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Understanding collimators

Posted: Sun Mar 06, 2011 8:00 am
by patrol-cz
Hi!
I would like to achieve better understanding, how the collimators are organized around the machine. When you look at the collimator fixed displays for beam 1 and 2, you can find some rules, but there are too many exceptions from the rules, that it's worth asking :-).
1) In IPs 1,3,5,6 and 7, both the displays show analogical collimators, but in the reverse order. It's probably because of the opposite beam travelling direction. So far, OK.
2) Collimators in IP2 for beam 1 seem to be mirrored in IP8 of beam 2, while the ones in IP8 for B1 are mirrored in IP2 for B2. Why ? Does it have a relation to the transfer lines, which are leading there ?
3) There is a very special collimator in IP2 for B1, TCDD.4L2, which doesn't have its counterpart in neither IP for B2. It is also the one with smallest gap (about 0.7/-0.7 all the time). What's this collimator for and why it's just in B1 ?
4) The gap values are permanently changing slightly for some coliis, without any apparent reason. Do they really move, or is it just some jitter in the position sensor ?
With regards,
Pavel

Re: Understanding collimators

Posted: Mon Mar 07, 2011 12:54 pm
by pcatom
patrol-cz wrote:Hi!
I would like to achieve better understanding, how the collimators are organized around the machine. When you look at the collimator fixed displays for beam 1 and 2, you can find some rules, but there are too many exceptions from the rules, that it's worth asking :-).
1) In IPs 1,3,5,6 and 7, both the displays show analogical collimators, but in the reverse order. It's probably because of the opposite beam travelling direction. So far, OK.
2) Collimators in IP2 for beam 1 seem to be mirrored in IP8 of beam 2, while the ones in IP8 for B1 are mirrored in IP2 for B2. Why ? Does it have a relation to the transfer lines, which are leading there ?
3) There is a very special collimator in IP2 for B1, TCDD.4L2, which doesn't have its counterpart in neither IP for B2. It is also the one with smallest gap (about 0.7/-0.7 all the time). What's this collimator for and why it's just in B1 ?
4) The gap values are permanently changing slightly for some coliis, without any apparent reason. Do they really move, or is it just some jitter in the position sensor ?
With regards,
Pavel
The TCDD is a mask that is installed just in front of the D1 magnet to protect it from showers coming from the losses around the TDI. In IP8 it is a fixed mask - it does not move - therefore it does not appear on the collimator page. In IP2 the TCDD has to be able to be retracted since it would otherwise interfere with the Alice ZDC detectors during collisions. It only has 2 positions (IN and OUT). When it is IN (during injection) it appears like it is fully closed (ie very small gap). This is an artifact of the collimator display since it understands positions, not IN/OUT binary states. A future version should take this into account i think.

The collimator positions appear to move slightly due to readings of the position. The jitter is at the 10um level. The collimator jaws themselves to not move.

Hope this helps.

Re: Understanding collimators

Posted: Mon Mar 07, 2011 4:08 pm
by patrol-cz
pcatom wrote:
patrol-cz wrote:Hi!
I would like to achieve better understanding, how the collimators are organized around the machine. When you look at the collimator fixed displays for beam 1 and 2, you can find some rules, but there are too many exceptions from the rules, that it's worth asking :-).
1) In IPs 1,3,5,6 and 7, both the displays show analogical collimators, but in the reverse order. It's probably because of the opposite beam travelling direction. So far, OK.
2) Collimators in IP2 for beam 1 seem to be mirrored in IP8 of beam 2, while the ones in IP8 for B1 are mirrored in IP2 for B2. Why ? Does it have a relation to the transfer lines, which are leading there ?
3) There is a very special collimator in IP2 for B1, TCDD.4L2, which doesn't have its counterpart in neither IP for B2. It is also the one with smallest gap (about 0.7/-0.7 all the time). What's this collimator for and why it's just in B1 ?
4) The gap values are permanently changing slightly for some coliis, without any apparent reason. Do they really move, or is it just some jitter in the position sensor ?
With regards,
Pavel
The TCDD is a mask that is installed just in front of the D1 magnet to protect it from showers coming from the losses around the TDI. In IP8 it is a fixed mask - it does not move - therefore it does not appear on the collimator page. In IP2 the TCDD has to be able to be retracted since it would otherwise interfere with the Alice ZDC detectors during collisions. It only has 2 positions (IN and OUT). When it is IN (during injection) it appears like it is fully closed (ie very small gap). This is an artifact of the collimator display since it understands positions, not IN/OUT binary states. A future version should take this into account i think.

The collimator positions appear to move slightly due to readings of the position. The jitter is at the 10um level. The collimator jaws themselves to not move.

Hope this helps.
Hi pcatom!
Many thanks for a very detailed and comprehensive explanation, it helped perfectly!
With regards, Pavel