NewSUBARU

Coherent THz radiation at NewSUBARU
Y. Shoji
NewSUARU, LASTI, University of Hyogo, Kamigori-cho, Ako-gun, 678-1205, Japan

Relative Power of CSR (dB)

NewSUBARU is a 1.5 GeV synchrotron radiation
ring at the SPring-8 site. Laboratory of Advanced
Science and Technology for Industry (LASTI) at theUniversity of Hyogo is in charge of its operation,
collaborating with SPring-8. The beam is injectedfrom the SPring-8 linac with 1.0 GeV of electronenergy. Three types of CSR (coherent synchrotronradiation) from three types of electron beam weredetected in the storage ring, NewSUBARU.
One was quasi-dc CSR (in other wards, steadystate CSR) from a low-current, short-bunched beam,
which is used for application experiment at BESSY-II.
The ring was operated in a quasi-isochronous mode(with a low momentum compaction factor), in which
it a short-bunched beam can be stored stationary. AtNewSUBARU a quasi-dc CSR was obtained at low
current of 1 pC/bunch and short bunch length of 3.4
ps FWHM. In this state, the longitudinal coherent
oscillation amplitude depended on the stored beam
current probably because of a burst of CSR by alongitudinal instability. Burst CSR could produce asudden energy loss and excite a coherent synchrotron
oscillation. Fig. 1 shows the FFT power spectrum ofthe pulsed CSR signal. This quasi-isochronous
operation requires delicate tuning and high stability ofthe storage ring.
1.5 µA/bunch
0.5 µA/bunch

0-10

2 fs fs-20-30-40-50-4-3-2-10

1 2f - fRF (kHz)34Fig. 1. Beam current dependence of the power
spectrum of CSR signal. The main peak (rf frequencyfRF) is normalized to 0dB.
The second type was a radiation pulse following
injection. A short-bunch linac beam, with a basewidth of 20 ps and a charge of 50 pC/bunch, emitted
short-pulse CSR in the storage ring. It was almost
impossible to store short and high-charged electron
bunch in a storage ring but the production of shortand intense bunch at a linac is not difficult. When theshort bunch is injected into an ideal isochronous ring,
the time structure of the bunch is frozen and it emits

short-pulsed radiation for every turn. It would supply
a strong coherent radiation pulse train in THz region
for beam lines in the storage ring. This plan will bepresented in the other presentation titled "Bunch
compression at the SPring-8 linac for successive
generation of THz pulse train in the isochronous ring"at the workshop.
The third type was a radiation burst from ahigh-density, single-bunch beam. Although its
radiation power is extremely high and easy to beobtained in any electron storage ring, this type is not
used for experiments. This is because the source of
the CSR burst is a fine time structure in the bunch
due to longitudinal beam instabilities, and is notstable. However, it could be used for some kind ofapplication experiments with an appropriate time
averaging. We investigated the time structure of theCSR burst using Schottky diode detector, which had ahigh time resolution. When we took an averaging
period of 10ms (=25250 revolutions), the fluctuationof the integrated power was about 10% (standarddeviation). In this time range, the relative fluctuation
decreases with the period length faster than the squareroot scaling law as shown in Fig. 2. It is thought that
the CSR burst can be obtained in most storage rings.
This type does not require fine and delicate machine
tuning as the quasi-isochronous operation does.
However for a user operation, two accelerator
techniques, an accurate bucket selection in theinjection process, and the top-up operation arerequired. These two techniques are not common at theexisting storage rings but coming to be a standard of
new rings.
fuluctation of averaged power(r.m.s / average)

Abstract

averaged time length (ms)0.001

100.01

0.1

110
-0.8

time length10.1

1

10100

1000

10000

averaged number of revolutions

Fig. 2. Fluctuation of time-averaged power ofCSR burst. The broken line is a guide which shows
the dependence of period-0.8.