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Re: IMPORTANT: size of diamond and goniometer frame for hallD



On Tue, 15 Jul 2008, Richard Jones wrote:

Dear Richard,

thanks for your quick reply.
I have some remarks below.

cheers elke


>
> Elke,
>
> Thanks for passing on these plots.  They show substantial improvement for
> our background conditions going from the green to the red curve.  In
> fact, the red curve seems to indicate that the Gaussian behavior goes out
> all the way to 6 sigma.  Within statistics, the entire red curve may be
> Gaussian.  Just looking at the real-space distribution without having the
> correlated momentum information, I cannot simulate what the background
> will be under the new scenario, but it looks substantially better.  The
> further out the Gaussian extends in the fit, in my eyes, the cleaner is
> the beam in terms of backgrounds.  Those beam particles in the Gaussian
> tails still belong to the same momentum distribution as the central ones,
> whereas those in the power-law tails of the green curve indicate
> substantial non-Gaussian components of the multi-dimensional phase-space
> distribution and have a broader momentum distribution as well.

I must admit I have a really hard time believing that there no tails.
That is really suprising at least if I use my experience from HERA, yes
Cebaf is not HERA, but there is synchrotron radiation and yes I know Cebaf
has less synchrotron radiation compard to HERA ((E2-E2)^4), because of the
lower beam-energy, but CEBAF keeps the radius in the arcs the same with
doubling the beam energy. I also know the number of turns is less
than in HERA, but again no tail. There is misalignement of the magnets
which widens your emittanceand their is rest gas, which actually inreases
because of the increased synchrotron radiation.
So all this effects populated tails, apart from the misalignement, this
increases the emittance and if it is really bad can increase the
synchrotron radiation.
So are we sure all this is really simulated. I will ask this to yves and
Leigh.

>
> I really don't see the point of trying to shift the beam off-center,
> perhaps to show that the improvements are not as big as they appear at
> first?  But what is happening between 5 and 5.2 mm is not going to
> produce huge background in the tagger.  This boundary of 5mm radius is as
> good as any other choice for defining the halo, but we cannot fixate too
> much on whether it is 5mm or 5.5mm, as though that were a critical
> thing.  What is important is what the shape is, extrapolating to larger
> radius, and what the momentum distribution is like in the tails.
>

yes, the exercise to off center the new calcualtion was only for
comparision. I totally agre with you that some definitions are not graved
in stone and can be slightly varied, but again I cannot believe this no
tail result.


> In any case, the thing I was responding to Yves about seems unrelated to
> these plots.  Yves was telling me that he has managed to reduce the sigma
> of the central peak by another factor of two, down to 0.5mm, and wondered
> if that would cause us problems with radiation damage in the crystal,
> from the beam spot on the radiator being too small.  At least, that was
> the question that I was responding to.  These two curves you showed me
> from Leigh have the same width of about 1mm in the central Gaussian.
>

yes, and I totally agree with your answer, but I think there is maybe a
bit more to understan on this calcualtions.

> Richard Jones
>
> Elke-Caroline Aschenauer wrote:
>
>  On Mon, 14 Jul 2008, Richard Jones wrote:
>
> Dear Richard,
>
> Leigh came yesterday to me and showed me also plots from the last studies
> from Casa
> http://www.jlab.org/~elke/DIMAD-vs-elegant.ppt
> His story about the new calculations was by far not as positive.
> You can see this also form the plots. In the old studies the beam was not
> centered at 0,0 now it is the tails have disappeared but the beam has
> become wider. This differences come from nothing else than changing to a
> new code, which they claim has been benchmarked against other ones.
>
> As some of this will be presented next week during the CD-3 review I would
> like to be a bit more careful and think one more minute about it. I also
> think it would be good and run a check with the MC implementing the red
> curve as beam profile.
>
> I happy to hear your opinions.
> cheers elke
>
>
>
>
>
>
>  Date: Mon, 14 Jul 2008 19:44:43 -0400
> From: Richard Jones <richard.t.jones@uconn.edu>
> To: yves roblin <roblin@jlab.org>
> Cc: Hall D beam working group <halld-tagger@jlab.org>
> Subject: Re: size of diamond and goniometer frame for hallD
>
> Yves,
>
> That's great.  The rounder we can make the beam at the radiator the better off we will be, I think.  reducing the
> beam sigma to 0.5mm in x to make it similar to sigma_y at the radiator, keeping it the same at the collimator,
> would be outstanding.  We would have to do more spot moves, but moving the crystal around is relatively quick.
> It also gives us more flexibility in terms of avoiding bad spots on a crystal, less sensitive to large-scale
> deformations of the crystal, and even smaller crystals would be usable.  I would not like to see the spot get
> smaller than 0.5mm in x or y, but making it close to round makes things simpler.  Thanks for the good news of
> these improvements.
>
> Richard Jones
>
> yves roblin wrote:
>
>       Richard,
>
>        Thank you for the information.
>
>
>
>       How small can I make the beam in sigma_x and  not reach a current density that would damage that
>       diamond  (Right now I have sigma_x=1mm and this is what I gave you last year too).
>
>       With our new optics (Double bend achromat), we reduced the emittance by roughly another factor of 2
>       which means  that I can make the beam 40 % smaller for the same quad settings (or relax the quads and
>       keep the beam to whatever size you need). The idea behind a smaller beam is that of course, we would
>       have even less halo.
>
>
>
>       Yves
>
>
>
>
>
>  ( `,_' )+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=
>   )    `\                                                  -
>  /    '. |                                                  +
>  |       `,              Elke-Caroline Aschenauer            =
>   \,_  `-/                                                    -
>   ,&&&&&V         Jefferson Lab                                +
>  ,&&&&&&&&:       HALL-D 12C / F381       121-A Atlantic Avenue =
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> |  |       ;--' | Mail:  elke@jlab.org    Mobil: 001-757-256-5224   -
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>    \_    |  |---' Tel.:  001-757-269-5352                             =
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>
>
>
>
>

 ( `,_' )+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=
  )    `\                                                  -
 /    '. |                                                  +
 |       `,              Elke-Caroline Aschenauer            =
  \,_  `-/                                                    -
  ,&&&&&V         Jefferson Lab                                +
 ,&&&&&&&&:       HALL-D 12C / F381       121-A Atlantic Avenue =
,&&&&&&&&&&;      Suite 8                 Hampton, VA 23664      -
|  |&&&&&&&;\     12000 Jefferson Ave                             +
|  |       :_) _  Newport News, VA 23606  Tel.:  001-757-224-1216  =
|  |       ;--' | Mail:  elke@jlab.org    Mobil: 001-757-256-5224   -
'--'   `-.--.   |                                                    +
   \_    |  |---' Tel.:  001-757-269-5352                             =
     `-._\__/     Fax.:  001-757-269-6331                              -
            +=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+=-+