BPM requirements in CLIC

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

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.

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.

References to concise papers will be given.

Main Linac and BDS

BPMs are used in the following procedures:

  • Beam Based Alignment (ML)
  • Continuous orbit correction (ML+BDS)
  • Tuning of Final Focus (BDS)

Beam Based Alignment in Main Linac


The ML will be aligned with the beam in various stages.

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. 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

One to One Steering

As one to one steering is only used to make the beam pass through the linac, no stringent requirements on the BPMs are needed.

Dispersion Free Steering (DFS) (or Ballistic Alignment)

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.

DFS with bunch compressor

Wakefield Free Steering / Girder Alignment

BPMs have no big impact in the girder alignment as it will be done with the wakefield monitors. BPMs are only used to keep the beam aligned.

Girder Alignment paper

Continuous orbit correction

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.

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:

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.

In addition, a BPM scaling error of 1% results in 0.5% luminosity loss.

Tuning of Final Focus

The CLIC Final Focus System (FFS), the last part (about 500m) of the BDS, is very non-linear and this needs a special tuning procedure. The current tuning algorithms require a BPM resolution of 10 nm.

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.

From the CDR:

BDS tuning and luminosity measurement with background

Failure and Redundancy

Preliminary studies have been done to determine the impact BPM failures wrt orbit correction. It has been shown that ML BPMs are quite redundant, while some BPMs in the Final Focus are crucial. BPM failure impact on orbit correction

Drive Beam (Decelerator)

In order to perform orbit correction in the Drive Beam Decelerator a sufficient number of Beam Position Monitors must be installed. The performance of the orbit correction depends on the accuracy and resolution of the BPMs, as well as the number of BPMs installed. If the BPMs are installed on all quadrupoles, the orbit correction algorithms reduce the beam envelope close to the minimum achievable, assuming a BPM accuracy of 20 Ám and a BPM resolution of 2 Ám, with performance mostly independent of quadrupole misalignment. If the BPMs are installed on every other quadrupole magnet, assuming the same BPM accuracy and resolution, the orbit correction performance would be degraded if the quadrupole misalignment is substantially larger than the baseline value of 20 um. As a compromise between cost, performance, and robustness, it is foreseen to install two BPMs per three quadrupoles. The BPM accuracy and precision must not deteriorate substantially for tune-up beams.

In addition, as the with the ML, the drive beam will need to be aligned with BBA and DFS. It has been shown that a BPM resolution of 2 Ám is adequate for that (paper).

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Topic revision: r1 - 15 Aug 2012 - JochemSnuverink

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