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

EXPERIENCE OF QUASI-ISOCHRONUS OPERATION AT NEWSUBARU
Y. Shoji, S. Hashimoto, A. Ando, LASTI, Himeji Institute of Technology, 678-1205, JapanWe define the linear and non-linear momentum
compaction factors (αn) as
L=L0(1+α1δ +α2δ2+α3δ3+. . .) .

(1)

α1=0.0000

1.5

α1=0.0014
10.5
Q3 Q40BEND-0.5-5-4-3

Q4 Q3

INV.BEND-2-1

BEND01s (m)2345Figure 1: Calculations of dispersion function ofNewSUBARU in one bending cell. The solid line is for
α1=0.0014 and the broken line for α1=0.
35polarity changeof Q3 family

302520150.7GeV

101.0GeV
50.00140.00120.001

0.00080.00060.00040.000200- 0.0002

2 EXPERIMENTS

2dispersion function (m)NewSUBARU [1] is a 1.5 GeV synchrotron radiation
ring at the SPring-8 site. Laboratory of Advanced Scienceand Technology for Industry (LASTI) at the HimejiInstitute of Technology is in charge of its operation,
collaborating with SPring-8. A bending cell in the ring isa modified DBA with an 8O invert bend between two 34O
normal bends. This facilitates the control of the linear
momentum compaction factor (α1) while keeping the cell
achromatic and with only a small change of naturalemittance.
The bunch length was measured with a streak camera
as a function of α1. The idea of bunch shortening isderived from a well-known expression that theequilibrium bunch length is proportional to √a1. It wasdemonstrated in some facilities in 1990s [2]. Recently M.
Abo-Bakur et al. reported that in BESSY the bunchlength decreased to 1 ps (FWHM) according to the √α1law, but with stored current of less than 1 µA per bunch[3]. On the other hand, Y. Shoji et al. theoretically
predicted the intrinsic bunch-shortening limit came fromlongitudinal radiation excitation [4]. At NewSUBARU,
that was 0.144 ps (FWHM) at 1.0 GeV. Approaching thislimit is one of the goals of the bunch-shortening study.
We report some experiences of quasi-isochronus
operation at NewSUBARU up to the present.

We control α1 by changing two quadrupole families atthe dispersive sections as shown in Fig.1. Themeasurement of the synchrotron oscillation frequency fS
confirmed a shift of α1. It is known that when we keep theRF acceleration voltage VRF constant, fS2 ∝ α1. The shiftof fS2 to the α1 of the lattice model is shown in Fig.2. Ourlinear lattice model had energy dependence and was notgood enough for the accurate calculation of α1. The α1
was estimated at one set-point from two kinds ofindependent measurements [5]. We estimated α1 at theother set from measured fS assuming that fS 2∝ α1.

2

1 INTRODUCTION

2.1 Control of α12

quasi-isochronus operation is one of the operation
modes of NewSUBARU, a 1.5 GeV VUV storage ring.
NewSUBARU has six invert bending magnets to control
the momentum compaction factor. The aim of thisresearch is to explore the extreme reduction of electron
bunch length by reducing the momentum compaction
factor. We experimentally reduced the momentum
compaction factor from 0.0014 down to less than 10-5,
keeping the beam in the ring. The second-ordermomentum compaction factor was adjusted to almost zero,
while keeping the third-order momentum compaction
factor positive. The ring was operated at 1.0 GeV. Using astreak camera, the shortest bunch length we observed was4 ps FWHM.

Here L is a circumference and δ is a relative energy
displacement defined by E=E0(1+δ). In this report
symbols with suffix “0” denote the values of the referenceelectron.

f S (kHz )Abstract

set-value of α 1

Figure 2: Measured fS2 vs. set value of α1 (calculation
using the present model). The solid line was measured at
the stored energy of 0.7 GeV and the broken line at 1.0GeV.

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

2.2 Control of α2
The second order momentum compaction factor α2
was controlled by changing one sextupole family SF at
the dispersive section. The smallness of α2 was confirmedby measuring fS varying ∆fRF. Fig. 3 shows fS vs. ∆fRF at
α1=7.5×10-6.
When α2 was not small enough for a small α1 there
existed a two stable buckets in one RF period. A beam in
one bucket was transferred to another bucket by changing
fRF as shown in Fig. 4.
1000

fS (kHz)

800
600Terms of higher order than α2 were not controlled
because the ring has no element to control them. The α3
was always positive. This finite positive α3 was essential
to keep the beam inside the ring aperture especially when
the ring parameter was moving. The smaller the α1, thelarger the energy displacement is, and the positive α3reduced the energy displacement for a mismatch of theRF frequency (fRF) to the ring circumference. Although abeam storage with smaller α1 value was not difficult, thesynchrotron oscillation was no longer linear. At
α1=7.5×10-6an energy displacement of 0.048%, which is astandard deviation of a natural energy spread, increases
the magnitude of α by about 10% because of the finite
higher-order terms.

2.3 Bunch length Measurement

4002000-50 -40 -30 -20 -100

10 20 30

∆fRF (Hz)Figure 3: Synchrotron oscillation frequency (fS) vs. theshift of RF frequency (∆fRF). Circles are measured and theline is a calculated assuming that the circumference is
proportional to the function: 1+7.5×10-6δ+0.9δ3 -180δ4+1000δ5 +6.4×106δ6 -6×10-8δ7. During this measurement
the RF acceleration voltage (VRF) was set at 114 kV.

For the bunch length measurements the streak camera(Hamamatsu C6860) was used in synchro-scan mode. The
set up of the camera was explained in the other article [6].
The fast sweep frequency was 83.3 MHz, 1/6 of the RF
frequency (500 MHz). The measured bunch shape was anaccumulation of signals emitted for 1.0 second. The
harmonic number of the ring was 198=6×33. We filled
the ring with 33 bunch trains of five filled and one
unfilled buckets in succession.
We reduced α1, keeping the VRF constant, to ensurethat the theoretically expected bunch length wasproportional to fS. The stored beam current was about
1µA per bunch with stored energy of 1.0 GeV. Fig.5
shows the measured bunch length with respect to themeasured fS. The bunch length agreed with theoretical
calculation in the range of α1=1.2×10-3 ~ 2.0×10-4. Fig.6
shows the observed bunch shape at α1=2.2×10-5, which
was fitted with Gaussian distribution. However at verysmall α1, the measured length was greater than thecalculated one. As yet, we have not reached any
conclusion about why this occurred.
We also observed a dependence on VRF, keeping α1constant, as shown in Fig.7. The bunch length was longer
at the highest VRF.
25
-3

Figure 4: Beam transfer from one bucket to anotherbucket. (a) shows a relation between ∆L (vertical axis )and δ (horizontal axis). The units are relative. (b) showsthe expected shift of fS to ∆fRF in the case of (a). (c) is thetime evolution of FFT spectrum. The vertical axis is timeflowing from the top to the bottom. The horizontal axis is
the frequency with full span of 10kHz. The centre line isthe fRF and the side lines are side bands corresponding tofS. We observed a jump of fS expected from thecalculation schematically shown in (b).

Bunch Length (ps, FWHM)

α1=1.27X10-6- 8X10

VRF=310 kV

2015calculation

10500

246fs (kHz)810Figure 5: Bunch length vs. fS. The circles are measured lengthand the line is a theoretical calculation. The RF acceleration
voltage was kept constant (VRF=310kV) while changing α1.

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

9 10 4-58 10 4 α 1 =1.36X1047 10
6 10 45 10 4
FWHM=3.8ps
4 10 443 10
2 10 4
1 10 40-1 10 41520
25time (ps)However the synchrotron oscillation side band was broadat small α1.
We observed a dependence of the synchrotron
oscillation amplitude on ∆fRF as shown in Fig. 8. The
effective bunch length was long for large synchrotron
oscillation amplitude as shown in Fig.9.
499955.480kH3035

Figure 6: Measured bunch shape by the streak camera,including the resolution of the system. The line is a
Gaussian distribution fitted to the data.
Bunch Length (ps, FWHM)

6499955.500kH

α1=0.0000081
VRF=130 - 310 kV

543499955.520kH

21
00.4

0.5

0.6
fs(kHz)0.7

0.8

Figure 7: Bunch length vs. VRF. The circles are measured
length and the line is a theoretical calculation.

Figure 8: Dependence of the synchrotron oscillation
amplitude on ∆fRF. The sharp peak at the left is fRF.
5000

2.4 Synchrotron Oscillation Amplitude(1)

relative strenghth

Coherent movement of the beam was detected via thepickup electrode in the storage ring, and timing
fluctuations appeared as side bands of fRF in FFTspectrum. The coherent synchrotron oscillation amplitudein time axis (∆tS) can be estimated from the peak ratio ofside bands (V[fRF±fS]) to fRF (V[fRF]) as∆tS = (1/2πfRF) (V[fRF±fS]/V[fRF]).

499955.480 kHz
499955.500 kHz
499955.520 kHz

4000
3000
2000
1000001020304050 time (ps)Figure 9: Measured bunch length for three cases of Fig.8.

Relative peak heightof sidebands of Sf (dB)-45

3 ACKNOWLEDGMENTS-50

We thank members of the SPring-8 accelerator
division for their support of NewSUBARU, especially Dr.
N. Kumagai, a head of the SPring-8 accelerator division,for his encouragement in this study.

-55
-60
-65
-70

4 REFERENCES
024

6810fS (kHz)

Figure 8: Relative peak height of the synchrotron
oscillation sideband to the main fRF peak in the FFTspectrum of beam signal.
Fig.8 shows the measured V[fRF±fS]/V[fRF] when we varied
α1. Two phase feedback loops of RF low level control
reduced the RF phase noise at low frequency, which
excited the synchrotron oscillation [7]. If the side bands
were narrow, V[fRF±fS]/V[fRF]=-70 dB means ∆τS=0.1 ps.

[1] A. Ando, et al., J. Synchrotron Rad. 5, 342-344 (1998).
[2] "Micro Bunches Workshop", AIP Conference Proc. 367.
[3] M. Abo-Bakr, et al., Proc. of 19th PAC, 2003.
[4] Y. Shoji, et al., Phys. Rev. E 54, R4556-4559 (1996).
[5] Y. Shoji, et al., "Estimation of Non linear RF bucket of
NewSUBARU", this proceedings.
[6] Y. Shoji, et al., "Quasi-Isochronus Operation at
NewSUBARU", SRI’03, to be published as AIP
conference Proceedings.
[7] Y. Shoji, et al., "Upgrading of NewSUBARU RF Low Level
Control", this proceedings.