System Block DiagramBeam phase monitor experiment on the SLC
Hardware has been installed on the SLC to measure the phase of the beam induced fields in the structure relative to the klystron RF phase as a test for the NLC beam phase monitor system. This system makes two measurements: The relative phase of the phase reference line and the klystron RF from the output of the structure, and the relative phase of the phase reference line and the beam induced RF in the structure with the klystron on standby. Each measurement is performed with a digital IF phase detector, and requires a single machine cycle.
System Block Diagram
This system uses a digital IF phase detection scheme: the 2856 MHz RF is mixed down by a 2846 MHz local oscillator to a 10MHz IF. The IF is directly digitized by a 40MHz, 10bit digitizer. The phase is measured by looking directly at the in phase and out of phase components relative to a mixed down signal from the accelerator phase reference line.
A switchable attenuator is used to compensate for the 40dB difference in signal strengths between the klystron RF and the beam induced RF, reducing the dynamic range to a reasonable level. The phase length of the 40dB attenuator is measured using separate calibration path with a step attenuator. Note that the mixer is protected from the high power from the klystron standby pulse by a microwave limiter (not shown).
Since the only difference in components between the klystron phase measurement and the beam induced field phase measurement is an attenuator, and its phase length can be calibrated on line, the resulting phase error should be very small. Phase measurements can be taken in a single pulse, essentially eliminating the effects of phase drifts in the accelerator phase reference.
System Schematic. (CanvasFormat)
Several measurements have been performed with this system. See: experimental results.
The phase measurement test setup differs from an actual NLC beam phase measurement system in several respects.
1. The NLC system would need to operate at X-band (11.424 GHz) rather than S-Band (2.856GHz). It is not clear to what extent this would increase the noise or drift of the measurement.
2. The NLC system needs a higher bandwidth so a much higher IF frequency would be used.
3. The signal processing for the NLC system would be much more sophisticated.
Page by Josef Frisch frisch@slac.stanford.edu 04/22/2002
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