1.  Mini-torus, reminder of results from last week: Current status (at our nominal current density of 470 A/cm² , which is about 80% of the maximum allowed value for water-cooled coils of ~ 560 A/cm²). :

bending down: Moller rate after secondary collimator ~ 10 kHz/nA

                         bends e-p’s by ~ -.36 degrees

                         displaces e-p peak down by 1.6 cm at entrance to main torus

http://www.phys.vt.edu/~jmammei/moller.html#plot2

bending up: Moller rate after secondary collimator ~ 100 kHz/nA

                    bends e-p’s by ~ .33 degrees

                    displaces e-p peak up at entrance to main torus

So bending up leads to unacceptably high rates (~ 1 MHz total rate) at 10 nA.  Bending down gives very reasonable rates, but potentially the 1.6 cm shift down puts it very close to the main torus support structures.

2. Further work on mini-torus:

• As a constraining assumption, we will assume that we don’t want to bend the e-p elastic electrons by greater than 0.4 degrees.
• Things we tried:
• Is the secondary collimator opening bigger than it needs to be?

     Answer:  NO; in fact it probably needs to be a little bit bigger; see

     http://www.phys.vt.edu/~jmammei/moller.html#plot1

• Can we move the defining edge of the secondary collimator further downstream by making the secondary collimator thinner (out of lead instead of concrete) so that the edge is further downstream.  This gives us more lever arm for a given bend.

                        Answer: Even under the most optimistic assumptions;  (mini-torus       

                                       coils only 5 cm from beam centerline, secondary collimator

                                       defined right at the exit of the current one, max. current density)

                                      things are okay, but these are somewhat unrealistic assumptions.

                     http://www.phys.vt.edu/~jmammei/moller.html#plot3

• Can we modify the field design so the high energy Moller electrons experience a greater B*dl than the e-p electrons do?

                         Answer: Couldn’t think of an easy way to do this, because some of the

                                        e-p electrons will experience a greater B*dl, as well.

3. Conclusion on mini-torus

        We would like to revisit bending down, since it is much more robust and easier to deal with.

• Easiest way to do it is to increase the mean polar angle of the experiment by about 0.4 degrees (from the current 8 degrees), then we bend down with mini-torus by 0.4 degrees so that it ends up in the same place at the entrance to the main torus.
•  Are there are any obvious objections to this before we consider it in more detail?  A quick look at the variation in figure of merit with a “point-like” target code yields the following:

Mean theta       A(ppm)            rate      Mean Q²          FOM= A²R      relative FOM

7.6                   -.181                8.2       .021                 .269                 .94

8.0                   -.208                6.8       .023                 .286                 1.00

8.4                   -.238                5.8       .026                 .329                 1.15

8.8                   -.270                4.9       .029                 .357                 1.25

9.2                   -.305                4.2       .031                 .391                 1.37

It looks like FOM would in fact improve; perhaps the focal plane shape gets worse, but I think (?) most of the improvement there came from trimming the phi acceptance.  So we will do a more realistic investigation of increasing theta, unless somebody sees some other potential show-stoppers with this scenario.