Proceedings of IBIC2012, Tsukuba, Japan

MOPA31DESIGN AND FABRICATION OF THE STRIPLINE BPM AT ESS-BILBAO

Abstract
A stripline-type BPM has been designed and built at
the ESS-Bilbao premises. The design is based on
traveling wave electrodes principles to detect thetransverse position of the beam enclosed within thevacuum chamber. In the design of stripline setup, it has
been considered to keep the comparison conditions withpreviously used pick-ups as similar as possible. Thelength of strip electrodes is 200 mm and the coverageangle is 0.952 rad. The structure is rotationally π/2
symmetric and the alignment of electrodes are π/4, 3π/4,5π/4 and 7π/4. The design is optimized for a frequency of
352 MHz, however it can function on a wide range of
frequencies out coming from the measurement results.
Striplines in general have well defined behavior even for
low beta and low intensity beams as well as functionality
at low and high frequencies. A report on the design andcharacteristics measurement of stripline is presented
which includes the frequency range, the effect ofinsulation of electrodes, the electrode response as well as
their sensitivity to beam power and position.

comprises the transitions and feedthroughs. The criteria ofmaximum sensitivity at the ESSB RF frequency of 352
MHz and the 50 Ω impedance for the elements are taken
as constraints to the design. The rigid N-type feedthroughwith long signal pin is chosen as also the signal feed out
from stripline to the electronics via coaxial cables. The
transition from the strip electrode to the feedthrough wassimulated and optimized in order to minimize the signal
reflection in both ways. The optimum length of electrode,for which the sensitivity of the signal to the beamdisplacement is maximum, occurs at a signal walk equal
to one quarter of wavelength of the exit beam. The
stripline tube inner diameter is 57 mm and the length of
strip electrodes is 200 mm, while the azimuthal coverageangle is 0.952 rad. Increasing the coverage angle could
result in signal integrity deterioration due to coupling
between adjacent strip electrodes. The assembly angles ofstrip electrodes are π/4, 3π/4, 5π/4 and 7π/4. This
corresponds to a π/4 rotation of the stripline block aroundthe beam axis in order to be fitted easily on the test stand.

INTRODUCTION
The ESS-Bilbao (ESSB) project comprises a light-ion
linear accelerator feeding a low-energy neutron source.
Beam Position Monitors (BPMs) are some of thediagnostic systems under current development. A firststage in such an endeavor was the full development of a
Button Pick-up BPM prototype [1] which attained full
performance specification. The main drawback of button
pick-ups concerns their weak sensing response at low
energy and low beta beams such are the ones alreadyunder consideration within ESS-Bilbao. To overcome
such difficulty, in collaboration with the Department ofElectricity and Electronics of the University of theBasque Country (UPV/EHU), we have designed, built andtested a stripline monitor consisting of four electrodes asschematically shown in Fig.1. The implementation of thisBPM system includes the pick-ups and stripline BPMs,
the test stand for simulating beam conditions, the analog
and digital electronic units and the control system [1].
The control system integrates the BPM system into theExperimental Physics and Industrial Controls System(EPICS) [2] network of the accelerator.

GENERAL DESIGN AND
CONSIDERATIONSThe electromagnetic structure of the stripline BPM was
separated into two smaller structures for the ease of theelectromagnetic simulation [3]. One structure includes thetube and electrode strips and the second structure

Figure 1: 3D schematic of the stripline BPM.

Figure 2: Stripline
configuration.

BPM

fabricatedelectrodesISBN 978-3-95450-119-9
Beam Position Monitor System

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○S. Varnasseri, I. Arredondo, D. Belver, F.J. Bermejo, J. Feuchtwanger, N. Garmendia,P.J. Gonzalez, L. Muguira, ESS-Bilbao, Leioa, Spain
V. Etxebarria, J. Jugo, J. Portilla, University of the Basque Country, UPV/EHU, Leioa, Spain

MOPA31

Proceedings of IBIC2012, Tsukuba, JapanSTRIPLINE MEASURED
CHARACTERISTICS-10-20To perform the measurements of the BPM system, atest stand has been developed at ESSB and UPV/EHU toallow assembling a stripline BPM block.

-40dB

BPM Test Stand-50

For the laboratory tests, a copper rode with a diameterof 9 mm was inserted inside the stripline block andlocated initially at the transverse center of the stripline
block. In the absence of real beam, this rode carries theRF signal and mimics the bunched beam at the specificfrequency. The relative position of the internal rodesimulating the beam can be changed with respect to theouter tube within a range of ∼20 mm for both X and Y
planes with positioning steps less than 10 µm.

Figure 4: Measured insulation between electrodes of thestripline.

Characteristics ImpedanceFrequency ResponseThe copper rode ends with a 50 Ω load. The upstream
ports of the stripline are terminated with a 50 Ω loadwhen not in use; while the downstream ports have shortcircuit terminations. Fig. 3 shows the Smith chart of one
strip electrode. Since the four electrodes are almost
identical, data shown in one graph also represents theother three electrodes. The red dot corresponds thelocation of working frequency of 352 MHz. It shows thecharacteristic impedance of around 50 Ω up to 1.5 GHz.

The response of the electrode strips to a 0 dBm
centered beam at different frequencies up to 3 GHz wasmeasured and it is shown in Fig. 5. The plot is expected toshow peaks at values corresponding to odd harmonics ofRF frequency (1st, 3rd, …) and minimum responses ateven harmonics of the RF frequency (2nd, 4th,…). The
measurement results also agree with a peak response (-14dBm) at 352 MHz and a minimum response (-38 dBm) at
the second RF harmonic.

-60
Adjacent S21
Opposite S21-70

-80

0500

1000

1500

Frequency (MHz)+j1.0(0-1.5 GHz)

10+j0.5+j2.00+j0.2-j0.2

-j5.0

Signal power (dBm)

5.0

2.0

1.0

0.50.0-10+j5.00.2

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○-30-20-30-40-50-j0.5-j2.0-j1.0

Figure 3: Smith chart for one stripline electrode.

Electrodes Insulation
In general terms, highly insulated electrodes should
translate into lower coupling and thereby higher accuracyof the beam position monitoring. In contrast, highcoupling values between electrodes increases the nonlinearity of the sensitivity curves, which complicates theextraction of accurate information on the beam position
from the electrode signals. The Fig. 4 shows that theinsulation between two adjacent electrodes is 21 dB andfor the opposite electrodes is 34 dB at 352 MHz. This canbe translated onto the values for the coupling between
adjacent and opposite electrodes at the corresponding
frequency. This corresponds to a 0.9% voltage coupling
between adjacent electrodes at 352 MHz [3].

-60

0500

1000

1500
2000
Frequency (MHz)

2500

3000

Figure 5: Stripline response to beam frequency variation.

STRIPLINE RESPONSE TO BEAM
CURRENT VARIATIONSIn order to test the linearity of the stripline response tobeam current variations, RF signals ranging from -40
dBm to 15 dBm were inserted into the central rode of thestripline which represents the beam on the test stand. TheFig. 6 shows the measured power from each electrode of
the stripline together with its linearity to beam current
variations. It is also worth mentioning that the striplineelectrode signal magnitude is strongly dependent upon its
distance to the beam center as well as to the beamfrequency. In real numbers, such a magnitude formeasurements carried out at our test stand has yielded avalue of -15 dBm for a 0 dBm, 352 MHz beam.

ISBN 978-3-95450-119-9
2Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japannot controlled. The gathered data are then analyzed using
procedures developed in Matlab® from where the longterm stability for horizontal/vertical planes and phase areextracted.
Nomeaningfulcorrelation
betweentemperature variation and the position stability ofreadings was found. The measurement was repeated forthe two reference frequencies of 352 MHz and 175 MHz.
At 352 MHz, the input beam power to the rode inside thestripline was -23 dBm, which corresponds to -35 dBm in
the Logarithmic Amplifiers (log amp) of the Analog
Front-End (AFE) unit, based on the data from Striplinecharacteristics measurement.

0-20

020-10-20-30-40-50

12.05

-60
Input rf power (dBm)

12Figure 6: Measured input and electrode signal variation.

LINEARITY OF X/Y POSITIONING(X,Y) Measured (mm)642-2-2

11.9

11.8
11.75

00.5

11.5
2Time (samples)

2.536x 10

Figure 8: Long term stability measurement of horizontal
position at 352 MHz.
For a total time of 11 hours running the system whichcorresponds to 3.3 M samples, the rms values for stability
in horizontal and vertical planes was found to be 9.9 µm
and 9.4 µm respectively (Fig. 8). With the same numberof samples, the rms for phase stability was 0.38o. For theshort term stabilities of the position and phase of thewhole system, 10 k continuous samples of each parameter
were also acquired. The rms values for stability inhorizontal and vertical planes was found to be 6.4 µm and5.7 µm respectively. The rms of phase stability was lessthan 0.1o at 352 MHz. The input power to the stripline
block was -12 dBm which corresponds to -24 dBm signal
level to Log-Amp of AFE unit.

ONCOMING PLANS0-4

11.95

11.85

The Fig. 7 shows the measured linearity of the position
reading in horizontal and vertical planes at the bunch
frequency of 352 MHz. The displacement data wasacquired only within one plane, while the beam position
was kept unchanged within the other plane at the sametime. This measurement scheme was repeated forhorizontal and vertical planes and relevant data pertaining
the instantaneous reading of beam position were taken.
The Fig. 7 shows the linear sensitivity curves near thetransverse center for X/Y planes. However the nonlinearity shows up as the distance from the center of thestripline is increased. The fitted values correspond to theslopes of the straight lines and yield 0.977 and 0.997 forhorizontal and vertical planes respectively.

-6352 MHz Long Term Stability

12.1

0246-4-6(x,y) Real (mm)Figure 7: Linearity in X and Y planes at 352 MHz.

STABILITY MEASUREMENTS
In order to perform long term stability measurements ofthe position measurement at horizontal/vertical planes andphase, the whole system is left operating overnight withina relatively quiet location, using the electronics systemdeveloped for the BPMs [4]. The room temperature of themeasurement laboratory was also recorded during themeasurement time; although the ambient temperature was

In order to characterize the behaviour of stripline typeand button pick-up type of BPM system, both types ofresponses to different beam conditions are being
measured and the results will be reported in a separate
publication.

REFERENCES[1] D. Belver, et al. “Overview of the BPM system of the ESSBilbao”, MOPD21, Proc. of IPAC’11, Germany, May
2011.
[2] P. Echevarria, et al. “Multipurpose controller based on a
FPGA with EPICS integration”, PAC 2011, NY, US,
March 2011.
[3] S. Varnasseri, et al. “Stripline BPM design and fabrication”
ESS-Bilbao internal report, January 2012.
[4] D. Belver et al., “Design and measurements of the Stripline
BPM system of the ESS-Bilbao”, MOPPR041, Proc.
IPAC’12, New Orleans, US, May 2012, pp. 870-872.

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

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○-40

X Position (mm)Electrode Signal Magnitude (dBm)

10-60

MOPA31