During a visit yesterday with Alexander M at Cornell, I came to realize
that the design of the E-166 positron transport was less than optimal --
in that there would still be considerable loss of particles that passed
through the momentum slit.
I eventually recalled that what we should be building is a
"nondispersive, translating spectrometer".
So old lore on this can be seen in the book of Steffen (DESY):
One way to fix things up would be to put an inverting lens between the two dipole magnets.
Since space is limited, a rather strong lens would be required.
Maybe we could do this with a solenoid....
A sketch of the desirable patterns of rays is
Steffen shows another design, based on 3 60 deg bend magnets (and no quad or solenoid).
This might also work for us.
In this scheme, the central ray is bent by 180 deg....
The present state of my drawing is: