Tagger and Vacuum Chamber Design Jim Kellie Glasgow University

Outline. Design considerations. Stresses and deformations. Mechanical assembly.

Design Considerations. The basic tagger parameters are: Main beam energy = 12 GeV. Magnetic field = 1.5 T – 12 GeV radius of curvature = 26.7 m. Momentum range of analysed electrons = 0.6 to 9.0 GeV/c. Intrinsic average focal plane momentum resolution = 0.05%. Decision to use an iron based magnet of reasonable size imposes a limit on the main beam bend angle of around 15 degrees.

Adopted design. Total main beam bend angle = 13.4 degrees. Total length of focal plane (25% to 95% of ) ~ 9 m. The focal plane detector package is divided into two parts. Set of 141 fixed scintillators spanning the full energy range with 0.5 % resolution. Movable microscope of finely segmented counters with 0.1% resolution spanning the coherent peak – tagged photon energies between 8.4 and 9 GeV for GlueX. Magnet configuration of two identical rectangular dipoles in series, in front of which there is a quadrupole to optimise the focal plane vertical focussing – the photon energies of interest to GlueX are analysed in the first magnet. For each: Gap width = 3.0 cm. Pole length = 3.1 m. Weight = 38 tonnes – heaviest single yoke piece ~13 tonnes. Coil power = 30 kW. A straight focal plane with optics which are not inferior to those for a single dipole tagger.

Vacuum chamber design. Since the tagger is broad-band it analyses electron energies from 25% to 95% of which are focussed along a focal plane ~ 9 m in length. The structure of the vacuum chamber should not intercept any of the analysed electrons, and the chamber should extend to within a few cm of the focal plane. The vacuum chamber should also allow the main electron beam to exit cleanly from the spectrometer. A long (~12.5 m) relatively narrow (~0.8 m) chamber with no internal supports is required. The design adopted uses the pole shoes of the dipole magnets as part of the vacuum system. The vacuum chamber fits around the pole shoes. Vacuum seals are made between a lip around each pole shoe and the top or bottom surfaces of the vacuum chamber. Compressed rubber O-rings form the vacuum seals.

The two identical magnets tagger Magnet 1 Magnet 2 Vacuum chamber The vacuum force is around 70 tonnes, so the vacuum chamber needs external support.

The electron entrance angle :5.9 degrees Main beam exit angle: degrees Main beam bending angle 13.4 degrees The angle between the photon exit beam and the focal plane: 9.94 degrees General view of the tagger showing the lay-out of the dipole magnets, focal plane and a selection of electron trajectories.

Length: 3.09 m. Width: 1.09 m. Height:1.41m. Weight: ~38 Tons for one magnet. Conductor area: 135 cm 2. Current density: 144 A/cm 2. Magnetic field: 1.5 T. Pole gap: 3 cm 1 Vertical section through one of the dipole magnets showing pole profile and coil geometry

Vacuum chamber 1 1 Top view Front view Right hand side view looking along output flange Pumping port

Vacuum chamber sections AA` and BB` O-ring Groove Weld Compression pad

Enlarged view of output flange (Electrons pass from back to front in the figure) For compression pad screws Vacuum window compression pad For compression fitting screws Main flange bolt hole Bevelled edge To manufacture the vacuum chamber: a. Weld together complete assembly. b. Skim those parts of the top and bottom surfaces used for the vacuum seals to make them flat and parallel.

Stresses and Deformations. For each magnet: magnetic force between the poles is ~ 150 tonnes, weight ~ 38 tonnes. Vacuum Forces. Total force on chamber~ 70 tonnes. This is supported by: Honeycomb strengthening of ~40 tonnes, 4 vertical struts from magnet 2 of ~15 tonnes, 3 vertical struts from magnet 1 of ~ 10 tonnes. Magnet 2 Magnet 1

Magnet deformation calculation with magnetic, vacuum and weight forces. (Maximum deformation in the pole gap is less than 0.21mm which is much smaller than the O-ring compression of ~6mm for the two O-rings. The calculation are based on a solid piece of iron. From measurements on the Mainz tagger the deflections are underestimated by a factor of ~1.5.) 3 point supports

Vacuum chamber deformation analysis – (for complete chamber). 1.Stainless steel – walls 15mm, ribs 20mm*160mm. Boundary condition: gap between pole shoes and vacuum chamber side walls allowed to vary by 0.1 mm.

Mechanical Assembly. Vertical section showing the arrangement for compressing the vacuum O-rings. Rods connected between yoke and vacuum chamber used to apply compression to the O-rings. Rubber O-ring.

Vertical sections showing how O-ring compression is defined. Back of vacuum chamberBottom pole shoe Spacer Compressed O-ring

Top yoke Top coil Bottom coil Top pole shoe Vacuum chamber Exit flange Support arms View with coils added.

Conclusions A design using 2 identical dipoles which are sample rectangular magnets is technically possible. The vacuum system design is very similar to that used with the Mainz tagger which has operated successfully for more than 10 years. The spectrometer can be assembled and tested in the factory of production, dismantled, and re-assembled in JLab. The maximum weight of any component is ~13 tonnes which simplifies the assembly.