1. Status of g ray generation at KEK-ATF ► Introduction ► Status of the cavity R&D ► Out Look French Labs. : LAL (Orsay) in Collaboration with CELIA (Laser lab., Bordeaux) and LMA (mirror coatings Lab., Lyon) Japanese Labs. : KEK, ATF group, Hiroshima University Tohru Takahashi Hiroshima University for 06 September 2012 POSIPOL 2012

2. Introduction ► Polarized e+ by laser Compton Scheme Ee~1GeV for 10MeV gammas easy to control polarization Toward the positron sources -> increase intensity of gamma rays Omori

3. Miyoshi PosiPol2010 dL ∝ /enhancement ~0.01nm for enhancement ~1000 L cav

4. Experiments at the KEK ATF 1.3GeV up to 10 bunches/train Cavity   s

5. Brief History ► 2007 2 Mirror cavities installed –  kW  rays generate ► 2010 French 4 Mirror cavity installed –  rays confirmed ► 2011 earthquake – No major damage to our equipment. – beam back in June 2011 ► 2011 KEK-Hiroshima 4 mirror cavity installed –  rays confirmed Reported FJPPL2010 Annecy

6. We should go to 3D 4 mirror ring cavity to get small sport size 2 mirrors is not stable for small spot size α 2d 4M has astigmatism 3D (or twisted) 4M ring cavity

7. 3 Dimensional 4 Mirror Cavity ► Resonates only for circular polarization – geometric phase due to twisted pass – cavity only resonates with circular polarization – usable for pol. switching circumference of the cavity left handed right handed transmitted light

8. Configration of test bed

9. calculation of spot size w/ test bench geometry minor axis major axis spot size  (  m) spot size is not sufficiently small with test bench geometry L(mm) L

10. new geometry

11. expected spot size w/ new geometry 421424426428 0 20 40 spot size  (  m) L(mm) laser spot size of 15  m is expected with new geometry Before optimization After optimization

12. New cavity to be installed into the ATF 45.6° 70mm

13. Two 4 mirror cavities are at the ATF KEK-Hiroshima installed 2011 LAL-Orsay installed summer 2010 relatively simple control system employs new feed back scheme sophisticated control digital PDH feedback

14. Installation of KEK Hiroshima cavity

15. Laser spot size 15  m achieved vertical position[  m]  ray yield [A.U]  =18  m electrons vertical deirection  e=10  m  L=15  m it was 30  m w/ 2 M cavity

16. Cavity length feedback with 3D feature + = 3D4M cavity resonate only with circularly polarized lasers differential signal for feedback cavity length must be L = n  with very high precision (for enhancement of 1900 dL<< 87pm while L = 1.64m)

17. 右円偏光 左円偏光 蓄積強度 Laser Power 2.6kW Time Jitter ＝ 8.0ps 14 th Sep 2012 Stored Laser Power in the cavity

18. ATF 2.16MHz ~2.6×10 8 /sec 5bunches/train 2970±20 MeV ⇒ ~120  s/train のガンマ線に相当 g ray Generation / electron e- laser  5.6ns E  [MeV]

19. 14 th Sep 2012 wave form from detector ◆～ 117/train ⇒ consistent w/ calorimeter measurement ◆ no bunch dependence ( yield is proportional to e- current) e- bunch monitor bunch/bunch measurement

20. Status and plan of KEK-Hiroshima cavity ► 2.6KW stored as of 25 May 2012 – 30  s / bunch -> 150  s /train – correspond to 3.3 × 10 8  /s ► BaF2 detector was employed to observe bunch/bunch generation – Planed to be replaced w/ a Lead glass counter ● used in TOPAZ barrel cal. at TRISTAN! ► Plan – more power enhancement ● 16600 enhancement (finesse 48,000) – digital feedback

21. Summary ► Optical Cavity at the ATF is in progress for Polarized positron source for the ILC – Good collaboration between France – Japan team ● information / technology exchange ► R&D of 4 mirror ring cavities are in progress – Sophisticated mechanism aming very high laser power enhancement,,, French team – Relatively simple but new cavity control practical experience w/ the ATF,,, Japanese team ► More to come – more laser power, more  rays – maturity toward the system