Beam Loss notes

Beam loss/collimation issues on 4GLS:

1. Parasitic loss should be localised deliberately rather than randomly - so we know where it's going. This appears to be the philosophy of the APS ERL proposal, and the Japanese ERL proposal. http://www.erl07.dl.ac.uk/Tuesday_Presentations/WG_2/Session_1/Xiao-ERL07-3.pdf
although admittedly they are using fewer collimators; but their beam power is about 10x greater (700 MW cf our 55 MW).
Also see:
http://www.aps.anl.gov/News/Conferences/2006/APS_Upgrade/apsmac/Borland_ERLPhysics.pdf

Japanese ERL proposal thinks about localisation too:
http://epaper.kek.jp/fls06/TALKS/WG221_TALK.PDF
but one of the things they are thinking about is trapping of very small losses (in the few Watt level) from gas scattering.

So I think that localisation is a sensible way to go. The collimator length does not depend much on the total power - the collimator lengths is just there to stop the shower coming out the end of the collimator with too high a mean energy. 200 mm Cu is the estimate for 550-750 MeV incident electrons.

2. We are worried about irradiating the undulators, and we have a lot of those. This is what XFEL worry about:
http://flash.desy.de/sites/site_vuvfel/content/e403/e1642/e1849/e1933/infoboxContent2164/TESLA-FEL-2007-051.pdf
(this is a very detailed paper that I have not digested yet!)
But I think at the moment that it is similar in principle to the BESSY-FEL collimation design:
http://web.elettra.trieste.it/fel2004/proceedings/papers/TUPOS02/TUPOS02.PDF
Remember that the beam power in the FELs is much lower than our 55 MW, so we should be as worried about collimation as they are!

3. At the moment, we don't know the best way to detect beam loss. Some issues:
a. Measurement of differential current monitors on the primary beam - I don't think this will be nearly good enough. The S/N isn't good enough - differential measurements are good enough for 'catastrophic' loss over ms, say, but not for ongoing losses.
b. Direct measurement of the loss, via scintillation or other techniques seems like the way to go. e.g. cable-based monitors like those used on ERLP. My favourite at the moment is a 'halo monitor':


c. Measurement of pressure rise sounds good (because it's cheap, and doesn't introduce extra components into the vacuum envelope that cause impedance), but I worry about the timescale of response of things like this, and what happens when they aren't working properly - we have lots of experience of that on the SRS!

d. Similarly, measurements of temperature rise are also not great, because you could for example get a mis-tuning of the beam optics which caused localised beam loss, and then not know about it until it was too late.

For all those reasons, I'm still in favour of collimators that will passively protect the other apertures as much as possible, hence have a fairly dense layout of them (regular spacing in phas advance), and to put monitors in as well.