Measurement of the Z boson cross-section in - Harvard University ...

Chapter 2: The Accelerator and the Experiment 39
two endcap sections. Together, they provide the magnetic field necessary for charged
particle momentum measurement over a volume of 12,000 m 3 . A schematic of the
ATLAS magnet system is shown in Figure 2.3.
The central solenoid provides a 2T axial magnetic field for the inner detector.
Charged particles bend in the r − φ plane. The winding is made of Al-stabilised
NbTi alloy, cooled to 4.5 K using liquid helium. The diameter and axial length of
the solenoid are 2.5 m and 5.3 m respectively, and stores 40 MJ of energy at the
full current of 7330 A. The material of the solenoid is equivalent to ≈ 0.66 radiation
lengths at normal incidence.
In the toroid, the magnetic field is along the φ coordinate, so that charged particles
bend in the r − z plane. The barrel toroid has eight racetrack-shaped coils arranged
symmetrically in the azimuth. It has inner and outer diameters of 9.4 m and 20.1
m respectively, and a length of 25.3 m. The endcap toroids each contain eight flat
coil units and eight keystone wedges bolted and glued together. Each unit is 5 m in
length. At the peak current of 20.5 kA, the barrel and endcap toroids provide average
fields of 0.5 T and 1 T respectively, and have a combined stored energy of 1.58 GJ.
In order to minimize multiple scattering, the toroids were designed with a air core
rather than using iron to enhance the magnetic field.
During operation, each coil of the barrel toroid experiences an inward Lorentz
force of about 1400 tons [98]. This force is balanced by Al alloy struts in between
the eight coils. In the case of the endcap toroids, the force is counteracted by stops
mounted on the eight barrel toroid coils.