### Numi EPB Magnetic shield

The NuMI extractions system of magnets is near the Recycler. Fringe fields from the extraction magnets pose a potential problem for the Recycler. Possible methods of shielding the recycler from these fields have been considered. For example, one may consider shielding the recycler and various elements. One may consider placing magnetic material somewhere between the extraction system and the recycler, or one may consider containing the field associated with the extraction magnets. The approach taken here is the later - to contain the stray fields.

The first step is to simulate the effects of the shielding. The POISSON program from the Los Alamos SUPERFISH set of programs has been used to study possible configurations. The fields outside an unshielded EPB have been measured. The first step was to compare these fields with those from the simulations. The agreement is approximate - but not exact. This is not surprising as the detailed permeability of the magnet steel as a function of excitation is not accurately included. Also, the fine details of the magnet are not yet included in the model.

A number of studies of possible magnetic shielding configurations have been studied. The current focus is to construct a prototype magnetic shield that consists of two layers of 0.25 inch thick steel separated from the EPB and then one another by 1.0 inches.

The drawing below is output from Poisson showing the configuration being studied.

Various magnetic shielding configurations may be parameterized by a shielding factor == SF . SF is defined as the ratio of the field in the region outside the magnet without the shielding to the field with the shielding. Note that with this definition, the shielding factor is > 1. The shielding factor is independent of the position outside the shielding. The table below shows the shielding factor for some of the configurations that have been studied.

```  Shielding Factors for Various Configurations
Note - the inner shield is 1 inch from the magnet
the second shield is 1 inch from the
outside of the first shield

Configuration        SF

1/4                  4.5     (no second shield)

1/8 + 1/8            8.5

1/4 + 1/8           23.0

1/4 + 1/4           35.3     (prototype)

3/8 + 1/8           37.9

```