DESIGN AND TEST OF A FAST FEEDBACKSYSTEM FOR ORBITCORRECTION AT TTF AND THE TESLA LINEAR COLLIDER.
K. Balewski, H.-T. Duhme, J. Klute, D. Keese, F. Obier, I. Reyzl, J. Rümmler, M. Schroeder,
DESY, Hamburg, Germany,
C. Magne, B. Phung CEA, Saclay, France
To achieve high luminosity in the TESLA Linear
Collider feedback systems will be needed to provide orbit
corrections within the bunch train. A prototype of thecomplete vertical feedback system has been installed inthe TESLA Test Facility at DESY. The use of digitalsignal processing techniques led to a fast and highly
flexible solution for the controller function. Additionalfeatures such as data logging and analysis allow easy
adjustment of the feedback parameters to achieve theoptimum performance of the system. An overview of thesystem will be presented as well as the results of firstmeasurements.

1 INTRODUCTION
The luminosity of a collider is proportional to the interaction rate, that next to the center of mass energy, is themost important parameter. TESLA aims for a luminosityof 6 ⋅ 10 33 cm −2 s −1 . Reaching this goal requires smalltransverse spot sizes [2] with a dynamically calm beam at
the interaction point (IP)
Unpreventable disturbances perturb the beam on its
way to the IP: e.g. misalignments and mechanicalvibrations of the focusing magnets scatter the bunch
centroids and can result in separations of the beam centers.
Consequently, control systems correcting the beam
trajectory are required in order to insure high beamstability and thus high luminosity. [1]The first prototype of a complete bunch-to-bunch orbit
correction feedback system has been built, installed andtested at the TESLA Test Facility (TTF) at DESY. TTF
offers the possibility to study the behaviour of real beam
motion and testing the performance of different feedbackstrategies.

2 FUNCTIONAL DESCRIPTION
Deviations in angle and position of a certain bunch canbe corrected by applying two consecutive kicks. Similarly
two beam position monitors can be used to measure thesame deviations from the ideal trajectory. Fig. 1 shows theprinciple configuration of a feedback system including
Fig. 1.

monitors and kickers at the collider. Since the bunches
are ultra relativistic the kicks to correct the trajectory ofbunch no. N can only be determined using the orbit
information of the previous bunches n < N. Thus thefeedback is only effective if the orbit information of
successive bunches are correlated. The correction
procedure consists of a feed forward and a feedback part.
If the orbit deviation of bunches have a similar patternwhich changes slightly from shot to shot a feed forward
controller can compensate these offsets. The update rateof this controller depends on the repetition rate of thecollider and is therefore relatively slow. In the case thatthe displacements of bunches within a bunch train differ
from shot to shot a feedback controller is applied. It isdescribed in section 4 in detail. The spectrum of the orbit
deviations extends fromf m = 0 to f m = 1 / 2tb . ( t b = bunch spacing)
In order to damp these frequencies the controller must
generate a correction that acts on the successor of alreadypassed bunches as a result of their motion. This implies
that the propagation delay of the feedback loop must be
shorter than the time interval between bunches e.g. 1µs at
TTF conditions. The typical propagation delay time ofsuch a feedback system, which is determined by cabledelays and signal processing, cannot be less than about
600ns. Therefore, if the bunch spacing of a certain
collider will be less, it may also be possible to act within atime interval of two times the bunch spacing. This will
decrease the system bandwidth with the result that the cut
off frequency of the damping performance is shifted down.
Fig.2 shows the theoretical reduction of orbit deviations
by a feedback loop that is able to act from bunch to bunch.
Fig 2
10

Reduction of Orbit Deviations [dB]

Abstract

01020304031 10

BPM 2

BPM 1

Detector
Electronics

Kicker 2

DSP Feedback Electronics

Kicker 1

Power Amp

41 10Frequency

1 10561 10

A common offset of all bunches from the design orbit
should not be treated by the feedback kickers. Trying todo this would easily drive the system into saturation.

Therefore applying an offset correction algorithm musteliminate such an offset. The required correction may bedirected to the steerer magnets to compensate theseoffsets.
Due to the fact that the system has to be compatible tovarious collider configurations i.e. TTF, TTF phase II,TESLA [2], it must be programmable with respect todifferent clock frequencies and algorithms.

3 BUNCH POSITION DETECTION
3.1 Re-entrant Cavity BPM
A broadband re-entrant coaxial cavity BPM serves as ahigh-resolution beam position detector. It consists of a
tube surrounding the beam-pipe thus forming a piece ofcoaxial transmission-line. The coaxial cylinder is excitedat the upstream end by the EM-fields of the bunchedbeam passing a concentric gap, which is shortened at thedownstream end. Position sensitive signals are sensed by
4 symmetrical arranged antennas at the upstream end of
the coaxial structure. Instead of operating the re-entrant
cavity at the narrow-band cavity dipole mode, we monitor
the evanescent fields of the beam excited fundamental
TE11 wave-guide dipole mode. The frequency of thismode is much lower (approximately half) and it offers avery large bandwidth (about 100 MHz), thus gives highsignal levels. Details on the theory of the re-entrant
coaxial cavity are found in [3]3.2 RF Detector Electronics
The RF electronics consists of a calibration box locatedclose to the BPM inside the tunnel and a synchronous
detection part, which is outside the tunnel. 180° hybrid
couplers are used to generate the beam position signals( ∆ X , ∆ Y ). The bunch current signal serves to scale the∆ signals. Fig 3 shows a principle sketch of this device.

Compared to feedback systems for circular accelerators,
in a linear collider the calculated correction information isneeded after a time equal to the bunch spacing rather thana full machine revolution. Therefore the required
processing rate is much higher, so that techniques likepiping can only be used very limited. Use of devices like
cable drivers and signal buffers must be minimized. To
achieve the optimum processing rate, a field
programmable gate array (FPGA) is used. It reaches
operation frequencies up to 100 MHz. FPGA chips may
have over 200 I/O pins, so that all peripheral devices canbe directly connected making it much faster than using adata bus. Contrary to microcomputers signal processing is
executed in parallel and is thus extremely fast. Complex
FPGA's have sufficient capacity to hold the entire
functionality of the feedback signal processing. Since
FPGA's are software-programmable high flexibility isachieved. Later modifications of the controller algorithm
may be easily down loaded. Taking all these
considerations into account leads to a single printed
circuit board design, one for each transverse direction.
The principle block diagram is shown in Fig. 4. The mainprocessing components are the FPGA with its directconnected data converters. This establishes the feedback
loop operation and has a total throughput time of about
200 ns. A sequencer is used to manage system set-up,
program download and real-time data logging into theRAM. The CPU, connected to a CAN fieldbus or VME
bus, performs host communication. For feedback
parameter set-up it is also connected to the sequencer.
Configuration settings are stored in an EEPROM enabling
stand-alone power-up.
Figure 4: Block diagram of the DSP board
Control Bus

VME INTERFACE

outside tunnel

180°
HybridRF∆Y∆YIF)∆YCPUSEQUENCERLO180°
Hybrid∆XLO∆XRF)∆X

TIMING, CLOCK
DELAY
GENERATOR

FPGA

Constant amplitude bursts are obtained using limiter
amplifiers, to get the phase dependent position signals
converted to the base band. [4] After filtering they can be
sampled by fast high-resolution analog to digitalconverters.

RAM1MB (512 words)

12 bit

ADC ∆ X ,BPM 2X PLANE
PROC.

IF

ADC ∆X ,BPM 1)

I bunch)ΣYIF

12 bit

ADC Σ X , BPM 1)

LORFΣXEEPROM)

ΣY

Signal preparationelectronics

re-entrantCavityMonitor

4.1 Printed circuit board layout

CAN INTERFACE

Figure 3: BPM with RF Detection Electronics
inside tunnel

4 DIGITAL SIGNAL PROCESSING

12 bit

ADC12 bit

DAC14 bit

DAC14 bit

EPROM

Stimulus

Excitation
Kicker 1
Excitation
Kicker 2
ExternalTiming4.2 Controller function
As shown in Fig. 5, to gain a beam current independent
deviation signal at first the bunch position information isscaled by the current information delivered by the BPM
electronics. The offset corrected orbit deviation signal isprocessed by the feedback path realized as a PI controller
and an integrating feed forward path to generate theoutput signal. The controller output signals are combined
according to beam optical requirements with the help of adecoupling matrix and then fed into the high power
amplifiers.

was closed over kicker #2 and BPM 2 while kicker #1
was used for the test pattern stimulus. Fig. 5 illustrates themode of operation. The first trace of the backgroundpicture shows 16 consecutive bunch trains with 10bunches. Seen is a certain offset, a fluctuating slope overthe bunch train and the stimulus ripple of 500kHz
detected by BPM#2.
Fig. 5 Open and closed loop operation.

Figure 5. Signal flow of the feedback controller.
11∆Yfrom BPM 1
ΣYfrom BPM 1I

X

TiI

beamP

ADC

ScalingX

αX
Offsetcorrection

ADC

Feedforward

Decouplingmatrixmmmm00∆Y

to kicker 101∆Y1∆Y
from BPM 2
ΣYfrom BPM 2IX

101
Ti

to kicker 2

11I

beam

P

ADC

ScalingX

αADC

Offsetcorrection

X

Feedforward

5 KICKER
The kicker prototype consists of a V2A sputteredceramic tube surrounded by two semi shell conductors
and four ferrite plates. This construction allows all kickerparts to be outside the vacuum, thus avoiding high voltage
feedthroughts and preventing vacuum contamination.
V2A seems to be the best compromise between
minimum kick-field shielding and sufficient heat
conductance. The ceramic tube is sputtered from the innerside with a coating thickness of 1µm. This leads to aneffective field attenuation of about 5%. The electricalcharacteristics are specified as a 50-Ohm system with abandwidth of 30 MHz. The tube type class-A driver
amplifier has an output power of 1 kW and 60dB gain.
[5,6]

6 FIRST EXPERIENCES AND RESULTSUp to now the system is operating under TTF testconditions.
Bunch spacing # Bunches Charge
Energy10..
202..4nC
200
MeV
1µs
In order to measure the response of various components
a test pattern is fed into the stimulus input of the feedback
electronics. For example by using a synchronized 500kHz
signal one is able to produce an alternating bunchdisplacement along the bunch train that can easily be
distinguished from other beam movements. This method
is used to adjust the feedback parameters and will aid tomeasure the whole frequency response of the closed loop.
As a first experiment a one dimensional feedback loop

The foreground picture shows 8 LINAC cycles when
the feedback loop was closed. Now the slope and theripple are compensated. (trace #1) The second traceshows the kicker output signal. The feed-forward part ofthe output signal is displayed by trace #3 and theIntegrator output by trace #4.

7 REFERENCES[1] Ingrid Reyzl “Feasibility Study of Feedback Systems
for Orbit Correction in the TESLA Linear Collider”,
Berlin 1999 ISBN 3-8265-4822-1[2] TESLA Design Report,
http://tesla.desy.de/cgi-bin/TDR-request[3] R. Bossart, “Microwave Beam Position Monitor Using
A Re-entrant Coaxial Cavity”, CERN PS 91-59 (LP)
CLIC Note 174[4] C. Magne, M. Juillard, M. Lalot, A. Mosnier, B.
Phung, DAPNIA/SEA, CEA-Saclay, France
Y. Lussignol, DAPNIA/SIG, CEA-Saclay, France
R. Bossart, CERN, Geneva, Switzerland“High Resolution BPM for future Colliders”[5] J. Rümmler, “A Kicker System for the TESLA
transverse Feedback”, Internal Report DESY M 00-03,
12/00[6] F. Obier, “TTF Feedback-Kicker”, Internal Report
DESY M 00-02, 12/00