RF BEAM CONTROL SYSTEM FOR THE BROOKHAVEN RELATIVISTIC HEAVY ION COLLIDER, RHIC Page: 3 of 3
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acceleration cycle to maintain constant closed-loop
dynamics as the synchrotron frequency changes.
The gains are obtained in a two step process. First an
LQR [**] optimization is done which weights heavily the
phase oscillation damping. From the resulting gains the
closed-loop poles are determined and Then by pole-
placement, using Ackermann's formula, analytic
expressions are derived for the gains as functions of the
pole values and the synchrotron frequency.
5 BEAM PHASE MEASUREMENT
The phase of every bunch is measured on every turn by
the phase detector, illustrated inkigure 3.
Figure 3. The bunch-by-bunch phase measurement
circuit. The I/Q components of the response of the Bessel
filter (Q=10) are demodulated with the components of the
reference signal at 28 MHz. Digital rectangular-to-polar
conversion supplies the phase and magnitude.
The bunch signal from the wall current monitor
longitudinal pickup is applied to a narrewband (Q=10)
bessel filter centered at 28 MHz. The response of this
filter, which is a burst lasting for about 500 ns, is
demodulated in a double-sideband circuit with the 28
MHz cavity-drive signal. The resulting I and Q baseband
signals are digitized to 12 bits, and a dedicated DSP chip
(Pythagoras, Plessy #1234) performs the rectangular to
polar transformation. The resulting angle is the phase of
the bunch with respect to the rf drive signal. The
rectangular-to-polar conversion provides full zero to 360
degree unambiguous phase results.
The magnitude is used in an Automatic Gain Control
circuit which keeps the signal levels in range for the
ADCs. When RHIC has the full complement of 120
bunches the bunch spacing is less than the ringing time of
the filter. So a parallel bank of eight filters is used and the
bunch signals are de-multiplexed with GASFET switches
into the eight channels. To reduce video feedthrough and
achieve high dynamic range the control signal is
bandlimited to a sin(x)/x-type shape so that any
feedthrough is below the lower band edge of the filter.
The average phase over one turn is calculated in the
phase detector electronics before the data is sent to the
beam control DSP. During the injection process the new
bunch is masked out of the average until the injection
damper has corrected any phase or energy errors.
6 SYNCHRONIZATION, INJECTION
RHIC is filled one bunch at a time from the four
bunches in the AGS at 30 Hz repetition rate. The AGS
cycles at 0.5 Hz, implying the nominal fill of 60 bunches
can be loaded in 3@.seconds. Single bunch transfer is
advantageous because any fill pattern of the 360 buckets
in RHIC is allowed. Typically the pattern is selected to
yield equal collision rates at all six interaction points.
The relative orientation of the gap between the two rings
is controlled by the collision synchronization system.
6.1 Bunch by bunch injection
The injection synchronization system provides the
triggers to the extraction injection kickers, and also a
reference signal to which the AGS bunches are locked.
Between each injection the phases of the kickers trigger
and the AGS reference signal must advance appropriately
to fill the desired bucket and provide that the AGS bunch
is at the kicker when it fires. The phasing of these signals
is effected by precise (2x 10-1*) control of the frequency of
the DDS with which they are generated. By switching to
a slightly (-30 Hz) offset frequency for a prescribed time
(a counted number of revolution periods) the bunches and
kickers trigger are aligned for filling the desired bunch
6.2 Synchronization for collisions
The two rings of RHIC accelerate independently and
when the beam in each ring reaches full energy the radial
feedback is replaced with a ring-to-ring synchronization
system. The two rings are first synchronized to a master
oscillator at 197 MHz . Then the missing bunch gaps are
aligned by advancing the phase of the synchronization
reference by the appropriate integral number of 197 MHz
[**]V115 Wave From Generators.
[**] D Boussard, Proc. CERN School on RF
Engineering, Oxford England, April 1991.
[**] H. Hartmann, RTDL BNL 63942
[xx] E. Onillon and J.M. Brennan.The new bnl ags
phase, radial and synchro loops.
[**] State Variables and Ackermann's formula, Digital
Control of Dynmaic Systems, Franklin, Powell, and
Workman, Addison-Wesley Publishing, 1990
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BRENNAN,J.M.; CAMPBELL,A.; DELONG,J.; HAYES,T.; ONILLON,E.; ROSE,J. et al. RF BEAM CONTROL SYSTEM FOR THE BROOKHAVEN RELATIVISTIC HEAVY ION COLLIDER, RHIC, article, June 22, 1998; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc709274/m1/3/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.