Parametric Current Transformer (Unser)
Direct Current Transformers (DCTs) are able to provide accurate and high 
precision measurement of circulating beam currents over a dynamic range of 10E5 
or greater. Sensitivities of better than 1microAmps are achievable and DCTs 
built at Fermilab have operated over periods of years with baseline drifts of no 
more than 5microAmps. A great deal of litterature is available describing the 
operation and design details of DCTs. High quality devices are commercially 
available. 
Since the dc beam current provides no time varying flux component to generate 
a signal by magnetic induction, an ac flux component is "brought to the beam" 
via the action of a magnetic modulator circuit. The operation of a magnetic 
modulator, and hence a DCT, is based on the non-linear characteristics of high 
quality tapewound magnetic cores.
Two toroidal cores are switched between 
flux saturation levels, first one polarity then the other, by counter-phased 
windings powered by an external source. In the absence of any dc beam current 
and to the extent that the two cores exhibit matched and symmetric B-H 
characteristics, sense windings of a common polarity around each core produce 
equal and opposite signals. The output of either winding is non-zero only during 
that time in which the flux in the cores is changing (i.e. non saturated). The 
sum of the two sense winding signals is zero. 

PCT 
schematic 
A dc beam current through the two cores biases each with flux of the same 
polarity, while the flux in one core due to the modulator drive remains out of 
phase with that in the second. Each core reaches its saturation flux level at a 
different point in the excitation cycle for one alternation than for the other 
alternation. The net result is a flux imbalance between the two cores, producing 
an outout signal at even harmonics of the excitation frequency. 
The magnetic modulator section of the DCT is essentially a magnetic "mixer", 
translating dc signals to a different location in the frequency spectrum. 
Functioning as a sampling device, it will produce aliasing of signal frequencies 
greater than half the excitation frequency. For example, a modulator operating 
at 1kHz drive frequency will produce an identical output for a 1kHz beam current 
component as for an equal magnitude dc current. This problem is usually avoided 
in a beam current monitoring DCT by coupling the dc modulator with an ac 
transformer in an hybrid network with a crossover frequency well below one half 
the excitation frequency. 
DCTs are normally designed to operate in a feedback configuration. Current is 
forced through a feedback winding on the cores to oppose the beam current and 
maintain a dc flux null. Instead of measuring this current, it is preferable to 
measure the voltage across a serie of high precision resistors, usually 
thermostabilized. 
Since you really measure the beam current passing through the toroids, this 
device provides an absolute measurement of the beam intensity. However 
the differences in the magnetic properties of the two cores makes the DCT 
inadapted to measure small currents with the same accuracy. Indeed you have to 
measure its offset before making any measurement; the offset measurement takes 
time because it is not stable (3uA drift + 250nA fluctuations).
The solution 
to this problem is to use the Unser monitor to calibrate another stable, linear, 
relative device, like a resonnant cavity. 

References:


A TOROIDAL DC BEAM CURRENT TRANSFORMER WITH HIGH RESOLUTION
K. 
Unser
IEEE Transactions on Nuclear Science, Vol. NS-28, No.3, June 
1981



BEAM INSTRUMENTATION: Beam current and charge measurements
Giovanni 
Gelato
Ed.J.Bosser, CERN-PE-ED 001-92, Nov 1994



THE ZERO-FLUX DC CURRENT TRANSFORMER, A HIGH PRECISION BIPOLAR WIDE-BAND 
MEASURING DEVICE
H.C. Appelo
IEEE Transactions on Nuclear Science, Vol. 
NS-24, No.3, June 1977



ACCURATE MEASUREMENTS OF DC AND AC BY TRANSFORMER
M. Groenenboom & 
J. Lisser
Electronics and Power, pp. 52-55, January 1977



CURRENT TRANSFORMER FOR THE ISR
CERN Courier, Vol.10, pp. 380-382, 
No.12, 1970