The Parametric Current Transformer, a beam current monitor developed for LEP

Toroidal transformers are used to measure the beam current in beam lines and accelerators. Placing such a transformer hi the feedback loop of an operational amplifier will increase the useful frequency range (active current transformer). A magnetic modulator can be added to extend the response to DC current, maintaining with a control loop the transformer core at a zero flux state. The magnetic modulator in the parametric current transformer gives not only the DC response but provides parametric signal amplification up to a transition frequency of about 500 Hz. The low frequency channel (magnetic modulator) and the high frequency channel (active current transformer) are linked together in a common feedback loop. A large dynamic range together with good linearity and low distortion is obtained. This arrangement protects the magnetic modulator from dynamic errors in case of a sudden beam loss, which could impair its zero stability. Dynamic overload protection is an important condition to obtain high resolution and good zero stability, even in applications which require in principle only a very limited frequency response.