Proceedings of IBIC2012, Tsukuba, JapanTUPB84

STORAGE RING TUNE MEASUREMENTS USING HIGH-SPEED
METAL-SEMICONDUCTOR-METAL PHOTODETECTOR
S. Dawson, D.J. Peake, R.P. Rassool, The University of Melbourne, Melbourne, Australia
R.J. Steinhagen, CERN, Geneva, Switzerland
M.J. Boland, Australian Synchrotron, Clayton, Australia

In any storage ring, the measurement and control of betatron tunes is an integral part of optimising the stability
and lifetime of the stored particle beam. This contribution presents a novel relative beam position measurement
system relying on a direct synchrotron-light detection using ﬁbre-coupled, high-speed Metal-Semiconductor-Metal(MSM) Photodetectors (PD) in a custom-made balanced
RF biasing circuit.
The system will be described along with its ﬁrst results
measuring the tunes for the storage ring at the Australian
Synchrotron. The results are compared to the existing
electro-magnetic BPM-based tune measurements taken.

INTRODUCTION
The Australian Synchrotron storage ring is a 3.0 GeVelectron ring with a circumference of 216 m, situated inMelbourne, Australia [1]. Its nominal characteristics aresummarised in Table 1.
Table 1: Nominal Characteristics of the Australian Synchrotron Storage Ring
CharacteristicEnergy
Circumference
Revolution Frequency
Betatron Tunes
Chromaticity

ValueUnit

3.02161.388(13.291,5.220)(403.9, 305.4)(3.5, 13)

GeVmMHz(Qx , Qy )kHz(ξx , ξy )Tune diagnostics systems, typically integrated into thebeam position monitoring (BPM) or transverse feedback
subsystems, are essential for any synchrotron and are used
to control the actual tune working point to prevent the onset of instabilities that can occur if the fractional tune approaches low-order resonances of the accelerator lattice.
The majority of tune measurement systems require excitation of the otherwise stable beam, and measure the resulting beam motion over time. The digitised position datais typically transformed into the frequency domain, where
any beam movement corresponding to the tune will show
up as a spectral peak. For most systems, a small amountof excitation is required to separate the tune signal fromthe measurement noise, therefore tune measurements often
cannot be performed during user time.

This contribution describes a high-bandwidth, low-noise
BPM utilising visible synchrotron radiation that has beensuccessfully tested at the Australian Synchrotron lightsource. The aim of this system is to utilise the high bandwidth of the system to allow for tune measurements of stable beam.

EXPERIMENTAL SETUP
The synchrotron-light based beam position monitor system (SL-BPM) described in this paper is built on the original Fill Pattern Monitor (FPM), which is based on a single high-speed Metal-Semiconductor-Metal Photodetector(MSM-PD) to measure the electron beam density in realtime [2].
Both systems use the synchrotron light that is emitted
by the electron passing through one of the main bending
magnets, separated from the X-rays using a vertical mirror above the beam axis, focused through a lens, and thenguided into an optical hutch where it is split to be used byeither system. The visible part of the synchrotron radiation was chosen since the system can be implemented using standard optical components such as mirrors and lenses
that also allow for rapid changes in the experimental set-up,
and to anticipate a possible future use of the system in theLarge Hadron Collider (LHC) that produces synchrotron
light with the peak power being in the visible range.
For the nominal SL-BPM system, the light is focussed
onto a set of length-matched optical ﬁbres that are grouped
to a diamond-type structure as illustrated in Figure 1, andeach being individually ﬁbre-coupled to its own MSM-PD.

Figure 1: Fibre diamond geometry in relation to the projected synchrotron-light beam spot size.
The SL-BPM uses the same MSM-PDs as the earlier
FPM [2, 3], that are robust, very low-noise, cost-effectiveand with a typical rise-time of 30 ps being ideally suited forhigh bandwidth measurements. Besides the given model,

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MSM-PDs are widely used in the telecommunication industry and MSM-PDs with bandwidths up to 375 GHz havebeen demonstrated [4].
Rather than placing the detector directly in thesynchrotron-light focal plane, the light is coupled throughﬁbres for two reasons:
1. The active area of the chosen PD is small (200 μm)
compared to the size of the given TO-5 case and cannot be closely packed within the synchrotron lightwithout large dead spaces which the use of ﬁbres
lessens, as illustrated in Figure 1. Also, once the lightis coupled into the optical ﬁbres, the relative locationsof the PDs is unimportant as long as the ﬁbres arematched in length. A further increase in coupling efﬁciency was achieved by removing the PDs’ protective
TO-5 cap and providing a custom ﬁbre-to-MSM-PD
coupling.
2. Since the MSM-PD’s electrical signal should ideallybe processed as close to the PD as possible to preserve
the bandwidths, signal levels and minimise electromagnetic interferences, longer ﬁbres allow to further
separate the ﬁbre-head from the electrical acquisition
system that can then be installed in a convenient placeelsewhere (e.g. in a low-noise, low-radiation Faradaycage).
The initial tests were performed using large core PMMA
plastic optical ﬁbres (diameter about 700 μm). In order
to increase the coupling efﬁciency between the optical ﬁbre and PDs, these were substituted with multi-mode ﬁbreswith a smaller core diameter of about 50 um. The usage ofmulti-mode ﬁbres is considered to be acceptable with respect to the targeted bandwidth (few GHz) and ﬁbre lengthsof less than a metre. Gradient-indexed ﬁbres are being investigated as an option, particularly if longer distances between the focal plane of the synchrotron light and MSMPD circuitry are required.
Since the beam spot is much larger than the active areaof the MSM-PD, small movements of the synchrotron-lightsource causes small variations of the incident photon ﬂux.
Using only a single PD (e.g. ’single pixel’), the measurement is indiscriminate of the plane of beam movement, depends on the actual beam spot size and speciﬁc beam toﬁbre alignment. A more robust position estimate is givenby the pair-wise difference-over-sum ratio of the incident
intensity signal of neighbouring ﬁbres as indicated in Figure 1. A specialised balanced biasing tee circuit has beenconstructed for the PDs to compute the incoherent sum ordifference in the analogue domain before being further processed or directly digitised. The schematic and 3D modelof the balanced bias tee is shown in Figure 2.
The balanced bias tee can operate in two modes;• With one MSM-PD biased to +10 V and one biased
to −10 V, the resulting signal output will correspondto the difference in the light intensity incident on thePDs.





(a) 3D Model(b) SchematicFigure 2: Balanced bias tee conﬁguration. I1 and I2 correspond with the incident photon ﬂux on photodetector 1 and2, respectively.
• With both MSM-PD biased to +10 V the output signal
will correspond to the incoherent sum of the signals oneach PD, allowing for intensity measurements.
The biased PDs can be considered as current-sources,
which can be easily combined and – in comparison to RF
hybrids – allow much higher bandwidths and better linearity. Also, it allows to compensate the common-mode bleedthrough due to off-centre beams or other intensity imbalances in the detector chain by simply varying the DC-bias
voltage of either PD.

TUNE MEASUREMENT
Using this set-up, the tunes for the storage ring at theAustralian Synchrotron have been measured and comparedto the existing tune measurement system. At the Australian
Synchrotron, the data for tune measurements is typicallyacquired using turn-by-turn data from the storage ring button BPMs while the beam is excited by a combination offour injection kicker magnets [5].
A comparison between the traditional button BPM-based
system and SL-BPM system is shown in Figure 3. Whilefor most measurements the output has been directly digitised using a fast sampling oscilloscope, the signal forthis speciﬁc measurement has been post-processed anddown-mixed using a simpliﬁed version of a Base-Band
Tune (BBQ) RF schottky diode detector (as described inRef. [6]). This signiﬁcantly reduced the requirements interms of bandwidth and resolution, as the resulting signal
wasbeing processed un-ampliﬁed and with a low-resolution(≈10 bit) oscilloscope.
Clear tune peaks at (Qx , Qy ) = (0.273, 0.227) canbe seen. The high vertical chromaticity visibly enhances
the synchrotron side-bands around the tunes, which is apparent in both sets of data. The datasets show a similar
signal-to-noise ratio for both the existing turn-by-turn ringBPM based system and the tested SL-BPM. This indicatesthat the sensitivity of the down-mixed SL-BPM is at least
equivalent to standard method. Since the MSM-PD’s darkcurrent is an order of magnitude smaller than the measured
signal levels, it is believed that the measured noise ﬂoor is
likely due to the digitisation noise of the acquisition system.

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Proceedings of IBIC2012, Tsukuba, JapanTUPB84

1.5
Turn−by−Turn Vertical
FPM25

10

Tunes: (0.29,0.22)
Tunes: (0.32,0.26)

0.5

4

100

Magnitude

Amplitude (arb. units.)

1−0.5

3

10

−1−1.5

00.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2Time (ms)(a) Turn-by-turn position data, taken simultaneously with the standard BPMs and the SL-BPM system.
Turn−by−Turn Vertical
FPM2

50
45

35

1

1.1

1.2

1.3

1.4

1.5

1.6

Frequency (frev)

1.7

1.8

1.92Figure 4: Two tune measurements utilising the SL-BPM
BPM system. Between the two measurements the storage
ring tunes shifted from (Qx , Qy ) = (0.29,0.22) to (0.32,0.26)

3025
2015
10
500

0.05

0.1

0.15

0.2

0.3

0.25

Frequency (f )

0.35

0.4

0.45

0.5

rev(b) The position data transformed into the frequency domain. Highchromaticity induces a splitting of the vertical tune lines.

Figure 3: A comparison between the optical tune measurement system and the existing turn-by-turn BPM system. Clear agreement between the different methods isevident. The fractional tunes for this measurement are(Qx , Qy ) = (0.273, 0.227)
Figure 4 is an overlay of two sets of data taken with theprototype SL-BPM system, utilising the balanced bias tee
conﬁguration. In this experiment, the storage ring tunes
were altered from (Qx , Qy ) = (0.29,0.22) to (Qx , Qy ) =(0.32, 0.26). Both sets of tunes lines have a high signal-tonoise ratio and were observed to move to the correct location after the tune alteration.

the achievable signal-to-noise ratio of the MSM-PD.
A future goal of this project is to leverage the high bandwidth position data to diagnose higher-order head-tail instabilities at the LHC. Head-tail instabilities occur whenthe motion of leading edge of a bunch is coupled to thetail and can lead to an increase in beam size or total beamloss. By coupling the MSM-PDs to a suitably high bandwidth data acquisition chain, the position of multiple slicesof a single bunch can be obtained and monitored over manyturns.
It is also anticipated that the system will ﬁnd applicationin next-generation electron accelerators such as CLIC andwill be tested at CTF3 in the near future.

REFERENCES[1] G.S. LeBlanc, M.J. Boland, and Y.-R.E. Tan, The Australian Synchrotron Project Storage Ring and Injection System Overview, EPAC 2004
[2] D.J. Peake, M.J. Boland, G. LeBlanc and R. Rassool, Measurement of the real time ﬁll-pattern at the Australian Synchrotron, Nucl. Instr. and Meth. A 589 (2008) p. 143[3] Hamamatsu, “Ultrafast MSM Photodetectors G4176 Series(GaAs), technical speciﬁcaton, http://www.hamamatsu.com

CONCLUSIONS AND FUTURE WORK

[4] Chen et al.,“375GHz-Bandwidth Photoconductive Detector”,
Appl. Phys. Lett. 59 (16), 14 October, 1991, pp. 1984

Preliminary results from the prototype optical beam position measurement device described above are promising.
The system has demonstrated the ability to track the temporal evolution of the tune and is in good agreement with theexisting BPM diagnostics. Future improvements will include a more reﬁned post-processing and higher-resolution
digitisation of the available analogue signals to fully exploit[5] M. Spencer, M. J. Boland, R. Dowd, G. S. LeBlanc and Y.R. E. Tan, Tune and Lifetime Studies at the Australian Synchrotron, APAC 2007
[6] M. Gasior and R. Jones, The principle and ﬁrst results of betatron tune measurement by direct diode detection, CERNLHC-Project-Report-853, 2005

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