Abstract The top-up operation established since 2010 at the Elettra third-generation synchrotron light source has solved the problems related to thermal drifts and beam current dependence, and a series of feedback loops acting on the machine optics and radio-frequency systems made easier to setup and operate the ring. Those systems together with the fast orbit feedback in operation since 2007, contributed to very high electron beam orbit stability. A description of the active systems and their performance, future perspectives as well as some still open issues will be presented and discussed

E Karantzoulis

G Gaio

M Lonza

S Cleva

S Krecic

CiteSeerX

BEAM ORBIT STABILITY AT ELETTRA 
G. Gaio, S. Cleva, E. Karantzoulis, S. Krecic, M. Lonza 
Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
Abstract 
The top-up operation established since 2010 at the 
Elettra third-generation synchrotron light source has 
solved the problems related to thermal drifts and beam 
current dependence, and a series of feedback loops acting 
on the machine optics and radio-frequency systems made 
easier to setup and operate the ring. Those systems 
together with the fast orbit feedback in operation since 
2007, contributed to very high electron beam orbit 
stability. A description of the active systems and their 
performance, future perspectives as well as some still 
open issues will be presented and discussed. 
INTRODUCTION 
Elettra, the 2.0/2.4 GeV Italian synchrotron light source 
operates for users since 1994. In order to keep the facility 
competitive in terms of the electron beam quality after the 
advent of more recent light sources, a series of machine 
upgrades have been made in the last decade. 
One of the major upgrades was the replacement of the 
old injector, a 1 GeV linac that did not allow full energy 
injection, with a 2.5 GeV booster. Before setting into 
operation the new injector in 2008, daily refills and 
energy ramping were a big concern for the complexity of 
machine preparation and tuning, and also very detrimental 
for the orbit stability and reproducibility. Drastic changes 
of thermal load on the vacuum chamber due the electron 
beam dump and subsequent refill induced large 
distortions with respect to the orbit measured before the 
beam dump [1].  
Another side effect of operating in decaying mode was 
the dependence of the Beam Position Monitor (BPM) 
readings on the current accumulated in the storage ring; 
an upgrade of the BPM system started in 2006 adopting 
digital detectors decreased the beam current dependence 
to 1m/10mA [1] but still leaving open issues with 
current decaying from 300mA to 150mA.  
The next major upgrade was the successful change of 
the operation mode to that of the top-up, although Elettra 
was not designed for it, that also solved as expected the 
majority of the problems related to orbit stability. The 
maximum decaying current in top-up regime is fixed to 
1mA, which requires an injection every 6 minutes at 2 
GeV or 20 minutes at 2.4 GeV. Moreover, the possibility 
to re-inject at full energy after a beam dump with no need 
of energy ramping has dramatically decreased the average 
time required to restore the beam and reduced the time 
needed by the vacuum chamber to reach the thermal 
equilibrium. As a consequence, the mid-term orbit 
stability, estimated as the rms of the difference orbit with 
respect to the reference, has been improved significantly 
(see Fig. 1) with respect to the previous situation [2]. 
 
Figure 1: Orbit rms after a storage ring refill. 
The improvements in the orbit stability by the top-up 
operation however revealed noise sources not seen or 
being important before. In particular, some beam-lines 
started to be sensitive to electron beam energy drifts due 
to slow horizontal orbit changes from tides and/or soil 
temperature gradients, eventually compensated by RF 
frequency adjustments.  
Several software applications were developed to 
guarantee the electron beam stability, requiring the 
intervention of the operator only for the machine 
setting-up. After a new refill, a slow orbit correction 
program restores the electron beam position and angle in 
the bending magnets and undulators. Then the following 
applications are turned on: 
• Top-up manager: keeps the current constant in the 
storage ring; 
• Energy feedback: changes the RF frequency in 
order to keep constant the  horizontal average orbit; 
• Tune feedback: changes the current of two 
quadrupole magnet families to keep constant the 
horizontal and vertical tunes; 
• Fast orbit feedback: corrects the global electron 
beam orbit at 10 kHz rate. 
In addition, a software application monitors the beam 
orbit stability by acquiring synchronous orbit positions 
and corrector magnet values at 10 kHz rate from the fast 
orbit feedback system and detects major sources of noise 
through spectral analysis.   
 FEEDBACK SYSTEMS 
Top-Up Manager 
A high level application manages the top-up operation: 
at every injection , it selects the storage ring buckets that 
5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing
ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-TUPRI075
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06 Instrumentation, Controls, Feedback & Operational Aspects
T05 Beam Feedback Systems
have to be refilled, regulates the charge accumulated in 
the booster by acting on the pre-injector gun voltage and 
finally fixes the number of shots to be injected by acting 
on the timing system. 
In order to minimize the injection time, single bunch 
injection is reserved only for particular filling patterns; 
usually a train of 60 bunches are injected at each shot for 
a total current of 0.25mA per shot.  It is possible to load 
any filling pattern and, in case of hybrid modes (one 
single bucket filled in the gap), a feedback loop 
measuring the storage ring filling pattern through an 
oscilloscope, keeps the current in the single bunch 
constant. Charge losses along the injection chain are 
continuously monitored; a prediction of the losses and the 
risk for injection inhibition by the personnel safety system 
is also provided.  
Energy Feedback 
The energy feedback keeps constant the horizontal 
average orbit by tuning the frequency of the RF master 
oscillator. The dispersive orbit is subtracted from the fast 
orbit feedback correction to avoid cross talks between 
feedbacks. The peak-to-peak day/night RF frequency 
excursion is typically less than 100 Hz. 
The energy feedback is effective in stabilizing slow 
drifts of the photon flux not only in the bending magnet 
beam-lines where the electron beam dispersion is high but 
also in the undulator beam-lines (see Fig.2). 
 
Figure 2: Photon flux intensity measured by the CIPO 
beam-line with energy feedback turned off and on (data 
acquired at 1 Hz). 
Tune Feedback 
An accurate control of the storage ring betatron tunes 
assures beam optics stability and constant injection 
efficiency. In fact, fractional tune shifts of the order of 
4% can reduce the injection efficiency by about 50%. 
 Undulator gap changes are the main source of tune 
shifts. While feed-forward correction systems based on 
trimming coils together with the fast orbit feedback 
minimize the effect on the beam orbit, fractional tune 
shifts caused by undulator gap changes can be of the 
order of 10%. 
The tune feedback can acquire tune values from the 
transverse multi-bunch feedback system or from the BPM 
system. In both cases the measurement is based on 
excitation and spectral analysis of the transverse electron 
beam position.  
The most accurate method is the one based on BPMs. 
The detectors are able to acquire up to 16384 turn-by-turn 
beam position samples. The acquisition is triggered by the 
same timing signal which acts on the injection kicker 
magnets allowing parasitic tune measurements during 
top-up injection. The tune peaks can be clearly identified 
by calculating the FFT of the acquired data. A simple 
algorithm avoids confusing the real tune peaks with 
possible artefacts.  
 The correction scheme is based on a 2x2 response 
matrix between the tunes (horizontal/vertical) and two 
quadrupole families.  
Fast Orbit Feedback 
A fast orbit feedback has been running at Elettra since 
2007 [3]. Twelve VME CPU boards collect position 
values at 10 kHz from 94 BPMs through Gigabit  
Ethernet links. The CPUs share the position data by 
means of a fibre-optic reflective memory, calculate the 
corrections and apply them to the 82 horizontal and 82 
vertical corrector power supplies. The correction scheme 
is based on the inversion of the response matrix (BPMs 
vs. correctors) by means of the Singular Value 
Decomposition (SVD), taking into account the first 23 
eigenvalues. As control algorithm a Proportional Integral 
Derivative (PID) regulator plus six harmonic suppressors 
centred at 50, 100, 150, 200, 250 and 300 Hz is used. 
In order to benchmark the fast orbit feedback 
performance with an “out-of-the-loop” sensor, a fast CCD 
acquiring images at 10 kHz rate has been installed in the 
Synchrotron Radiation Profile Monitor (SRPM) using 
bending magnet radiation. The results are depicted in 
Fig. 3.  
 
Figure 3: Spectra of the photon beam position and beam 
size acquired by a fast CCD on the SRPM with fast orbit 
feedback off and on. 
5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing
ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-TUPRI075
06 Instrumentation, Controls, Feedback & Operational Aspects
T05 Beam Feedback Systems
TUPRI075
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An offline analysis eventually discriminates photon 
beam motion from beam shape/intensity fluctuations. 
STABILITY MONITORING SYSTEMS 
After twenty years of service the Elettra storage ring is 
facing problems related to aging of its systems. In 
particular the magnets power supply electronics suffer 
from electrolytic capacitors drying out, causing an 
increase of the noise induced on the electron beam at 
50 Hz and its harmonics. Even mechanical vibrations 
induced by anomalous water/air cooling fluxes or 
sporadic malfunctioning of single BPMs when the fast 
feedback switched on, sometimes affect the orbit stability.  
A software application has been developed to 
continuously analyze the spectra of orbit positions and 
corrector currents provided by the fast orbit feedback 
system (see Fig. 4).  
 
Figure 4: 2D spectra of the horizontal/vertical correctors 
driven by the fast orbit feedback. 
 
Table 1: Example of spectral analysis of the feedback 
corrector currents showing noise components above 
threshold 
Corrector Frequency Hz Amplitude mA 
corr_s2.4 V  100 2.11 
corr_s2.3 V 100 1.93 
corr_s2.5 V 100 1.69 
corr_s3.1 H 100 1.55 
corr_s3.2 H 100 1.37 
The spectra are continuously analysed and in case of 
abnormal increase of a noise component with respect to 
normal values, an alarm is sent to the general alarm 
system to warn operators in the control room (see Tab. 1). 
OPEN ISSUES 
The air conditioning control system originally designed 
to keep the machine and service gallery air temperature 
stable within +/- 0.5°C, nowadays due to aging, doesn’t 
always guarantee the thermal stability required in order to 
keep the long term orbit drifts in the sub micron range. In 
particular, the BPM electronics seems to be one of the 
most important systems suffering from temperature 
changes. Day/night temperature excursions, although 
mitigated by the energy feedback, may change the orbit 
rms by 6 m during a period of 24h (Fig. 5). The 
installation of BPM detectors in temperature-controlled 
cabinets is under discussion while a complete 
refurbishment of the air-conditioning control system is 
foreseen in the near future. 
 
Figure 5: rms orbit error during three days. 
SUMMARY 
Several feedback systems control the main beam 
parameters and greatly simplify the machine operations. 
The orbit stability performance achieved is also 
confirmed by measurements carried out on the 
beam-lines. 
Efforts are going on for better machine monitoring and, 
when possible, further improvement of the beam orbit 
stability. 
REFERENCES 
[1] E. Karantzoulis et al.,”Topup operational experience 
at Elettra”, THPC027, IPAC’11. 
[2] D. Bulfone et al., “A Fast Orbit Feedback for the 
Elettra Storage Ring”, RPPA20, ICALEPCS’07. 
[3] M. Lonza et al., “Commissioning of the ELETTRA 
Fast Orbit Feedback System”, MOPAN024, PAC’07. 
5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing
ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-TUPRI075
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06 Instrumentation, Controls, Feedback & Operational Aspects
T05 Beam Feedback Systems


BEAM ORBIT STABILITY AT ELETTRA

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BEAM ORBIT STABILITY AT ELETTRA

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