Tutorial on beam current monitoring Page: 4 of 20
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Impedance measurements of a ceramic gap actually installed in the Fermilab 8 GeV
beam transport line are shown in Figure 3. The curve labeled "open" is the impedance
of the gap as found. It shows that, in this case, a dc path with an inductance of about
1.luH shunts the gap. A plot of the impedance of 1.luH is overlaid for comparison.
The measurement was not designed for quantitatively accurate results above a few
tens of megahertz, but does serve to show that resonances can occur below the
frequency where the gap capacitance begins to dominate. The "shorted" curve displays
the result of placing shorting braids, long enough to reach outside and around a
proposed beam current transformer, across the gap.
.0 t d . . l . . . . . . . . . .
.1 1.0 10.0 100.0 1000.0
FIGURE 3. Impedance measurement of installed ceramic gap with and without a somewhat long
shorting strap attached across the gap. Impedance of 1. luH inductor included for reference.
Although a gap provides the required window to the beam, it may cause undesired
side effects. The beam currents, typically with a broad frequency spectrum, will drive
the gap impedance. Significant beam induced voltage can result if the gap
environment is left uncontrolled. This voltage feeds back on the beam itself and can
cause instabilities in circulating beams. The gap voltage and the currents taking paths
external to the gap may couple into neighboring circuits and at high frequencies
produce radiating fields. A beam current of 500mA at 10Mhz through the impedance
of the gap of Figure 3 will, for example, induce about 30 volts. This is insufficient to
corrupt the beam in a small number of turns, but large enough to present itself as a
'noisy neighbor' to nearby circuits. Narrow, high impedance resonances can result in
very high gap voltages and potentially serious beam disruption.
An example configuration to control side effects of gap impedance and improve the
environment of a beam current monitor is shown in Figure 4. Straps or a full housing
around the transformer and gap serve to reduce the effective impedance presented to
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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/m1/4/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.