[Sbs_gems] [EXTERNAL] GEM operating voltage

[Kondo Gnanvo]

Dear all,
Please find attached the preliminary analysis on 500k replay (Thanks Sean) of two run where we were comparing the efficiency with nominal GEM HV (red) and with the HV lowered 25 V (blue) on all the chambers.

For all layers, the difference in efficiency is within 1% ==> it is even higher for the UV GEM Layer 2 at the lower HV setting (which might be due to the opposing effect of the less current drop in the divider)

Conclusion is that it does not seems to be any significant difference in the performances.
Xinzhan post the same results with 50k replay (less statistic) and I am going to post a log entry for this slides as well

Our plan now is to have a run at -50 V when we move back to LD2 production and to compare once again and after we can make a final decision

Best regards
Kondo



From: Bogdan Wojtsekhowski <bogdanw at jlab.org>
Sent: Thursday, December 16, 2021 11:09 AM
To: Kondo Gnanvo <kagnanvo at jlab.org>; Xinzhan Bai <xb4zp at virginia.edu>; Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>
Subject: Re: [EXTERNAL] GEM operating voltage

I came to the same idea. We must reduce HV today!
________________________________
From: Kondo Gnanvo <kagnanvo at jlab.org<mailto:kagnanvo at jlab.org>>
Sent: Thursday, December 16, 2021 11:07 AM
To: Xinzhan Bai <xb4zp at virginia.edu<mailto:xb4zp at virginia.edu>>; Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>; Bogdan Wojtsekhowski <bogdanw at jlab.org<mailto:bogdanw at jlab.org>>
Subject: RE: [EXTERNAL] GEM operating voltage


Hi Xinzhan,

I agree with you and I am making this case for quite a while now and I send a slide about it a few days ago.

Using the track based efficiency as only way to decide everything on how to operate the GEMs is a huge mistake

That is why the data taken at -25 and -50 V need to be analyze to see what we loose in efficiency. I am sure we don't loose anything at -25 V and very small at -50 V.



Best regards

Kondo



From: Xinzhan Bai <xb4zp at virginia.edu<mailto:xb4zp at virginia.edu>>
Sent: Thursday, December 16, 2021 10:47 AM
To: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>; Kondo Gnanvo <kagnanvo at jlab.org<mailto:kagnanvo at jlab.org>>; Bogdan Wojtsekhowski <bogdanw at jlab.org<mailto:bogdanw at jlab.org>>
Subject: [EXTERNAL] GEM operating voltage



Dear Nilanga and Kondo,



Regarding the newly found dead sector, see HALOG entry:

https://logbooks.jlab.org/entry/3964165



I checked around, I think that it is possible that this is due to discharge from a large ionization.

in the attached two papers (I think similar to our application):

Page 4 of "J_D_Swift_1969....pdf",

and Page 6 of "HV_discharge_acceleration_by_....pdf"



I can see a clear increase of the discharge probability when applying a higher

voltage upon a fixed distance.



Maybe we don't need to lower 50 V for all chambers, but lowering 20 ~ 30 V can also make a big difference in reducing the discharge probability. I think reducing voltage can improve the lift-span of our GEM chambers under high beam current operation.



To me, I am not 100% sure we are losing real tracks considering we have fake tracks. The efficiency from tracking is only around 50%~70% suggests that the fake tracks are more than 20%.



Best,



Xinzhan






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