Solenoid Design for DVCS

Design options for DVCS Solenoid

Email from Michel Garcon 30-Apr-02

The DVCS experiment has considered successively

a) a simple coil (see proposal)

b) a coil with iron flux return (Latifa, Lassiter...)

c) a coil surrounded by a second one powered in reverse direction in order to kill the fringe field on the torus (Saclay)

d) eg1 as a back-up

e) other Helmholtz coil design ?? (Latifa...)

Basically, we have 3 criteria:

i) guide the Moller electrons in the central hole of the Inner Calorimeter

ii) not go over a limiting value of fringe field on the torus coils

iii) leave free acceptance for particles up to 60 degrees going toward CLAS.

To the best of my knowledge, c) is the option that can best answer all three criteria.

Here is the info you requested, for this option:

1) central clearance: about 28 cm diameter. There no flat field portion. Coil length at 17 cm radius about 18 cm, at 39 cm radius about 32 cm

2) B(z) vs. Z

z=0	B=4.9T
z=5 cm	B=4.5 T
z=60 cm	B = 0.12T
z=80 cm	B = 0.05T

(with some variations depending on the exact geometry still under study).
You may find one field map of option c) in the e1-dvcs secure web page, under michel/dvcs_as_map35.dat (not the best one yet)
Note added by SK: You can find very nice 3D plots of this solenoid here .

3) For Moller, a) might be best. But a) is out because of ii).
Several of us have made simulations and/or tracking of Moller electrons. The most complete results are by Cole (see on the secure e1_dvcs web page - his Saclay 3 option corresponds to the above field map). We assume that the luminosity limitation for CLAS will be comparable or slightly higher than for eg1, while checking the count rates on the IC.

It seems to me that the choice iii) of detecting DVCS recoil protons in CLAS is incompatible with the option of a long target. Another choice for DVCS would have been to build an inner detector for the high angle protons, but it was felt (I think rightly so) that this would bring us too far in the future. We have not looked at options of ultra-thin solenoid which protons could go through.