The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

STATUS OF THE BEAM ACCUMULATION AT KSR
T. Shira~ Y. !washita, H. Tongu, T. Sugimura, A. Morita, A Noda,Institute for Chemical Research, Kyoto University, Uj~ Kyoto 611 -0011, Japan
T.-Y. Lee
PLS, POSTECH, San 31, Hyoja Dong, Pohang, Kyungbuk 790-784, Korea

Abstract
KSR (Kaken Storage Ring) is a compact electron ringin Kyoto University. The injection and the maximum energy are 100 MeV and 300 MeV, respectively. It is usedfor not only an electron storage ring but also a stretcher.
The present stored current is 65 rnA at 100 MeV. It islimited by an ion trapping effect and longitudinal single
bunch instability. The beam lifetime is 300 sec at 100
MeV.

Table 1: Basic parameters ofKSR
Maximum energy
Injection energy
Circumference
Straight length
RF frequency
RF Volta e

1 INTRODUCTION
A compact electron ring (Kaken Storage Ring, KSR) isworking at Institute for Chemical Research, Kyoto
University [1 ]. The basic parameters are shown in table 1
and the schematic view of the ring is shown in Fig.l. KSR
has two operation modes, the storage mode and thestretcher mode. In the former case, the injected beam isaccelerated up to 300 MeV. In the latter mode, the beam
is extracted by the slow extraction and the beam energy islOOMeV.
The beam commissioning had started from September
1999. Figure 2 shows the history of the stored electron
current during the commissioning stage. At first, only a
low current ( <10 rnA) was accumulated. Mter theshutdown to install the remaining vacuum chambers andthe devices for the slow extraction, we restarted thecommissioning from March 2000. The pumping system

KickerFigure 1: Layout of KSR

was also improved at the same period. The stored current

reached to 65 rnA at August 2000. Since the beginning of2001, we started a normal operation of KSR. In this paper,
we describe some phenomena to limit the beam current,
which was found in the commissioning period.
In the year of 2001, the major experiment in KSR is aslow extraction of the electron beam using a RF
perturbation [2]. The experiment of a Laser-Thomson
scattering has also been carried out. To produce hard Xray, the electron energy is 100 MeV and the laserwavelength is 532 nm [3].

300MeV
100MeV25.689 m
5.62m
116.724MHz
30kV

70 r----------------------- ------.

° FirstBeam

6FirstAcceleration

50~5 40ct\)~ 30u~ 20

r:o102 STORED BEAM CURRENT ANDTHE LIMITATION

0The electron linac typically injects a beam pulse at thecurrent of 80 mA and the pulse width of 10 or 20 nsec.
Table 1 shows the principal beam parameters at theoperation of 100 MeV.

- i~· -." ...,g -..." ::sti ~ ~ ...,,· -.

~ 0'iS.

"'Year 1999.;

Ci~·:>,

3:>,"3Year 2000

Figure 2 History of the stored current of KSR-184-oli~

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Betatron tune
Storage current
Momentum compaction factor
Damping time ("tx. 'ty, 'ts)
Chromaticity (sx. sv)(2.300, 1.275)
100 rnA (Design value)
0.106(3.4, 1.7, 0.67) sec(-2.5, -8.1) (Natural)

Horizontal Emittance

15.4 :rt.nm.rad (Equilibrium)
160 :rt.nm.rad (at 10 rnA)

Vertical Emittance
Energy spread
ressure8.23 X 10-5 (Equilibrium)
3.3 X 10-4 (at 10 rnA)
0.63 nTorr

100MeV(2.285, 1.255)
65mA(-3.1, -6.4) (Natural)(0.0, 0.1) (with Correction)
130 :rt.nm.rad (at 10 rnA)
6.4 :rt.nm.rad (at 10 rnA)

1.1 nTorr

2.1 Lifetime
The stored current (I) is determined by the beam·
lifetime (t) in the simplest case,

I= r[,eplo(1)where t;.._P is a repetition of the injection and 10 is a storedcurrent per one injection. 10 is typically 2 or 3 rnA and f.ep
is 0.3 Hz, which corresponds to the horizontal dampingtime.
Because the RF voltage is 30 kV, the quantum lifetime
is enough long. The beam lifetime is limited by thescattering with the residual gas and by Touschek
scattering. Touschek lifetime is generally a severe
limitation in a small ring. But the horizontal emittancebecomes much larger than the natural emittance becausethat the injection energy is low and the effect of theintrabeam scattering (IBS) is strong. This effect extends
the Touschek lifetime.
Figure 3 shows the measured lifetime bellow 40 rnA
without any instability and the calculated Touschek
lifetime which includes IBS effect. It shows that thepresent major limitation comes from the residual gas
scattering. Even in the present lifetime, the expected
stored current is higher than 100 rnA from eq. (1) but
actually it is limited by following reasons.

2040

60

80

100

Beam Current (rnA)

Figure 3 Measured lifetime without any instability
and the calculated Touschek lifetime, which
includes IBS effect.

2.2 Head-tail instability
The natural chromaticity is very large in KSR because
the curvature of the bending magnet is small and the ~function at the end of the straight section is large. It isdifficult to accumulate the beam without a chromaticity
correction. Even after the correction, the beam instability
was found around the stored current of 15 rnA when thechromaticity was slightly below zero.
Figure 4 shows the variation of the stored current
measured by DC current transformer (DC-CT). The beam
was injected every 4 seconds. The horizontal and verticalchromaticity was 0.3 and -0.5, respectively. The sudden-185-20 sec/div
Figure 4 Stored beam current measured by DCCT. The beam was injected every 4 seconds. The
horizontal and vertical chromaticity was 0.3 and-0.5, respectively

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

beam loss was found and the beam current could notexceed 20 rnA. After the chromaticity is changed to 0.0and 0.1, the beam loss disappeared. This sensitivity to thechromaticity means that it comes from the head-tailinstability.

30mA

2.3 Ion trapping instability
Due to the low injection energy (100 MeV), the beam
trapping is remarkable in KSR, even though there are 2ion clearing electrodes to apply 500 V. Figure 5 shows theDC-CT signal, when all RF buckets are filled with thebeam bunches. At three seconds after the injection, thebeam lifetime becomes extremely short (about 15seconds). When the beam current decreased below 8 rnA,
it recovered rapidly to about 1000 seconds. At the sametime, the vertical tune shift of 0.026 is also observed. The
following relation is known between the tune shift (6vy)and the ion density (di) [4],6vy

OmA

2 sec/div
Figure 5 Stored beam current, when all RFbuckets are filled with the beam bunches. At
three seconds after .the injection, the beam
lifetime becomes extremely short.

=J d;{Jy ds1+ ay(2)where (3y is a vertical 13 function and a is a beam size. The
measured result is consistent with this relation.
The ion trapping effect can be reduced by empty
buckets. When the filled buckets is less than half, thedrastic ion trapping is not observed. But it seems that theweak effect still exists because the measured beam
lifetime in Fig.3 is shorter than the calculated one fromthe average vacuum pressure.
Frequency (20 kHz/div)

2.4 Longitudinal single bunch instability
In small electron rings, a strong longitudinal single
bunch instability was reported because the revolution
frequency is high. In KSR, a similar phenomenon isobserved. Figure 6 shows a frequency spectre of a pickup
signal from an electrostatic monitor. The centre peakcorresponds to the revolution frequency and the side two
peaks are synchrotron oscillation sidebands. It shows theexistence of the longitudinal instability. The sideband
sometimes grows up and leads to the beam loss. Thisinstability does not depend on the number of the beambunch.

Figure 6 frequency spectre of a pickup signal
from an electrostatic monitor. The centre peakcorresponds to the revolution frequency and theside two peaks are synchrotron oscillation
sidebands

group in JAERI and KEK. We want to present our special
thanks to late Mr. Fujita for his great contributions toKSRproject

3 CONCLUDING REMARKSThe present stored current is 65 mA and the beam
lifetime is about 300 seconds at 100 MeV. They are
limited by the longitudinal instability and the weak ion
trapping. To clarify the situation and increase the lifetime,
we continue the aging by the synchrotron radiation. The
present beam lifetime at 300 MeV is also reported inanother contribution paper [5].

ACKNOWL EDGEMENT
The authors would like to present our thanks to thesupport of the HIMAC group in NIRS and the accelerator

REFERENCES[1] A.Noda, et a]., "Electron Storage and Stretcher Ring,
KSR", Proc. of European Particle Accelerator
Conference, Barcelona, 451-453 (1996)[2] T.Sugimura, et al., "Stretcher Mode Operation ofKSR", Proc. of European Particle Accelerator
Conference, Vienna 1002-1004 (2000)[3] A. Yamazaki, et al., "X-ray J>roduction with LaserThomson scattering at KSR", Proc. of ISEM, Tokyo,(2000) to be p,ublished.
[4] A.Wrulich, 'Single-beam Lifetime", Proc. of theCERN Accelerator School, CERN 84-15, 1984
[5] H. Tonguu, et al., "Upgrade of the Vacuum System inKSR", Contribution paper to this symposium-186-