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

EXPERIENCE OF QUASI-ISOCHRONUS OPERATION AT NEWSUBARU
Y . S h o j i , S . H a s h i m o t o , A . A n d o , L A S T I , H i m e j i I n s t it u t e o f T e c h n o l o g y , 6 7 8 - 1 2 0 5 , J a p a n

A b stra c tquasi-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 Hie 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 a
streak camera, the shortest bunch lenglfli we observed was4psFWHM.

2.1 C ontrol o f a r
We control ax by changing two quadrupole families atthe dispersive sections as shown in Fig.l. The
measurement of the synchrotron oscillation frequency f s
confirmed a shift of av It is known that when we keep theRF acceleration voltage
constant,// oc av The shift of/ / to the ax of the lattice model is shown in Fig.2. Ourlinear lattice model had energy dependence and was notgood enough for the accurate calculation of av The ax
was estimated at one set-point from two kinds of
independent measurements [5]. We estimated a t at theother set from measured/ 5 assuming that f s 2<« av

1 IN T R O D U C T IO NNewSUBARU [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 8° invert bend between two 34°
normal bends. This facilitates the control of the linear
momentum compaction factor (cq) 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 av The idea of bunch shortening isderived from a well-known expression that theequilibrium bunch length is proportional toIt wasdemonstrated in some facilities in 1990s [2]. Recently M.
Abo-Bakur et al. reported that in BESSY the bunch
length decreased to 1 ps (FWHM) according to thelaw，but with stored current of less than 1 |jA 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.
2 E X P E R IM E N T S

We define the linear and non-linear momentum
compaction factors (a„) asZ^=L0(l+ a 1d+a2d2+a3d3+.

Here JL is a circumference and 6 is a relative energy
displacement defined by E=E0(l-hd). In this reportsymbols with suffix
denote the values of the referenceelectron.

( 1)

s(m)
Figure 1:Calculations of dispersion function ofNewSUBARU in one bending cell. The solid line is for
apO.0014 and the broken line for ay=0.

s oo① 00o og g g g q s s0 0 0 0 ^ 0 0
set-value of aioo

Figure 2: Measured / / vs. set value of ax (calculationusing 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 20032.2 C ontrol o f a 2
The second order momentum compaction factor Oj
was controlled by changing one sextupole family SF at
the dispersive section. The smallness of Oj was confirmedby measuring f s varying Af^. Fig. 3 shows f s vs. Af即 ata^l.SxlO'6.
When a2 was not small enough for a small at there
existed a two stable buckets in one RF period. A beam in
one bucket was transferred to another bucket by changing
Jrf as shown in Fig. 4.

Terms of higher order than a2 were not controlled
because the ring has no element to control them. The a3
was always positive. This finite positive a3 was essential
to keep the beam inside the ring aperture especially when
the ring parameter was moving. The smaller the au thelarger the energy displacement is, and the positive a3
reduced the energy displacement for a mismatch of the
RF frequency (fe) to the ring circumference. Although abeam storage with smaller ^ value was not difficult, thesynchrotron oscillation was no longer linear. At
ai=7.5xl0"6, an energy displacement of 0.048%, which isa standard deviation of a natural energy spread, increases
the magnitude of a by about 10% because of the finite
higher-order terms.
2 J B unch length M easurem ent

AfeF (Hz)Figure 3: Synchrotron oscillation frequency (fs) vs. theshift of RF frequency (Afup). Circles are measured and theline is a calculated assuming that the circumference isproportional to the function: 1+7.5x10'66+0.963-18064+1000d5+6.4xl06d6-6xl0 867. During this measurement
the RF acceleration voltage (V^) was set at 114 kV.

(a)\\一-2

一- 1 .5o.

Vハ:ALfs(b)0 .2- 1 - 0 . 5- 0 .1- 0 .2\ 。
3s 1F\。
3- 0 .4- d .2I ^/rf0 .2

0 .4

0 .6

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 M H z,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=6x33. We filled
the ring with 33 bunch trains of five filled and one
unfilled buckets in succession.
We reduced al9 keeping theconstant, to ensurethat the theoretically expected bunch length wasproportional to f s. The stored beam current was about
IjxA per bunch with stored energy of 1.0 GeV. Fig.5
shows the measured bunch length with respect to themeasured f s. The bunch length agreed with theoretical
calculation in the range of a t=1.2xl0'3 〜2.0xl0'4. Fig.6
shows the observed bunch shape at a 1=2.2x l0"5, which
was fitted with Gaussian distribution. However at verysmall al9 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 V^, keeping atconstant, as shown in Fig.7. The bunch length was longer
at the highest V^.

xFigure 4: Beam transfer from one bucket to anotherbucket, (a) shows a relation between AL (vertical axis)
and d (horizontal axis). The units are relative, (b) showsthe expected shift of f s to 4 4 f 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 istheand the side lines are side bands corresponding tof s. We observed a jump of/ 5 expected from the calculation
schematically shown in (b).

afs(kHz)

Figure 5: Bunch length vs. fs. The circles are measured lengthand the line is a theoretical calculation The RF acceleration
voltage was kept constant (Vjy^lOkV) while changing av

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

However the synchrotron oscillation side band was broadat small a ト
We observed a dependence of the synchrotron
oscillation amplitude on Af^p as shown in Fig. 8. The
effective bunch length was long for large synchrotron
oscillation amplitude as shown in Fig.9.

o4o4
o4o4
o4o4
o4o4o4-iio4152025time (ps)3035

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.

rfs(kHz)

8: D: idence of the synchrotron oscillation\fM. The sharp peak at the left isId
eFigure 7: Bunch length vs. V^. The circles are measured
length and the line is a theoretical calculation.

52 J Synchrotron O scillation A m p litu d e(1)、

Ats= (1/271/^)c

Coherent movement of the beam was detected via thepickup electrode in the storage ring, and timing
fluctuations appeared as side bands of ん in FFT spectrum.
The coherent synchrotron oscillation amplitude in time
axis (Ats) can be estimated from the pe^k ratio of sidebandsto/ 砂 (Wfo]) as

20304050 time(ps)Figure 9: Measured bunch length for three cases of Fig.8.
ACKNOW LEDGM ENTS

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.
6 REFERENCES
fe(kHz)rigure 8: Relative peak height of the synchrotron
oscillation sideband to the main
peak in the FFTspectrum of beam signal.
Fig.8 shows the measured VffnF土fsVVU^] when we variedcq. 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,土ム]/F|/ 脱]=-70 dB means Axs=0.1 ps.

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[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).
[51 Y. Shoji, et al” ’’Estimation of Non linear RF bucket ofNewSUBARU' this proceedings.
[6] Y. Shoji, et al” ”Quasi-Isochronus Operation at
NewSUBARU", SRT(B， to be published as AIP
conference Proceedings.
[7] Y. Shoji, et al，"Upgrading of NewSUBARU RF Low Level
Control", this proceedings.