Experiment Proposal - opera - Infn

Externally, the magnet is 8.75 m wide, 10.0 m high and 2.64 m long. The length is given by the
two walls, each of 0.82 m, withafreespaceof1.0 m in between allowing the insertion of the Precision
Trackers. This free space is defined to optimise the momentum resolution. Each wall is made of 12 iron
layers 50 mm thick, with 11 gaps 20 mm thick to house the Inner Trackers. Each iron layer is made of
7 iron plates 50 × 1250 × 8200 mm 3 to be assembled in situ, for a total of 168 plates per spectrometer.
The plates are precisely milled along the two short sides (1250 mm). They include holes for bolting and
lifting as well as slots for cabling.
The total weight of one magnet is about 950 ton: 270 ton for the top and bottom flux return paths
and 680 ton for the walls.
The plates are bolted together with bolts (36 mm in diameter 820 mm long) spaced by about 1 m,
resulting in a solid structure which is used as a base for the emulsion target supports. The nuts holding
the bolts also serve as 20 mm spacers between the plates. The plates are also bolted at the top and
bottom edges to the return flux paths. This design permits a fast and clean magnet assembly, alternately
mounting in iron layers and inner tracker planes.
We envisage the use of a spin casted unalloyed quality steel with magnetic properties of the type Fe
GM01 UNI 3595. The permeability is 7 × 10 −4 Hy/m, for an average magnetic field of 1.55 T ,well
below saturation. The first tests performed on a sample of this iron indicate magnetic properties about
3% better than nominal.
A field map calculation performed with the ANSYS 5.3 code has shown that the homogeneity of the
wall magnetisation is within 5% or better over most of the plates. The fringe field is about 10 G at
10 cm distance from the edge. Fig. 56 shows the distribution of the magnetic field in the 12 iron layers
at their centre (left) and at the connection with the top flux return iron (right).
In order to simplify the construction and the operation, the coils have low current density and air
cooling. Two symmetric coils run around the top and the bottom flux return paths. The turns are made
of copper bars (type Cu-HCP UNI 5649-1) 80 × 15 mm 2 ; their resistivity is 0.017 Ω × mm 2 . The
current density is 1 A/mm 2 for a total current of 1200 A. The total number of turns is 46 (23 per coil).
The total length of the conductors is 810 m and their weight 10 ton. The total resistance is 0.011 Ω,
the voltage 13.2 V and the power 15.8 kW .
The two flux return paths (top and bottom path) and the multilayer vertical walls are bolted together
so they constitute a self supporting structure. The bottom flux return path is bolted to the floor by means
of 50 bolts (30 mm in diameter) cemented to the floor concrete. The vertical iron plates are bolted to
the bottom return paths. Each multilayer wall consists of plates bolted together. The nuts, screwed on
the bolts between the plates, act as spacers for the trackers and tighten the plates together. The bolts
and the nuts prevent sliding of the plates making the multilayer wall, together with the return path, a
compact structure self supporting and usable, in addition, as a support for the target modules.
A prototype magnet is presently under construction at the Frascati Laboratory. It has full height but
a reduced width and wall thickness. It is made of two walls each 8.6 m long and 2.5 m wide, for a total
weight of about 70 ton of iron. It will be equipped with RPC inner detectors. Tests will be performed
on the mechanical properties, on the installation procedure and on the operation of the RPC’s, as well
as measurements of the magnetic field.
76