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

HIGH BEAM CURRENT OPERATION OF NEWSUBARU RF
Y. Shoji, A. Ando, LASTI, Himeji Institute of Technology, 678-1205, Japanand H. Ego, T. Ohshima and Y. Kawashima SPring-8, 679-5198, JapanAbstract
The maximum stored beam current reached to theradiation safety limit (500mA) at 1.0GeV in
NewSUBARU. The parameter of the RF was one of thekeys for the achievement. A change of coupling
coefficient (β)of the input coupler from 2.8 to 5.6 pushedup the Robinson’s stability limit. A change of a time
constant of ALC feedback control reduced theenhancement of the coherent synchrotron oscillation atlow synchrotron tune. An adjustment of temperature ofthe cavity and an adjustment of the Robinson’s tuning
angle reduced the beam blow-up by HOMs.

1 INTRODUCTION
The synchrotron radiation facility NewSUBARU [1] isan EUV and Soft X-Ray light source at the SPring-8 site.
Laboratory of Advanced Science and Technology forIndustry (LASTI), at the Himeji Institute of Technology isin charge of its operation, collaborating with SPring-8.
The main parameters of NewSUBARU are listed in Table1.
Table 1: Main Parameters of NewSUBARU in July 2003.

Ring Parameter
EnergyCircumference
Betatron Tune νX / νY
Chromaticity ξX / ξY
Momentum Compaction Factor α
RF Parameter
Number of RF Cavity
RF Frequency fRF
Harmonic Number
Shunt Impedance Rs
Q value
Coupling Coefficient of Coupler
Beam Parameters at 1 GeV
Maximum Current ;single bunch;multi bunch
Acceleration Voltage VRF
Radiation Loss Per Revolution U0
Synchrotron Oscillation Frequency
Natural Emittance εX
Natural Energy Spread
Linear Coupling
Damping Time τH / τε0.5 ~ 1.5 GeV
118.731 m
6.30 / 2.233.2 / 5.80.0014
1499.956 MHz
1985 MΩ
28000
5.550 mA
500 mA
120 kV33 keV6kHz
38 nm
0.047%
1%
22 ms / 12 ms

The ring has two operation modes for users. In the 1.5GeV mode, the beam is accelerated to 1.5 GeV and stored,
while in 1.0 GeV top-up mode, the beam current is kept at
250+0.15 mA by an occasional injection with the gaps ofundulators closed.
NewSUBARU has one KEK-PF type RF cavity [2],with SiC absorbers. It is powered by a 180kW/500MHzklystron. The shunt impedance of the cavity was much
lower than the design value because of an accident at theinitial commissioning.
In November of 2001 the permission of a new
condition of radiation safety raised the safety limit for thestored beam current from 100mA to 500mA. Some
improvements and parameter adjustment followed thepermission and we soon achieved a highest beam currentof 300 mA in May of 2002, by adjusting chromaticities.
However in order to realize a stable operation with higher
current than 200mA and to reach to the limit current,
500mA, the adjustments of parameters of the RF cavity
was essential. The maximum stored beam current of500mA was achieved in June 2002.
Here we report our experience of some improvements
and some data on the stable operation of the RF cavity
with high beam current

2 CHANGE FOR HIGH BEAM CURRENT
2.1 Coupling Coefficient of the Input-Coupler
In March 2002 the stored beam was not stable over
200mA. A fluctuation of power reflected from the cavity
rapidly increased with the stored beam current over
200mA (Fig.1), which was lower than the Robinson’s
stability limit [3]. The synchrotron oscillation frequency
became too low by the beam loading. An easy way tostabilize the beam was to increase the RF acceleration
voltage (VRF) or cavity power (Pc) as shown in Fig. 2.
However this was not accepted because the increase ofVRF reduced the phase acceptance of RF bucket and madethe injection efficiency worse [4]. The change of thecoupling coefficient (β) of the input-coupler was the best
way to overcome the beam loading.
In May 2002 the β was changed from 3.4 to 5.5. This β= 5.5 was the maximum coupling as far as we use thepresent coupler. This change pushed up the coherentsynchrotron oscillation frequency (fs) and made the beamstable at 200mA. The shift of fS by the beam loading isshown in Fig. 3.

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

(ALC) would enhance the phase modulation end excite
the coherent synchrotron oscillation [5].
The time constant of the ALC feedback loop waschanged. The frequency response of the loop, at before
and at after the change are shown in Fig. 4 [6]. Thefeedback module of NewSUBARU is the same as that ofSPring-8, described by T. Nakamura et al [7]. The
capacitance C of the Figures was named as CL in ref.[7].
With the larger C the increase of the synchrotron
oscillation amplitude at over 250mA disappeared asshown in Fig.5.
00

β = 3.4

3

Pc=2.03kW
Pc=2.74kW
Pc=3.44kW
Pc=4.08kWPr (kW)2.5
2β = 5.5

1.5

Pc=2.22kW
Pc=2.53kW
Pc=2.97kW

1ALC loop phase (degrees)

3.5

C=24.7µs

C=24.7µs-10
ALC loop gain (dB)Figure 1: RF power reflected from the RF cavity. Here
β=3.4 and VRF=117kV. The reflected power was notstable at over the current with minimum reflection power.

C=125µs-20-30

C=125µs-80-120-40-160-50 01010121010frequency (Hz)310410010

121010frequency (Hz)- 30500

synchrotron oscillation
frequency (kHz)

Figure 2: RF power reflected from the RF cavity (Pr)with β=3.4 and β=5.5 for various cavity power(Pc).

65.5
5
4.5
4
3.5
3
2.5
20

Pk[fs]/Pk[frf] (dB)

200300400
stored beam curent (mA)

C=24.7 µF- 404- 45

3
C=125 µF0β=3.450 100 150 200 250 300
stored current (mA)

Figure 3: Synchrotron oscillation frequency shift by thebeam loading. The open circles with β=3.4 and the shaded
circles with β=5.6, both with VRF=107 kV.

2.2 Feedback Parameter
The next problem was the time constant of the lowlevel feedback control. Because the fS was still low at thehigh beam current, the automatic voltage level control

5fs- 50

β=5.54fs (kHz)100

106- 35

03

Figure 4: ALC feedback loop gain and phase withdifferent capacitance, fast (C=24.7µF) and slow
(C=125µF) case. The C is CL in ref.[7].

0.50-4050

100

150 200
I (mA)

250

2300

Figure 5: Relative amplitude of the synchrotronoscillation side band to the main RF acceleration
frequency. The open circles are fs. The shaded triangles
and the circles were relative amplitude of fs side bands, atbefore and at after the change. With the small capacitance
C the synchrotron oscillation amplitude increased with fs< 3 kHz. At this time VRF=116kV and β=5.5.

2.3 Cavity Temperature
In fall of 2002 we found a correlation between thestored beam current and the cooling water temperatureshown in Fig. 6 (a). In January 2003 the temperature wasraised up to 36 degrees. The amplitudes of TM010 mode
HOM, frequencies of 790MHz and 795MHz, decreased
with the rise of the temperature (Fig. 6 (b)). This change
enabled a beam accumulation up to 500mA.

400(a)-55

350

HOM amplitude (dBm)

Beam current at the failure (mA)

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

300250200150100
30 31 32 33 34 35 36water temperature [cavity out](o C)

3 SUMMARY(b)-60795.4MHz-65790.4MHz-70-75
24 26 28 30 32 34 36
water temperature [caviy in]( o C)

Figure 6: The effect of the water temperature.
(a) Stored beam current vs. temperature of coolingwater of the cavity. (b) Water temperature vs. HOM
frequency.

2.4 Tuning Angle
The next problem was the horizontal instability maybe
by HOM of 990 MHz or 790MHz. The HOM made thebeam injection unstable at 50~100mA and caused anabrupt beam loss when the RF voltage was raised for theacceleration or during the acceleration. A small jitteringof Pr is observed in Fig.1 at 50~100mA. A change oftuning angle of the cavity, from -5O to -21O eliminated theloss. Fig. 7 shows examples of the beam current during
the acceleration from 1.0 GeV to 1.5 GeV. With the newangle parameter, we started the user operation at 1.5GeV
at over 300mA.

Some parameter tunings in these two years improved
the beam behaviour at high beam current. However there
still be a sudden and small beam loss at a high current for
about once a month, which we do not have identified thereason.
In this summer we installed another remote controllable
tuner, we call it HOM-tuner, and the beamcommissioning with it has just started.

4 ACKNOWLEDGEMENT
The authors wish to thank Mr. T. Takashima and Dr. Y.
Ohashi of SPring-8 for their help in preparation of the
low-level control system. We also thank Mr. T. Nakamura
of SPring-8 for his suggestion on the ALC feedback.

5 REFERENCES[1] A. Ando et al., J. Synchrotron Rad. 5, 342-344(1998).
[2] T. Koseki, M. Izawa and Y. Kamiya, Proc. of 1995
PAC, Dallas, pp.1794~1796.
[3] P. Wilson, SLAC-PUB-2884 (1991).
[4] Y. Shoji, “Estimation of Non linear RF bucket of
NewSUBARU”, this proceedings.
[5] T. Nakamura, sast’99, pp.245-247.
[6] Y. Shoji, sast’99, pp.245-247.
[7] T. Nakamura, T. Ohshima and T. Takashima, sast’99,pp.242-244.

850

−5 o

400

−21 o
1.5GeV

350

800
750

300

700

250

650

200

600

150

550

100
5005001.0GeV

510 15 20time (min.)20

510 15 20time (min.)

Magnet Current (A)stored beam current (mA)

45045025

Figure 7: The stored beam current with the tuning angle of -5O (left) and -21O (right). The broken lines are
current of bending magnet. The solid lines are examples of stored beam current during the acceleration.