Tutorial on beam current monitoring Page: 16 of 20
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buckets in the machine are filled. This structure is visible on scope traces in Figures 16
and 17. The beam rf signal picked up by the cables is radiated from unshielded gaps
and ceramic kicker beam tubes in the machine in a manner as described earlier in the
ceramic gap discussion. Even the beam makes noise, so it is proper to conscientiously
shield all gaps!
A SUCCESSFUL BEAM CURRENT MONITOR INSTALLATION
As a working example, a beam current monitor installation used in the Fermilab
8 GeV beam transport line is depicted in Figure 18. The system provides accurate
measurement of the charge in each 1.6usec beam pulse injected into the transport line.
High fidelity representation of the macropulse shape is required, but resolution of the
individual 53Mhz bunches is not. A system bandwidth of only a few megahertz is
acceptable to meet signal observation requirements. Ultimately the signal is processed
by an analog integrate and hold circuit before presentation to the control system.
The current transformer itself is a commercially available model featuring an
isolated output winding and a metallic case for shielding. Twinax cable is utilized to
transport the signal about 50 meters to the processing electronics. (Noise reduction
core is optional.) The twin signal conductors, totally contained within a Faraday
enclosure formed by the transformer case, cable shield, and metal electronics chassis,
are grounded only at the differential receiving amplifier input. The amplifier circuit
(see Fig. 19), based on the Analog Devices AD830, is designed with suitable
bandwidth to meet signal processing requirements and good common mode rejection
to further attenuate noise. Note that the amplifier is not DC coupled to the transformer.
If this seems inappropriate, remember that the transformer does not DC couple to the
beam in any case, so nothing is lost! The measured frequency response and common
mode rejection ratio of the amplifier circuit are given in Figures 20 and 21.Twinaxial cable ith
grounded shield
Shielded transformer
with floating output
and grounded case
Bypass straps across beam tube gap
around outside of transformerFIGURE 18. A successful beam current monitor installation example.
AD830
True differential receiver
amplifier in grounded
electronics chassis
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Webber, Robert C. Tutorial on beam current monitoring, article, June 22, 2000; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc712084/m1/16/: accessed May 5, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.