Electron Cloud Effects in the KEK-PS and the
KEK/JAERI Joint Project
T. Toyama*, Y. Me*, S. Kato*, K. Ohmi*, C. Ohmori*, K. Satoh*, M. Uota*
N. Hayashi#
*KEK, Oho, Tsukuba, Ibaraki 305-0801, Japan
#
JAERI, Tokai, Naka, Ibaraki 319-1195, Japan
Abstract. An e-p instability is potentially a serious problem for proton rings in JKJ, which forces us to study the e-p
instability in several high-intensity proton rings. This work informs JKJ whether we have to take measures to cure the
instability. A TiN coating on the chamber surface is one of remedies. Results of SEY measurements performed at KEK
are discussed. The observation of electron cloud candidates at the KEK 12 GeV PS Main Ring is also presented.
ring, electrons move under an electric potential
generated by a rigid "half-sin" proton beam. Space
charge force between electrons is neglected because
the average neutralization factor is less than 0.1 in the
rings discussed here. The magnetic fields are also
neglected in the whole ring for simplicity.
The amplification factor (Ae\ the number of
multiplied electrons divided by the number of primary
electrons per one bunch passage, is calculated for
several stages of the relevant rings. The results are
shown in Table 1 and also in Figure 1. At every bunch
passage a peak is formed by trailing-edge
multipacting. Equilibrium is reached in 5 - 10 bunches
passage. The neutralization factor strongly depends on
several parameters: the beam size, chamber size,
bunch length and bunch spacing.
The beam stability is evaluated based on a wake
field approach and a coasting beam approximation and
the instability may be fast enough regardless of the
synchrotron oscillation. Both the proton beam and the
electron cloud are assumed to have a rigid Gaussian
distribution. By linearizing the coupled motion, the
proton motion can be considered to be a forced
oscillation with the wake field that is generated by the
proton beam passing through the electron cloud.
Including the damping effect due to electron
oscillation frequency spread, the coupling impedance
is reduced by Fourier transformation of the wake field.
Making use of this coupling impedance, the dispersion
relation, U=l, is obtained. For U>1, the beam is
unstable. In Table 1, two values of UH and UL are
listed. They are unstable criteria for the peak and
bottom values of the neutralization factor,
respectively. For ISIS, the slippage factor and the
synchrotron tune are assumed to be the same as PSR.
PSR is unstable. On the other hand, ISIS and AGS are
stable. These results qualitatively agree with the
observations. The rings of the joint project are inbetween.
INTRODUCTION
A high-intensity proton accelerator facility has
been proposed in Japan as a joint project of KEK and
JAERI (JKJ). The facility would be equipped with two
proton rings: a 3 GeV rapid cycling synchrotron and a
50 GeV proton synchrotron. The bunch population,
which would be 4xl013, compares with that of LANL
PSR, RAL ISIS, BNL AGS. The e-p instability is
potentially a serious problem for these two rings of
JKJ. Not all high-intensity proton rings suffer from an
electron cloud instability. It is worth comparing those
proton rings from the viewpoint of the electron cloud
instability.
It is important to know the secondary electron yield
efficiency (SEY) not only as a candidate of remedies,
but also as an input of a computer simulation. The
results of ongoing measurements of SEY at KEK are
discussed for several materials. The observation of
electron cloud candidates at the KEK 12 GeV PS Main
Ring is also discussed, although not yet confirmed.
ELECTRON CLOUD FORMATION AND
INSTABILITY
Production rate of the primary electron generated
due to the residual gas ionization by the proton beam
(Yi?i), due to proton beam loss (Yu), at a stripping foil
for H-minus charge exchange injection (Yi?fou) are
evaluated as Yu = V.VxlO'9 e~/(m-p), Yu = 4.4xlO'6
e~/(m-p), Yi?foii = 1.9xlO~5 e~/(m-p), respectively.
Electrons at the stripping foil will be swept by the
leakage field of bump magnets.
Electron cloud formation is estimated by tracking
the transverse 2D motion of electrons produced by the
primary and secondary electron emission. Primary
electrons are produced at the chamber wall with
energies of 10±5 eV. The secondary electron yield is
approximated by true secondary electron yield of
Ys(max)=2.1 at Emax=200 eV. At position s along the
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
364
Although there seems to be no instability, the
following experiments were performed to clarify the
source of the baseline drift. Figure 3 shows the peak
voltage of the baseline drift at the transition energy
with seven, eight or nine successive bunches, with the
same bunch population of 2.8xlOu protons. The
baseline drift was saturated by applying more than -40
V of positive bias. This saturation level decreased with
increasing bunch gap. The baseline drift was not
detectable if the bunch number was less than 6. At a
fixed bunch number of 9, the peak voltage of the
baseline drift was measured with solenoid currents of
0, 10, 20 and 30 A, as shown in Fig. 4. At 30 A no
baseline drift was observed.
SEY MEASUREMENTS
A series of measurements of the secondary electron
yields (SEY) were made using an electron beam of
(|)0.5 mm with 100 - 5000 eV and of some tens nA, or
using an argon ion beam with a raster-scanned size of
a few mm2, 5000 eV and some tens nA. Surfaces of
sample materials were analysed with x-ray
photoelectron spectroscopy or Auger electron
spectroscopy.
Figure 2 shows the dependence of the SEY on the
primary electron energy with normal incidence at the
as-received surfaces and after sputtering with argon
ions. Although a titanium as-received sample showed
the highest yield, the yields of the others, except an
isotropic graphite, were close to the peak of titanium.
However, the yields of the carbon materials and the
TiN film showed lower. The yields of the all materials
were reduced after the argon ion sputtering. Since the
carbon and oxygen impurities in the TiN film reached
their saturation and remained even after sputtering of a
thickness of 65nm, those impurities may have been
included in the film, itself, during its preparation.
Their reduction may reduce the yields of TiN further.
SUMMARY
Electron cloud build-up and beam stability were
evaluated for high intensity proton rings: JKJ 3 GeV
RCS, 50 GeV MR, PSR, ISIS and AGS. The number
of primary electrons is amplified by trailing-edge
multipacting. The amplification factor strongly
depends on the secondary electron yield, beam shape,
and chamber geometry. Then, using the neutralization
factor obtained by the simulation, the single bunch
stability was evaluated using a coasting beam model.
The obtained stabilities agree qualitatively with
observations in the existing machines.
Including both the elastic reflection in SEY
estimation and the space charge in force evaluation
is a subject for future study.
ELECTRON CLOUD IN THE KEK-PS
In the KEK-12 GeV-PS Main Ring, baseline drifts
were observed around the transition energy and around
the beginning of the flat top on newly installed
electrostatic pick-ups and a previously installed
exciter. Four electrodes were directly connected to the
center control room on trial with a high impedance
termination. The baseline drifts implies a charge-up of
the pick-ups with a negative current of a few
microamperes.
REFERENCES
1. References in T. Toyama, Y.Me, S. Kato, K. Ohmi, C.
Ohmori, K. Satoh and M. Uota, KEK Preprint 2002-53.
TABLE 1. Basic parameters of the proton rings.
Variable
circumference
Lorentz factor
Bunch population
Number of bunches
Harmonic number
Rms beam size
Bunch length
Rms energy spread
Slippage factor
Synchrotron tune
Beam pipe radius
Amplification factor
Amplification factor
Neutralization
Neutralization
Stability (bottom)
Stability (peak)
symbol
L(m)
y
13
Np(xl0 )
nb
h
Or(cm)
w
OE/E(%)
^
vs
R(cm)
Ae(bottom)
Ae(peak)
rj (bottom)
rj(peak)
UL
UH
Joint project
3GeVRCS
inj.
348.3
1.4
4.15
2
2
1.9
110
0.6
-0.48
0.0058
12.5
42.0
87.6
0.020
0.042
0.07
0.23
PSR
SOGeVMR
ext.
inj.
348.3
1567.5
4.2
4.2
4.15
4.15
2
8
2
9
1.2
1.1
82
82
0.7
0.7
-0.047
-0.058
0.0005
0.0026
12.5
6.5
18.0
9.4
62
136
0.0067
0.0035
0.023
0.05
0.23
0.11
0.78
1.6
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54
4.15
8
9
0.35
16
0.25
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0.0001
6.5
0.13
6.9
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0.0005
0.02
1.2
90
1.85
3
1
1
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65
0.25
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118
236
0.034
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1.6
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366