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

Tue Apr 30 19:25:54 EDT 2019

	Hi Michael,

Here at Berkeley we are looking at the idea similar to the BPM pickup. This could be done with a quadrupole cavity or in theory with a non-resonant pickup (e.g. striplines, pixels). The problem in the latter case is signal-noise ratio but if we are still evaluating if it is doable with the existing hardware. There is a company in San Diego that makes quadrupole cavities — it’s a spinoff from an R&D done at SLAC. The caveat is that a single cavity does not measure the beam size (ie. <x^2+y^2>) — you measure quadrupole modes, which are <x^2-y^2> and <xy>. But this may be good enough for a constraint on the beam size asymmetries, especially since <x^2> would dominate due to synchrotron emission in the bends. SLAC guys did publish a paper that claims that one could measure the full emittance of the beam with 6 quadrupole cavities positioned at different betatron phases. 

The other option that could potentially work is to have a sufficiently pixelated synchrotron light monitor, and de-correlate variations in energy from it. Silicon pixel sensors would get fried quickly, but one could consider diamond sensor technology that is just being developed for high-luminosity colliders. 

Yury

> On Apr 30, 2019, at 1:16 PM, Michael Tiefenback <tiefen at jlab.org> wrote:
> 
> 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 <mailto:benesch at jlab.org>>
> Sent: Tuesday, April 30, 2019 2:30 PM
> To: moller at jlab.org <mailto: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%7Cmoller%40jlab.org%7C11b60a14f8be481df1c008d6cdc33a38%7Cb4d7ee1f4fb34f0690372b5b522042ab%7C1%7C0%7C636922635710455783&sdata=ClfzjW0dWjCNEN92Yu4LAT4goThvLSjAWQgyL4zS5JI%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%7Cmoller%40jlab.org%7C11b60a14f8be481df1c008d6cdc33a38%7Cb4d7ee1f4fb34f0690372b5b522042ab%7C1%7C0%7C636922635710455783&sdata=ClfzjW0dWjCNEN92Yu4LAT4goThvLSjAWQgyL4zS5JI%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.
> 
> ------------------
> 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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