[Sbs_gems] [EXTERNAL] RE: [New Logentry] Brief Voltage Spike on uva_xygem_2

Mon Dec 6 18:45:01 EST 2021

Dear all,
Following a quick phone conversation with Xinzhan, I produce a table with possible sources of sparks with GEM in the experiment (based on my understanding of how these detector operate).
I listed the most likely sources, where it affects the most in the chamber, and what we can do about them.

To summarize, with the BB GEMs been operational for more than 2 months now, only the spark caused by large ionization and the one caused by defects on the GEM foils are in my view the one we should expect during the runs.

  1.  The first one with large ionization is likely to cause no damage as long as it is not happening on a week spot of a GEM foil. It also likely going to happen on the third GEM foil where the concentration of charge around the holes is huge. The probability of this to happens increase with beam current and with HV on the GEMs. I am pretty sure this is what we are seeing right now. Setting an HV trip will do nothing other than forcing us to ramp up very frequently the GEMs (should be avoided)

  1.  The second is if there is a local defect on the GEM foils. This defects comes with the fabrication of the GEM foil and are normally cured during the assembly process. But there is no certainty that all the defect can be eliminated. So an avalanche around this area will trigger the spark (even with low ionization event and low beam current) and the spark have a high probability to short the sector. There is nothing we can do about this one if I happens. A current trip limit in operational condition will not do us any favor. If the spark happens, it will do the damage it want to do, whether you shot the HV down or not. I believe this is what happens with INFN GEM as well as UV layer 0 HV sector.

The other causes are the one in our hands i.e avoid humidity in the chamber, operating with clean gas mixture, and knowing the HV divider etc … Considering the chambers has been running for quite sometime now, I would not worry too much about these causes.

In any cases, we can reduce the spark rate probability by lowering the HV on the GEMs and / or the beam current. So there is a compromise between safety and long term stability and the performance in term of high rate capability and efficiency and I think this is a discussion we need to have within the GEM experts of the collaboration.

Hope that helps
Kondo

From: Sean Jeffas <sj9ry at virginia.edu>
Sent: Monday, December 6, 2021 4:06 PM
To: Gnanvo, Kondo (kg6cq) <kg6cq at virginia.edu>
Cc: David Flay <flay at jlab.org>; Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>; sbs_gems at jlab.org
Subject: Re: [Sbs_gems] [EXTERNAL] RE: [New Logentry] Brief Voltage Spike on uva_xygem_2

Hi All,

I scrolled through the logbook and found we had sparks causing > 20 uA jumps three times total in the past week of running. Therefore it seems like a rare spark to me, and should not cause any issues, and does not need any action. We chose our current operating voltages at the very beginning of the plateau so I think it would be a significant change to decrease any of them by 50 V.

Best,
Sean

On Mon, Dec 6, 2021 at 3:51 PM Gnanvo, Kondo (kg6cq) <kg6cq at virginia.edu<mailto:kg6cq at virginia.edu>> wrote:
Hi all,
I think we should be clear on what we are discussing here!

  *   What I saw on the logbook looks like a spark from me and I don’t really think it is something to worry about. We expect that with GEMs unless there is a specific pattern with that spike and in that case we should take ab look and try to understand. I would suggest that GEM expert look at the chart and come with a clear conclusion on that

  *   Reducing HV for 50 V because of APV saturation is a discussion that we had yesterday , so I will not repeat the argument here but let’s try not to mix issues and solution we apply to try to address.

Best regards
Kondo

From: Sbs_gems <sbs_gems-bounces at jlab.org<mailto:sbs_gems-bounces at jlab.org>> On Behalf Of David Flay
Sent: Monday, December 6, 2021 3:46 PM
To: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>; sbs_gems at jlab.org<mailto:sbs_gems at jlab.org>
Subject: Re: [Sbs_gems] [New Logentry] Brief Voltage Spike on uva_xygem_2

Thanks Nilanga,

Zeke and I will do that before we start the next run.

Best regards,
David

----------------------------------------------------------------
David Flay, Ph. D.
Hall A/C Staff Scientist
Thomas Jefferson National Accelerator Facility
Office: CEBAF Center, C119
----------------------------------------------------------------

________________________________
From: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>
Sent: Monday, December 6, 2021 3:37 PM
To: David Flay <flay at jlab.org<mailto:flay at jlab.org>>; sbs_gems at jlab.org<mailto:sbs_gems at jlab.org> <sbs_gems at jlab.org<mailto:sbs_gems at jlab.org>>
Subject: [EXTERNAL] Re: [Sbs_gems] [New Logentry] Brief Voltage Spike on uva_xygem_2

Hmmm, that is a bit worrying.

I think, until we understand this a bit more, we should back down the HV on just this one module by 50 V.

I expect the reduction of efficiency to be very small because of this change. So I think we should do this soon.

Best

Nilanga

________________________________
From: Sbs_gems <sbs_gems-bounces at jlab.org<mailto:sbs_gems-bounces at jlab.org>> on behalf of flay at jlab.org<mailto:flay at jlab.org> <flay at jlab.org<mailto:flay at jlab.org>>
Sent: Monday, December 6, 2021 3:20 PM
To: sbs_gems at jlab.org<mailto:sbs_gems at jlab.org> <sbs_gems at jlab.org<mailto:sbs_gems at jlab.org>>
Subject: [Sbs_gems] [New Logentry] Brief Voltage Spike on uva_xygem_2

Brief Voltage Spike on uva_xygem_2 <https://logbooks.jlab.org/entry/3957495>

  *   Edit<https://logbooks.jlab.org/entry/3957495/edit?destination=email/send>
  *   Delete<https://logbooks.jlab.org/entry/3957495/delete?destination=email/send>

Lognumber 3957495<https://logbooks.jlab.org/entry/3957495>. Submitted by flay<https://logbooks.jlab.org/user/flay> on Mon, 12/06/2021 - 15:17<https://logbooks.jlab.org/entries?start_date=1638818252&end_date=1638825452&book=HALOG>.
Logbooks:
HALOG<https://logbooks.jlab.org/book/halog>
Entry Makers:
flay

Just catching up on this after the DAQ has settled back in and the HCAL HV was power cycled.

At around 13:50 we had a HV spike on uva_xygem_2, see attached.
[https://logbooks.jlab.org/files/styles/plentybig/public/2021/12/3957495/uva_xygem_2_spike_12-06-21.png]
_______________________________________________
Sbs_gems mailing list
Sbs_gems at jlab.org<mailto:Sbs_gems at jlab.org>
https://mailman.jlab.org/mailman/listinfo/sbs_gems
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/sbs_gems/attachments/20211206/61587db8/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Spark_with_tripleGEMs.pdf
Type: application/pdf
Size: 136562 bytes
Desc: Spark_with_tripleGEMs.pdf
URL: <https://mailman.jlab.org/pipermail/sbs_gems/attachments/20211206/61587db8/attachment-0001.pdf>