Re: [Fwd: RE: question] (fwd from Leigh Harwood)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Tue, 15 Jul 2008 22:27:16 -0400
From: Leigh Harwood <harwood@jlab.org>
To: Elke-Caroline Aschenauer <elke@jlab.org>
Cc: roblin@jlab.org, halld-tagger@jlab.org, Rolf Ent <ent@jlab.org>
Subject: Re: [Fwd: RE: question]

Elke,

You're most welcome.  Irrespective of what some people may say, I try to
be open and transparent.  I consider those part of being a
good/responsible scientist and being a good team member...and, in my
humble opinion, we're all a team working together toward doing good
physics.  :-)

I don't consider your questions out of line, impolite, or anything else
negative.  You have responsibility for insuring that the research
program(s) in Hall D can be done.  If you see something that doesn't
mesh with your previous experience...or reasonable extrapolations from
it....I consider it completely appropriate for you to ask questions to
get a better understanding.

That being said......

I was pretty surprised that the new calculations had no "fuzz ball"
around the core.

One correction to something you wrote:  There ARE non-Gaussian shoulders
on the new distributions.  I have seen graphs (I don't have them; Yves
showed them to me today) that show this to be true.  What the new
calculations don't have are the clearly identifiable "wings".

Indeed, misalignment, residual steering errors, and non-linearities in
the magnets' fields should lead to distortions in the beam's phase space
and "halo".  The synch rad makes the emittance...and thus the beam
size....larger for 12 GeV than we have at 6 GeV.  That emittance growth
also depends on energy (more with higher energy), bend radius (more with
smaller radii) the optics (combination of beta functions and dispersion
function), and total path length in the dipoles.  I haven't looked at
HERA but each of those items are rather different between HERA and
CEBAF.  I don't know the differences between the two machines' for all
those (emittance growth, steering, field quality) so I can't comment on
the relative scales of the contributors to halo OR to what is to be
expected.
Gas scattering can also contribute.

However...again...I would expect there to be some "fuzz" around the core
of the beam.  Why one set of calc's show it and the other doesn't.....I
don't know.
What I can say is that all the effects are included in the simulations:
non-perfect steering, non-perfect fields, and synch rad increases in the
emittance.  And I believe that the optics include some "errors" as
well...but I can't swear to that.  We do not include gas scattering,
however, since a previous analysis showed that there isn't enough gas to
be a problem in a one-time-through machine like CEBAF.

I hope this is helpful...or at least informative.

cheers,

Leigh

PS
Yves, can you do a graph of the energy distribution as Elke asks?
Thanks in advance.


Elke-Caroline Aschenauer wrote:
> On Tue, 15 Jul 2008, Leigh Harwood wrote:
>
> Dear Leigh and Yves,
>
> first of all let me thank you both for keeping Hall D / GlueX so nicely
> informed about these new calculations. It is highy appreciated.
>
> Actually I have a couple of some questions to the new calcualtiosn, which
> indeed look very very promissing.
>
> I must admit I have a really hard time believing that there no
> non-gaussian 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, this should increase the synchrotron radiation
> more than just what you would expect from the energy difference. I also
> know the number of turns is less than in HERA, but again no non
> gaussian tails at all. 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, because a lot of particles go of center through
> the quad fields.
> So my question is are all this effects simulated.
>
> Please don't misunderstand my mail I don't want to be negative on the
> results, but woud like to understand a bit better how the simualtions are
> done. I think it would be also nice to have the energy spectrum going
> together with this new spatial distributions of the beam.
>
> Thanks and have a nice evening
> elke
>
>
>
>
>> Date: Tue, 15 Jul 2008 21:04:51 -0400
>> From: Leigh Harwood <harwood@jlab.org>
>> To: elke Aschenauer <elke@jlab.org>
>> Subject: [Fwd: RE: question]
>>
>> Elke,
>>
>> I asked Yves to calculate the number of particles outside the diamond
>> (+/- 5mm) and outside the inned edge of the frame (said to be +/- 7.5mm)
>> The results are below.
>>
>> I'm not sure I believe them!
>>
>> L
>>
>> -------- Original Message --------
>> Subject: 	RE: question
>> Date: 	Tue, 15 Jul 2008 11:31:29 -0400
>> From: 	yves roblin <roblin@jlab.org>
>> To: 	'Leigh Harwood' <harwood@jlab.org>
>> References: 	<487C2CD2.7080605@jlab.org>
>> <000801c8e688$34bdbdd0$9e393970$@org> <487CC0D1.9000806@jlab.org>
>>
>>
>>
>> Leigh,
>>
>>
>>   Here are the numbers :
>>
>>
>>                                     DIMAD
>>   ELEGANT
>>
>> Past 5mm
>>                      35150                                            1504
>>
>> Past 7.5mm
>> 10583                                            0
>>
>>
>> At first glance, looking at the plot it did not look that it would be
>> that much in DIMAD. However,
>>
>>  I checked in the file and there are many bins and they are in the
>> dozens or hundreds counts..
>>
>>
>> The counts were calculated for each plot independently (not aligned
>> together).
>>
>>
>> Yves
>>