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EPIC Far-Forward Weekly Meeting
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Description
Wiki: https://wiki.bnl.gov/EPIC/index.php?title=FarForward
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https://bnl.zoomgov.com/j/1602996456?pwd=NHlISDV4bWpDTlZVZjFWNzdkMEtBZz09
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Meeting topics:
Roman pots alignments - past experiences at LHC and RHIC, and what we need for the EIC
Roman pots alignments - past experiences at LHC and RHIC, and what we need for the EIC
Attendees: Wlodek Guryn, Alex Jentsch, Carlos Munoz Camacho, Prithwish Tribedy, Tonko Ljubicic, Yulia Furletova, Sebouh Paul, Michael Pitt, Yuji Goto, Raphael Dupre, Michael Murray
Experience with alignment of Roman pots at STAR/pp2pp (Wlodek Guryn)
- Only vertical pot stations, on both sides of STAR, about 55m from the IP.
- Accelerator coordinate system used for position and fields for the magnets.
- RP packages are surveyed w.r.t. position of the magnets in the same coordinate system.
- Central detector surveyed the same way --> precision may not be very high (things move over time).
- Alex: is it initial surveying that is less-precise, or is it just the movement over time?
- precision for the RP is ~ 10um, whereas central detector ~ few hundred um.
- Alex: is it initial surveying that is less-precise, or is it just the movement over time?
- Survey results are corrected using data taken from the run.
- For detector packages, precision pins are used to align each single-sensor frame with its neighbors --> overall package (four sensors) has tooling balls with survey targets used to align the detector w.r.t. the accelerator coordinate system.
- We need to think of where these will be located for the EIC Roman pots since our whole detector package is in vacuum --> perhaps something on the motor system?
- Basic Alignment steps:
- Align detector planes with each other using pins, then initial alignment of full packages with survey.
- Calibrate the motion and position readouts using Linear Variable Differential Transformer (LVDTs).
- Elastic scattering data used to provide corrections using collinearity of elastic scattering --> we won't have this in EIC.
- Standard candle process we can use in e+p to do this? What about e+A?
- Corrections obtained from data to the survey alignment are on the order of 20-30um.
- Lessons from RHIC: 1) BPMs need to be accessible so they can also be surveyed. 2) accelerator needs relative alignment, which uses beam-based alignment. 3) accelerator design has to incorporate these experimental alignment needs --> BPM requirements, absolute beam position.
- Prithwish: fiducial volume to ensure track goes through all layers in EIC vs. RHIC --> why not at EIC? Answer, at RHIC we have elastic scattering so we have a proton on both sides.
- BPMs give you some information about absolute beam position so a fill-by-fill correction offset can be applied --> very important for EIC since we won't have the collinearity constraint.
- Yulia: Precision of stepper motors at RHIC? --> Will have to look it up and get back to you. (very likely < 10um/step precision).
- Wlodek: back to Prithwish's question about fiducial volume. At one point the beam pipe was used to help with a fiducial volume correction using sharp edges of position of RP behind triplet, with less < 1mm precision.
Experience with alignment of Roman pots at LHC (Michael Pitt)
- Two experiments: CMS Precision Proton Spectrometer (PPS) & ATLAS Forward Proton (AFP)
- This discussion focuses on CMS experience.
- CMS PPS related to TOTEM.
- Located ~200m from IP, can measure 85% to 97% total proton momentum (e.g. XL).
- Vertical detectors do xL > 80%, and pT ~ 0.1 to 2.0 GeV/c (special LHC optics)
- Horizontal detectors operated during normal running with more-constrained acceptance.
- Proton reconstruction is done using linear optics with MAD-X info from machine.
- Alignment in the global coordinate system.
- In standard optics elastic scattering doesn't land within detecter acceptance (basically still within the beam).
- High-beta* optics (90m) allowed for low-pileup running with high acceptance of scattered proton.
- Beam based alignment.
- Use same optics as normal run, but with only 2 colliding bunches in the machine.
- Align collimators (done by machine) -- extract relative position of collimator to beam center.
- Move RP toward beam in small steps until acute activity is seen in beam loss monitors. --> placed right after the RPs.
- Location measured in local coordinate system, transform determined from collimator position information.
- Vertical position of beam located using beam halo (seeing two portions of ellipse on vertical pots), horizontal from diffractive line --> intersection is beam center.
- This is only for the calibration run to get a rough position.
- Fill-by-fill corrections are performed using data from normal running conditions (see slides). --> corrections on the order of 100um.
- Wlodek: 1) Beam based alignment at RHIC only refers the magnet; has a broader meaning. 2) reiterate need for a physics process "standard candle" to help with this procedure.
- Yulia: How often is the single bunch running done for calibration? Once per year, during commissioning, unless some major optics change happens in which case it must be done again, or switching ion species, etc.
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