Study of High Intensity Multi-Bunch  -ray Generation by Compton Scattering ATF TB 28/May/2006 presented by Tsunehiko OMORI (KEK) on behalf of Cavity-Compton collaboration

Cavity-Compton Collaboration

Two ways to get pol. e + (1) Helical Undurator (2) Laser Compton e - beam E >150 GeV Undulator L > 150 m

Two ways to get pol. e + (1) Helical Undurator (2) Laser Compton e - beam E >150 GeV Undulator L > 150 m Our Proposal

Why Laser Compton ? ii) Independence Undulator-base e + : use e - main linac Problem on design, construction, commissioning, maintenance, Laser-base e + : independent Easier construction, operation, commissioning, maintenance iii) Low energy operation Undulator-base e + : need deccelation Laser-base e + : no problem i) Positron Polarization.

ILC requirements 2x10 10 e + /bunch (hard) 2800 bunches/train (hard) 5 Hz (we have time to store e + s) Strategy New: Design for cold LC (ILC) make positrons in 100 m sec. Electron storage ring, laser pulse stacking cavity : Re-use !!! positron stacking ring. Old: Design for warm LC make positrons at once. both electron & laser beams: throw away Basic Idea: K. Moenig P. Rainer T. Omori et al., NIM A500 (2003)

Conceptual Design for warm LC T. Omori et al., NIM A500 (2003) Ne + =1.2x10 10 /bunch Old design

Electron storage ring laser pulse stacking cavities positron stacking in main DR Re-use Concept to main linac Compton ring New design

Laser Pulse Stacking Cavity Input laser (YAGlaser) Energy 0.75 mJ / bunch nsec bunch spacing train length = 50  sec Cavity Enhancement Factor =1000 Laser pulse in cavity 750 mJ/bunch single bunch in a cavity Fabry-perot Resonator

Schematic View of Whole System

R/D items (1) Compton ring (2) Laser Pulse Stacking Cavity (Optical Cavity) (4) Laser (3) e + stacking in DR simulation study hardware R/D for bunch length modulation (optional / in future) experimental R/D We need cooporation with companeis. Progresses of lasers are very rapid.

R/D items (1) Compton ring (2) Laser Pulse Stacking Cavity (Optical Cavity) (4) Laser (3) e + stacking in DR simulation study hardware R/D for bunch length modulation (optional / in future) experimental R/D <- this proposal (Cavity-Compton collaboration) We need cooporation with companeis. Progresses of lasers are very rapid.

Laser Pulse Stacking Cavity is a key. a) One of the most uncertain parts of the current design. b) The efficiency of whole system highly depends on the cavity design. c) The reqirements to other parts highly depends on the cavity design. cavity design : enhancement factor, laser spot size, and collision angle Simulation alone is not effective in desiging cavity. We need experimental R/D.

Plan: Exprmntl R/D at ATF. Make a fist prototype single cavity Put it in ATF ring Hiroshima-LAL-IPN-CERN-Kyoto-Waseda-KEK L cav = 420 mm

Points of R/D Points for high enhancement factor Points for small spot remove/suppress vibration establish feed back technology 2  - L cav --> +0 all are common in pol. e + and laser wire good matching between laser and cavity parabola mirrors (option) Achieve both high enhancement & small spot (less stabile) & (less stabile)

Points of R/D (continued) Number of g-rays strongly depend of crossing angle This in NOT common in pol. e + and laser wire Achieve smaller crossing angle 10W, 357MHz crossing angle Counts/crossing --> Small crossing angle is preferable --> constraint in chamber design ATF e - bench length = 9 mm (rms) Ne = 1x10 10 /bunch

. Collision point is at between QM13R and QM14R (s = 40 m) Collision Point

DR North Straight Section by T. Okugi e - beam optics

alpha_x = m beta_x = m eta_x = 0 m alpha_y = m beta_y = m eta_y = 0 m s = 40 m (=s0) （ between QM13R and QM14R) eps_x = 1.0E-09 m eps_y = 0.5E-11 m Assume e- beam spot size sig_x (s0) = 78 um sig_y (s0) = 6 um Stay almost constant in S = +- 1 m Twiss Parameter e - beam optics and spot size

Mirror R (mm) rms laser spot size (micron) L R Laser stacking cavity with Two Spherical Mirrors Choice of R and spot size our choice for 1st prototype L = mm

Preparation and Schedule Sep/2005 X-ray generation cavity achieved Enh.~1000 But large spot size ~ 100  m Oct Install prototype cavity into ATF ring Nov-Dec test apparatuses using parasitic running Apr/2006 We started assemble of test cavity (Not compatible to install ATF) Aug Start fabrication of prototype cavity Sep Complete prototype cavity Jan/2007 First gamma-ray generation test

Expected Number of  -rays Number of  -rays/bunch Electron :Ne = 2x10 10 (single bunch operation) Laser : 10 W (28 nJ/bunch) Optical Cavity: Enhancement = 1000 N  =1300/bunch X-ing angle = 10 deg N  = 900/bunch X-ing angle = 15 deg Number of  -rays/second Electron :Ne =1x10 10 (multi-bunch and multi-train operation) Electron 20 bunches/train, 3 trains/ring Laser : 10 W (28 nJ/bunch) Optical Cavity: Enhancement = 1000 N  = 8.5x10 10 /sec X-ing angle = 10 deg N  = 5.7x10 10 /sec X-ing angle = 15 deg

2nd & 3rd Prototype Cavities? Longer cavity L cav = 840 mm, 2100 mm Cavity using parabolic mirrors Possible candidates (not decided yet)

Goal of the Project Results of prototype cavity(s) Design cavity of ILC Compton Ring(CR) Decide number of cavities, laser power, number of lasers used in CR. Design Compton Ring & other details.