Proceedings of IBIC2012, Tsukuba, JapanTUPB48BEAM INSTRUMENTATION FOR THE COSY ELECTRON COOLER
E. Bekhtenev, V. Bocharov, M. Bryzgunov, A. Bubley, A. Denisov, G. Karpov,
V. Panasyuk, V. Parkhomchuk, V. Reva , BINP SB RAS, Novosibirsk, Russia
V. Kamerdzhiev, L. Mao, K. Reimers, FZ-Jülich, Germany
J. Dietrich, TU-Dortmund and HIM Mainz, Germany
The report deals with beam instrumentation of theelectron cooler for COSY storage ring. The electroncooler is an electrostatic accelerator designed for beamenergy up to 2 MeV and electron current up to 3 A inenergy recovery mode. The electron beam is immersed inlongitudinal magnetic field so the electron motion isstrongly magnetized. The control electrode in the electron
gun is composed of four electrically isolated sectors.
Applying AC voltage to one sector allows tracing ofmotion of that particular part of the beam. The electron
beam shape is registered with the combination of 4-sector
electron gun and the BPMs. This method allows
observing both dipole and quadruple (galloping) modes ofelectron beam oscillation. Compass probe for measuringand tuning the direction of magnetic field in the cooling
section is described. A monitor based on a few small
Faraday cups for measuring the electron beam profiles ispresented.

INTRODUCTION
The 2 MeV electron cooler for COSY was built at
BINP [1]. The design energy and good performance of thesubsystems were demonstrated during electron beamcommissioning in Novosibirsk. Further conditioning withelectron beam is required to achieve electron currents
above 200 mA at high energy. It will be done after theinstallation in COSY.
The electron beam is transported to and from thecooling section by two transport lines (see Fig. 1). This isdue to the requirement of the HV system being installedoutside the accelerator radiation shielding.

Lossless beam transport is essential for operation in theenergy recovery mode. Beam position measurement andcorrection are necessary for beam loss minimization andaccurate beam alignment in the cooling section. The
design of the cooler includes numerous beam diagnosticsystems. 12 beam position monitors (BPMs) provide
position information in the electron transport lines and in
the cooling section. The electron gun and its high voltage
power supplies were designed having beam diagnostics inmind. Only electron gun features directly related to beamdiagnostics are discussed in this paper. A Faraday cup
array is used to measure the profile of the electron beam
at low average current. Periodic verification of themagnetic field straightness in the cooling section isrequired to ensure best possible cooling.

ELECTRON GUN
Beam position measurements require modulation of theelectron current. This is done by applying AC voltage at
3 MHz and up to 10 V to the gun control electrode (seeFig. 2). Furthermore, the control electrode of the electron
gun is composed of four electrically isolated sectors [2].
Applying AC voltage to one sector at a time allows theuse of BPMs for tracing the corresponding part of theelectron beam. Comparing the positions of each sectors
from BPM to BPM or the sector positions in a single
BPM at different currents in the corrector coils it ispossible to analyze the optics in the transport line.

Figure 2: Diagram of the gun modulation electronics.

Figure 1: Layout of the 2 MeV electron cooler.

The modulation electronics is mainly located inside
the HV vessel at potential of up to 2 MV. Modulation
signal is transmitted to high voltage part by means of light[3]. Laser diode ADL-66505TL mounted on a viewportoutside the HV vessel is used as optical transmitter. Its
optical output power is stabilized at the level of 20 mW.
Silicon PIN photodiode BPW34 located at 0.3 m distancefrom the laser diode is used as optical receiver.
Combination of a low noise amplifier (LNA) and acomparator allows to have stable modulation voltage atISBN 978-3-95450-119-9
Miscellaneous and others

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

TUPB48

Proceedings of IBIC2012, Tsukuba, Japanthe control electrode even if the optical transmitter ismisaligned by up to 10 deg. Measuring the modulation
voltage at the LNA output by means of a detector and an
ADC allows adjustment of transmitter angle for
maximization of optical power coming to the receiver.

BEAM POSITION MONITORSBPM signal processing electronics includes apreamplifier and a signal processor consisting of analogand digital boards [3]. The preamplifiers with high input
impedance are located close to the pickups. Mainparameters for the BPM system are shown in Table 1.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 1: Main Requirements for the BPM System
Electron current
0.1-3 A
Modulation amplitude
0.3-1.5 mAof electron current
Proton current
0.1-2 mA
Position measurement error< 100 μm
Measurement rate
0.1-1 sThe principle of operation is shown in Fig. 3. The
preamplifier is connected to the pickup via four 75 Ohm
1.2 m long cables. The switches used in the preamplifiersprovide 4 connection combinations between 4 BPM
electrodes and 4 signal processing channels. Thisapproach allows eliminating measurement error caused byinequality of the channel transmission coefficients. After
amplification the BPM signals with frequency FSIGN aremixed with reference frequency FSIGN + ΔF.

The ΔF = 23 kHz signals are sampled by a 14 bit ADC
after low pass filtering and amplification. The digital
signals are sent to an FPGA where further processing isperformed. It includes synchronous detecting andaccumulation. A 3 MHz calibration signal can be applied
to the preamplifier inputs via S1 switch. The BPM
processor occupies one 1U 19” chassis. Each processor
can serve two BPMs. The system was built and tested. It
was in operation with electron beam at BINP sinceNovember 2011.
Table 2: Measured Accuracy of the BPM SystemPosition variation for different beam
modulation, IMOD = 0.3-1.5 mA
Resolution, IMOD = 0.3-1.5 mA
Temperature dependence

4 μm< 1 μm
2 μm/°C

To determine the position accuracy a sinusoidal 3
MHz test signal was used. It was applied via four-waysplitter to four preamplifier inputs. Signal amplitude waschanged in the range 0.2-1 mV corresponding to beam
current modulation range 0.3-1.5 mA.

Figure 4: Beam positions measured by three different
BPMs: BPM2 (red), BPM4 (green) and BPM10 (blue).
The gun modulation was applied to one sector at a time.
The results of experimental evaluation for
KX=KY=43 mm are presented in Table 2. Figure 4
illustrates the electron beam shape measured at threedifferent locations while driving the gun in the single
sector mode as described in the previous section [3].

PROFILE MONITOR
A detector consisting of six Faraday cups located inthe transport line is used to measure electron beamprofiles.

Figure 3: Block diagram of the BPM system.

Figure 5: Design of the Faraday cup detector for profile
measurements (left, 5 of 6 cups are shown) and its
location within the beam aperture (right).

ISBN 978-3-95450-119-92

Miscellaneous and others

Proceedings of IBIC2012, Tsukuba, JapanTUPB48The electron beam is deflected by a corrector magnet
onto the detector and does not reach the collector. In orderto minimize the load on the HV generator the cooler isoperated in pulsed mode (5 μs pulse at 20 Hz repetitionrate). The Faraday cup signals are recorded together withhorizontal and vertical corrector currents while the beamis scanned across the detector.

MAGNETIC FIELD STRAIGHTNESS

12

42

I
3IIIV

Laser
Electronic unit
Preamplifier

Integrator

Synchr.
detector

Y

Preamplifier

Integrator

Synchr.
detectorпред-Current sensors

Array

Modulation

XпредIII

Figure 6: Illustration of the operating principle of thesystem monitoring magnetic field straightness. Magneticsensor (1), corrector coils (2), beam splitter (3), photodetector (4).

Figure 7: Horizontal magnetic field in the coolingsolenoid before (1) and after (2) coil adjustment (left).
Photograph of the magnetic sensor (right).

SUMMARY
The 2 MeV electron cooler was designed, built andcommissioned with electron beam at BINP. The design
energy was demonstrated. Obtaining high electron
currents at nominal energy requires further conditioning
which will continue after the installation of the cooler in
the COSY ring. Beam instrumentation is a vital part of thecooler. Several electron beam modulation techniques areimplemented in the gun electronics enabling beam
position and shape measurements. The novel gun design
was shown to be very useful from the point of view ofbeam diagnostics. The four-sector technique allows
observing both dipole and quadruple modes of electron
beam oscillation. The BPM system was tested withelectron beam. The position accuracy was determined tofulfil the requirements. A laser based system formeasuring straightness of magnetic field in the cooling
section without breaking vacuum was successfully used tooptimize the field quality in the cooling section. A
detector consisting of 6 Faraday cups was used tomeasure electron beam profiles at low average current.
Beam instrumentation was extensively used duringcommissioning at BINP. Good performance of thesubsystems described was shown.

ACKNOWLEDGMENT
The authors would like to thank the mechanical design
team, especially V.A. Poluhin, A.M. Kruchkov, A.A. Putmakov, V.K. Gosteev and the machine shop team who
manufactured the cooler components, B.F. Chirkov, E.S.
Ruvinsky. We are also grateful to the members of theelectronics department A.S. Medvedko, E.A. Kuper andV.V. Kolmogorov.

REFERENCES[1] M. Bryzgunov et al., “High Voltage Electron Cooler”
RuPAC 2012.
[2] V. Reva et al., “High Energy Electron Cooling”, HB 2012.
[3] E.A. Bekhtenev et al., “Beam Position Monitor System for2 MeV Electron Cooler for COSY” RuPAC 2012.
[4] V.N. Bocharov et al., “Precise Measurements of aMagnetic Field at the Solenoids for Low Energy Coolers,”
COOL’05.

ISBN 978-3-95450-119-9
Miscellaneous and others

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The efficiency of the cooling process strongly depends
on the quality of longitudinal magnetic field in thecooling section. Magnetic field straightness of the orderof 10-5 is required. That’s why the cooling solenoid iscomposed of numerous short coils. The angle ofindividual coils can be adjusted. In order to measuremagnetic field straightness a highly sensitive magnetic
probe was designed and built. The basic idea of this probe
is similar to the ones used at BINP and Fermilab for thesame purpose [4]. However, the compass-based probe isinstalled on a guiding rail inside the vacuum chamber ofthe cooling section. This is done to improve theavailability of the cooler. The light beam generated by alaser is reflected by a mirror attached to the compass. The
reflected laser beam is then detected by a 4-segment
photodiode. A feedback system using two differential
signals from photodiode segments, controls currents inhorizontal and vertical corrector coils. The value of thiscurrent corresponds to the transverse magnetic field in thecooling section. Figure 6 illustrates system operation. The
important feature of the probe described is operation invacuum. Therefore the gimbal suspension and jewel
bearings were used instead of an organic polymers thread.
This engineering solution decreased the accuracy of theprobe but improved the mechanical strength of the device.
The permanent magnets were replaced by needles madefrom a soft magnetic material. The field to be measured isstrong enough so there is no loss of sensitivity.
Furthermore, the probe should withstand baking at 200 ºC
without losing its characteristics. Soft magnetic materialis suitable for this purpose. For mechanical stability of themirror surface it was made from polished molybdenum
without any additional reflective layer.
Figure 7 shows the horizontal magnetic field in thecooling solenoid before and after coils adjustment. The
rms ripple of the magnetic force line was decreased from6∙10-4 to 2∙10-5.