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

UPGRADING OF LOW LEVEL CONTROL OF NEWSUBARU RF
Y. Shoji, LASTI, Himeji Institute of Technology, 678-1205, Japan
T. Takashima, Y. Kawashima, SPring-8, 679-5198, Japan
Abstract
We installed additional feedback loops in the RF lowlevel control system, which effectively reduced thevoltage ripple of the klystron DC power supply. The
additional PLL loop almost eliminated the low frequency
beam ripple. This was an important step for a time
resolving experiment, which requires a small timing jitter.

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 Table
1. The ring has two operation modes for users. In the 1.5
GeV mode, the beam is accelerated to 1.5 GeV and stored,
while in 1.0 GeV top-up mode, the beam current is kept at250+0.15 mA by an occasional injection with the gaps ofundulators closed
NewSUBARU has another special operation mode, aquasi-isochronus operation adopting extremely small
momentum compaction factor α [2]. The small α means
short bunch length and low synchrotron oscillation
frequency. In that case an experiment would suffer for aklystron noise-induced coherent synchrotron oscillation.
We have reported that a main source of the coherent
phase oscillation at NewSUBARU was the klystron noise[3]. Its amplitude was ±2ps of synchrotron oscillation(5kHz) and ±1ps of 120Hz. There were other weakeroscillations with harmonic frequencies of 60Hz, thefrequency of primary AC power line. It was not harmful

in the normal operation with bunch length of 40ns(FWHM). However it was not acceptable when we
shortened the bunch length down to a few ps.
In the summer of 2002 we added an additional
Automatic voltage Level Control (ALC) and Phase Lock
Loop (PLL), to compensate the ripple of the klystron
output power in addition to the existing ALC and PLLloops. We refer the new feedback loops by klystronfeedback loops and the existed feedback loops by cavity
feed back loops. Here we report the effectiveness of theklystron feedback loops.

2 NEW FEEDBACK LOOPS
Fig.1 shows a schematic view of the low-level control
system. The thick lines in were the added loops. The
existed cavity feedback loops were to cancel the beamloading. The frequency range of the feedback loops wasdifferent for the cavity and for the klystron as shown in
Fig.2. Their gains go down by 3dB at 12Hz (cavity ALC),
70Hz (cavity PLL), 11kHz (klystron ALC) and 10.3kHz(klystron PLL). The frequency range of the cavity ALC
was set low because it works as a positive feedback ofphase with heavy beam loading [4].
The noise reduction by the new loops was measured
without electron beam (Fig.3). The reduction of thevoltage level noise was efficient (roughly -20 dB) in therange from 100 Hz to a few kHz. The reduction of thephase noise worked in more wide range, from a few Hz toa few kHz. The 120 Hz noise was reduced by about -34
dB. This is the frequency range of the synchrotron
oscillation with small α.
dummy load

500MHZ
Phase
Shifter

Phase
Shifter

RFSW

RFmodulator

DC VFBCFBCcavityklystron

DC V

FBC

FBC

RF DetTuner ContBPF

BPF

BPF

RF DetPhase Det.

Phase Det.

Phase Det.

DC V

VPCFreq. Conv.

Freq. Conv.
Freq. Conv.

Figure 1: Feedback loops of the low level control system of NewSUBARU. The thick lines are the new loops
introduced in the summer of 2002 to reduce beam ripple due to the klystron noise.

000-10-10-60-20-60-120-30-180-40-240

Cavity ALC-50-60110100f (Hz)1000gain (dB)-30-180-40-300-50-360-600Cavity PLL
1-30010100f (Hz)

1000-3600-60-10-60-20-120-20-120-30-180-40-240

klystron ALC-50-60110100 1000 104
f (Hz)

gain (dB)0-30-180-40-240-300-50-360
105-60

klystron PLL
1-300

100 1000 104
f (Hz)

10phase (deg)0-10phase (deg)gain (dB)-20phase (deg)0phase (deg)gain (dB)

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

-360
105-40-60

kly-ALC OFF-80
kly-ALC ON-100-120
1101001000

410f (Hz)-40720Hz120Hz

Power of Phase Ripple Signal (dBV)

60Hz-20120Hz
200Hz240Hz

Power of Amplitude Signal (dBV)

060Hz-2020240Hz

Figure 2: ALC feeedback loop and PLL feedback loop gains for the cavity feedback and for the klystron feedback. The
solid lines and dotted lines represent gain and phase, respectively.

kly-PLL OFF
-60

-80

-100
kly-PLL ON-1201101001000

104f (Hz)Figure 3: ALC and PLL noise signal with the klystron feedback loops turning on and off. The frequency range of theklystron feedback is wider than that of the cavity feedback. The voltage signal (ALC) of 1V corresponds to 100kV. Thephase signal (PLL) of 1V corresponds to 20 degrees.
Full scale: 100dB
Full span: 50kHz

Full scale: 100dB
Full span: 1kHz

Figure 4: Side bands of the RF frequency of the beam signal. The left plots are before and the right plots are after theinstallation of the new loops. The vertical scale is 10dB/div. The horizontal axes are the same for the right and for theleft. The highest narrow peaks are the RF frequency. The broad peaks in the above are synchrotron oscillation sidebands.

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

3 OSCILATION OF STORED BEAM
3.1 Picked Up Signal
The signal of coherent electron bunch oscillation in thestorage ring was obtained from a pick up electrode. The
electrode is set at the dispersion free section, so that thesignal is sensitive to the timing arrival and the verticalmovement. The signal was FFT analysed and the timing
oscillation, in other wards a phase ripple, appeared as side
bands of the acceleration frequency (fRF) signal. Fig. 4
shows the side band structure at before and at after theinstallation. The main low frequency ripple peaks
disappeared. The phase ripple amplitude of frequency∆fBEAM, that is an amplitude of coherent oscillation in timeaxis, was calculated from the peak ratio of side bands of
the RF frequency fRF as

Figure 5: Streak camera image of the beam. The fullrange of the vertical axis (fast sweep axis) was 220 ps andthe full range of the horizontal axis (slow sweep axis) was
0.5 ms. The image was different for a different shot. Thetwo images were extreme two cases, with large oscillation
and with no oscillation.

∆φBEAM[deg.]= (360/2π) (V[fRF+fRIP]/V[fRF] ),here fRIP is an frequency of the ripple. The phaseoscillation amplitudes of beam for the main ripple
frequencies are listed in Table 1. The oscillations camefrom the klystron power supply was reduced to theacceptable level.
Table 1: Main frequency components and amplitudes
phase oscillations of beam at before and at after theinstallation of the new feedback loops.
frequencyside band (dB) beam ripple (degree)(Hz)

before afterbeforeafter60
120
240
720

-62-50-64
-65

0.046
0.181
0.036
0.032

0.003

-85

Figure 6: PLL signal with stored beam current of 250mA.
The horizontal full scale was 2msec, and the vertical fullscale was 0.8 degree (4.4 ps).

4 CONCLUSION
New klystron feedback loops reduced the ALC andPLL noises in low frequency region by about -20dB and 30dB, respectively. The coherent beam oscillation wasdrastically reduced. However we still observe large
oscillation at some instance.

5 ACKNOWLEDGEMENT
The authors thank members of SPring-8 accelerator
division especially the RF group. They helped us in many
situations, in trouble and in the improvements. We also
thank Prof. A. Ando, a head of NewSUBARU for hissupport.

3.2 Measurement with Streak Camera
The coherent movement was also observed with streak
camera during the user operation time. The camera wasoperated in double sweep mode. The fast sweep
frequency was 1/6 of the fRF. Two beam signals arerecorded in the same frame with the opposite fast timeaxis. We occasionary observed a coherent synchrotron
oscillation as large as 27ps p-p (left image of Fig.5). The
PLL signal at that time (Fig.6) did not have such a large
oscillation signal, neither the FFT spectrum did.

6 REFERENCES[1] A. Ando et al., J. Synchrotron Rad. 5, 342-344 (1998).
[2] Y. Shoji et al., “Quasi-Isochronus Operation at
NewSUBARU”, SRI’03, to be published as AIPproceedings;
Y. Shoji et al., “Experience of Quasi-Isochronus
Operation at NewSUBARU”, this proceedings.
[3] Y. Shoji et al., Proc. of PAC’01, pp.1219-1221.
[4] Y. Shoji et al., “High beam current operation ofNewSUBARU RF”, this proceedings.