Difference: BeamDynamicsBPMrequirements (4 vs. 5)

Revision 504 Jun 2013 - JsnuverinkSnuverink

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BPM requirements in CLIC

This page will discuss the CLIC BPM requirements from Beam Dynamics.

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The CDR states an accuracy / resolution of 5 Ám / 50 nm for the Main Linac (ML) BPMs, 5 Ám / 20-50 nm for the Beam Delivery System (BDS) and 5 Ám / 3 nm for the final doublet (last two quadrupoles), and 20 Ám / 2 Ám for the Drive Beam decelerator BPMs. In addition, there are four horizontal and four vertical beam size laser wires per BDS line in its diagnostic section. The vertical laser wires must resolve a 1 Ám beam with 1% resolution.
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The CLIC CDR states an accuracy / resolution of 5 Ám / 50 nm for the Main Linac (ML) BPMs, 5 Ám / 20-50 nm for the Beam Delivery System (BDS) and 5 Ám / 3 nm for the final doublet (last two quadrupoles), and 20 Ám / 2 Ám for the Drive Beam decelerator BPMs. In addition, there are four horizontal and four vertical beam size laser wires per BDS line in its diagnostic section. The vertical laser wires must resolve a 1 Ám beam with 1% resolution.

The Main Linac BPMs have the following specifications (CDR):

Parameter Value
Quantity 4096
Nominal Current 1.2A
Bunch frequency 2GHz
Single bunch charge 600pC (3.72e9)
Beam aperture 8mm
Position resolution 50nm rms
Temporal resolution 10ns
Accuracy 5 um
Stability 100 nm
  Many simulation studies have been performed with different algorithms, parameters for misalignment, resolution etc. and different degrees of realism. In general it is impossible to give single requirements for certain parameters, as there is a complicated interplay between all parameters. Requirements can be loosened by requiring tighter requirements on others.
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  • Continuous orbit correction (ML+BDS)
  • Tuning of Final Focus (BDS)
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Beam Based Alignment in Main Linac

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Introduction

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 The ML will be aligned with the beam in various stages.
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Beam based alignment techniques are used to minimise the emittance growth due to residual misalignment of beam line elements after the initial pre-alignment done with a wire system. At the start of the ML, the (normalised) emittance is 10 nm and the emittance growth budget for static misalignment is 5 nm. First, simple one-to-one steering is used to make the beam pass through the linac without significant beam loss. Then dispersion free steering is used to optimise the position of the beam position monitors and quadrupoles. This can also be done with ballistic alignment. Next, the offsets of the accelerating structures relative to the beam are determined using the wakefield monitors and minimized using the movers on which the acceleration modules are installed. Finally, emittance tuning knobs are used to further reduce the emittance growth. These knobs cancel the wakefield effects globally by moving accelerating structures at various locations until the emittance measured at the end of the linac is minimized.
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Beam based alignment techniques are used to minimise the emittance growth due to residual misalignment of beam line elements after the initial pre-alignment done with a wire system. At the start of the ML, the (normalised) emittance is 10 nm and the emittance growth budget for static misalignment is 5 nm. First, simple one-to-one steering is used to make the beam pass through the linac without significant beam loss. Then dispersion free steering is used to optimise the position of the beam position monitors and quadrupoles. This can also be done with ballistic alignment. Next, the offsets of the accelerating structures relative to the beam are determined using the wakefield monitors and minimized using the movers on which the acceleration modules are installed. Finally, emittance tuning knobs are used to further reduce the emittance growth. These knobs cancel the wakefield effects globally by moving accelerating structures at various locations until the emittance measured at the end of the linac is minimized.
  overview paper
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  As one to one steering is only used to make the beam pass through the linac, no stringent requirements on the BPMs are needed (440 nm).
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Dispersion Free Steering (DFS) (or Ballistic Alignment)

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 Simulations show that a BPM resolution of 100 nm will result in an acceptable emittance growth. Position errors of the BPM with respect to the RF structure is a crucial part here, with a quoted requirement of 10 Ám rms. This will be delivered by the pre-alignment wire system.
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  DFS with bunch compressor
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  BPMs have no big impact in the structure alignment as it will be done with the wakefield monitors. BPMs are only used to keep the beam aligned.
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  Girder Alignment paper
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  Girder Alignment paper
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Continuous orbit correction

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 Realistic dynamic simulations (including ground motion, active and passive stabilisation and a realistic orbit and IP feedback) have been performed. This IPAC11 paper is a good introduction to those simulations.
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These simulations and the orbit feedback algorithm has improved in recent years and for the current baseline, these simulations show the following correspondence between BPM resolutions and luminosity loss:

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These simulations and the orbit feedback algorithm has improved in recent years and for the current baseline, these simulations show the following correspondence between BPM resolutions and luminosity loss:
 
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  Previously, a BPM resolution requirement of 50 nm in the ML was thought to be necessary, but recent studies show that 100 nm should be sufficient.
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  However, currently these procedures do not result in the required performance (90% chance of reaching 110% of luminosity). It is expected (hoped) that a new BDS lattice and/or improved algorithms will obtain the required performance.
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From the CDR:
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From the CDR:
  BDS tuning and luminosity measurement with background
 
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