQ; no other devices at STAR need to be on except for the slow controls of TPC outer sector anode voltages
• Gated Grid fully open (Alexei & Jim know how to do this)
• Collision rate somewhere around 10 kHz - 20 kHz
• ~20 minutes duration
• Anode voltages will be brought up from ~zero gain slowly and currents will be kept below the 1.5 μA seen in pp510, but will be brought to where currents are in excess of 1.0 muA to replicate loads that are at least comparable; notable tripping would be cause to cease the test
• The test would be conducted on at least (and probably only) outer sector 20

The risks of the test are worth some discussion. While not truly quantifiable, here are some comments on the risk:

• The test will not expose anodes to the same current loads seen in past operations - there is no intent to operate the detector under never-before-seen conditions of loading.
• Given that both the reason for the tripping under even mild loads before 2018 and the drastically reducing tripping of anode channel 20-5 in 2018 are unknown, it is not possible to quantify the risk for channel 20-5 to begin tripping easily again after the test. That risk is generally perceived to be non-neglible, as is also the risk that it begins tripping easily again even without the test.
  • While the tripping of 20-5 was triggered by high load in the past, there is clearly something different this year.
  • This is only 1 channel out of 192 anode channels that is perceived to have a non-negligible risk, although that 1 channel's operation likely dictates whether outer sector 20 by itself can be used for physics analyses in the absence of inner sector 20 (iTPC test unit).
• No anode channels (including 20-5) tripped during the 20 kHz collisions, 1.5 kHz DAQ operations which significantly increased the load from our standard isobar operating conditions, implying that we are not operating on the edge of a cliff for channel 20-5.
• The BUR for Run 18 was presumably planned based on past data, for which sector 20 was unusable (that it would be usable was only learned after we had real data in hand). Flipping the argument to say that running without sector 20 is an unacceptable risk implies that the BUR was not properly planned for the reality. That said, having sector 20 does give the experiment additional margins particularly for multi-particle analyses.
• If the iTPC test unit (inner sector 20) becomes available to physics, it should easily mitigate any loss of anode channel 20-5, as there would still be a large number of TPC hits usable for tracking even if the outer rows of inner sector 20 and inner rows of outer sector 20 are blacked out.
  • It appears that the iTPC test unit will likely be read out in every event for the remainder of Run 18.

 
-Gene