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CLIC Luminosity Monitoring with Beamstrahlung for Fast Re-Tuning

This paper discusses the challenges in luminosity tuning at CLIC due to coherent pairs and proposes using beamstrahlung photons for faster monitoring and re-tuning. It covers the concept of post-collision beam lines and suggests ideas for measuring beamstrahlung photons. The text elaborates on the angular distribution, energies, and rate of beamstrahlung photons, emphasizing the need for a fast signal to track changes in beam parameters. Various layouts, background considerations, and open questions related to tuning using beamstrahlung are explored, along with practical solutions for monitoring and adjusting luminosity efficiently.

michelleandrews
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CLIC Luminosity Monitoring with Beamstrahlung for Fast Re-Tuning

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  1. CLIC luminosity monitoring/re-tuning using beamstrahlung “?” 1. Beamstrahlung etc. – from D. Schulte 2. Conceptual design of a CLIC post-collision beam-line (“spent beam”) – from A. Ferrari 3. How could one measure beamstrahlung photons - ideas from E. Bravin 4. Possible layout, background, open questions

  2. 1. Beamstrahlung etc. – from D. Schulte

  3. luminosity tuning: performed in BDS, using laser wires etc. ... a tedious, long procedure ... luminosity monitoring and re-tuning: ILC uses incoherent pairs – CLIC has problem: many coherent pairs

  4. -> keep track of luminosity … "fast" signal needed (there will be changes of beam position, angle, waist …) … and correct for these changes -> possible signal: beamstrahlung

  5. beamstrahlung photons: energies: from <1 GeV to <1.5 TeV (max. at > 1 TeV) rate: 2.4 photons per electron (positron) in beam, i.e. 1E+10 photons per bunch x 311 bunches 3E+12 photons per 207 ns pulse (repetition rate 50 Hz) angular distribution: ± 50 mrad (full width) (2005 parameters) NB. all numbers for nominal collision parameters !

  6. 3.

  7. 20 mrad crossing angle – horizontal plane beam separation in the horizontal plane [m] distance from IP [m] H

  8. V distance from IP [m] 4 extraction magnets (- 3.2 mrad) 4 “C” magnets (+ 3.2 mrad) vertical displacement for 1.5 GeV beam [m] beam dump 16 quadrupoles (huge aperture !)

  9. V distance from IP [m] somewhere around here... ? 4 extraction magnets (- 3.2 mrad) 4 “C” magnets (+ 3.2 mrad) vertical displacement for 1.5 GeV beam [m] wrong sign particle diagnositcs / dump E > 200 GeV

  10. A. Ferrari, CLIC note 704

  11. 3. How could one measure beamstrahlung photons - ideas from E. Bravin (18 July 2007 meeting) 3E+12 photons in 207 ns, E up to 1.5 TeV  better use a “thin” detector – “fast” (get information on the number of photons, can not get information on their energy) basic principle: converter + OTR monitor g -> e+e- (optical transition radiation) question: layout, backgrounds, etc. etc.

  12. typical OTR monitor arrangements: e.g. “intensity” and profile e.g. “intensity” only CCD optics PM polished tube works at >10E+11 part. good pos. resolution (+ size of beam) “rad. hard” cameras exist... very slow almost “single counting”; very fast (< 1 ns) “radiation hard” sensitive to “direct hits” ATTENTION: No absolute calibration for the intensity !!

  13. one of the “standard” OTR systems at CERN (different screens for different beam intensities)

  14. Pair Production (nuclear field)in 1 mm graphite (0.005 Xo ) 4.4 E+9 part. Signal B.G. pair production probability 1.8 E+9 part. photon energy [MeV]

  15. 4. Possible layout, background, open questions > 130 m

  16. A. Ferrari, CLIC note 704

  17. top view < 1.5 TeV . . . . . > 200 GeV view through last C-magnet in beam direction (dimension in cm, from CLIC Note 704)

  18. side view last C-magnet

  19. background No. 1: synchrotron radiation photons pair production -> < 50 MeV particles solution (?): 10-3 Tm magnetic field (-> 15 mrad at 20 MeV) (if possible, sweep l.e. particles in H-plane, observe OTR light in V-plane) use “small” OTR screen at 5 m from converter (e.g. diameter 30 mm OTR screen) background No. 2: scattered electrons/positrons of all kinds -> to be studied background No. 3: neutrons (stay far away from IP and from dumps) -> to be studied

  20. side view field 10-3 Tm last C-magnet z=100 m z=95 m

  21. open questions: ->strength of C-magnets (separation beamstrahlung from particles > 100 mm (?)) -> investigate beamstrahlung distribution for variousnon-perfect conditions(e.g. Fig. 20 in CLIC note 704) -> introduce “realistic” monitor into simulations (?) and test the “tuning knobs” + everything overlooked or forgotten !

  22. 5. Summary The option of using beamstrahlung photons for “fast” feedback and luminosity tuning at CLIC appears still valid. The technique using a converter plus OTR has several interesting features (e.g. change converter thickness for lower intensity running, different options for OTR detection, etc. etc.). Assuming that CLIC-Note 704 is the reference design for the post-collision beamlines, the location at about 100 m from the interaction point could be reasonable. “... affaire à suivre ...”

  23. extraction magnet (“window-frame”) C - magnet A. Ferrari, CLIC note 704

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