Meeting to Discuss Laser Cavity Design for Photon Linear Collider - Daresbury, UK Jan 10 th 2006

1. Meeting to Discuss Laser Cavity Design for Photon Linear Collider - Daresbury, UK Jan 10th 2006 Present: Mark Oxborrow National Physical Laboratory Graeme Hirst Central Laser Facility RAL Guido Klemz DESY/Zeuthen Klaus Moenig LAL-Orsay/DESY-Zeuthen Andrew Rollanson Keele University Ken Strain Glasgow University Valery Telnov Novosibirsk David Walker Zeeko Ltd. David Miller UCL Aleksander Filip Zarneki Warsaw Alexander Finch Lancaster University Steve Maxfield Liverpool University LCWS06 Bangalore

2. Background to Meeting “THE PHOTON COLLIDER AT TESLA”, V.Telnov et. al “ Design study of an optical cavity for a future photon-collider at ILC “ G. Klemz , K. Mõnig , I. Will “Thoughts on R+D for Gamma Gamma Optical System” Josef Frisch “Optical cavity for ILC g-g collider: feasibility and development “Mark Oxborrow “Additional comments on R+D for Gamma Gamma Optical System “ Ken Strain “Photon Linear Collider Laser Cavity Requirements. “ Andrea Freise Jan 10th Meeting to discuss all the above… LCWS06 Bangalore All documents are available at:http://www.hep.lancs.ac.uk/LaserCavity/

3. Essence of a Photon Collider (from G.Klemz talk) LCWS06 Bangalore

4. Conclusions from Compton Scattering… (from G.Klemz talk) LCWS06 Bangalore

5. Optical design parameters (from G.Klemz talk) LCWS06 Bangalore

6. Why use a cavity? • There are ~1010 electrons in a bunch • Need ~ 1019 photons in laser for efficient Compton conversion ( 5 Joules) • Less than 1 in 109 photon used. • Can reuse the laser pulse, which means • Need a (much) lower powered laser LCWS06 Bangalore

7. Basic Design Criteria for Cavity LCWS06 Bangalore

8. Proposed design… LCWS06 Bangalore

9. Logical layout of cavity… LCWS06 Bangalore

10. Optimizing the size of the mirrors LCWS06 Bangalore

11. Result of optimisation… LCWS06 Bangalore

12. Conclusions from Klemz et.al paper • A realistic design exists • Mirrors need a diameter of around 1.2m • Fairly insensitive to displacements transverse to the beam • Very sensitive to change in length of the cavity (as power enhancement is lost). • Accuracy to less than 1nm required • Adaptive optics • Power deposit on mirrors appears to be below damage threshold of materials LCWS06 Bangalore

13. Comments on Klemz et al paper.. From Joe Frisch, Mark Oxborrow, Ken Strain, and Andreas Freise. Executive Summary: Looks OK on paper (i.e. no-one spotted “show stopper”) so far as it goes (“statics”) but Could it be made to work in practice ? (especially “dynamics”) LCWS06 Bangalore

14. Summary of Joe Frish’s comments • Can the cavity be kept stable? • Optical damage effects are not known for pulsed high energy laser. • Other effects of high energy pulsed laser ? • Can a feedback system be designed? • Drive laser is still difficult, even with a cavity. LCWS06 Bangalore

15. Summary of Ken Strain’s comments • Gravity wave experience suggests longitudinal stability problem is soluble. • Similarly angular control ( e.g. with preheating) • Passive adaptive correction may help. • Pulsed power effects seem less difficult than with Gravity wave detection. LCWS06 Bangalore

16. Summary of Andreas Freise’s comments • A full numeric model which includes typical aberations and deviations from specification can be used to understand the feasibility of the proposed topology better. • A more detailed proposal for the mirror suspension and • control scheme would be helpful. LCWS06 Bangalore

17. Summary of Mark Oxborrow’s comments(1) Mechanical/infrastructure Identify sources of vibration and reduce them. Cavity stability At the required λ/100 precision will the sag of the optical cavity’s mirrors be a problem? Pulsed power effects Could the performance be affected by photomechanical shock (outside the bandwidth of any servo)? Feedback/locking/mode-matching Is the cavity compatible with Pound-Drever-Hall locking (of a worthwhile servo bandwidth) as it is commonly implemented? How exactly can one measure the laser beam’s profile. LCWS06 Bangalore

18. Summary of Mark Oxborrow’s comments(2) Adaptive Optics Can the mirrors be moved fast enough in view of their mass? Can information from a low-power CW laser, be used to steer the high-power pulsed-mode. How would an adaptive wavefront corrector be implemented? Who Moves? The driving laser’s output to track the optical cavity, or vice versa; and/or the electron beam Modularization and assembly Should optical cavity be designed separately from the drive laser? How to match the laser’s natural output onto the optical build-up cavity? LCWS06 Bangalore

19. Result of the discussion on Jan 10th. (Extremely valuable to have a large range of expertise together!) • Off the wall comments/questions: • Are the linear collider parameters really a given, for example the time structure? (Answer Is probably YES but it is important to ask the questions!) • Is it definitely best to have separate laser and optical cavity? (Answer not clear, needs to be seriously studied as well) • How about having mirror with a hole in it for the electron beam to pass through? (Probably radiation damage is a problem) LCWS06 Bangalore

20. Result of the discussion • In response to worries about manufacturing the mirrors: • This is similar to the next generation of photolithography optics, so should not be a problem. • In response to questions about the viability of the adaptive optics required: • Well within the current state of the art in telescopes. Not done with pulsed lasers, would need to average over several pulses. • parabolical mirrors (or any other shapes ) are not a problem • Optical damage. • No results in the literature using pulsed lasers. LCWS06 Bangalore

21. Results of Discussion (3) • Pulsed power effects: • Photon pressure effects should not be a problem • Thermal distortions • taken care of with adaptive opics - • how do the mirrors get cooled? other materials , e.g. aluminium or silicon carbide could help • getting hot should not be a problem • must be taken care of in the design. designed for working temperature. • Compensating optics • Can measure the aberration of the system, and put a compensating optic to correct it. • relaxes tolerance on rest of system. LCWS06 Bangalore

22. Way forward • Continue networking! • The few experts at this meeting were already able to give valuable input • Need an “End to End” simulation of the dynamics of the design. This will help to identify which are the critical elements. • Need to investigate damage threshold issues further using pulsed lasers, may need R+D if no-one else has studied it. • Learn as much as possible from other related projects such as the work done at Orsay for the polarimeter. LCWS06 Bangalore

23. Conclusions • Hard to summarise such a large number of comments and questions. (Especially as a particle physicist). • Lots of experts can find lots of things to worry about! • Some of the experts’ worries were dismissed by other experts! • No-one laughed out loud, and said it couldn’t be done. • Lots of these “worries” might reduce efficiency of the cavity, and so potentially luminosity. There is no known effect at present that would prevent it working at all. • Alternative designs need to be looked at in at least as much detail. • Still a lot to be done, but (for me at least) the path is becoming clearer. LCWS06 Bangalore