(1)
ELETTRA
ELETTRA Photon
Photon Source
Source Beam
Beam Stabilization
Stabilization(1)
Steve
Steve N.
N. Thanos
Thanos and
and Rodger
Rodger H.
H. Hosking
Hosking
Pentek, Inc.
Pentek, Inc.
Upper Saddle River, New Jersey, USA
Upper Saddle River, New Jersey, USA
Abstract. This paper outlines the technical issues involved in configuring an electron beam
Abstract. This paper outlines the technical issues involved in configuring an electron beam
transverse multi-bunch feedback system for the Elettra Synchrotron in Trieste, Italy. By
transverse multi-bunch feedback system for the Elettra Synchrotron in Trieste, Italy. By
measuring
the electron beam with fast A/D converters and then analyzing the error in beam
measuring the electron beam with fast A/D converters and then analyzing the error in beam
position
to the
the beam
beam to
to damp
damp
positionusing
usingmultiple
multiple digital
digital signal
signal processors,
processors, aa corrective
corrective kick
kick is
is applied
applied to
itsitsinstabilities.
Commercial
data
acquisition
and
DSP
products
were
successfully
deployed
to
instabilities. Commercial data acquisition and DSP products were successfully deployed to
achieve
achievereal
realtime
timeoperation.
operation.
INTRODUCTION
INTRODUCTION
ELETTRA
in Trieste,
Trieste, Italy.
Italy. The
The
ELETTRAisisaathird-generation
third-generation synchrotron
synchrotron radiation
radiation facility
facility in
facility
provides
the
scientific
community
with
photons
in
the
range
of
a
few
electron
facility provides the scientific community with photons in the range of a few electron
volts
corresponding to
to the
the spectral
spectral
volts(eV)
(eV)totoseveral
several tens
tens of
of thousands
thousands (keV),
(keV), the
the latter
latter corresponding
domain
of
soft
X-rays.
The
main
characteristics
of
the
synchrotron
radiation
are
very
domain of soft X-rays. The main characteristics of the synchrotron radiation are very
high
brightness,
polarization
and
wide
range
of
tunability
in
wavelength.
high brightness, polarization and wide range of tunability in wavelength.
Figure
light source
source is
is composed
composed of
of
Figure11isis an
an aerial
aerial photograph
photograph of
of the
the facility.
facility. The
The light
three
transfer line
line and
and the
the
threeparts:
parts:aalinear
linear accelerator
accelerator (commonly
(commonly known
known as
as aa linac),
linac), aa transfer
storage
storagering.
ring.
Figure1.1. The
TheELLETRA
ELLETRA facility
facility in
in Trieste,
Trieste, Italy
Italy (Courtesy
(Courtesy of
Figure
of ELETTRA)
ELETTRA)
CP648, Beam Instrumentation Workshop 2002: Tenth Workshop, edited by G. A. Smith and T. Russo
© 2002 American Institute of Physics 0-7354-0103-9/02/$19.00
248
The storage ring is filled once a day by the linac with 0.9 GeV electrons whose
energy is eventually increased up to 2 - 2.4 GeV. Synchrotron radiation is produced
when electrons traveling at relativistic speeds are deflected in magnetic fields. The
brightness of the photon beam is derived from the small transverse size and divergence
of the electron beam, which is called emittance.
Low emittance, stability, reproducibility and long lifetime of the stored beam are
the main requirements for the light source. These properties are a function of the beam
environment, e.g. the magnetic field lattice, the accelerating electric fields and the
quality of vacuum in the vacuum chamber.
Beam instabilities have different time scales ranging from milliseconds to months
and different techniques are used for their suppression and control. Although much is
done to passively control instabilities, active feedback systems working at different
frequencies and with different bandwidths are needed. Given the small size of the
beam (typically tens of microns), stabilizing it requires exceptional performance in
position measurement and correction.
SYSTEM OVERVIEW
The electron beam stored in a synchrotron light source is not a continuous beam. It
has a "bunched" structure produced by the effect of one or more RF cavities installed
in the ring and used to replenish the energy lost by the electrons as electromagnetic
radiation on each turn. The reason for this bunch structure is that only electrons
arriving at the right time will be accelerated, while the rest are lost.
At ELETTRA, the RF frequency of the four cavities is 500 MHz, so the electron
bunches are spaced 2 nsec apart. As currents in the ring are increased, the very high
electromagnetic field associated with each bunch interacts with the surrounding
vacuum chamber and can start resonating, in one or more storage ring positions, until
the next bunch arrives.
As a result, the bunches are no longer independent, but they become coupled to
each other by the action of their respective electromagnetic fields. The result is that
bunches start oscillating at a characteristic frequency, the betatron frequency (in the
order of a few hundreds of kHz) but with different phase relationships among them
resulting in different "modes".
There are as many normal modes as there are bunches. In the case of ELETTRA,
there are 432. Such coupled-bunch instabilities can be cured by the use of active
feedback systems. The approach used at ELETTRA involves a digital feedback
system that samples the bunch positions in the storage ring and applies corrections
computed by digital signal processors.
249
RF FRONT
END
A/D
CONVERTER
DIGITAL
PROCESSING
FOUR
CHANNEL
DSP
DMA
DSP
DSP
DSP
DSP
DSP
D/A
CONVERTER
RF
AMPLIFIER
Electron bunches
BPM
2 nsec
KICKE
R
FIGURE 2.
2. Feedback
Feedback System
System Block Diagram
FIGURE
As shown
shown in
in Figure 2, the system includes a 2-axis "Beam
As
“Beam Position Monitor"
Monitor”
(BPM) and
and aa position
position corrector,
corrector, appropriately
appropriately called
called aa “Kicker”.
"Kicker". One
One BPM
BPM is used to
(BPM)
produce the
the vertical
vertical and
and horizontal
horizontal position
position error
error signals,
signals, while two stripline kickers
produce
are used
used for
for correcting
correcting in
in the two planes. The wideband signals are demodulated by
are
the RF
RF front
front end
end to
to produce
produce aa 0–250
0-250 MHz
MHz baseband signal that represents the position
the
error
of
the
bunches
as
they
pass
through
the BPM
BPM at
at aa frequency
frequency of
of 500
500 MHz.
MHz.
error of the bunches as they pass through the
The output
output analog
analog signal
signal is
is sampled
sampled by
by aa fast 500 MHz A/D converter and passed
The
onto aa bank
bank of
of DSPs
DSPs that calculate
calculate the required correction. The correction is then
onto
converted into
into analog
analog form
form by
by aa 500
500 MHz
MHz D/A
D/A converter.
converter. The
The analog
analog signal
signal represents
converted
the correction
correction that
that has
has to
to be
be applied
applied to
to the
the bunches
bunches as
as they
they pass
pass through the kicker. A
the
RF power
power amplifier
amplifier supplies
supplies the
the necessary
necessary power
power to
to drive
drive the
the kicker.
kicker.
RF
SIGNAL PROCESSING
PROCESSING
SIGNAL
The ELETTRA
ELETTRA Transverse
Transverse Multibunch
Multibunch Feedback
Feedback consists
consists of aa wideband bunch-bybunch-by The
bunch system
system where
where the
the position
position errors
errors of
of the
the 432 bunches, separated by 2 nsec of
bunch
beam travel
travel time,
time, are
are individually
individually corrected.
corrected. After
After demodulating
demodulating the wideband signal
beam
from the
the BPM,
BPM, the
the baseband
baseband xx and
and yy signals
signals are
are sampled
sampled by
by 8-bit
8-bit 500 MHz
MHz A/D
from
converters. One
One converter
converter is
is used
used for
for the vertical signal and another one for the
converters.
horizontal.
horizontal.
250
A/D BOARD
FPDP In
Analog
In
A/D
D
E
M
U
X
FPDP I/F
VIM-4
MODULE
PENTEK 4290
Mez
FIFO
C6000
DSP
Mez
FIFO
C6000
DSP
From other
five FPDP I/F
and processor
boards
FPDP
To other
five FPDP I/F
and processor
boards
Global
Bus
C6000
DSP
D/A BOARD
Mez
FIFO
C6000
DSP
Mez
FIFO
SYSTEM
CONTROLLER
Analog
Out
D/A
FPDP Out
TIMING BOARD
EMBEDDED CPU
Ethernet
to Workstation
M
U
X
FPDP
CLOCK
GENERATOR
500 MHz
Storage Ring RF
TRIGGER
GENERATOR
VMEbus
FIGURE 3.
3. Signal
Signal Processing
Processing Block
Block Diagram
Diagram
FIGURE
We will
will address
address the
the vertical
vertical position
position signal
signal processing
processing first.
first. As
As shown
shown in Figure 3,
We
the 500
500 Mbyte/
Mbyte/ sec
sec data
data flux
flux isis first
first demultiplexed
demultiplexed into
into six
six 32-bit
32-bit FPDP
FPDP (Front
(Front Panel
the
DataPort)
Port) channels.
channels. The
The data
data from
from each
each channel
channel is
is then
then sent
sent through a custom VIM-4
Data
mezzanine module
module designed
designed by
by ELETTRA
ELETTRA to
to meet the needs of this project. It
mezzanine
providesaaFPDP
FPDP interface
interface with
with the
the demultiplexer
demultiplexer to
to distribute
distribute the
the A/D data to the four
provides
DSPs
of
the
Pentek
Quad
'C6201
board.
DSPs of the Pentek Quad ’C6201 board.
The data
data coming
coming from
from one
one FPDP
FPDP channel,
channel, which
which corresponds
corresponds to
to 72 bunches, is
The
passed
to
one
DSP
for
online
diagnostics
and
signal
analysis.
Concurrently,
the data is
passed to one DSP for online diagnostics and signal analysis.
split
evenly
over
the
remaining
three
DSPs,
each
of
which
executes
the
feedback
split evenly over the remaining three DSPs,
algorithm
on
its
respective
24
bunches.
Thus,
each
DSP
is
responsible
for
processing
algorithm on its respective 24 bunches. Thus, each DSP is responsible
the samples
samples of
of aa given
given group
group of
of bunches
bunches all
all the
the time.
time. To
To process
process all
all 432 bunches, six
the
Model4290’s
4290'scontaining
containing24
24’C6201’s
'C6201'sare
areused.
used.
Model
The FPDP
FPDP input
input interface
interface receives
receives 32-bit
32-bit words
words and
and writes
writes them to the BI-FIFO of
The
the DSP
DSP to
to which
which they
they are
are specifically
specifically assigned
assigned for
for processing.
processing. At
the
At the
the same
same time,
time, the
the
VIM-4 mezzanine
mezzanine reads
reads the
the BI-FIFO
BI-FIFO which
which contains
contains the
the calculated
calculated data
data words
words and
VIM-4
and
sends them
them to
to the
the FPDP
FPDP output
output interface.
interface. The
The FPDP
FPDP interfaces
interfaces act
act as
as bidirectional
sends
bidirectional
programmableswitches
switchesfor
for each
each incoming
incoming and
and outgoing
outgoing word
word and
and the
the switching
switching rules
programmable
rules
aredownloaded
downloadedininaatable
tableatatthe
thebeginning
beginningof
ofsystem
systeminitialization.
initialization.
are
Theoutput
outputdata
datafrom
from the
the six
six custom
custom FPDP
FPDP mezzanines
mezzanines are
are multiplexed
multiplexed by
by the
the D/A
D/A
The
board and
and converted
converted to
to analog
analog form
form by
by the
the 8-bit
8-bit 500
500 MHz
MHz D/A
D/A converter.
converter. The
The entire
entire
board
process isis synchronized
synchronized by
by the
the timing
timing electronics.
electronics. All
All the
the electronics
process
electronics required
required to
to
stabilize
the
beam
in
the
vertical
direction
are
housed
in
one
VME
cage
in
the service
service
stabilize the beam in the vertical direction are housed in one VME cage in the
area. One
One additional
additional VME
VME cage
cage with
with identical
identical electronics
electronics is
is used
used for
for the
area.
the horizontal
horizontal
direction.
direction.
251
CRITICAL ISSUES
'C6201 is a fixed-point DSP clocked at
at 200
200 MHz
MHz (5
(5 nsec instruction
instruction cycle).
cycle). Its
The ’C6201
VLIW architecture allows it to execute up to 88 instructions per clock cycle. The
requirement here
here is to execute all the necessary operations in one beam revolution, or
requirement
(2 nsec x 432 bunches). With a highly optimized code written in assembly
864 nsec (2
language, the time needed to execute the required code of aa 5-tap FIR filter, which is
on the
the 432 bunches,
bunches, is
is 600
600 nsec
nsec which
which is shorter
shorter than the
used as the feedback element on
beam revolution
revolution time.
time.
Another critical issue is the time required for data transfers between the DSP and its
BI-FIFO. To minimize
minimize the time
time data
data stays
stays on the board, thorough use of the Pentek
BI-FIFOs, interrupts and DMAs allows very efficient
efficient data transfer without interfering
with the algorithm execution.
execution. The feedback system
system relies
relies on
on very strict timing. The
A/D converter must sample
sample the analog
analog signal
signal synchronously
synchronously to the bunch crossing at
the BPM and the D/A converter must generate the analog correction signal in phase
same bunch
bunch passing
passing through
through the
the kicker.
kicker.
with the same
start A/D and D/A triggers must start the conversions in a
In addition, the start
deterministic and
and repeatable
repeatable way
way with
with respect
respect to the bunch structure in order to let
deterministic
every DSP work with a known group of bunches.
RESULTS
Figure 4 consists of two synchrotron beam profile images: the one on the left is
with the feedback loop open, while the one on the right was taken with only the
closed. The photographs
photographs show
show how effective the control is at
vertical feedback loop closed.
stability.
improving beam stability.
FIGURE 4. Synchotron beam profiles: left, without feedback; right, with
stabilization. (Courtesy
(Courtesy of ELETTRA)
ELETTRA)
only vertical axis feedback stabilization.
252
ACKNOWLEDGEMENTS
The authors wish to express kindest acknowledgements to Dr. Daniele Bulfone and
Dr. Marco Lonza of ELETTRA who are responsible for conceptualizing the system.
Their dedicated engineering team overcame many system integration challenges which
ultimately brought this remarkable system into reality. We are extremely grateful to
them for their time in sharing system details and in reviewing and editing the technical
content of this paper.
REFERENCES
(1) This article originally appeared in Pentek Pipeline Newsletter, Volume 10, No. 3,
Fall 2001 and is reprinted with permission of Pentek, Inc. An on-line version of
this article entitled "Pentek DSPs Keep the ELETTRA Photon Beam Stable and
Bright", is available from Pentek's website at:
http://www.pentek.com/pildocs/6982/pipelines/PIPE103.PDF
(2) More information about the ELETTRA facility in Trieste, Italy, is available from
the facility website at: www.elettra.trieste.it
253