Re: Beam termination and possible effects on tagger (fwd from Pavel)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Tue, 08 May 2007 12:46:25 -0400
From: Pavel Degtiarenko <pavel@jlab.org>
To: Elton Smith <elton@jlab.org>
Cc: may <may@jlab.org>, halld-tagger@jlab.org, welch@jlab.org
Subject: Re: Beam termination and possible effects on tagger

Hi Elton,

Highly focused 3 uA beam is capable to produce local damage in materials
at the point of impact, especially if thermal conductivity of the
material is low. So I think we should not let the beam hit the concrete
wall directly, even if it is a rare event. Better to have an iron block
at the place of possible impact at the wall.

Same considerations are valid for the magnet and the vacuum chamber
walls. Beam excursions during fast shutdowns, or due to the tagger
trips, generally may lead to short beam hits there. The materials should
be capable to withstand corresponding local thermal stresses. (They most
probably are).

In this respect, may be it's worthwhile to consider an option to join
consecutively the power supplies of the tagger and of the magnet couple
that elevates the beam in the tunnel. If they trip simultaneously, the
beam remnants in the machine will hit the tunnel stub upstream, instead
of a place in the tagger area.

Regards,
Pavel

Elton Smith wrote:
> Hi Bob,
>
> I am copying this message to the halld-tagger group in case anyone has
> additional comments.
>
> My assumption has been that beam operation would be interlocked with the
> tagger power supply, and in the (infrequent) event that beam was lost due
> to the loss of power and or other circumstances in the beamline that the
> detectors would be able to handle the instantaneous exposure until the
> fast shutdown turned the beam off. However, it is probably true that a
> full analysis of the effect on the detectors has not been carried out.
>
> The permanent magnet is in place to prevent the electron beam from being
> transmitted down the photon line for any reason. In case of the tagger
> magnet supply failure, the beam would be dumped below the photon beam and
> into the berm which again we assume can handle an instantaneous dump of
> the electron beam.
>
> If you want to discuss this further, just let me know.
>
> Cheers, Elton.
>
>
>
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite # 16
> Newport News, VA 23606
> elton@jlab.org
> (757) 269-7625
>
> On Tue, 8 May 2007, may wrote:
>
>
>> Elton,
>>
>> Pavel and I were talking about the accidental beam loss situation where
>> electron beam transport to the point of the permanent magnet results in
>> beam offset to the tagger area east wall and what, if anything, should
>> be placed to intercept the beam. We were also wondering about what the
>> effects would be on the tagger (detectors, pmts, etc.) if a high power
>> beam loss occurs at some point in the tagger.
>>
>> As beam is terminated by the fast shutdown system, it seems to "wander".
>> For example, we often see a spike in ion chambers in a hall associated
>> with a fast shutdown event somewhere in the CEBAF accelerator. Whatever
>> is occurring at the moment of fast shutdown, the result seems to be beam
>> offset, an increase in energy spread perhaps, or a combination of this
>> and other things. I don't know if there has been any discussion about
>> how this might affect tagger detector hardware if momentary beam offset
>> from a fast shutdown event result in some beam loss in the tagger. Do
>> you think we should meet to talk about this?
>>
>> Bob
>>
>>
>>