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

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

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

Present:

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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

“Additional comments on R+D for Gamma Gamma Optical System “

Ken Strain

“Optical cavity for ILC g-g collider: feasibility

and development “Mark Oxborrow

“Photon Linear Collider Laser Cavity

Requirements. “

Andrea Freise

Jan 10th Meeting to discuss all the above…

All documents are available at:http://www.hep.lancs.ac.uk/LaserCavity/

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Essence of a Photon Collider

(from G.Klemz talk)

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Conclusions from Compton Scattering…

(from G.Klemz talk)

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Optical design parameters

(from G.Klemz talk)

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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

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Basic Design Criteria for Cavity

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Proposed design…

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Logical layout of cavity…

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Optimizing the size of the mirrors

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Result of optimisation…

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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

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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”)

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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.

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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.

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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.

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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.

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Summary of Mark Oxborrow’s comments(2)

Adaptive OpticsCan 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?

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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)

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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.

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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.

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Way forward

1. Continue networking! The few experts at this meeting were already

able to give valuable input

2. Need an “End to End” simulation of the dynamics of the design. This will help to identify which are the critical elements.

3. Need to investigate damage threshold issues further using pulsed lasers, may need R+D if no-one else has studied it.

4. Learn as much as possible from other related projects such as the work done at Orsay for the polarimeter.

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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.