CERN - European Organization for Nuclear Research

Minutes of the CLIC Meeting - 17 June 2005


Agenda:
Luminosity Tuning Bumps in the CLIC Main Linac
Speakers:
P. Eliasson, D. Schulte

Abstract

Preservation of beam emittance in the CLIC main linac is a challenging task. This requires not only beam-based alignment of the beam line components but also the use of emittance tuning bumps. Here, the potential use of luminosity tuning bumps is explored and compared to emittance tuning bumps.

While a direct measurement of the luminosity would be ideal for the bump optimisation, it may be too slow. Alternative signals are presented that may be used for the optimisation of tuning knobs.


Peder reported on Luminosity tuning bumps in the CLIC main linac.
After BBA, still 380% emittance growth (target <100%) persists from residual structure misalignment. So emittance/luminosity tuning bumps have to be used for correction. Two accelerating structures are moved transversally (2 sets of rthogonal bumps) to compensate the integrated wakefields and the effect is measured downstream. With realistic 5 point scans for each bump, the luminosity can be almost fully recovered in the ideal case. With 3% noise, the luminosity is ~3% lower. Including a BDS simulation, a level of 90% can be reached with 5 pairs of bumps, which is good compared to the target value of 70%. A 'large' laser wire to simulate the other beam could be used as a tuning signal.
copy of Peder's transparencies (pdf format)

Daniel gave Some Remarks on Luminosity Signals.
Luminosity measurement is critical to be able to optimise machine. Conventional signals (like radiative Bhabhas or large angle Bhabhas) may be not visible or too slow for efficient luminosity optimisation. Alternative signals that could be used for tuning are:
- Incoherent pairs: their energy is proportional to luminosity and a good signal to use but could be spoiled by coherent pairs.
- Beamstrahlung: the fact that it is minimal for head-on collision without offset can be exploited.
99% of the luminosity can be obtained using these signal with luminosity tuning bumps but the sensitivity of tuning procedures to small fluctuations in the beam parameters needs to be understood
copy of Daniel's transparencies (pdf format)

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Frank Tecker  - Last updated  06-07-2005