LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
below the 1-micron level. The raw position signal from a cavity BPM is proportional to beam
charge, so that a normalization procedure will be necessary to extract position information.
Figure 8.36 Conceptual design for a combined button- and cavity-type position monitor
Recent research at Stanford, supporting the NLC project, has produced an X-band cavity-
BPM design, which solves the fundamental mode problem by a clever arrangement of the output
waveguides [48]. In this design, a second cavity is also integrated in the design with its
fundamental mode frequency set to match the position sensitive cavity’s mode frequency. This
second cavity can then be used to extract the phase (sign) information, in addition to providing
charge normalization.
The incorporation of conventional button- and cavity- type pickup electrodes in the same
assembly affords several advantages over either system alone. It is relatively simple to calculate
the sensitivity (e.g. volts per micron) of a conventional button-type position monitor. The
analogous sensitivity for the cavity monitor, though potentially much higher, depends on such
things as cavity Q, waveguide coupling efficiency, etc., and as such may not be as easily
determined a priori. A cross-calibration of the two with beam immediately resolves any
uncertainties in the determination of length scales. Similarly, the cavity monitor has a very easily
determined and highly accurate electrical center, reproducible at the sub-micron level, while
conventional electronics have always performed poorly in this regard. By steering to the cavity
null and recording the readbacks from the conventional electronics attached to the buttons,
uncertainty in electrical center stability of the conventional electronics is significantly reduced.
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