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Next: 3.4 The D0 dipole Up: 3 The magnetic architecture Previous: 3.2 The ALEPH solenoid

3.3 The UA1 dipole magnet

(In collaboration with W. Flegel PPE/EC)

The central ALEPH magnet is complemented on both sides by a dipole magnet with a horizontal field of B = 0.7 T orthogonal to the beam. We propose to re-use the flux return yoke of the UA1 magnet for these two magnets, see Fig. 3.2. The magnets will be arranged as the existing UA1 magnet, but each with half of the available number of yokes. To adapt to the required length of the new magnet, two short flux return yokes have to be added. This requires completely new windings which, in addition, have to provide a large opening of 1.8 m diameter around the beam axis at the end closest to the interaction point (see Fig. 3.3). At the opposite end, the beampipe is of 20 cm diameter, it fits between the two halves (see Fig. 3.3) and needs no special shaping of the winding. A support and moving system similar to the one for the UA1 magnet has to be constructed for each magnet.

  
Figure 3.3: A view of one-half of the UA1 magnet, which will be located next to the central ALEPH magnet.

3.3.1 Mechanical arrangement of the magnet system

The new magnets will consist of two halves which are mirror symmetric about a vertical plane containing the beam axis, as is the case for the UA1 magnet. Each half consists of 4 rectangular, C-shaped flux return yokes of UA1 plus an additional one to adapt to the length of the new magnets. The opening of the C-shaped yoke is facing the beam. Each yoke is constructed of 16 low-carbon steel plates of 50 mm thickness and separated by 17 mm gaps. For UA1 they were instrumented with scintillators for hadron calorimetry, but they will not be so equipped for FELIX. Each magnet will be placed onto a support system which permits one to horizontally separate the two magnet halves orthogonal to the beam, in order to gain access to the magnet volume for detector installation without dismantling the beam pipe. In addition, in its open position it has to allow for the axial displacement of the ALEPH end caps.

3.3.2 Winding and coil partition

The new coils were calculated and designed to obtain the required acceptance angle and the necessary bending field. In order to avoid a strong saturation of the iron in the return yoke, a field value of B = 0.7 T is envisaged. For reasons of cost, aluminium is proposed as the conductor material with an acceptable current density of s = 3.2 A/ . Finally, the pressure drop of the cooling water in the underground hall of ALEPH has to be kept well below 10 bar. These considerations then lead to a similar conductor cross-section as in UA1. Each magnet half will contain two coils with the same number of turns. The coils near the beam axis on the side facing the interaction point provide a large opening of 1.8 m diameter for particle acceptance. Fig. 3.2 shows in a perspective view the C-shaped return yokes of one-half of a magnet half and its pair of coils in its position next to the ALEPH magnet. Each coil is built of 7 pancakes, each with two layers of 5 turns. A list of the parameters of the proposed magnet layout is given in Table 3.1.

 

 


Table 3.1: Parameters of the proposed UA1 magnet.

3.3.3 Coil optimization

One way to reduce the cost of the magnet is to re-use the existing material and equipment from the UA1 magnet. Additional iron is, however, needed to adapt to the required length of the new magnets. Because of their complex shape, the coils will be a major cost item. A first optimization of the costs for coil manufacture versus operation cost has been performed. The resulting current density determined the choice of the conductor cross section. The following costs were assumed for this calculation:

In addition, the actual cross section chosen takes into account the space constraints given by the iron yoke and the required magnet aperture. Further optimization of the design will follow through consultation with industry.

3.3.4 Field calculations with TOSCA

The combined field of the ALEPH solenoid and the UA1 dipole magnet at nominal current has been calculated with the TOSCA [2] programme. For these calculations, the main field component of the dipole magnet is in the x-direction, that is in the horizontal direction orthogonal to the beam axis. The main field component of the ALEPH solenoid is in the z-direction, that is the beam axis. Fig. 3.4 shows the horizontal component of the field. In the dipole region, the asymmetry in the field is due to the coil asymmetry between upstream and downstream currents. The interference between the ALEPH solenoid and the UA1 dipole has also been checked. Figs. 3.4 and 3.5 show the x and z field components, respectively, resulting from both magnets. In the region of ``ALEPH cap & UA1 coils'' the main fringe field effect in the z-component is from the dipole, while in the horizontal direction, the biggest effect is due to the radial component of the field in the solenoid. In this simulation, the shims and grooves, currently made in the ALEPH end-cap in order to reduce the field inhomogeneity, have not been taken into account  [1].

 

 


Figure 3.4: component for the total field: the dotted curves correspond to different emittance angles from the interaction point in the horizontal plane; the solid curve is the field along the z-axis.

 

 


Figure 3.5: component for the total field: the dotted curves correspond to different emittance angles from the interaction point in the horizontal plane ; the solid curve is the field along the z-axis.


next up previous
Next: 3.4 The D0 dipole Up: 3 The magnetic architecture Previous: 3.2 The ALEPH solenoid

V.A.