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Meeting to Discuss Laser Cavity Design for
Photon Linear Collider - Daresbury, UK Jan 10th 2006
Present:
Mark Oxborrow
National Physical Laboratory
Graeme Hirst
Valery Telnov
Novosibirsk
David Walker
Zeeko Ltd.
Central Laser Facility RAL
Guido Klemz
David Miller
UCL
DESY/Zeuthen
Aleksander Filip Zarneki
Klaus Moenig
Warsaw
LAL-Orsay/DESY-Zeuthen
Alexander Finch
Lancaster University
Andrew Rollanson
Keele University
Steve Maxfield
Liverpool University
Ken Strain
Glasgow University
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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”
“Additional comments on
R+D for Gamma Gamma
Optical System “
“Optical cavity for ILC
g-g collider: feasibility
and development
“Mark Oxborrow
Ken Strain
Josef Frisch
“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
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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

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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
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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
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Gravity wave experience suggests
longitudinal stability problem is soluble.
Similarly angular control
( e.g. with preheating)
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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 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?
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Result of the discussion on Jan 10th.
(Extremely valuable to have a large range of expertise
together!)
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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
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In response to worries about manufacturing the
mirrors:
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In response to questions about the viability of the
adaptive optics required:
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This is similar to the next generation of photolithography
optics, so should not be a problem.
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)
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Pulsed power effects:
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Photon pressure effects should not be a problem
Thermal distortions
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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
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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!
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2.
3.
4.
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.
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Conclusions
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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.
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