A simulation with MAFIA for the HERMES transverse target magnet
- Results from calculations done in May/June 2002

Introduction

Due to the fact that the fringe field of the HERMES magnet affects the transformers downstream it is necessary to install a shielding plate. The aim of the simulations with MAFIA was to find the best position for the shielding plate and to optimize the geometry of the plate so that the effect on the field homogeneity of the magnet is minimal.

The shielding plate

In May 2002 a first layout for a shielding plate was proposed by Gunar Schnell and Maurice Bouwhuis. A sketch of the geometry is displayed in figure 1. The thickness of the plate is 5 mm, the iron is EBG. For the first simulation run the plate was located at z = 341 mm between the magnet and the transformers.
Figure 1: Sketch of shielding plate Figure 2: Homogeneity with and without shielding

Figure 2 shows the curves of the field homogeneity along the beam axis with and without a shielding plate. The influence on the field uniformity is not acceptable. All calculations use a current of I = 545 A.

In a second calculation the shielding plate was moved 6 mm nearer to the magnet i.e. to z = 335 mm. However this procedure produced no improvement.

In a third step a second shielding plate was added in 5 mm distance to the first plate as shown in figure 3. Figure 4 shows the influence to the field homogeneity of the double shielding.
Figure 3: Layout of the double shielding plates Figure 4: Field uniformity in comparison

The representation in figure 4 shows clearly the disadvantage of a second shielding plate. The field uniformity is much worse than that calculated for a single shield. Therefore another calculation was done with a circular shielding plate positioned in the direct neighborhood of the transformers at z = 430 mm.
Figure 5 displays the geometry of the round shielding plate. Figure 6 shows the field uniformity with and without a circular shielding plate.

Figure 5: Layout of the circular shielding plate Figure 6: Field uniformity with and without circular shielding plate

Figure 7 shows the distribution of µ in the iron of the circular shielding plate.


Figure 7: µ-distribution in the iron of the circular shielding plate

Examination of the fringe field

Of great interest is in any case the fringe field in an area between 400 and 550 mm from the center of the magnet outside the yoke. Here are the sensitive transformers located. The following images show the fringe field at a z-poition of 450 mm with and without a shielding plate. Please click on the thumbnails to get a postscript file of each plot. The field is displayed in a range between 0.0 and 0.04 T.

Figure 8: Fringe field without shielding plate Figure 9: Fringe field with single shielding plate Figure 10: Fringe field with double shielding plate Figure 11: Fringe field with circular shielding plate

Figure 12 shows the fringe field for all calculated versions at the same position at z=0.450 m, y=0.170m and x=0.0 m. It can be seen that for the circular case the shielding works best.

Figure 12: Fringe field with and without shielding plate

Summary

The simulations done with MAFIA should help to find out if the fringe field of the transverse HERMES magnet could be reduced by using a shielding plate. The location and the layout of the shielding plate was also of great interest, because the field uniformity of the magnet should not be influenced by a shielding plate. The calculation results show that the effects of the plates destroy the field homogeneity considerably which is required to be deltaB/B less than or equal to 1.4x10-4 along ± 200 mm in beam direction. Best results were achieved with a circular shielding plate.

Author: Michaela Marx, DESY-Hamburg, Group MPY, 22603 Hamburg, Germany