JAERI-Conf 95-021

PB29
BUNCH LENGTHENI NG OF THE Nlfl-IV
M. Yokoyama, M. Kawai, K Yamada*, N. Sei*, S. Hamada, S. Sugiyama·, H. Ohgaki*,T. Mikado*, T. Noguchi*, R. Suzuki•, M. Chiwaki*, T. Ohdaira• and T. Yamazaki•

Kawasaki Heavy Industries, Ltd. 118, Futatsuzuka, Noda, Chiba, 278, Japan*Electrotechnicall..aboratory, 1-1-4, Umezono, Tsukuba-shi, lbaraki, 305, JapanAbstract
The bunch length and the beam sizes in thesingle bunch operation on the storage ring NIJI-IV
dedicated to FEL were measured and analyzed byusing theories of the potential well distortion. The
measured bunch length were in good agreement withthe theoretical fitting curve with the longitudinal
coupling impedance of 14 Q , and the microwave
instability was not found at beam current of below 8
rnA The measured beam sizes in the both directions
were constant at below 8 rnA It was convinced fromthe measured bunch length and the horizontal beam
size that the energy spread is not increased at the beamenergy of 310MeV.

1. Introduction
The Nlll-IV is a compact storage ring dedicatedto free-electron lasers (FELs). Lasings of FEL at 595
nm and 488 nm in the visible wavelength region and at
352 nm in the UV region were achieved up to1994[1,2,3].
Generally, the potential well of the vacuum
chamber occurs the bunch lengthening. On somestorage rings, the microwave instability brings about
the turbulence bunch lengthening with increase ofenergy spread above a threshold current. Since theseeffects decrease peak current and FEL gain, it isimportant that these effects are investigated.
In this paper, the beam current dependencies of thebeam size and the bunch length are discussed taking
into account the distortion of the potential welltheoretically. The threshold current of the microwave
instability was not found at the beam current of below
8rnA A longitudinal coupling impedance I Zt/n I
obtained from the theoretical fitting curve of thedistortion of the potential well was about 14 Q . The
large widening of the beam size measured was notfound in both directions in the single bunch operation.
The energy spread is not increased.
2. Theory of bunch lengthening
The bunch lengthening was studied by Hansen[4],Sacherar[5], Chao[6] and others. In single bunch
operation, the coupled bunch instability does. not occur,
so only the potential well distortion should be taken
into account as a cause of the bunch lengthening.
Below a certain threshold current, the bunch
lengthening due to the distortion of the potential wellby the vacuum duct is given by~( R)(o, ) (o,) Iae\ZI.,Ji;;v
30 10-0 10-0E0 10

3=O( 1),

where E is the beam energy, o 10 is the standard
deviation of the natural bunch length, a 1 is thestandard deviation of the bunch length, R is theaverage radius of the storage ring, e is the electronis the longitudinal coupledcharge, I Z/n I
impedance, v 0 is phase oscillation wave number ais momentum compaction factor[4]. The criterion ofthe threshold current I th of the microwave instability
is described by';?..fjit(JI ~((JE)

IfhR

2\Z I nJe E(2).

The energy spread a E increases above the thresholdcurrent as follows.

_1 !!j_( ~)(J EO J2(3),010

where o Eo is the energy spread determined from theradiation damping and the quantum excitation.
3. Measurement of bunch length
Time structure of the synchrotron radiation from anelectron bunch at a bending section was observed byusing a fast avalanched photodiode (ANIEL-270, risetime below 90 ps) and a sampling oscilloscope (HP5490, sampling head 6740 with the frequency
response of 50GHz). The bunch forms obtained withdecrease of beam current were analyzed and theirstandard deviations were evaluated. Figure 1 shows
the current dependence of bunch lengthening
measured in the single bunch operation. The thresholdof the bunch lengthening was not found in themeasured beam current of below 8rnA The dash line-329-JAERI-Confresult is reasonable for the estimated value. Thismeans that the energy spread is not increased under
8mA It is important that the degradation factor f[1] of FEL gain is not decreased.

300•- - -250

ena.
:c
....

200

c:

150

Ol~

..... _...

....-100.r::.
uc::J_...%.......

Measurement data
Fitting data

4. Measurement of the beam size-- --- -----··

I(mA) = (2. 94 x 10·5 p 3-Q. 275 p ,) /al50

I Z/n I

IZ/nl= 14.3Q00

246Beam current(mA)

810Fig. 1 The current dependence of the bunch length in the single
bunch operation . The squares show measured values at the beamenergy of310MeV. The dash line shows calculated values of aequation in this figure and is obtained by fitting to EQ(l).

was a fitting cuxve by the EQ(1). The longitudinal
coupling impedance I Z11 In I obtained from thefitting cuxve was about 14 Q. A longitudinal coupling
impedance I Z11 In I of a smooth vacuum duct witha finite conductivity a con is written byz" _I- iR~Z0b2na •••n+i ~(_!_+In~)
f3y 24(4),awhere y is the relativistic energy, 13 is the ratio of
the electron velocity to the light velocity, ~ isthe free space impedance 377 Q , b is the radius of thevacuum duct cross section, a is radius of thetransverse beam size, n is mode number[7]. The
longitudinal impedance of the bellow is written by

zfl = -i~ 1n b' total

95-021

length the bellows(5),nbcircunference
where b' is outer radius of the bellows[7]. Generally,
the longitudinal coupling impedance of smoothvacuum duct is much smaller than one of the bellows.
On the NITI-IV, if the all vacuum chambers except forthe bellows are smooth, its impedance is below 1 Q by
EQ(4). The impedance of the bellows is about 2 Q by
EQ(S). The longitudinal impedance 14 Q obtained
from bunch lengthening measurement is much larger
than the estimated one. In practice, the chambers withthe screen monitor( screen monitor inputs and outputs
owing to obtaining the position of the injection beam)
are unsmooth and there are 6 screen monitors in thering. It is not large compared with the values of theother rings, for example 20 Q for the SRS[8], 30 Q
for SPEAR- I [9] and 40 Q for SOR[10]. Under theobsexved current( below 8mA), turbulence bunch
lengthening was not obsexved. The threshold current
estimated by EQ(2) is about 18mA. The measured

The beam size at the bending section was obtained
from the synchrotron radiation through a focusing lensby a CCD(charged coupled device) camera. The
focusing length, the length between the radiation point
and the lens, and the length between the lens and theCCD camera are O.Sm, 1m and 1m, respectively. The
beam profile with the same scale was displayed on thecamera. It is convenient that the camera has linearity
on the relation between the light intensity and theoutput signal. This relation was investigated by using
the some neutral density filters and the synchrotron
radiation from the bending section. The beam profile
is kept for long time at low beam current of 0.1mA andcan be used as a standard light source. Figure 2 shows
the relation between the light intensity and the output
signal. The output signal of the camera is proportional
to the light intensity. Figure 3 shows examples of thelight intensity in horizontal and vertical direction. The
beam profiles in the both direction can be fitted by theGaussian distribution. Figure 4 shows the relation
between the beam current and the beam sizes a' x anda' Y in both directions in the single bunch operation.
The large widening of the beam size was not found inboth directions. It means that the energy spread is notincreased and the microwave instability does notoccur at below 8mA It is shown that the emittance
growth due to the effect of the intrabeam scattering[11] also is very small. The emittance growthestimated from the theory of the intrabeam scattering
is increase of a few percent at 8mA-330-200::jroQ) 150
OlJ!!

0.:: 100::3.9::l050... ...

..

..

..

... ..

Filter(%)

Fig. 2 Output voltage of the CCD versus Beam intensity

JAERI-Conf(a) Vertical profile of the beam
150-120

irgp.
.~'::ieli

Q) 90
01J!!

D Clg 60~-,e.

86
I .

;:,;:,

0' &30-.......

0 !lilt""~
0100

J'l

200..,_,..__.,

400

500

300

600

Length(a.u.)(b) Horizontal profile of the beam150r-~-r~--r-~~--~~--~--~~-

bunch operation on the storage ring NIJI-IV dedicatedto FEL were measured and analyzed by using theoriesof the potential well distortion. The threshold current
was not found at the beam current of below 8mA andthe microwave instability does not occur. The
longitudinal coupling impedanre I z,;n I obtained
from the theoretical fitting curve was about 14 Q . The
impedanre of the bellows is about 2 Q . The
longitudinal impedanre 14 Q obtained from themeasurement is much larger than the estimated one. It
is considered that the differenre of the impedanre isdue to the unsmooth chambers. The large widening of
the beam size measured was not found in both
directions in the single bunch operation. The energy
spread is not increased and the. microwave instability
does not occur at below 8mA. The emittanre growth
due to the effect of the intrabeam scattering also wasvery small. The emittanre growth estimated from thetheory of the intrabeam scattering is increase of a few
percent at 8mA.

120::ieli

Referenres

01[1) T. Yamazaki, K. Yamada, S. Sugiyama, H. Ohgald, N. Sei, T.
Mikado, T. Noguchi, M. Chiwaki,R. Suzuki, M. Kawai, M.
Yokoyama, K. Owald, S. Hamada, K. Aizawa, Y. Oku, A. Iwataand M. Yoshiwa Nucl. Instr. and Meth. A331, 27 (1993).
[2] T. Yamazaki, K. Yamada, S. Sugiyama, H.Ohgaki, N. Sei, T.
Mikado, T. Noguchi, M. Chiwald, R. Suzuki, M. Kawai, M.
Yokoyama S.Hamada and A.Iwata Jap. Apl. Phys., 27 (1994).

Q) 90

J!!

g 60

-,e.
;:,;:,

030~ML~~~~~~
o---600
500
400
300
200
100
0Length(a.u.)
Fig. 3 Examples of the light intensity in horizontal (a) and verticaldirection (b). The dot line in this figure is a fitting to the Gaussian
distribution.

0.80 r----r-~---.---~-.--~--.---.-r-----. 0.45

E 0.75~ 0.70·filE 0.650000 0 0 0 0•Verticil

0HoltzOIUIOCJ0 0~ 0.60
~ 0.55-~ 0.500J:95-0210.45
0'40 20.40
0.35 ~
0.30 _2
0.25m

0.20 ~

12.
[3) T. Yamazaki, K. Yamada, N. Sei, H. Ohgald, S. Sugiyama, T.
Mikado, R. Suzuki, T. Noguchi, M. Chiwald, T. Ohdaira, M.
Kawai, M. Yokoyama, S. Hamada, and A. Iwata, Nucl. Instr. andMeth., A358(1995) 353.
[4) S. Hansen, H. G. Hereward, A. Hofmann, K. Hubner and S.
Myers, IEEE Trans. Nucl. Sci. NS-22(1975)992.
[5] F. J. Sacher, IEEE Trans. Nucl. Sci. NS-24(1977)1393.
[6] A. W. Chao, AlP Conf. Proc. 105(1982)352.
[7) V. G. Vaccaro, Nucl. Instr.and Meth. A239{1985)65.
[8] T. Garvey, IEEE Trans. Nucl. Sci. NS-32(1985)2544.
[9] M.A. Allen, et al., Proc. 9th Int. Conf. High Energy Accel.,
Stanford {1974)352.
[10) S. Asaoka, G. Isoyama, A. Mikuni, Y. Miyahara and H.
Nishimura, IEEE Trans. Nucl. Sci. NS-30(1983)2663.
[11] J.D. Bjorken and S. Mtingwa, Particle accelerators, 13 {1983)
115.

0.15 ~••3••••••

45••
6• •

0.10

7s0·0532..

Beam curent (rnA)
Fig. 4 The current dependence of the beam size at the bending
section in the single bunch operation •5. Conclusion
The bunch length and the beam sizes in the single-331-