Proceedings of IBIC2012, Tsukuba, Japan

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MODULAR LOGARITHMIC AMPLIFIER BEAM POSITION MONITORREADOUT SYSTEM AT THE UNIVERSITY OF HAWAI’I∗

Abstract
High brightness electron beams for inverse Compton
photon sources driven by thermionic microwave guns require real-time position measurements in order to achieve
the spatial and temporal coincidence necessary to ensurestatistically measurable signals. True logarithmic amplifiers are more adequately suited to signal comparison thanare sigma-delta methods. A low-cost, modular and scalable
readout and data acquisition system for strip-line beam position monitors utilizing the AD640 log-amp is being developed at University of Hawai’i MkV Linear Accelerator and Free Electron Laser Lab. Initial measurements andprototyping of the hardware is complete with commissioning and deployment of the system currently ongoing. We
present the methodology and early results of this project.

INTRODUCTIONThe University of Hawai’i (UH) MkV Linear Accelerator facility and Free Electron Laser (FEL) Lab utilizes athermionic LaB6 cathode electron source in a microwave
gun injector followed by a single section of traveling wave
S-Band linear accelerator to produce a ∼200 mA average
macro-pulse current 40 MeV electron beam. This beam
drives a hybrid NdFeB planar undulator and Michelson interferometer phase-locked resonator based infrared FEL.
Constructed on UH campus and occupying roughly onethird of the first floor of the physics department, the accelerator beamline was commissioned by 2009 and the lab
produced first laser light in 2010; current experiments arefocused on initial demonstration of inverse Compton xray photon production via FEL laser output and electron
beam collision. A pico-second resolution x-ray detector
and multi-gigabit per second sampling electronics are also
being developed in parallel by collaborators in the University of Hawai’i Instrumentation Development Lab (IDLab)for measurement of the resulting micro-bunch x-ray train.
The electron beam transport system configuration forthese experiments requires strong focusing to achieve smalltransverse size of the e-beam (and similarly for the optical beam used in the collision) in order to achieve optimum x-ray flux. One of the machine upgrades underway
in support of the objective of a well-centered beam in thequadrupole magnets of the transport system and measurement of the electron beam transverse position for injection
into the x-ray interaction point (IP) is the instrumentation∗ Work supported by DHS Agreement Number:
AR1045-02, Amendment 2

2010-DN-077-

of the beam position monitors (BPM’s) installed along the
UH MkV beamline.
Two varieties of BPM’s are installed along the MkV
beamline, a stripline type and a wall current variety. The
stripline BPM’s (shown installed on beamline in Fig. 1)
consist of a stainless steel body with four copper electrodes
oriented with π2 symmetry mounted on standard 2 34 inch
conflat flanges and are surplus from the SPEAR project at
SLAC[1]. These stripline BPM’s are installed in four locations along the diagnostic chicane transport system, whichtransports the e-beam from the linac to the FEL, as well as
injects the e-beam into the scattering chamber where thex-ray IP is located.
The so-called “tin-can” wall current BPM’s contain an
FR-4 printed circuit board (PCB) sandwiched between apair of conflat-type knife edge flanges that make vacuum
seal against the copper plating on the board, routed withtraces which carry induced wall current at four cardinal positions around the beam pipe with a minimum of temporal
distortion to a 4-port connector block at the perimeter of the
PCB. A suitably placed toroid provides a large inductancethat forces all of the image currents that otherwise mightflow along the outside of the beam pipe to instead flow
along the impedance-matched PCB traces, insuring that all
the components of the wall currents so sampled will emerge
on the inner conductors of the coaxial output ports of the
BPM. The frequency response of these wall current BPM’sis nearly flat from below 100 kHz to beyond 20 GHz. Figure 2 shows one of the two wall current beam position monitors that have been installed in the MkV beamline (one at
the linac output and one at the input to the FEL). Further
description of these devices will be published separately.

Figure 1: Stripline BPM installed on University of Hawai’i
MkV FEL beamline.

ISBN 978-3-95450-119-9
Beam Position Monitor System1

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○B. T. Jacobson, M. R. Hadmack, J. M. J. Madey, P. Niknejadi
Department of Physics & Astronomy, University of Hawai’i, Honolulu, HI 96826, USA

MOPA17

Proceedings of IBIC2012, Tsukuba, Japanan essential mathematical operation in the signal comparison chain. The log-amp and supporting circuitry are implemented in a modular way on pluggable cards with a compatible multiple slot chassis that provides for system expansion, component replacement, and maintenance access.
The data acquisition (DAQ) system collecting the position
signals derived from the log-amp detector cards consists ofa dedicated analog to digital converter (ADC) and a computer display in the operator control room.

BPM SIGNAL HETERODYNE SYSTEM

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The beam position monitor readout system utilizes heterodyne mixing of the raw BPM outputs in order to convert
the strong 2.856 GHz signal to the more manageable HFBand. A sample BPM output signal chain is shown in Fig.
3, which is repeated for each output of each BPM. All connections are SMA and components are in-line devices, asseen installed in Fig. 1.

Figure 2: Photo of the so-called “tin-can” wall current
beam position monitor installed in two locations on the
University of Hawai’i MkV FEL beamline.

BPM SIGNAL READOUT SCHEME
The timing structure of the accelerated electron beam exhibits both a micro-pulse and a macro-pulse structure. The
short micro-pulses are a result of the thermionic microwave
electron gun source filling each RF bucket in the linac andthe temporal compression imparted by an alpha-magnet [2]integrated into the injector system. In practice, the alphamagnet field gradient and S-band drive frequency of 2.856
GHz yields 1-2 ps long micro-pulses separated by 350 ps.
High power RF is applied to the accelerator and microwave
gun for a macro-pulse length of 4.5 μs at a 4 Hz repetition
rate duty cycle. This micro-bunching in the electron beam
produces strong signal at the accelerator RF frequency inboth types of BPM’s.
In order to avoid the usual difficulties in BPM signal
comparison (namely the phase match required to directlycompare two RF voltages, detector and electronics lifetimes in intense radiation fields found near the beamline,
and signal degradation and loss in long cables) the readout
strategy is to mix the BPM signals with a local oscillator RF
reference in the accelerator vault, resulting in an intermediate frequency (IF) between 10 - 20 MHz. The IF signal
from each output port of each BPM is carried through longruns of LMR-400 coaxial cable from the accelerator vault
to the operator control room.
The Analog Devices AD640 logarithmic amplifier
serves as both the detector used to measure the mixer outputs for each BPM port as well as the means to provide

Figure 3: Block diagram of heterodyne concept for typical
BPM signal channel.
A 50 Ω 10 dB attenuator helps to provide an impedance
match between the stripline electrodes and the signal cables and other components (not required for the tin-canwall current BPM’s which have internal 50 Ω stripline
traces). A band pass filter (Mini-Circuits VBF-2900+) ensures that no higher order harmonics (for stripline BPM’s)and other frequency components down to DC (for wall
current BPM’s) enter the doubly balanced mixer (MiniCircuits ZX05-42MH-S+). Any high frequency leakageout of the mixer is blocked by the low pass filter (MiniCircuits VLF-190+) on the mixer IF port.
Each mixer requires +13 dBm (20 mW) input to the local oscillator port close to the accelerator frequency, andtherefore a local oscillator (LO) distribution network is being assembled, which is capable of delivering the +19 dBmof power to each BPM mixer station. A 4-port power splitter (Mini-Circuits ZN4PD1-50-S+) divides the LO at each
BPM mixer station to each of the four mixers. Additional
power splitters will distribute the LO to each BPM, with thereference provided by a single HP-8616A RF signal generator followed by a 40 dB amplifier (Mini-Circuits ZHL4240).

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

BPM LOGARITHMIC AMPLIFIERREADOUT SYSTEMThe IF mixer output signals from each beam position
monitor are transmitted over long cables from the accelerator vault to a patch panel in the operator control room.
The readout electronics for each BPM are implemented ona dedicated PCB card which is installed into the readout
system chassis slots via a 44-contact card-edge connector
Each PCB card has four input channels, and hence is capable of instrumenting a single beam position monitor.

MOPA17the output of the difference amp (combining Eq. 1 and Eq. 2)
 R
is:
Vin
,
(3)
Vout = G × Vslope log
LVinwhich necessarily vanishes for a centered electron beam.
A BPM detector card PCB known as Electron Stripline
Monitor Analog ReadOut Log-Difference Analyzer (ESMAROLDA) containing two copies of the circuit indicatedin the block diagram has been designed, prototyped, tested,
and finalize in a production run. Initial bench test results
are shown in Fig. 5 and in Fig. 6. In each case, a 20 MHz
signal source is divided by a balanced 2-way 3 dB power
splitter with one output sent directly to an ESMAROLDA
input and the other splitter output being passed through avariable attenuator before connection to the opposing ESMAROLDA input channel. Testing shows a logarithmic response from the log-amp and correct overall transfer function and position error signal output.
Log−amp Response Measurement

Vout = Vslope log

Vin
Vref


,

(1)

where the constants Vslope and Vref are controlled,
trimmed, and set at the chip foundry to be uniform amongdevices. Each log-amp output voltage signal is bufferedand sent to a monitor card for time-resolved signal monitoring.
The sample and hold block (S&H) consists of an externally gated analog switch (Fairchild CD4066) and lowleakage polypropylene film capacitors. The external gate
is generated by a Berkeley Nucleonics Model 505 gate anddelay generator and timing governed by the electron beam
master trigger. A programmable gain difference amplifiercompares the DC signals present on the S&H output andderives the position error signal. The transfer function of
the difference amplifier is:R
L− Vout),Vout = G × (Vout(2)

where the sensitivity G is adjusted by changing the pluggable gain resistors on the PCB. The transfer function at

1.1
1
0.9
0.8
0.7
0.6

0

2
4
6
8
Single Channel Attenuation (dB)10

Figure 5: Response of typical BPM signal detector circuit,
AD640 log-amp and AD844 current to voltage buffer, as alinear function of input attenuation.
ESMAROLDA Output Response Measurement
0
−100
−200
−300
−400
−500
0

2
4
6
8
Single Channel Attenuation (dB)10

Figure 6: Output response of typical horizontal or verticalplane signal comparison circuit as a linear function of single channel input attenuation.

ISBN 978-3-95450-119-9
Beam Position Monitor System3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○A simplified block diagram for calculating the beam position in one coordinate plane (in this case, the horizontal
position) is shown in Figure 4. The log-amp block contains
the AD640 logarithmic amplifier and the 60 MHz bandwidth fast slewing AD844 op amp current to voltage buffer
circuit. The transfer function of the log-amp block is:

ESMAROLDA G = 1 Outpout (mV)

Figure 4: Block diagram of ESMAROLDA beam position
monitor readout cards.

Log−amp Output Voltage

1.2

MOPA17

Proceedings of IBIC2012, Tsukuba, Japan

The purpose of the Beam Position Monitor Graphical
User Interface (BPM-GUI) is to provide an interactive tool
for the BPM hardware and software system. The BPM
mixer stations and detector cards readout the beam offset,
in both x and y directions, as a positive or negative voltage. The BPM-GUI is part of the DAQ system and displaysthese voltages in the control room where the accelerator operator can use the BPM-GUI readout to center the e-beamby adjusting the beam transport system as to null the readouts.
The BPM hardware interface is the 14-channel ADC ina LabJack UE9. The LabJack UE9 has all analog input(AIN) channels on its DB37 connector, which mates witha custom cable from the BPM patch panel chassis, and isused to read the horizontal and vertical offset signals fromeach BPM. The nominal input range for each AIN is ± 5
volt. When using several channels at once there is a 12 microsecond delay for 12 bit-streaming from channel to channel. The pulse rate in the accelerator is 5Hz. For accurate
readouts the BPM GUI is set to read every 100ms (meetingthe minimum of 2 readout per cycle). Hence the delay inthe channel readouts can be neglected.
The LabJack software (LabJackPython) was implemented in the GUI code written in wxPython to provide thefirst version of BPM-GUI, shown in Fig. 7. Currently thisdevelopment version of the BPM-GUI only provides thereadout for each BPM in volt (using read only display text
boxes). The button “Read Once” retrieves the voltages only
once as a test at the start of the run and the “Read Continuously” retrieves and displays the voltages until the programis stopped. The “Restart” button zeros all the entries and is
only functional when the program is not in continuous run
mode. The “Exit” button stops the reading and closes the
BPM-GUI.
Upon complete installation each BPM will be renamed
to identify its position along the beamline. Finally thenext version of the BPM-GUI will have a bullseye display for each BPM that will illustrate for the operator how
off-center the beam is and will facilitate the centering process and also display a graphical illustration of the electron
beam centroid’s evolution along the length of the machine.

EARLY BEAM TEST MEASUREMENTS
Unexpected machine down-time, maintenance, and apartially assembled local oscillator network have postponed full-up operation of the BPM readout system, however initial testing has been preformed using the fully assembled chassis and signal cable network with one BPM
active at a time, for each of the stripline and wall current
varieties.
Beam test was done with a single stripline BPM locateddownstream of the first bend magnets in the MkV diagnostic chicane. The accelerated beam parameters for the
run were 170 mA average macro-pulse current and a beam

Figure 7: Screen shot of BPM GUI display.
Measured Vertical Displacement Response
G = 1 Difference Amp Output (mV)

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○BEAM POSITION MONITOR
GRAPHICAL USER INTERFACE

150

100

50

0
−2

−1
0
1
2
3
Screen Beam Position and Size (mm)4Figure 8: BPM readout response as a function of verticalelectron beam position (with beam size shown by horizontal error bars) on downstream transition radiation screen.
energy of 37 MeV. The beam was imaged on a copper transition radiation (TR) screen downstream of the beam position monitor and centered in the beamline quadrupoles (andtherefore roughly centered in the BPM) by minimization ofbeam deflection from increase in quadrupole gradient. The
beam was then given kicks independently in both the vertical and horizontal directions upstream of the BPM andoffset voltages from the readout system recorded. In thisinstance, the difference amplifier gain is set to unity.
By knowing the pixel to size calibration for the recorded
TR screen images, the beam displacement can be calculated. Figure 8 shows the BPM readout offset signal asa function of the measured beam position at the screen.
The beam centroid offset is estimated by simple geometricmeans from the relative location of the dipole kicks, BPM,
and TR screen and plotted in Fig. 9.
After testing the stripline BPM, the LO was moved to
one of the wall current BPM’s and the accelerator restarted.

ISBN 978-3-95450-119-9
4Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, JapanSUMMARY
We have constructed a modular logarithmic amplifier
beam position monitor readout system at the University of
Hawai’i MkV Linear Accelerator and Free Electron Laser
Lab and tested this system on both a stripline BPM and a“tin-can” wall current BPM. Favorable initial results havebeen met and final commissioning of the system for readoutof all 6 BPM’s expected in October. A basic graphical userinterface is available with a clear upgrade and development
path outlined.

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0

ACKNOWLEDGMENT
−1
0
1
2
3
BPM Beam Position and Size (mm)4Figure 9: BPM readout response as a function of estimatedelectron beam position (with beam size shown by horizontal error bars) at BPM location.
Testing demonstrated that signal levels were more than adequate to measure, however it was also discovered that theangular orientation of the BPM as presently installed isskewed by a large angle from the vertical and horizontal
directions in the lab. Once this rotation is corrected, thesystem will be re-evaluated.

We would like to acknowledge the dedication, cooperation, and support of the rest of the UH FEL group members:
J. M. Kowalczyk, E. B. Szarmes, and I. Howe. Additionally we would like to thank our collaborators in the UH
IDLab: G.S. Varner, M. Z. Andrews, and K. Nishimura.

REFERENCES[1] Chang, A K and Kang, H K and Larsen, R S, “Position andIntensity Monitoring System for the SPEAR Transport System at SLAC,” PAC’73, San Francisco, March 1973, p. 624[2] John M. J. Madey and Glen A. Westenskow, “Microwave
Electron Gun,” United States Patent 4,641,103 (1984)

ISBN 978-3-95450-119-9
Beam Position Monitor System5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○G = 1 Difference Amp Output (mV)Measured Vertical Displacement Response−2

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