Dynamic Tune Measurements
Introduction
Recent measurements using a Gated Tune Analyzer allowed us to study the
change in betatron tune during the 16 ms store time of the beam. The dynamic
behavior of the betatron tunes gives us some insight into the collective
effects as they relate to the changing beam size and charge density, as
well as ion trapping effects.
Instrumentation
An HP3589A Spectrum/Network Analyzer with a gated sweep option was used
to aquire the tune data during a 0.6 ms long window that opens at a predetermined
time after injection.
The swept spectrum measurement samples many beam pulses, but always
in a narrow time interval and at a fixed time with respect to injection.
The beam is transversely excited through the damping ring "growler"
system and is driven from the output of the network analyzer after first
being mixed down to the 0-mode betatron sideband frequency. The amplitude
response of the beam from a BPM stripline is detected at the lower betatron
sideband of the 2nd revolution harmonic.
Triggers
A standard trigger from an STB module in the control system provides a
trigger at extraction in the ring. An appropriate single beam code
for the trigger (BC 10) was used to select just one of the multiple pulses
the damping rings presently handle (PEP II HER, e+ production, FFTB).
The gate can be delayed in the HP3589A after the trigger,
which provides a convenient way of varying the time after injection when
the measurement is made. Since the initial trigger is at extraction,
we delayed the gate by 100 ms - 16.6 ms to give us the injection
time of the next injected pulse.
The gate length is adjustable from the HP3589A, but was kept at its
shortest value of 640 microseconds for these measurements. (The minimum
gate length is a function of the resolution bandwidth used.)
Results
The tune frequency and amplitude of the excited betatron sideband were
measured in 2.5 ms steps from injection to just before extraction.
Plot
of excited tune band frequency and amplitude vs time.
Analyzer settings:
RBW=1.2 kHz
Gate Trig EDGE
Gate Polarity POS
Gate Length 640 micro sec
Gate Delay Manual
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If you have comments or suggestions, email Patrick Krejcik at pkr@slac.stanford.edu