The beam current sensor consists of 5 strip wound toroids with separate magnetic cores packed together in toroid assembly. In the toroid assembly cross section, shown on figure2.2.2.3 one sees 5 cores T1 to T5. T1 is magnetic modulator, T2 excitation generator, T3 demodulator, T4 ripple feedback compensation transformer and T5 zero flux transformer.
The current measurement is based on very precise compensation of the magnetic field created by the beam current. The magnetic modulator in the center of the assembly is very a sensitive magnetometer that determines the resolution and zero drift of the PCT. The modulator is sensitive to external magnetic fields. Therefore these fields should be attenuated by extensive magnetic shielding. The sensitivity to axial external magnetic field is 1 A/G and to radial magnetic field 100 A/G. Even shielding by a factor of 500 to 1000, is not adequate for many applications. In addition high permeability shields are easily saturated by a strong eternal magnetic field, and one has to take this into consideration. The magnetic field of the dipole magnet in the electron arm had no detectable effect on the Unser when powered with a 1500 A current. However, one should test again for possible effects when the magnet carries a 1800 A current and the magnets iron becomes saturated or when the Unser is closer to the dipoles of the spocrometer. In our test the Unser and the electron arm dipole were on opposite side of Hall A.
The Unser beam position dependence is smaller than 0.1 A over 50 % of inner diameter.
The Unser should be calibrated, by passing through it a known current, from the current supplier located in the counting house. The combined absolute accuracy of that unit and the digital voltmeter used for the Unser calibration is about 0.1 %, if Unser calibration is performed at 100A.