[Moller] Noninvasive LHC transverse beam size measurement using inelastic beam-gas interactions

[Michael Tiefenback]

Valeri Lebedev was considering using such an imaging device here many years ago, adapted from proton ring practice.  The reason he dropped the idea was the strong ionization of background gas by VUV Synchrotron Radiation.  He concluded that the background would be unworkably large.  This may or may not be so, depending upon how close the device is to the final dipole, and how much or little S.R. is generated in the focusing fields of quadrupoles.


In that ionization detector imaging, I think that a transverse magnetic field was desirable to preserve the spatial structure, the electrons being low-energy and "tied" to the field lines.  In such a way, it was possible to keep the phosphor somewhat distant from the beam while preserving optical fidelity.


Alternate 1) Optical Transition Radiation is quite prompt.  One might be able to use this on an occasionally inserted target, although not for the continuous monitoring supported by the ionization detector.


Alternate 2) Another alternative might be placement of a wire array (say, eight BPM-ish pickups) on a 1 cm radius circle and routing the signals out on coax into one of the nonlinear-processing devices to obtain estimates of beam size.  We've toyed with such an idea, but the size of the BPMs we have is large.  I expected the aspect ratio to suppress the achievable resolution.  But if such an array could be placed close to the beam, possibly as an adjunct to a "halo target," it might work out well.

This seems to me to be a potentially workable path.  I included the whole "moller" group in reply, as Jay included them in his suggestion.

Michael Tiefenback

________________________________
From: Jay Benesch <benesch at jlab.org>
Sent: Tuesday, April 30, 2019 2:30 PM
To: moller at jlab.org
Cc: Michael Tiefenback
Subject: Noninvasive LHC transverse beam size measurement using inelastic beam-gas interactions

https://gcc01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fjournals.aps.org%2Fprab%2Fabstract%2F10.1103%2FPhysRevAccelBeams.22.042801&data=02%7C01%7Ctiefen%40jlab.org%7C0a8294e5122a4d6c5f7108d6cd99fef4%7Cb4d7ee1f4fb34f0690372b5b522042ab%7C1%7C0%7C636922458627310032&sdata=Q0rqTs0MSVeIOD%2BFYt2xj8kXrFKsdpPMn3o8dWWt580%3D&reserved=0<https://gcc01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fjournals.aps.org%2Fprab%2Fabstract%2F10.1103%2FPhysRevAccelBeams.22.042801&data=02%7C01%7C%7C8fddd4334a104a57724b08d6cda8becb%7Cb4d7ee1f4fb34f0690372b5b522042ab%7C1%7C0%7C636922521962397034&sdata=6EicRc2Ps6yKisdOavCOiGzfnLpi3KEfiwchogmblVY%3D&reserved=0>

Noninvasive LHC transverse beam size measurement using inelastic
beam-gas interactions
A. Alexopoulos et al. (The BGV Collaboration)
Phys. Rev. Accel. Beams 22, 042801 – Published 11 April 2019

The beam-gas vertex (BGV) detector is an innovative instrument measuring
noninvasively the transverse beam size in the Large Hadron Collider
(LHC) using reconstructed tracks from beam-gas interactions. The BGV
detector was installed in 2016 as part of the R&D for the
High-Luminosity LHC project. It allows beam size measurements throughout
the LHC acceleration cycle with high-intensity physics beams. A
precision better than 2% with an integration time of less than 30 s is
obtained on the average beam size measured, while the transverse size of
individual proton bunches is measured with a resolution of 5% within 5
min. Particles emerging from beam-gas interactions in a specially
developed gas volume along the beam direction are recorded by two
tracking stations made of scintillating fibers. A scintillator trigger
system selects, on-line, events with tracks originating from the
interaction region. All the detector elements are located outside the
beam vacuum pipe to simplify the design and minimize interference with
the accelerated particle beam. The beam size measurement results
presented here are based on the correlation between tracks originating
from the same beam-gas interaction vertex.

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Seems like it should be easy (i.e. several dissertations) to extend this
to measuring electrons at 11 GeV, 249 MHz, helicity correlated, to 10ppm
desired by MOLLER.  Nice reference list.
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