[Halld-tagger] Sasha's note, meeting on Monday

[Alexander Somov]

Hello Richard,

I am preparing the final plots for the geometry
where the microscopes are positioned in the focal
plane; I will update the note shortly.

The quadrupole field is not yet optimized. Though,
the vertical resolution does not look bad even with
the non-optimal quadrupole field, sigma_y = 0.6 cm which
is smaller than a half counter width. Focusing in Y direction
can defocus slightly in X. But I would expect  a small impact
on the energy resolution due to the defocusing in X. Our
detectors are placed close to the parallel-to-point focal plane
where we see a large contribution to the energy resolution
from to the electron beam spot size. Anyway, we can certainly
optimize the quadrupole field.

I think that it's also important to generate a new field map
for the narrower magnet with ANSYS ot TOSCA and go through the
final simulation once again. I checked the fields of the
default-width and narrow magnets using a Poisson program -
the field profiles look very similar, but it's better to have
a realistic filed map.

Cheers,
        Alex




On Fri, 13 Nov 2009, Richard Jones wrote:

> Dear colleagues,
> 
> This is a reminder of the regular biweekly gluex photon beam working
> group meeting on Monday at 11:00 EST.  Top on the agenda is a discussion
> of the latest results from Sasha.  The following questions stand out to
> me in looking at the latest figures.
>  1. What happens when, as we agreed last week, the microscope is moved
>     onto the focal plane and the fixed array and exit window are moved
>     inward be the same amount?  This should improve the resolution in the
>     microscope a little, and at the same time reduce the size (and hence
>     cost) of the vacuum chamber.
>  2. So far we have only considered the focus in the dispersion (xz)
>     plane.  Where is the focal surface in the y direction?  As I
>     remember, the focal surface in the xz plane is mainly due to the
>     dipole field and weakly depends on the quadrupole.  However this is
>     not true for the y-focus, which comes about entirely from the
>     quadrupole field.  Without the quad, the y-focal length is negative.
>     Dan Sober once showed using TRANSPORT that the y-focal surface is not
>     parallel to the x-focal plane, so the two can be made to cross
>     anywhere in energy simply by dialing the strength of the quadrupole
>     field.  In Geant we can do this by placing an imaginary cut in the
>     field map upstream of the dipole and rescaling the field there by
>     some factor of order 1 such that the two focal planes cross in the
>     region of the microscope.   Now that we have changed the dipole
>     optics, we need to repeat this exercise to optimize the
>     y-resolution.  The latest figures from Sasha show that with the
>     reduced pole width, the y-focus is no longer optimal.
> Richard Jones
> 
>