Tutorial on beam current monitoring Page: 3 of 20
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attenuate magnetic fields propagating to the exterior by 53dB. This is sufficient to
clobber the sensitivity of a practical beam current monitor, but not so much as to
render the beam signal invisible to a sensitive radio receiver! The shielding
effectiveness of copper is 55dB better than that of the same thickness of stainless steel.
1 KHz 10 KHz 100 KHz 1 MHz 10 MHz
Copper 2.1 mm 0.66 mm 0.21 mm 0.066 mm 0.021mm
302 Stainless Steel 13.3 mm 4.2 mm 1.3 mm 0.42 mm 0.13 mm
FIGURE 1. Skin Depth Chart.
Since the magnetic field of the beam is severely attenuated outside a continuous
conducting vacuum chamber, a practical beam current monitor requires a "window to
the beam". The monitor must be placed within the vacuum chamber walls or the
conducting path in the chamber must be broken. To minimize mechanical
complications associated with inserting a device into the vacuum, a non-conducting
material, often ceramic, may be inserted electrically in series with an otherwise
continuous beam tube section. This interruption of the conduction path along the beam
tube forces wall currents to find a new path, potentially under the instrument
designer's control, outside the vacuum chamber.
Conducting beam tube Zgap
FIGURE 2. Typical ceramic gap installation in accelerator environment and equivalent circuit model.
Figure 2 depicts a ceramic break or ceramic gap installation in a typical beam tube.
The break provides a local region suitable for placing a beam current monitor through
which, at low frequencies at least, only the beam current flows. Wall currents take
whatever paths, controlled or uncontrolled, that are presented across the gap. At low
frequencies the lowest impedance path is often well out around the local region. The
effective gap impedance is a parallel combination of the inherent gap capacitance and
all external impedance, including beam tube capacitance, grounds, cable shields,
straps or housings placed across the gap, and even the current monitor itself. Elements
comprising the external paths may be intentional or incidental, local or distant, but will
always be present. The beam will supply whatever gap voltage is required to drive the
image currents through the effective gap impedance. A gap voltage
Vgap (w)= Zgap (w) - Iwa (W) =Zgap (w) - Ibeam(W) (4)
will be generated.
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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/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.