Proceedings of IBIC2012, Tsukuba, Japan

TUCC01

ELECTRON STORAGE RING AS A SINGLE SHOT LINAC BEAM
MONITOR*
Y. Shoji#, K. Takeda, University of Hyogo, Ako 678-1205, Japan
Y. Minagawa, Y. Takemura, S. Suzuki, T. Asaka, JASRI, Sayo 679-5198, JapanThe SPring-8 linac has been used as an injector to theelectron storage ring, NewSUBARU. There exists a shotto-shot fluctuation in the injection efficiency during topup operation. In order to understand the source of thefluctuation, we have developed single shot diagnostics of
the injected beam using the visible light beam line of thering. The techniques utilized the phase rotation ofbetatron or synchrotron oscillation in the ring. The timeresolving visible light monitor in the ring, a streak cameraor ICCD gated camera, records the profiles of the injectedlinac beam over multiple revolutions. The time profile
recorded at the point of injection reveals the bunch
structure in the 1 ns macro pulse, which contains threelinac bunches. The time profile at after 1/4 of thesynchrotron oscillation period gives the energy profile ofa pulse. The bunch-by-bunch spatial profiles overconsecutive several revolutions after the injection can beused to reconstruct the transverse emittance of each bunch.

INTRODUCTION
The electron storage ring NewSUBARU [1] has used
the SPring-8 linac [2] as an injector for top-up operation
since 1998. However, fine parameter tuning is required
for stable injection because of the small ring acceptance.
For that process, single shot linac beam monitors werenecessary to measure beam parameters with the injection
efficiency. Although the feedback control of steering
magnets using BPMs along the transport line [3]improved the stability, the injection efficiency still
fluctuates due to the injected linac beam. We havedeveloped single shot diagnostics utilizing the phaserotation in the ring, providing an opportunity tounderstand fluctuations in the injection efficiency.
We used the time resolving visible light profile
monitors of the electron storage ring. The time profile atthe instance of the injection, recorded by a streak camera,
gives the bunch structure in a pulse. The time profile after
1/4 of the synchrotron oscillation period gives the energy
profile. The dual sweep streak camera can also be used torecord the spatial profile of several turns. The fast sweep
separated linac bunches in a macro pulse and the slow
sweep separated profiles from different turn numbers.
The betatron oscillation in the ring produced a phaserotation allowing the reconstruction of the beamemittance. Here the beam emittance means not only thearea but the ellipse in the transverse phase space.
The stability of the linac beam was evaluated from the____________________________________________*Work supported by "comprehensive support program for thepromotion of accelerator science and technology" by KEK.
# shoji@lasti.u-hyogo.ac.jp

shot-to-shot variation. We could also see differences ofbeam parameters in the bunches of the same macro pulse.
The measurements detailed here required dedicatedtime at the facility because the stored beam should notexist in the ring. This meant that storage of the injectedbeam was not necessary. The ring parameters, such as thebeta functions, betatron tunes, etc. could be fully
optimized for the measurement. This also means that abooster synchrotron of any facility can be used as amonitor if it has a light extraction port.
In addition to the two aforementioned monitors, wewill discuss the possibility of using an ICCD gatedcamera, capable of recording several spatial profiles,
allowing analysis of the H/V coupling of the injected
beam.

PARAMETERS OF DAILY OPERATION
Averaged Beam Parameters
Figure 1 shows the layout of the SPring-8 linac, thebooster synchrotron, and NewSUBARU storage ring.
Table 1 shows the main parameters for the linac.

Figure 1: Layout of the 1 GeV SPring-8 linac, the booster
synchrotron, and the NewSUBARU storage ring.
Table 1: Main Parameters of the Linac
Electron energy

1 GeV

Rf frequency

2856 MHz

Common pulse rate

1 Hz

Common pulse width

1 ns

FWHM Bunch length; front/middle

10 ps / 14ps

Full energy spread; front / middle

0.4% / 0.6%

2

Transverse emittance (FWHM )100  nmrad.

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

TUCC01

Proceedings of IBIC2012, Tsukuba, JapanThe macro pulse width of the linac beam is normally 1
ns, which enables a single rf bucket injection of the ring,covering three rf buckets of the linac [4]. The 1 ns beam
is obtained by applying 200V rectangular pulse to the gridof the thermionic electron gun.
The listed transverse emittance in Table 1 wascalculated from the average over many shots, measuredby Q-scanning at the beam transport line. The
longitudinal parameters were obtained from themeasurements of the synchrotron oscillation in the ring
just after the injection [5]. The listed bunch length and theenergy spread were obtained from the average of 10 shots.

six typical bunch structures. They are grouped and named
A-F for convenience. The vertical axis in the images is
the ring rf phase and the horizontal axis shows the
progressively increasing number of revolutions. The time
structure at the injection imprints a bunch structure in apulse. The time profile at 1/4 of the synchrotron
oscillation period (41 μs) gives the energy profile.
Figure 4 shows the bunch structure of each group andthe corresponding energy profile of the middle bunch.
The energy profile had two peaks, separated by 0.3-0.4%.
These two peaks were intentionally produced at thebunching process using two bunchers [6].

Setup of the Visible Light Beam Line
F
E
D
C
B
A

rear

middle400

300

energy profile(middle bunch)contributionof the front
is added

Intensity (arbit. unit)
500

contribution of the front

bunch structure

front

Figure 2 shows the schematic layout near the injection
point and the monitor line, named SR5. The dove prismon the line switches transverse measurement axis for the
streak camera, in this case we selected the vertical with90 degrees rotation.
The Twiss parameters at the light source point are x= 0.7 m, y = 17 m and the dispersion function is  =0.052 m. The longitudinal and transverse radiation
damping time are 12 ms and 22 ms, respectively.

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

200

100

-0.4

Figure 2: Layout of the visible light monitor line SR5.

CORRELATION OF INJECTIONEFFICIENCY WITH BUNCHSTRUCTURE
Bunch Structure and Injection Efficiency

-0.2

0
0.2
time (ns)

0.4

0
-1

-0.5
0
energy displacement (%)

0.5

Figure 4: Bunch structure of three bunches and thecorresponding energy profile of the middle bunch. The
part of the energy profile, that contains a contribution
from another bunch was plotted with a dotted line. The
labels A-F corresponds to the images of Figure 3.
The injection phase of a bunch was stable with a small
jittering of the synchronization system between the 2856
MHz linac rf and the 500 MHz ring rf [7]. On the other
hand the timing jitter of the gate pulse was roughly 0.3 ns,
much larger than the expected (30 ps FWHM [8]). The
actual gate pulse length was thought to be 0.7 ns or less.

The bunch structure was fluctuated by a jittering of
the gate pulse timing of the electron gun. Figure 3 shows
six typical dual sweep images of the streak camera with

Figure 3: Typical dual sweep images. Because of the non-linear momentum compaction factor, the movement of thefront bunch is slower at the initial 50 μs.
ISBN 978-3-95450-119-9
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Overview and Commissioning of Facilities

Proceedings of IBIC2012, Tsukuba, Japan

TUCC01Figure 5 shows the movement through 1/4 of thesynchrotron oscillation period for images A and F. It
explains the non-linear movement in Figure 3. The rf
bucket model was based on the measurement of nonlinear chromaticity of synchrotron tune [9]. This showedthat the acceptance of the rf bucket was large enough forthe linac beam. Another experimental survey about thelongitudinal ring acceptance was consistent with themodel.
Figure 6 shows the injection efficiency of 35 shotswith the ring parameters characteristic of daily operation.
Shots shown in images A and B had a comparatively
worse injection efficiency. These two results suggested
that a dependence of transverse parameters caused
fluctuations in the injection efficiency.
Injection Efficiency (%)

95

Data Analysis of a Typical Shot

90

85
A
B
C
D
E
F

80

75
0.22

parameters because it was easier than that to measure
horizontal parameters.
The linear ring lattice was the same as those of daily
operation because the vertical beta function was large at
the light source point. Some other parameters weredifferent from those of daily operation during themeasurement. The injection pulse bump was optimized sothat the injected beam had a small dipole oscillation andmaintained almost 100% injection efficiency. The verticalchromaticity was reduced to 0 in order to eliminate thechromatic betatron phase spread over multiple revolutions.
The linear H/V coupling was reduced to less than 1%.
The very small beam size of the stored beam was used tocalibrate the vertical spatial resolution. It was 0.3 mm
FWHM at the light source point with y = 17 m.

0.24

0.26

0.28

0.3

0.32

Pulse Charge (nC)

Figure 6: Injection efficiency for different bunch structurelabeled A-F in Figure 3.
Here, we will mention that we could have blocked thelight from the other bunches at after 41 μs, if we needed
accurate energy profile. Setting an appropriate mask at apoint of the light source image could have blocked themusing a displacement by 0.5% energy difference, althoughsacrificing the time profile at the injection timing.

BUNCH-BY-BUNCH EMITTAMNCEMEASUREMENT

Here we demonstrate our method of analysis using a
representative shot (#12, image B) as an example. Its total
charge, measured by fast CT on the beam transport line,
was 0.23 nC.
Figure 7 shows the dual sweep image of the streak
camera frame and the projection to the fast sweep axis.
The vertical axis is the horizontal position in space andthe ring rf phase in time. The upper part of the figure
corresponds to the head of the linac pulse with the tail inthe lower part of the figure. The horizontal axis is thevertical position in space and the ring revolutions in time.
The six spots correspnd to profiles from the 4th through9th turns after the injection. Profiles at the initial 3 turns
were out of the camera slit because of a large horizontal
displacement. The horizontal profile of each spot in theimage corresponds to the vertical spatial profile in thering.
Figure 8 shows the profiles of the 6 turns of the two
respective bunches. They were fitted with Gaussianfunctions. We derived the peak position y and adivergence 2 as a function of the revolution number N.

Setup of the Ring
We wanted a bunch-by-bunch transverse parametermeasurement. As the first step, we measured verticalISBN 978-3-95450-119-9
Overview and Commissioning of Facilities3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○(A)(F)
Figure 5: Calculated movement of bunches in the ring rf bucket for the images A and F of Figure 3. The black solid lineis the separatrix. The red spots show the bunches at the injection and the blue spots at after 41 μs. The origin of the time
axis is the synchronous phase

TUCC01

Proceedings of IBIC2012, Tsukuba, Japan-[ y' +(y/y) y ]/y (E-3)

The beam ellipse at the injection point was calculatedusing the phase advance to the light source point, 0.15 rad.
Figure 10 shows the calculated beam ellipses at the
injection point.

Figure 7: Dual Sweep image of the injected beam of theshot #12.
100

80

0.1

middle

front

0
-0.1
-0.2
-0.2

-0.1

0

0.1

Figure 10: The calculated beam ellipses in the normalized
phase space at the injection point.

Shot-to-Shot Fluctuation
We recorded the profiles of 20 shots in order to see
the shot-to-shot fluctuation. Figure 11 shows the bunchcharge for the 20 shots. They are grouped A-E, according
to the charge of the front bunch.

60

40

bunch charge (nC)

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

front
20

0

0.2

y // y (E-3)

middle

front

9th
8th
7th
6th
5th
4th

0.2

-2

-1

0

y (mm)

1

2 -2

-1

0

y (mm)

1

2

Figure 8: Vertical beam profiles from turns 4 through 9 of
the front bunch and the middle bunch.
As shown in Figure 9, the data were fitted with thefollowing sinusoidal functions(1)
y = A0 + AS sin(2 y N) + AC cos(2 y N)(2)

 2 = B0 + BS sin(4 y N) + BC cos(4 y N)

Here y =0.224 is the sub-integer part of the verticalbetatron tune. The dipole oscillations of two bunches
were obviously different. The oscillations of the widths
were almost the same except for one data point, the 6thturn of the middle bunch.
y (mm)

2

rear
A
B
C

0.1

D

0.01

E
0.001

0

5

10

shot#

15

20

Figure 11: Bunch charge for the highlighted 20 shots.
The parameters quantifying the dipole oscillation, AC
and AS, of the middle bunch are plotted in Figure 12. The
fluctuation was correlated with the group and took placeat the phase AS/AC = 1.3. We added data points of thefront bunch of groups A–C to the plot. The differences inthe same pulse were roughly of the order of thefluctuation in phase.
As (mm)
0.3

2

 ( mm )0.3

middle

0.2

0.3

0.1

0.2

0.25

0.1

0.2

0

0.15

-0.1

0.1

-0.2
-0.3
3

4

5

6

7

turn

0.05
front
middle
0
8
9 10
3

0

middleA -0.1B
C -0.2
D
E
-0.2

4

5

6

7

turn

8

9

10

Figure 9: The dipole oscillation of the peak position (left)
and the quadrupole oscillation of the divergence (right).
The dotted line on the right indicates the spatial resolution.

-0.1

0

0.1

Ac (mm)

0.2

0.3

0.4

-0.3
-0.2 -0.1

00.1

Ac (mm)

A
B
C
D

front
E
A
B
C

0.2

0.3

0.4

Figure 12: Shot-to-shot fluctuation of the dipole
oscillation of the middle bunch (left) and those of thefront bunch (right). The oscillations of different bunches
in the same shot are connected with lines.

ISBN 978-3-95450-119-9
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Overview and Commissioning of Facilities

Proceedings of IBIC2012, Tsukuba, Japan2

BS ( mm )0.2

middle
A
B
C
D
E

front
A
B
C
D
E
A
B
C

0.15

0.1
0.05

-0.2

-0.15

-0.1

2

-0.05

BC (mm )

0

0
-0.2

-0.15

-0.1

2

-0.05

0

BC (mm )Figure 13: Shot-to-shot fluctuation of the quadrupole
oscillation parameters of the middle bunch (left) andthose of the front bunch (right).

TRANSVERSE EMITTANCEAt the present, the ICCD gated camera is used tomonitor the small transverse oscillation and confirm theidentification of the number of revolutions. However it
has a potential to reconstruct a transverse fourdimensional profile and identify the H/V coupling of thelinac beam. Figure 14 shows an example of single shot
profiles from turns two through six in the frame of the
ICCD gated camera.
In this case the dipole oscillation was adjusted to asmall amplitude. However, an intentional dipole
oscillation, both in horizontal and vertical directions,would separate profiles at defferent turns. Too small
horizontal beta function at the light source point is theother setting parameters, which should be changed to
improve the accuracy of horizontal beam parameters.

Figure 14: Injected beam profiles from turns two throughsix taken from the ICCD gated camera.

also depended on the bunch structure. The bunch-bybunch emittance measurement showed a difference inbeam parameters for different bunches.
Our measurement showed that the energy stabilization
using ECS [8], which certificate the energy stability of
0.02%, is not enough. The improvement of the timing
jitter of the grid pulser was another important factor of
the beam energy. We are considering to add re-trigger
using the master clock for the improvement.
In order to have much injection tolerance, the linac
beam energy was raised by 0.4% after the measurements.
We are preparing system for the independent tuning of
the beam energies, that for the booster and that for
NewSUBARU.
The data at our disposal from the new diagnostics
provide an opportunity to address and solve problems in asystematic way. However, there still exists scope for
improvement. Our research is still underway for more
improvement based on better understanding of theinjection process.

ACKNOWLEDGMENT
We thank members of the SPring-8 linac group formany suggestions on the measurements and discussions
on our results.
The visible light beam line SR5 was constructed under
the supervision of Prof. Mitsuhashi of KEK. He aided
with the basic design of the beryllium mirror, themeasurement of its deformation by synchrotron radiation
heat using a Hartmann mask, as well as the elimination ofmonitor halo from edge scattering.

REFERENCES[1] A. Ando, et al., Jour. Synchro. Rad. 5 (1998) 342.
[2] S. Suzuki, et al., "Recent Improvements in SPring-8
Linac for Early Recovery from Beam Interruption",
LINAC'12.
[3] K. Yanagida, et al., "Bean Instrumentation Using
BPM System of the SPring-8 Linac", Proc.
LINAC'04.
[4] T. Asaka et al., "Performance of Electron Gun for
SPring-8 Linac", Proc. PAC'97.
[5] T. Matsubara, et al., PR ST-AB 9, 042801 (2006).
[6] A. Mizuno, linac group of SPring-8, private
communication.
[7] Y. Kawashima, et al., PR ST-AB 4, 082001 (2001).
[8] H. Hanaki, et al., "Beam Stabilization in the SPring-8
Linac", Proc. APAC'04.
[9] Y. Shoji, "Estimation of Non-Linear RF Bucket of
NewSUBARU", Proc. SAST'03,
http://conference.kek.jp/sast03it/WebPDF/2P040.pdf.

SUMMARY AND DISCUSSION
We have presented some single shot data from theSPring-8 linac obtained at NewSUBARU. The shot-toshot fluctuation in the injection efficiency was related tothe bunch structure, originating from the time jittering of
the gate pulse. The energy profile and the vertical orbit

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The parameters for the quadrupole oscillation, BC andBS, of the middle bunch are plotted in Figure 13. No clear
dependence on the group was observed. However, it
seems to be no accident that the difference of two
bunches satisfied 2BS  BC > 0 for all 8 shots.

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