Progress in tune, coupling, and chromaticity measurement and feedback during RHIC run 7

Abstract

Tune feedback was first implemented in RHIC in 2002, as a specialist activity. The transition of the tune feedback system to full operational status was impeded by dynamic range problems, as well as by overall loop instabilities driven by large coupling. The dynamic range problem was solved by the CERN development of the Direct Diode Detection Analog Front End [1]. Continuous measurement of all projections of the betatron eigenmodes made possible the world's first implementation of coupling feedback during beam acceleration [2,3], resolving the problem of overall loop instabilites. Simultaneous tune and coupling feedbacks were utilized as specialist activities for ramp development during the 2006 RHIC run. At the beginning of the 2007 RHIC run there remained two obstacles to making these feedbacks fully operational in RHIC - chromaticity measurement and control, and the presence of strong harmonics of the power line frequency in the betatron spectrum [4]. We report on progress in tune, coupling, and chromaticity measurement and feedback, and discuss the relevance of our results to LHC commissioning.

Full text

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NATf ONAL LABORATORY
Progress in tune, coupling, and chromaticity
measurement and feedback during RHIC run 7
P. Cameron, J. Cupolo, W.C. Dawson, C. Degen,
A. DellaPenna, L.T. Hoff, Y. Luo, A. Marusic,
R. Schroeder, C. Schultheiss, S. Tepikian
Presented at the 22nd Particle Accelerator Conference (PAC)
Albuquerque, New Mexico
June 25 - 29,2007
Collider-Accelerator Department
Brookhaven National Laboratory
P.O. Box 5000
Upton, NY 11 973-5000
www.bnl.gov
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under
Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the
manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,
irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others
to do so, for United Statcs Government purposes.
This preprint is intended for publication in a journal or proceedings. Since changes may be made before
publication, it may not be cited or reproduced without the author's permission.

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DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the
United States Cov
ent. Neither the United States Government nor any
agency thereof, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness, or any
third party's use or the results of such use of any information, apparatus, product,
or process disclosed, or represents that its use would not infhnge privately owned
rights. Reference herein to any specific commercial product, process, or service
by trade name, trademark, manufacturer, or otherwise, does not necessarily
constitute or imply its endorsement, recommendation, or favoring by the United
States Government or any agency thereof or its contractors or subcontractors.
The views and opinions of authors expressed herein do not necessarily state or
reflect those of the United States Government or any agency thereof

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ions. At injection many power supplies are close to
minimum current, where regulation is poor. As the ramp
begins and currents increase the anomalous response
diminishes. Additionally, this anomalous response was not
seen during beam studies of near-integer working points.
Efforts to tune the P1D loop of the tune tracker at
injection revealed a discontinuous behavior of tune
tracking with small changes in loop parameters. As
proportional gain was increased there was a sudden and
dramatic improvement in the amplitude and phase signals,
and a somewhat smaller improvement in tune mcking.
Our undepstanding is that higher gains prevent jumping
between the multiple peaks.
The anomalous beam response at injection may have
contributed to the excessive noise that appeared in tune
tracking when the tune and coupling feedback loops were
closed. in previous years, the additional noise and tune
dither resulting from closing the loops was barely
perceptible. In the present run the tune dither increased
significantly when the loops were closed. This was cause
for some concern regarding overall loop stability, but
experience suggests that the effect of the large tune dither
is minimal for tune and coupling feedback.
Unfortunately, the anomalous beam response is not so
benign with regard to chromaticity feedback. The
measurement needed for this feedback is accomplished by
modulating the beam momentum, and measuring the
amplitude of the resulting tune modulation. With tune
feedback on the tune modulation is suppressed, and the
chromaticity information is extracted from the quadrupole
correction strength. The tune modulation places additional
demand on the tune tracker. With normal loop gains the
tune modulation causes the tracker to be dragged between
peaks of the beam response. The combination of the
anomalous beam response, noise that appears when the
loops are closed, and the additional stress of tune
modulation requires exceptionally high loop gains for
reliable chromaticity measurement. Tune dither becomes
so Iarge that overall loop stability is a real concern.
TUNE SCALLOPING
During Run 7 we observed a phenomenon that we calsll
'tune scalloping'. Figure 2 is a typical example.
Figure 2. Tune scalloping on the ramp
The red trace is the vertical tune as measured by the
tune tracker during a portion of a ramp. The green dots are
tune as measured by the conventional kicked tune system.
The excursions of the tune tracker measurement are
clearly visible.
n/9ains Harmonics
H n RHIC the betatron resonance is excited hy high
harmonics of the power line frequency. This excitation is
much stronger than in similar large synchrotrons (for
instance, the Tevatron or the SPS). D ~ n g
resulting spectral Iines extend as much as 70d5 above the
noise floor of the tune tracker. Reliable tune tracking
requires that kicker excitation be -100 times stronger that
what would be needed without this interference. When we
turn down kicker excitation, the scalloping stops.
Improved Tune Tracker Performance
During Run 6 we tried to filter out the mains
harmonics. The resulting large phase shifts limited tune
tracker loop gains, without giving satisfactory results. The
mains harmonics are simply too large and closely spaced
to be effectively filtered. For Run 7 we gave up on this
approach and opened up the filter. This permitted much
larger loop gains and greatly improved tune tracking.
When we turn down loop gains, the scalloping stops.
Poor Chromaticity Control
The third ingredient in the recipe for tune scalloping is
small chromaticity. Our intent had been to commission
Run 7 with chromaticity feedback, but the controls
infrastructure needed for sextupole control was not ready.
Even if it had been ready, the anomalous beam response at
injection would have been problematic. Without this
feedback, it was not unusual to have small (or even
wrong-signed) chromaticity during ramp development.
The Combined Egect
The combination of excessive kicker excitation, precise
tune tracking, and poor chromaticity control results in
tune scalloping. The tracker preferentially excites a small
subset of the tune distribution, a dot in the middle of
phase space. With small chromaticity and large excitation,
this subset is driven to large amplitude. It experiences
amplitude-dependent tune shift, and the tracker follows it
out of the tune distribution, exciting it to progressively
larger amplitude and tune deviation, until it starts
dribbling out of the dynamic aperture and is depleted. The
tracker then falls back to the middle of phase space, grabs
another dot and drives it out of the distribution, . . .
Figure 3 shows horizontal and vertical tunes and in-
phase and quadrature signals during a ramp with
feedbacks on. The blue vertical bars in the images are
'stepstones'. In this ramp tune and coupling were being
continuously controlled by the feedback loops, but
chromaticity was open-loop, being adjusted at the
stepstones. In the upper pane, the amplitude of the vertical
response takes off at a stepstone shortly before mid-ramp,
then begins to oscillate as slices are driven up,
ramping the

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Figure 3. Tune Scalloping during Ramping with Tune and Coupling Feedbacks
depopulated, driven up,. . . The oscillations stopped when
loop gains and kicker excitation were manually lowered.
After -30 sec they were restored to slightly less than their
original values, and the oscillations took off again,
stopping when gains and excitation were again lowered.
The tune fluctuations, despite the effect of the feedback
loop, are visible in the tune traces. The quality of the
coupling feedback was also affected.
Tune scalloping puts the tune feedback effort in a tight
box. It damages operational reliability. Measures to
ameliorate the scalloping also damage reliability. The
problems of mains harmonics and chromaticity control
have found a particularly bothersome way to manifest.
CONCLUSION
Despite the difficulties outlined here, the tune and
coupling feedback effort made a useful contribution to
Run 7 ramp development, and has also proven to be an
essential tool for machine studies.
The robustness of the tune and coupling feedback in
crossing transition bodes well for LHC commissioning.
One of the unexpected problems (tune scalloping) would
disappear if the mains harmonics were not so strong. For
the other unexpected problem (anomalous beam response
at injection), the best candidate explanation so far is again
power supply related. If these power supply problems are
absent i n the LWC, then the field will be clear there to
deal with the problem of chromaticity control. This
control is also an issue at RHIC, and efforts are ongoing
to implement chromaticity feedback.
REFERENCES
[I] M. Gasior and R. Jones, CERN-LHC Project Report
853, 2005, available at ~docura~ents.cersn~.c"w
~-~ee:m
2005-853
(21 Y. Luo et a!, PRST-AB 9, 124001, Dec. 2006.
133 R. Jones et al, DIPAC 2005, Lyon.
[4] P. Cameron et al, C-AD AP Note 253, available at
cgi-
&~~i=iI~c~roi
&dl?
-indcx.html
[5] P. Cameron et al, PRST-AB 9, 122801, Dec. 2006.