ENERGY FEEDBACK SYSTEMS AT THE KEKB INJECTOR LINAC
K. Furukawa, A. Enomoto, N. Kamikubota, T. Kamitani,Y. Ogawa, S. Ohsawa, K. Oide and T. Suwada
High Energy Accelerator Research Organization (KEK)
Oho 1-1, Tsukuba, Ibaraki, 305-0801, Japan
Abstract
While commissioning the KEKB
8-GeV /
3.5-GeVlinac the beam handling system was greatly improved since
stable operation of the linac is required to achieve a higher
luminosity. One of the newly installed sub-systems is anenergy feedback system.
The feedback system comprises an energy monitor anda tuner. The monitor measures the linac beam energy using
beam position monitors installed at a location with a large
dispersion. Noise to the measurement caused by beamorbit fluctuation is eliminated by a simple beam optics calculation around the monitor locations.
The tuner changes the beam energy using microwave
phase shifters at two adjacent klystron stations simultaneously to maintain the energy distribution in a beam bunch.
A generalized graphical operator interface to the system
is also developed employing script languages in order tomanipulate and monitor the feedback parameters in realtime.
This type of feedback system is installed at six locations along the linac and is used for four beam operationmodes. The same software is applied to beam-orbit feedback and accelerator equipment feedback loops. They havealso greatly improved the stability of linac operation. This
report describes the current status of linac feedback systems with emphasis on the beam-energy stabilization.
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INTRODUCTION

The KEK/
linac has been reinforced to inject fullenergy beams to the KEKB asymmetric B-factory with 8GeVand 3.5-GeV .
Operation of the KEKB electron linac became much
more different from that in the previous project, TRISTAN.
The main reason is that the required electron and positron
beams are of higher quality in terms of energy, charge
per bunch, emittance and stability. The energy and beam
charge are also quite different between four beam operation modes, namely the KEKB , , PF
and AR-PF[1].
Each component of the linac was designed to achieve therequired quality. However, it was found that the stability of
the linac sometimes did not satisfy the requirements. Thiswas partially caused by the synchronized temperature orelectric-power variation over the linac, or interim hardware
failures. Not all sources have been identified.
Because of these reasons, several software feedback
loops were installed during the commissioning. Some

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of these were installed after some instabilities and theirsources were identified.
However, in most cases the source of a variation could
not be found, although we made every effort to find sourceby employing a statistical method and singular value decomposition (SVD)[1]. Also in some cases, the source wasassumed to be at the linac injection part, where many kindsof equipment were installed with many parameters. Evenso, certain parameters were chosen to tune a monitor value
and to form a feedback loop.
Instead of simple feedback, more sophisticated methods like downhill simplex[1] or a global orbit correction
were studied and gave promising results. However, sincethese methods are multi-parameter to multi-parameter corrections and interpretation of results is often difficult, they
may hide important issues in the linac, which should becorrected or improved. Therefore they are not routinelyused.

2

LINAC BEAM PARAMETERS

It was realized that keeping the beam parameters stable isquite difficult because four beam modes had to be switchedfrequently. Since some beam parameters, such as charge
per bunch, are more than ten-times different between thebeam modes the operation point of each piece of equipment
must be adjusted to meet the beam-acceleration scheme.
Although each variation is within the design value, the accumulated variation sometimes exceeds the expected value.
For example, about 60 high-power microwave generators (klystron) are installed in the linac, and the cumulative
variation of many parameters, such as microwave phases
and SLED (SLAC energy doubler) phase flip timing at each
klystron station, affects the beam energy.
Other equipment parameters may affect the beam parameters like the orbit, emittance or energy spread. These beam
parameters are predefined at the injection lines to the rings.
If they exceed predefined values, KEKB luminosity may be
degraded or some equipment may be even damaged.



3BEAM MONITORS AND TUNERS

Since the reinforcement for KEKB, the linac has beenequipped with several computer-readable beam monitors,
which were not available in the previous project.
In order to diagnose the beam parameters, various typesof beam instrumentation and their readout systems weredeveloped or are being developed: strip-line beam position monitors (BPM)[2, 3], streak cameras as beam

bunch monitors[4, 5], and wire scanners as beam profile
monitors[6].
These monitors are utilized in routine operations as wellas accelerator studies. Currently, most beam feedback
loops employ BPM’s because of their straightforward interpretation as described below.

Read Beam Position (x and δx)
at Reference Points
SP-BPM SP-BPM

BeamFluctuation

Klystron

BeamKlystron––> Convert into RF Phases
for two Klystrons
Energy Knob
Beam Positionsat Measurement Points

3.1

Linac Energy Monitor

The linac energy can be determined by measuring the beam
position displacement at a location with a large dispersion.
This value is linear with the beam energy and is intuitive
to the operator. We also tried to find the Linac energy atthe straight section by measuring the betatron wavelength,
however, the result was not reliable enough.
Information of beam energy derived from the beam positions may include noise, a part of which is caused by electric noise induced by high-power klystrons; another frombeam-orbit fluctuation.
The former can be reduced by integration. Using a BPM
resolution of about 100 micrometers, a data acquisition frequency of 1 or 5 Hz and the dispersion function we may determine the integration constant depending on the requiredstability.
For the latter the betatron orbit can be isolated by measuring the beam-orbit displacement at the previous straightsection of the linac, which is then subtracted.
Along the linac there exist six locations where the dispersion is designed to be large. At all of these locations
feedback loops are installed. For some locations the beam
charge or the beam energy varies depending on the beam
operation modes.

3.2

Linac Energy Tuner

The beam energy can be tuned by modulating the upstream
microwave system. Normally, we vary the microwave
phases at the klystron stations, since both the amplitude
and the phase vary if we change the klystron voltage.
If the phase value at only one klystron station is changed,
the energy spread becomes large, which is critical for theKEKB ring injection. We thus change the microwave
phases at two adjacent klystron stations simultaneously in
opposite directions relative to the phase crests, as shown inFig. 1. Actually, the energy tuners are packaged in what iscalled energy knobs, which are sometimes used independently.

4SOFTWARE

A feedback loop was implemented as a client to the linac
control system[7]. It was written in the Tcl script language
in order to maximize the flexibility[4]. The software is actually modified several times a week, as the commissioning
advances. The graphical user interface (GUI) was designedwith Tk widgets and the operator can change the parameters in real-time.

SP-BPM's– Beam Fluctuation––> Convert into Energy
Average (Integration)
x Gain – Offset
Energy Feedback

Figure 1: Process flow for beam-energy feedback.

4.1

Structure

The software has five parts. The first part tests conditions,such as the beam operation mode and the beam charge
limit.
The second part determines the monitor value. It reads
information through a direct Tcl interface to the control
system or a shell command, where Tcl post-processing canbe specified. Time-weighed averaging and windowing areapplied before it is passed to the next step. Optionally,
beam-orbit fluctuation is isolated for energy feedback.
The third part calculates the feedback value, where theconversion factor from the monitor to the tuner and thefeedback gain are multiplied, and the frequency can bespecified. If the result exceeds a minimum feedback value,a dead-band, it is sent to the next part.
In the fourth part the value is applied to the tuner throughthe control system, where Tcl pre-processing can be specified, if it goes in between the lower and upper limits.
The last part controls the information flow, monitors processing errors, draws graphs and logs the feedback information. It accepts controls from other applications, suchas a global orbit correction and a feedback status viewer,
through interprocess communication.
Almost all parameters and Tcl pre/port-processing codes
can be specified through GUI at any time, as shown inFig. 2. These codes are structured into hierarchical orders:a general package; energy, orbit and device feedback packages; and applications.

5OTHER FEEDBACK LOOPS

For a beam-orbit correction, the response from a steering
magnet to BPM’s in one betatron wavelength is measured
beforehand. The weighed average over the BPM’s is used
as a monitor and it is fed back to the steering magnet. Currently, two steering magnets in a section with a 90-degree
phase advance are chosen to make a loop.
This adaptive feedback is effective, since the wake field
generated by a high-current beam is difficult to predict under the present circumstances. If the global orbit correctionthat the amplitude spreads over a wide range in frequency
domain. We thus plan to install a faster readout system, atleast for important locations. Since the feedback is slightly
affected by fast electric noise, even a 50-Hz average with aslow feedback may help.
Betatron orbit isolation is not yet sufficiently mature,
more studies on wake fields are required.
The above-mentioned feedback loops are all linear.
However, there are many accelerator parameters whichshould be optimized, but have quadratic behavior. We optimize those parameters, such as the microwave timings andphases, when we start the accelerator operation using correlation plots. However, some parameters are often foundto be shifted off the optimum.
The beam energy spread is one of the most crucial parameters. If the energy spread becomes large, energy feedback is useless. In order to suppress it, a wire scanner,which is being installed[6], can be utilized. Upstream microwave systems may be tuned so as to optimize the value.

7 CONCLUSIONS

Figure 2: Graphical user interface to the feedback system
built using Tcl language and Tk widgets. This is an interface for energy feedback at the R0 section.
becomes sufficiently robust to be repeated continuously, it
may replace feedback loops.
After the commissioning began, some parameters of theaccelerator equipment were found to fluctuate over predefined ranges. Although the hardware or local-controller
feedback was planned to be installed, as a quick fix,
software feedback loops were installed. They cover thevoltages of electron guns, amplitudes and phases of subharmonic bunchers (SHB).

6DISCUSSIONSIt should be noted that the graphical user interface (GUI)should be improved so as to avoid human mistakes.
Although the beam repetition of the linac is normally
50Hz, the current data acquisition system is limited to1Hz[2]. Thus, beam feedback loops cover the frequency
domain below 0.1–0.5Hz. Currently with 0.5Hz, our energy feedback systems suppress the energy fluctuation by
one half or one third, which is just below the predefinedconstraint. The sum of the energy variation and energy
spread sometimes exceeds the imposed limit.
We observed a fluctuation of the linac beam energy using
an experimental BPM readout system at 50Hz and foundLinac energy feedback systems are installed at six locations
along the linac and are utilized routinely to stabilize injection. They will be improved with a reinforcement of thebeam instrumentation. Twelve orbit feedback systems andsix equipment feedback systems are installed as well. They
could all installed or modified quickly because of the script
language, thus have greatly helped the linac commissioning.

8REFERENCES[1] Y. Ogawa and LCG, “Commissioning Status of the KEKB
Linac”, Proc. PAC’99, New York, U.S.A., 1999.
[2] N. Kamikubota, T. Obata, K. Furukawa and T. Suwada,“Data Acquisition of Beam-Position Monitors for theKEKB Injector-Linac”, in these proceedings.
[3] T. Suwada, N. Kamikubota, K. Furukawa and H. Kobayashi,“New Data-Acquisition System of Beam-Position and WallCurrent Monitors for the KEKB”, Proc. APAC’98, Tsukuba,Japan, 1998.
[4] K. Furukawa, A. Enomoto, Y. Ogawa, S. Ohsawa and T.
Kamitani, “Control System for a Bunch/Profile Monitor atthe KEK/Linac”, Proc. Linac’94, Tsukuba, Japan,1994.
[5] Y. Ogawa, K. Furukawa, T. Kamitani, S. Ohsawa, A.
Enomoto, T. Suzuki, S. Abe and K. Iwazaki, “New StreakCamera System for the KEKB Linac”, Proc. APAC’98,
Tsukuba, Japan, 1998.
[6] Y. Funakoshi, N. Iida, T. Suwada, T. Kawamoto and M.
Kikuchi, “Beam Tests of a Wire Scanner for the KEKB Injector Linac and Beam Transport Line”, Proc. APAC’98,
Tsukuba, Japan, 1998.
[7] K. Furukawa, N. Kamikubota, K. Nakahara and I. Abe,
“Upgrade Plan for the Control System of the KEK/Linac”, Proc. ICALEPCS’91, Tsukuba, Japan, 1991, p. 89.
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