W-9
BEAM SIGNALS FOR RF PHASE CONTROL OF KEK PROTON SYNCHROTRON
K.Muto, S.Takeda and M.Kondoh
National Laboratory for High Energy Physics[KEK]
Abstract
The harmonic number of KEK Proton Synchrotron[PS] is nine. So the beam
revolution frequency is one ninth of accelerating frequency[RF]. To eliminate
the phase distortion by cross-modulation, it is better to compare both phases
at RF rather than at revolution frequency in the beam-RF phase-locked loop[PLL].
For this, a system was developed, in which nine wall current monitors[WCMs]
were installed on the beam duct around PS at the same intervals. The system
works well for both single and nine pulse acceleration modes.
1.

General Description

In principle, one monitor head is enough to get the beam signal for PLL at
RF in nine pulse mode. But, up to now, due to the beam loss after the injection,
the beam pulse of the first injection decreases several times smaller than theninth(last) pulse before acceleration(see Fig.4(a)). So, if only one monitor isused, the beam signal is deeply modulated in amplitude at revolution frequency.
It becomes an origin of phase error. Moreover, on the single pulse mode, a frequency multiplier of nine times should be needed(or one should compare the beam
signal with one ninth of RF). The nine WCM system does not bear such problems.
The block diagram of the system is shown in Fig.l. PS is composed of equally spacing 56 subcells. Fortunately, the beam ducts are coupled with electrically insulated flanges at the fixed position in each of the subcells. By using
these electrical gaps, the WCMs cam be made easily. Though the beam path lengthes from a pair of flanges to the next pair are exactly the same around PS, 56/9
is not an integer. So, nine WCMs could not be located in equal spacing. Thelocation errors are noted in Fig.l as errors in RF phase. These errors are reduced to one seventh by adjusting the cable lengthes to get all beam signals inphase at 7 MHz(RF varies from 6 to 8 MHz during the acceleration). Then themaximum location error becomes less than 4°(at 6 and 8 MHz).
The beam signals from nine WCMs are mixed at the OR circuit and, throughthe tracking filter, only the RF Fourier component of the beam signal is fed tothe phase detector. This means that RF is phase-locked approximately to thecenter of charge.
2.

Details of Apparatus
2.1.

Monitor Head(WCM)

Figs.2 and 3 illustrate a WCM. It picks up the wall current induced by
the beam along the duct. Two 3300 pF capacitors are inserted to reject the induced voltages by dB/dt and by the earth potential differences among nine WCMs.
One 47 ohm resistor is for the impedance matching to suppress the standing
waves along the cable. Fig.4(a) shows the beam signal observed at the RF control station.
2.2.

OR Circuit

As shown in Fig.3, it is a diode OR circuit.

The nine signal are mixed
in the manner that the maximum amplitude is dominant. Then the output signal
is nearly the same for single and nine pulse modes. The merit is that there isno need of switching between these two modes. Fig.4(b) shows the output of thecircuit.
2.3.

Tracking Filter

The tracking filter is a current controlled resonance circuit. The control
current is generated by the reference input from RF oscillator. The output isa sinusoidal wave.
1812.4.

Phase Detector

The phase detector is composed of two saturable amplifiers and a balanced
mixer. The use of a balanced mixer is advantageous because the output is zero
when the phase difference is ±90°(this means that the beam is on 0° or 180° inRF phase).
Acknowledgement
We wish to thank Mr. M.Toda for his help in preparing the manuscript.

TRACI'\ING

PHASE

FILTER

DETECTOR.----.'\;\; ....----.

Fig.l Block diagram of the system with WCM location errors in RF phase.

(a) Output of WCM.

c___ _ _ _W C M _ _ _ ____.

Fig.2 WCM installed at the insulated flanges.

WCM

OR CIRCUIT

Fig.3 Circuit of WCM and OR circuit.

182(b) Output of OR circuit.
Fig.4 Beam signals.