Proceedings of IBIC2012, Tsukuba, Japan

TUPA46STREAK CAMERA MEASUREMENTS AT ALBA:BUNCH LENGTH AND ENERGY MATCHING
U. Iriso and F. Fernández, CELLS, Cerdanyola, Spain
EXPERIMENTAL SET-UP

This report describes the electron beam longitudinalstudies performed at ALBA Storage Ring using the streak
camera. We first show the usual studies involving precise
bunch length measurements and related beam parameterslike energy spread. Next, the studies to match the injectedbeam in energy and phase are reported and compared withsimulations.

INTRODUCTION
Since the beginning of the ALBA Storage Ring commissioning in March 2011, a beam diagnostics beamline(BL34) is operational. It uses the visible part of the synchrotron radiation and is mainly devoted for longitudinalbeam studies using the Streak Camera (SC), an Optronis SC-10 model, with synchroscan frequency working at
250 MHz to distinguish the beam bunches spaced by 2 ns.
The SC converts the incoming photons into electrons,
which are swept to transpose its longitudinal (time) structure into a transverse footprint. The sweeps are performed
by a fast and a slow unit. We always use the fast sweep
in the “synchroscan mode”, which sweeps continuously asinusoidal field of 250 MHz at different amplitudes, allowing sweep speeds of 15, 25 and 50 ps/mm. The slow unit
provides sweep speeds from 660 ps to 5 ms/mm, triggeredusually at 50 Hz and synchronized with both the storage
ring (SR) revolution frequency and the injection repetition
rate (3 Hz). The working principles of SC are thoroughly
explained in Ref. [1].
Table 1: ALBA Storage Ring Main Parameters
Parameter

Value

energy, E [GeV]
hor emittance,  x [nm-rad]revolution time, T [ns]harmonic number, hrf freq., frf [MHz]dipole field, B [T]synchrotron freq., f s [kHz]rf voltage (max), V[MV]

3.0
4.6
896
448
499.6
1.42
6.5
3.6

In this report, we first describe the SC characterizationand its sweep speed calibration. Next, the bunch length
measurements of the stored beam are reported including theenergy spread measurements. Finally, we show measurements of the injected beam, including booster bunch lengthand longitudinal damping time. Table 1 lists the ALBA
Storage Ring (SR) parameters relevant for these studies.

The light used at BL34 is extracted from the first bending after the injection section. At 8.5 m from the source, welocate an in-vacuum mirror (VMIR) whose purpose is to reflect only the visible part of the synchrotron radiation spectrum. We do so by moving up the mirror (to about 2 mrad),and so avoiding the x-ray part of the spectrum circulatingin the orbit plane – see Fig. 1. The light is then directed
perpendicular towards the other side of the shielding wall,
already in atmospheric pressure. At this point (and at 9 mfrom the source), we locate a 4m lens in order to focalize
the light into the experimental set-up. Next, 6 conventional
mirrors direct the light through the shielding wall performing a vertical chicane to prevent radiation outside the SR
tunnel. Finally, the light arrives to the experimental setup
in BL34. The light optical path and optical elements areshown in Fig. 2, and more details are explained in Ref. [2].

Figure 1: Sketch of the in-vacuum mirror (VMIR) andholder. The mirror is kept up around 2 mrad to avoid
the x-ray part of the synchrotron radiation. The mirror isequipped with 3 thermocouples (not shown in the sketch)
to prevent damages induced by possible overheating.

STREAK CAMERACHARACTERIZATION
The “synchroscan sweep” is a sinusoidal electric field of
250MHz whose amplitude is varied to allow three different
deflection speeds: 15, 25, and 50 ps/mm. The sweep is synchronized with the machine rf system working at 500 MHz.
In order to crosscheck the linearity of these sweeps, we
compared the bunch phase wrt sweep phase controlled by
the SC. While the non-linearity is negligible for the 15 and25 ps/mm speeds, this is not the case for the slowest verticalsweep: 50ps/mm (see Fig. 3). The total range swept withthis speed corresponds to 990ps, which for the 250 MHz

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

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Proceedings of IBIC2012, Tsukuba, Japan

Figure 2: Optical path from the source point to the SC.
Note the position of the lens ( f =4 m), and the iris, that
limits the incoming light aperture at the SC.
Figure 4: Calibration set-up.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○synchroscan frequency corresponds to about ±22o in phase.
In order to guarantee the linearity along the whole ramp, a
multi-linear calibration is applied.

troids distance for both even and odd bunches. We consider
an experimental error bar of about 1% in the centroids measurement. The result shows that 1 mm at the optical tablecorresponds to 3.67 ps ±6%, which is about 10% larger
than the one corresponding to the speed of light (3.33 ps).
This is about a 10% larger than the theoretical value, but
still acceptable if we consider the experimental error bars.

Figure 3: Linearity using the 50ps/mm sweep speed.
We also crosscheck the manufacturer time-to-space conversion factor, for which we prepared the calibration set-up
shown in Fig. 4. The synchrotron light arrives at an optical splitter that divides the light into two pulses A and B
going in two perpendicular branches of (initially) the samelength. Each of these pulses finds a full mirror that reflects
the light and sends it back to the splitter, where they are
merged together and sent to the SC. By moving one of themirrors a distance ∆x from the initial length, we can see thetwo pulses in the SC separated by a distance 2·∆x. The distance ∆x is controlled with a precise micrometer, and thencrosschecked with the SC result.
Because the SC frequency is 250 MHz, we sweep the
odd bunches using the upward part of the sinewave (at 0ophase), and the even bunches with the downward (at 180o ).
This is why the SC images show the odd bunches in theupper half image, and the even bunches in the bottom. The
image of the calibration method is shown in Fig. 5. The
distance between lobes ∆x is obtained by inferring the cen-

Figure 5: Images of lobes A and B for odd (top) and even(bottom) bunches. The image correspond to an almost full
SR revolution. The filling pattern is composed by 8 trainsof 112 ns, each train consisting in 45 consecutive bunches
followed by 22 ns of gap.

STORED BEAM MEASUREMENTS
The SC settings need to be well tuned in order to perform
precise bunch length measurements. First, we see enlargements of about 25% if the focusing lenses are not perfectly
tuned, which we did with the help of the manufacturer [3].
The most significant effect is due to the transverse beamspot, which produces bunch length enlargements of about
50% (even by closing the SC slit). We control this beam
spot with a CCD camera located next to the SC. The lightISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

TUPA46

follows the relationsσz =2παC Eσ2E
hV cos φ s ω2rev,(1)

where αc is the momentum compaction factor (given by themachine optics), E is the beam energy, h is the harmonic
number, φ s is the synchronous phase and ωrev is the angular SR revolution frequency. Both measured and modelledvalues are shown in Fig. 9, where the measurement errorbar
considers the overestimation measured in Fig. 6.
Figure 6: Linear fit to the distance between lobes for different micrometer position. The linear term provides thecalibration in terms of ps/mm.

Figure 7: Transverse beam spot arriving to the SC as seenby the CCD (left) and SC itself (right). The multiple spots
are suspected to be due to multiple reflections at the 6 conventional mirrors. Precise bunch length measurements are
performed if we collimate the spot using the iris.

Figure 8 shows a typical example of the bunch length
measurements at ALBA. The filling pattern at SR is madeof 8 trains spaced by 11 buckets (22 ns), each train containing 45 consecutive bunches. The image at Fig. 8 (left)
corresponds to an almost full SR turn (896 ns), where wecan see almost completely the 8 trains. As usual in thestreak camera, the odd bunches are at the image top, thebottom bunches are at the image bottom. The bunch lengthis obtained after the vertical projection of the selected ROI.
As seen in Fig. 8 (right), the projection follows a Gaussian distribution, from which we obtain a beam size of
FWHM=52 ps (σz = 22.1ps).
Figure 9 shows the evolution of the bunch length σz byvarying the rf voltage V, which we can see it consistently

Figure 8: Example of a bunch length measurement.

Figure 9: Measured bunch length wrt rf voltage.
A proper bunch length measurement allow the calculation of the energy spread σE , following:σz =αC· σE ,
2π f s(2)where f s is the synchrotron frequency, controlled with theSpectrum Analyzer after varying the rf voltage. The linear
relation obtained is shown in Fig. 10, with a σE =0.83%,
consistent with the expected values.

INJECTED BEAM MEASUREMENTS
In order to perform longitudinal studies at injection, we
first “unclose” the injection bump to dump the beam circulating in the machine and keep the injected beam (which arrives after the 3 Hz repetition rate from the injectors). This

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○is directed to both devices using a light splitter. Figure 7
shows the image arriving at the SC as seen by a CCD camera (left), and as seen by the SC itself (right) in focusing
mode (no sweeps). Precise bunch length measurements are
only performed if we collimate the beam spot using the iris
located in front of the SC (see Fig. 2). We suspect the reason for the peculiar shape of this spot stems in the use of
conventional mirrors, whose reflection is produced by thefront surface, but they can also add residual reflections at
the back surface. We are currently investigating this effect.

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Proceedings of IBIC2012, Tsukuba, Japan

Figure 10: Energy spread measurement.

allows to keep the SR intensity ∼0.2 mA and so to workwith the same SC settings. In these conditions, we checkthe longitudinal matching between the Booster and SR bymeasuring and correcting the phase and energy offsets, aswell as measure the injected bunch length and dampingtime.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Phase and Energy Match
Since the radiation damping eventually damps the injected beam to the natural SR bunch length and energy
spread, the details of the longitudinal dynamics (like energy and phase match) are usually ignored as long as there
is an adequate capture of the injected beam. However, inview of the future top-up operation at ALBA, we started acampaign to properly match the energy and phase from the
Booster to the SR.
From the settings used in normal operation, the simplest
thing is to check the rf phase. This is done by comparing
the phase of the injected beam wrt the stored beam. Figure 11 shows the images of the beam before (left) and after(right) the phase matching. After injection (at ∼10µs), theleft hand side pictures shows a phase jump of 139ps, corresponding to a phase offset of 25o . After applying this phaseshift to the RF system, we can see the injected beam phaseis well matched with the stored beam phase, and the oscillation starting at ∼10µs are due to injection energy mismatch,which we correct next.
To correct the energy mismatch, we prepare the Booster
to extract at three different energies: the setting used during
normal operation (which we call hereafter “Reference”),
and at ±1% of the nominal energy. We do so by editing
the Booster magnets ramp curves. Because the SC images
the odd and even bunches at different places, we measurethen the oscillations of the odd and even bunches separately. This is then the input for a tracking simulation,
which allows to identify the energy offset corresponding
to the phase difference. The results are shown in Table 2.
We conclude then that we were injecting with an energy
offset of -18 MeV (or -0.68% in energy), and so we adjust
the Booster settings to ramp at an energy of +0.68%. After several iterations with small adjustments in energy and

Figure 11: Streak images before/after rf phase matching.
Note the time goes from right to left, and injection occurs
at ∼10µs. The SC phase has been matched to superimpose the odd and even bunches at the same phase, but when
the injection occurs, the injected odd bunches go upwards,
injected even bunches go downwards, and the remaining
stored beam remains at the same phase.

phase, the image of the injected beam is shown in Fig. 12.
It corresponds to an energy difference of 1.5 MeV and aphase difference of 10 ps (1.8o ). Note the (small) centroid
oscillations wrt Fig. 11. The centroid and bunch length oscillations are explained in the next section. The phase spacetracking simulations corresponding to the different energies
tried in this exercise are shown in Fig. 13.
Table 2: Top/bottom oscillation at the SC for three different
Booster ramps, and corresponding energy difference after
the tracking simulations.
BO ramps
Reference -1%
Reference
Reference +1%

top/bottom osc.

∆E

353/262 ps
127/111 ps
62/72 ps

-43 MeV-18 MeV+10 MeV

Injection Dynamics and SR Damping Time
Once we tune the injection to the right energy, we could
analyze the beam coming from the Booster, which allows
us to study the longitudinal damping time and the injection
dynamics.
In Fig. 12, we see that the (small) centroid oscillation period coincides with the synchrotron period, while the bunch
length oscillates at half this period, as it is expected because the bunch length performs two cycles for each centroid turn. Note the bunch length oscillations has different minima and maxima at different cycles, due to the nonsymmetric shape of the phase space trajectory (see Fig. 13).
Following the injection, the initial beam distribution
from the Booster begins to filament until it eventuallydamps to the natural distribution of the SR [4, 5]. Since
our theoretical damping time is τ =3 ms, we record the in-ISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

Figure 14: Streak image used to infer the longitudinal
damping time (top), and evolution of centroid (middle)
and bunch length (bottom). Injection occurs at about t =2.9 ms. The bunch length is fitted to an exponential decay, the damping time obtained from the fitting results isτ =3.2 ms.

from the Booster, which is used to obtain an insight viewof the injection dynamics.

ACKNOWLEDGMENTS
We are thankful to the RF and Operation teams for their
help with the experiments involving the RF and Booster
ramps. We appreciate the help and support from P. Summ
(Optronis) with the Streak Camera.
Figure 13: Matching simulations in phase space.
jection transient over 14 ms. From fitting the bunch length
decay during this time we can infer the damping time – see
Fig. 14. In this case, the bunch length oscillation observedin Fig. 12 cannot be resolved because the bunch length performs one oscillation over half the synchrotron period (notethe different horizontal time scale at both pictures). The fitting result shows τ = 3.2 ms, consistent with the theoretical
expectations [6].

CONCLUSIONSThe SC at ALBA has been operational since 1 year ago.
In this time, we performed several studies to characterize
the SC, with the aim of ensure the linearity and crosscheckthe time to space conversion factor. All this allows us toperform precise bunch length measurements and related
machine parameters like energy spread. Furthermore, we
use the SC to match in energy and phase the beam coming

REFERENCES[1] K. Scheidt, Review of Streak Cameras for Accelerators: features, applications and results. Proc. of EPAC 2000.
[2] F. Fernandez and U. Iriso, Visible Synchrotron Radiation
Front End and Beamline for Longitudinal Beam Diagnostics.
ALBA Internal Note, AAD-FE-DI-VMIR-01 (2011).
[3] www.optronis.com[4] J. Byrd and S. De Santis, Longitudinal injection transientsin an electron storage ring. LNBL Note, 2001. http://
escholarship.org/uc/item/8k6677nw[5] H. Moshammer, Application of the Fokker-Planck Equation
to particle beams injected into e- Storage Rings, SLAC-PUB5825, 1992.
[6] M. Munoz and D. Einfeld, Optics for the ALBA Lattice, Proc.
of PAC’05.

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 12: Injection after matching the energy and phase.
Note the centroid oscillations are in the order of 10ps (middle plot), and it oscillates at the synchrotron frequency (period of ∼150µs), while the bunch length (bottom plot) oscillates at twice the synchrotron frequency (period of ∼75µs).
In this case, the injection occurs at about t = 200µs.

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