Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

COHERENT SYNCHROTRON RADIATION BURST FROM ELECTRONSTORAGE RING UNDER EXTERNAL RF MODULATION
Y. Shoji#, K. Takeda, NewSUBARU/SPring-8, University of Hyogo, Japan,
T. Takahashi, Research Reactor Institute, Kyoto University, Japan
Abstract
We quantitatively investigated the time structure ofcoherent synchrotron radiation (CSR) burst to find out
how unstable it is. When the rf phase was modulated with2fs (twice of the synchrotron oscillation frequency), theburst structure was modulated with 2fs and the fluctuationof radiation power was reduced.

INTRODUCTION
It is known that a high peak current beam in electron
storage ring emits burst of coherent synchrotron radiation(CSR) in THz region [1-7]. In comparison with the othermethods to produce CSR in electron storage ring, thismethod supplies higher power and the CSR is easy to beobtained with no special expense. The high peak current
is normally obtained in a single bunch or a few bunch
operations. Despite these merits of CSR burst, it is not
used for experiments. This is because the origin of theCSR burst is a fine time structure in the bunch due tolongitudinal beam instabilities, therefore, the burst is notstable. We investigated the time structure or fluctuation of
the CSR burst to know how unstable the burst is. We
hope that the bursts would have an application with anappropriate time averaging.
The burst is qualitatively understood as so-called sawteeth instability. The time structure in a bunch wasproduced by a beam induced field (wake field) or CSR
itself. The instability (oscillation condition) has athreshold peak current, which depends on the energy
spread and others. When the beam condition is above thethreshold, a fine time structure grows in a bunch andemits CSR. At the same time this enlarges the energy
spread, and stops the instability and CSR. After the burst
the radiation damping gradually reduces the energy
spread and bunch length until the next burst starts.
We have already reported the time structure of CSR
with the beam current dependence in normal conditionand bunch compressed conditions, reducing themomentum compaction factor (α) or raising the rf
acceleration voltage (Vrf) [8]. The measurements of CSR
from one bunch showed that with an average period of 10
ms (comparable with the longitudinal damping time, 12ms) the fluctuation of averaged power was about 10%.
The fluctuation ratio had small dependence on beamcharge, rf acceleration voltage and momentum______________________________________________*Work supported by the Grant-in-Aid for Scientific Research from
Ministry of Education, Science, Sorts and Culture Government of Japan.
#shoji@lasti.u-hyogo.ac.jp

compaction factor. The successive bursts had acorrelation because the beam had a memory of former
bursts. This worked to reduce the fluctuation in long
period.
This article reports CSR under the rf phase modulation,
where a bunch in one rf bucket is separated to three [9]. In
our case, the three bunches synchrotron oscillate in an rfbucket.
Beside the easiness of production, there are two key
accelerator techniques of operation for users. The first isan accurate bucket selection in the injection process,which keeps the high current single bunch. The second isthe top-up operation, which roughly keeps the constantbeam current. This is important because the high peakcurrent beam has a short Touchek lifetime. Thesetechniques are not common but to be introduced at many
existing facilities and will be standards of synchrotron
radiation ring.

MEASUREMENT SETUP
Storage Ring
The electron storage ring for the measurement wasNewSUBARU (NS), 1.5 GeV synchrotron radiation ringat the SPring-8 site. It was operated with top-up mode at
1.0 GeV during this measurement. The maximum stored
beam current at the single bunch operation is 70 mA forthe circumference of 119 m. Normally the rf acceleration
voltage (VRF) at 1 GeV is 100 kV and the linear
momentum compaction factor (α) is 0.0012. The unloaded synchrotron oscillation period is 0.2 ms.
In order to give modulations to the CSR burst, weapplied a modulation to the phase of the rf acceleration
field, with frequency of twice of the synchrotron
oscillation frequency (2fs=10.2kHz).

CSR Radiation Detector
We used commercially available Schottky diodedetector (WR5.1ZBD; Virginia Diode Inc.). It wassensitive to a radiation of 0.14 - 0.22 THz, at just above
the cut-off of the light extraction port. The signal from thediode was amplified (17dB; 10kHz - 1GHz band) andrecorded by a digital oscilloscope (10Gs/s; 2GHz).
With a single shot, the signal for 10 ms (=25250revolutions) was recorded. The pulse width was 1.2 ns
FWHM, which was much longer than the electron bunch
length. The pulse area for each revolution was calculated
at off-line analysis and applied non-linearity correction.
The results gave relative radiation power for every-737-

Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

revolution from the same bunch. Fig. 1 shows examplesof the results. The frequency (=number of pulses)
distribution of pulses for CSR power was obtained frommeasurements of 10 or 6 shots.

RESULTS
Previous Measurement
In the previous study, sets of measurements took placeat stored beam current (I) of 5 mA, 10 mA, 20 mA, 40mA, and 60 mA with normal VRF and α. The current
dependence of the time structure showed typical behavior,
which has been observed at other facilities. The radiation
burst was observed at above the threshold current (about 3mA at normal condition of NS). The plot of the averaged
radiation power against the beam current showed thedependence between the linear and the parabolic, which
was also observed at other facilities.
At lower current of 5 mA and 10 mA, we observed aquasi-periodic burst. The integrated power in one burst
had weak correlation with the successive time interval to
the next burst. This was reasonable that a stronger burst
ended with larger energy spread and then the beamrequired longer time for dumping to the threshold level.
With higher beam current, the number of bursts in 10 ms
increased and the burst width became narrower. At above
20 mA, weak burst appeared between strong bursts andthe quasi-periodic structure disappeared into a chaoticburst.
We calculated the fluctuation of time averaged CSR
power. σ of the averaged powers with fixed period,normalized with the total average of 10 shots, was plottedagainst the period length (Fig. 2). When we took theaveraging period of 10ms, the fluctuation of the integrated
power was about 10%. It was not strange that thefluctuation decreases rapidly with the period length at thelength close to the time interval for quasi-periodic burst.
Two other parameters of our interest were VRF and α.
They are often used to compress the bunch and reduce thethreshold current. We investigated the effect of theseparameters on the fluctuation of the CSR burst. Both ofthem raised the radiation power for the same beam charge
and gave slight improvement of the fluctuation.

CSR burst under RF modulation
The third figure of Fig. 1 shows the typical time
structures of CSR burst under the modulation. The burst
timing was roughly synchronized with the modulation
signal. Fig. 3 shows the typical bunch structure and theburst in a modulation period. The bursts took place after
about 1/4 of the modulation period. Fig. 2 shows thefluctuation of the averaged power. The fluctuation wasimproved, but not so much as we hoped.
Fig. 4 shows the jittering of the bunch crossing timing(bright points in Fig. 3(a)) and the burst timing (averageof a pulse like that in Fig. 3(b)). The jittering of theaveraged radiation timing to the phase of the external
modulation signal was what we did not want. At least oneof the sources of it was the shift of the bunch crossingtiming.

Figure 1: Examples of observed time structure of CSR burst in 10ms with normal ring parameters. The above two
are the typical quasi-periodic burst at low current and the chaotic burst at higher current. The bottom trace shows thestructure under the rf phase modulation (the modulation period is roughly 250 revolutions).
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Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

burst peak (µs)

1050-5-10-15-45-40-35-30-25-20

bunch zero-crossing (µs)
Figure 4 Bunch crossing timing and the burst peak timing
to the external modulation phase.
Figure 2: Fluctuation of time-averaged CSR power.
The arrows with Ts and τL indicate the synchrotronoscillation period and the longitudinal damping time,respectively. The shaded rectangles show the interval of
the quasi-periodic burst shown in Fig.1.

SUMMARY
The fluctuation of the time-averaged power of CSR
burst was improved by applying the rf phase modulation
with 2fs. The periodic structure with 2fs was not only
amplitude modulated but also phase modulated.
The improvement was not enough, however, more
improvement would be possible with optimization ofparameters, such as synchrotron oscillation.

radiation power(per pulse; arbit. unit)

REFERENCES
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revolution
Figure 3: Three bunches in a bucket (above image) andthe CSR timing (below). The vertical axis of the above, adouble-sweep streak camera image, is the rf phase.

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