-------- Original Message --------
Subject: Energy monitor
Date: Thu, 12 Sep 2013 18:19:30 -0700
From: Wienands, Uli <uli@slac.stanford.edu>


Hi John,

Here is an update on my work on this energy monitor business:

I made a (very conceptual) sketch. Please find attached a pdf. Making
this sketch brought it home that I have no idea what the KEKB tunnel
dimensions are. At some point in time these become a relevant
restriction. While I put some dimensions on it and it is basicaly a
scale sketch; things like the dipole are obviously wrong. I will need
more input from KEK. But it may give you an idea.

I also discussed the whole scheme with Bill White, our laser expert
(who has also been closely involved with the thoughts at SLAC about
an Inverse Compton gammar-ray source). Bill had several suggestions:

1: Think about using a YAG laser rather than a CO2 laser. Apparently
YAG's produce much cleaner and more monochromatic light. However,
longest practical wavelength is 1.55 mum. This would require much
more shallow angles than I assumed so far, to keep the gamma energy
in the few MeV regime. YAGs are small and reliable and could live in
the tunnel.

2: With a YAG laser one may employ a two-colour scheme and use a
grating to produce the two angles. This would make the setup much
more robust.

3: There will almost invariably be optics that has to accomodate both
laser angles. Much easier in case of the 1.55 mum YAG.

4: I need to estimate what diffraction is doing to this scheme. Maybe
spoiling it completely at 10 mum wavelngth, certainly better at 1.55
mum. This is a serious issue I need to follow up on.

Using YAG lasers in the tunnel could be a considerable cost savings
and simplification. I am begnining to build a budget and while the
laser costs are not the dominant factor, the light guide is (using
recent SLAC experience at FACET). If we could avoid that, it'll be
significantly cheaper (and we are talking a factor of maybe 2/3
here).
Bill actually seemed to be excited about this thing and offered more
help as needed. He will be out for 10 days, though, after which I
will be out for 10 days.

I hope to see you tomorrow @ our meeting.

Uli