442
F. Bordry et al.
8.6.2.2 AC Beam Transformers
In order for a transformer to interact with the magnetic field of the beam it has to be
placed over a ceramic gap in the vacuum chamber. The wall current accompanying
the beam along the metallic wall of the vacuum chamber is deviated around the
transformer using a low impedance radio-frequency bypass. A beam current, I B , can
be considered as the primary winding of the transformer, with the output voltage
from the secondary windings given by V α L dI B /dt, L being the transformer
inductance. An ideal transformer would give a differentiated response, with the
integrated charge being zero, which is not of much use as a measuring device. In
reality the secondary windings of the transformer have some capacitance and are
terminated by some finite resistance. This leads to signals that closely resemble the
beam intensity distribution, with the added inconvenience of a DC offset due to the
transformer droop. This DC offset can be corrected for either electronically or by
software treatment of directly digitised data. Bandwidths from 200 Hz to 1 GHz
are now available, using ferromagnetic cores wound with high permeability metal
tape to avoid eddy currents. Such transformers are capable of resolving the charges
contained within single short bunches, acquired either by direct digitisation or using
fast integrator electronics. Resistive or inductive wall current monitors can also be
used to provide such signals, but again require DC offset compensation [126].
8.6.2.3 DC Beam Transformers
In storage rings and accelerators with cycle times of several seconds a DC beam
Current Transformer (DCCT) can be used to measure the total current. Such an
instrument has a bandwidth that extends from a few kHz down to DC (hence its
name) and was developed for the CERN-ISR (Intersecting Storage Rings), the first
machine to sustain beams for hours [127].
A DC transformer is based on a pair of matched, toroidal, ferromagnetic cores,
which are driven into saturation by a modulation current at frequencies of up to a
few kHz. The principle of operation makes use of the hysteresis loop of the toroid.
If an equal but opposite modulation current is applied to both cores with the beam
not present, then the voltage induced in the detection windings on each core will
be equal but opposite. When, however, there is a beam current present, the starting
point in the hysteresis loop for zero modulation current is offset due to the static
magnetic field generated by the beam current. Since the modulation is opposite in
each toroid, the time spent in saturation will be different for the two branches of
the hysteresis loop. This results in the generation of voltage pulses at twice the
modulation frequency when the induced voltage in the detection windings on each
core is combined. The demodulation of this signal gives a train of pulses, with the
width of each pulse being a direct measure of the beam current, i.e. by how much
the hysteresis curves are offset.
In the “zero flux detector” implementation of the DC beam transformer, the result
of the demodulation is fed back through the pair of transformers by a compensating
F. Bordry et al.
8.6.2.2 AC Beam Transformers
In order for a transformer to interact with the magnetic field of the beam it has to be
placed over a ceramic gap in the vacuum chamber. The wall current accompanying
the beam along the metallic wall of the vacuum chamber is deviated around the
transformer using a low impedance radio-frequency bypass. A beam current, I B , can
be considered as the primary winding of the transformer, with the output voltage
from the secondary windings given by V α L dI B /dt, L being the transformer
inductance. An ideal transformer would give a differentiated response, with the
integrated charge being zero, which is not of much use as a measuring device. In
reality the secondary windings of the transformer have some capacitance and are
terminated by some finite resistance. This leads to signals that closely resemble the
beam intensity distribution, with the added inconvenience of a DC offset due to the
transformer droop. This DC offset can be corrected for either electronically or by
software treatment of directly digitised data. Bandwidths from 200 Hz to 1 GHz
are now available, using ferromagnetic cores wound with high permeability metal
tape to avoid eddy currents. Such transformers are capable of resolving the charges
contained within single short bunches, acquired either by direct digitisation or using
fast integrator electronics. Resistive or inductive wall current monitors can also be
used to provide such signals, but again require DC offset compensation [126].
8.6.2.3 DC Beam Transformers
In storage rings and accelerators with cycle times of several seconds a DC beam
Current Transformer (DCCT) can be used to measure the total current. Such an
instrument has a bandwidth that extends from a few kHz down to DC (hence its
name) and was developed for the CERN-ISR (Intersecting Storage Rings), the first
machine to sustain beams for hours [127].
A DC transformer is based on a pair of matched, toroidal, ferromagnetic cores,
which are driven into saturation by a modulation current at frequencies of up to a
few kHz. The principle of operation makes use of the hysteresis loop of the toroid.
If an equal but opposite modulation current is applied to both cores with the beam
not present, then the voltage induced in the detection windings on each core will
be equal but opposite. When, however, there is a beam current present, the starting
point in the hysteresis loop for zero modulation current is offset due to the static
magnetic field generated by the beam current. Since the modulation is opposite in
each toroid, the time spent in saturation will be different for the two branches of
the hysteresis loop. This results in the generation of voltage pulses at twice the
modulation frequency when the induced voltage in the detection windings on each
core is combined. The demodulation of this signal gives a train of pulses, with the
width of each pulse being a direct measure of the beam current, i.e. by how much
the hysteresis curves are offset.
In the “zero flux detector” implementation of the DC beam transformer, the result
of the demodulation is fed back through the pair of transformers by a compensating