The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

MEASUREMENT OF THE BEAM BUNCH LENGTH IN THE COMPACTSUPERCONDUCTING STORAGE RING “AURORA” USING AN
AVALANCHE PHOTODIODE
Y. Yamamoto, I. Sakai*, K. Yadomi**, T. Hori* and H. Iwasaki,

Synchrotron Radiation Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan*Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hirosima,Hirosima 739-8526, Japan**Engineering Dept. Osaka University RCNP representative SHI Accelerator Service Ltd.,Ibaraki, Osaka 567-0047, Japan
AbstractThe beam bunch length in the compact superconducting
storage ring AURORA at Ritsumeikan University wasmeasured using the avalanche photodiode. The radiation
beam from the bending magnet was detected by theavalanche photodiode and a frequency spectrum wasrecorded using a spectrum analyzer. To compare thebunch length obtained by the present method, the bunch
length was measured by streak camera. The bunch lengths
that determined by the present method agree with thestreak camera method.

INTRODUCTION
The beam bunch length in an electron storage ring is an
important parameter for beam dynamics and there areseveral methods for its measurement. The most frequently
adopted is the one employing a streak camera in which an
instantaneous image is taken of the bunch running in theelectron orbit. We present in this paper an alternative
method, in which the frequency spectrum of radiation
emitted from the storage ring is measured using anavalanche photodiode and the bunch length is derived by
Fourier transformation of the frequency spectrum. It has
been shown that the method yields the value in substantial
agreement with that measured using the streak camera.

SREXPERIMENTAL PROCEDURES
Measurement was made at the compact electron storage
ring AURORA at the Synchrotron Radiation Center at
Ritsumeikan University. Details in the structure andperformance of AURORA are found in [1,2]. Figure 1
shows schematically the layout of the measurement
system constructed at the beam extraction port BL-9 [3,4].
The first mirror that is made of beryllium and cooled bywater reflects the visible SR lights. The quartz window
seals in vacuum of storage ring and passes the lights. Theradiation detector employed was an avalanche photodiodemodule APDM (MATSUSADA R263VF), the sizes ofthe detector being 55 mm (Width) × 45 mm (Height) × 90mm (Depth). Since the avalanche photodiode is a detector
with a very quick response, it is possible to measure thefrequency structure of the intensity of radiation. Signals
from the detector were transferred to a spectrum analyzer(HP E4403B) and analyzed. To compare the bunch length
obtained by the present method with that by theconventional method, the streak camera (HAMAMATSU
C1370-01) was placed behind the movable mirror and theradiation beam was accepted with the mirror out ofposition.

Be Mirror

Quartz Window
Movable Mirror

Streak Camera

Al Mirror

APDM
Spectrum Analyzer
Figure 1: Layout of the bunch length measurement system at BL-9.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Cq γ 2§σε ·,=¨
¸
Js ρ
© E ¹

EXPERIMENTAL RESULTS

2

Bunch length determination

where α is momentum compaction factor, Cq is aconstant, Js is dumping partition number, γ is ratio of
total energy to rest energy and ρ is electron trajectory
radius. Ω s is synchrotron oscillation frequency given byΩ 2s = 2πf rf2eV gα cos φ shE0,(4)

where φs is synchronous phase angle.
80Bunch Length (mm)Amplitude (arb. units)

Figure 2 shows the frequency spectrum measured at astored beam energy E of 575 MeV and a gap voltage Vg of
100 kV. The fundamental frequency frf is 190.86 MHz
that is RF frequency of AURORA. It can be seen that thespectrum contains higher harmonics up to the sixth orderin addition to the fundamental one and their amplitudedecreases with increasing order. Harmonics higher than
the seventh order could not be recorded due to the cut-off
frequency 1200 MHz of the present measurement system.
If the bunch were so short that it could be regarded as apoint, a series of harmonics up to infinite order would beseen with the same amplitude. The spectrum was recorded
at various gap voltages.

(3)

APDM
Streak Camera
Calculation

75
70
65
60
55
50
45
40
50

60

70

80

90

100 110120130

140 150

Gap Voltage(kV)
0.0

190.9

381.7

572.6

763.4

954.3

Figure 3: The gap voltage dependence of the bunch length.
The curve is calculated one, squares and triangles
represent experimental data.

1145.2Frequency (MHz)Figure 2: Frequency spectrum with fundamental
frequency of 190.86 MHz.
Assuming a Gaussian profile for the bunch, theamplitude of the n -th harmonics is expressed asª(n ⋅ f rf )2 º
An = C exp «−» ,2«¬ 2(c (2πσ L )) »¼(1)

where σ L is the standard deviation of the profile andtaken here as the bunch length. c is the velocity of light.
C is the quantity dependent on the stored beam current
and the efficiency of APDM. σ L was determined byfitting eq. (1) to the observed order dependence of theamplitude of the harmonics. The bunch length thus
determined is plotted as a function of the gap voltage inFig. 3. In the figure the bunch length determined using thestreak camera is also plotted. Solid curve represents thecalculated bunch length on the basis of the formula givenby§ α · §σε ·¸¸ ⋅ ¨ ¸ ,© Ωs ¹ © E ¹

σ Lcalc = c ⋅ ¨¨(σε/E) is given by(2)It is seen that the bunch length determined using theavalanche photodiode is only a little smaller than thevalue from the streak camera method and almost inagreement with the calculated one, indicating thereliability of the present method. It is simpler and lessexpensive than the streak camera method and will have awide range of application. The method developed here
may be called a method of measurement in the Fourier
space.

Bunch diagnostics
The method present here can be used to observe achange in the beam bunch length. Figure 4 shows thespectrum when the operation condition of the ring ismodified to shorten the bunch length. Comparing withthat shown in Fig. 2, the amplitude decrease of theharmonics with increasing order is less remarkable, inagreement with the result by the streak camera. The bunch
length determined by fitting equation (1) is 33mm, whilethat determined by using streak camera is 36mm.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Amplit ude (ar b. unit s)

ACKNOWLEDGMENT
We would like to thank Mr. H. Miyade and Dr. H.
Tsutsui of Sumitomo Heavy Industry for usefuldiscussion.

REFERENCES

0.0

1 9 0.9

3 81 .7

5 7 2 .6

7 63 .4

95 4 .3

1 14 5 .2

Fr equeny (MHz)Figure 4: Frequency spectrum for the operation condition
to shorten bunch length.

Amplitude (arb. units)

Figure 5 shows the spectrum under the operation
condition to induce instability. Irregular dependence ofthe harmonics intensity on the order has appeared,suggesting that the bunch loses the Gaussian shape andchanges into a twin form. This is also consistent with thebunch profile measured by the streak camera.

0.0190.9

381.7

572.6

763.4

954.3

1145.2Frequency (MHz)Figure 5: Frequency spectrum for the operation condition
to induce instability.

[1] N. Takahashi, Nucl. Instrum. Methods Phys. Res. A
24/25, 425 (1987).
[2] H. Iwasaki, Y. Nakayama, K. Ozutsumi, Y.
Yamamoto, Y. Tokunaga, H. Saisho, T. Matsubara,and S. Ikeda, J. Synchrotron Radiat. 5, 1162 (1998).
[3] I. Sakai, Y. Yamamoto, T. Mitsuhashi, D. Amano, H.
Iwasaki, Rev. Sci. Instrum. 71, 1264 (2000)[4] I. Sakai, Y. Yamamoto, T. Mitsuhashi, D. Amano, H.
Iwasaki, Nucl. Instrum. Methods A 480, 121 (2002)