TEST OPERATION OF THE PF LINAC RF SYSTEM UPGRADEDFOR THE KEKB INJECTOR
H. Hanaki, S. Anami, A. Enomoto, H. Katagiri, S. Michizono, Y. Otake, Y. Saito
National Laboratory for High Energy Physics
Oho, Tsukuba, Ibaraki, 305 JapanThe RF source of the PF 2.5-GeV linac has been
reinforced by installing SLEDs and replacing the existing30-MW klystrons with new 50-MW ones. Conditioningof the upgraded high-power units is proceeding and thedischarge in the process has been analyzed. The averagedenergy gain and energy multiplication factor of theupgraded units in operation is 163 MeV/unit and 1.93,
respectively. A signal-analyzing system for diagnosing
the RF source was developed and is now being examined.

1 INTRODUCTION

2 RF CONDITIONING
2.1 Procedure and History
Since the maximum RF power fed into an accelerator
guide had been about 8 MW before the upgrade,
multiplied microwaves greater than 70 MW cannot be fed
in a short term. It is necessary to gradually increase theRF power so that gas molecules adsorbed on the surface

4-7 Conditioning
1 0- 340
Pkly(MW)
IPK-B
IPL30

1 0- 4201 0- 5

101 0- 6

IP Vacuum (Pa)The B-physics project at KEK (KEKB) started in 1994,
and construction will be completed in 1998. The PF
electron linac, which will be an injector for the KEKB
collider rings, will have its energy upgraded from 2.5 to 8
GeV by increasing both the RF power and the total
accelerator length[1, 2].
The present klystron modulators have been upgraded[3]in order to increase the output power and to extend thehigh-voltage pulse width up to 4 µs on its flat top. The30-MW Klystrons have been replaced by a new 50-MW
type[4]. A SLED, which is an RF-pulse compressor, hasbeen installed in every high-power RF source[5], except
for two or three which require severe phase and amplitudecontrol.
At present, eleven SLEDs have been installed as of thisspring. The conditioning of seven of them has almost
been completed, and they are actually being used for beam
acceleration. The remaining four are being conditioned.
For a good understanding of the linac condition it isvery important to quickly detect any problem or symptom
concerning any problem regarding the RF system, such asa defect in the RF-wave form or variations in the vacuumof the wave guides. Therefore, an intelligent RF monitor
system[6], which is now being examined, will be
introduced in each two high-power RF amplifiers.

of the RF components may not be released drastically andmay not initiate a discharge.
If the vacuum in the waveguides or the accelerator
guides exceeds an alarm level during the conditioning, acontrol system immediately cuts off the RF pulses so as
not to cause any serious damage due to discharging.
When a SLED system is set up, the first conditioningof the high-power RF system is carried out according tothe following steps:
1) The SLED cavities are detuned and RF pulses are fed
into the accelerator guide while gradually increasing
its power from 0 to 30 or 40 MW.
2) The SLED is turned on. The operating klystron
output power is from 0 to 10 MW, mainly forconditioning the SLED cavities. It normally takes 4
or 5 days.
3) The klystron output power is from 10 to 40 MW or
more, mainly for conditioning the accelerator guides.
The period of this stage strongly depends on anaccelerator guide, and is more than two weeks.
Although the above processes are presently carried out
by manually operating a local controller with monitoring
the vacuum, it will become automatic this summer.
Figure 1 represents the conditioning history of thenumber 4-7 unit as an example. In this case, theprocesses were completed in relatively short term.

Klystron Output (MW)Abstract

00510Day

151 0- 720

Figure 1: Conditioning history of the 4-7 unit. Plottedsymbols of IPK-B and IPL represent the rough pressuresin the waveguide and the accelerator guide, respectively.

2.2 RF-Pulse Reflection Caused by Discharge
The RF pulse reflected in the accelerator guide is observedwith a long delay time (0 ~ 1 µs) on an oscilloscope,discharges around the input or output couplers areremarkable. This result is consistent with the fact thatthe reflection from both couplers has been clearlyobserved. Note that the events around the input coupler
may contain discharges in the waveguide.
.
12108

Eventssince the group velocity of the microwaves in theaccelerator guide is very slow, and the filling time of theguide is about 0.5 µs in our case. That is, the delay time
can give the discharge position in the accelerator guide.
Here, we focus on a multiplied part (width w = 0.6~1µs) of a SLED RF pulse, and assume the following forthe analysis. Suppose that an RF pulse, which travels inan accelerator guide, causes a discharge because of its highfield strength at some position in the guide. Then a partof the travelling RF pulse, filling the accelerator guidefrom the upper stream to the discharge position, isreflected backward, and the other downstream part of it
goes forward.
When a reflected RF pulse with a width of t 2 i sobserved, the pulse appears with a delay time of t1, whichis equal to the traveling time of the RF pulse to and fromthe discharge position (L ). We can then find thefollowing equations, which give the discharge timing (T)of an RF pulse and the normalized position (L) in an
accelerator guide:

64
2
0-0.2

00.2

0.4

0.6

0.8

1

Discharge Position

T = w − t2Figure 3: Discharge-position distribution of RF pulsesfor the 4-7 accelerator guide. The negative positions werecaused by errors in reading t1 or t2 .

t −T

1ln(1 − g) L = 1 − exp 1g
 2 Tf
where Tf is the filling time of the accelerator guide and g
is defined as 1 – e -2τ (τ : attenuation parameter) for aconstant -gradient structure. The positions of L =0 and 1
correspond to the input coupler and the output coupler,
respectively.
In reality, the shapes of the reflected pulses were not as
ideal as that assumed above, and it was not easy todetermine the Ts and Ls. However, the analysis wasperformed according to the equations in order tounderstand the general characteristic of the discharge.
Figure 2 and 3 are examples of the T and L distributionsof the discharges which occurred in the 4-7 accelerator
guide.

3 ACCELERATION TEST
Beam-acceleration tests were perfomed in order to obtain
the energy gain of the acceleration units and the energy
multiplication factors of the installed SLED. Figure 4
illustrates the distribution of the obtained values in casethat the SLED cavity charging time was 3 µs. It showsthat the energy gains are around a specified value of 160
MeV/unit, and the average is 163 MeV/unit (21.6MeV/m). The multiplication factor is 1.93 on theaverage, which agrees with the computed value of 1.96.
Note that the klystrons outputted different power levels tothe SLEDs.

142.2

122.1
Energy Multiplication Factor

Events10864
20-0.22.0

Computed Value

1.9
1.8
1.7
Specified Value
1.6

00.2

0.4

0.6

0.8

1Discharge Timing (µs)

Figure 2: Discharge-timing distribution of RF pulses forthe 4-7 accelerator guide.
Figure 2 explains that the growth of a discharge takes
time, which is well-known. Figure 3 shows that

1.5100120

140160180200

Energy Gain (MeV/unit)

Figure 4: Distribution of the energy gains of theacceleration units with SLED operation and their energy
multiplication factors.

The shape of the RF pulse ejected by the SLED does
not have any flat part, and the pulse width must be as
short as possible in order to obtain a high multiplication
factor. This means that the stability of the energy gainstrongly depends on the RF timing.
We experimentally and theoretically examined theenergy loss caused by the timing shift, as shown in figure
5. Both results show rather good agreement.

displayed on it and refreshed more than 30 times persecond.
As practical operation of all monitor stations, it can be
expected that a poor reliability of the hard disks of each
VXI system may spoil the reliability of the total monitor
system, because the number of stations will be more than30. Therefore, a complex system comprising a server anda couple of diskless VXI systems will be undergoing test
operation this summer.

0EthernetVXI-Bus System

Energy Loss (%)-2

Wave Form
Controller
Digitizer

ADCDIO-4

Selector

Calculated Data
-6

DC Monitor
Pulse
MonitorMeasured Data

DC Monitor
Pulse
Monitor-8High PowerRF Unit 1-10
1.5

1.55

1.6

1.65

1.7

Peak Power
Meter

Phase
Detector

High PowerRF Unit 2

1.75

RF Selector

RF Pulse Timing (µs)

Figure 5: Energy loss as a function of the timing of theRF pulses. The SLED cavity charging time was 3.3 µs
and the width of the multiplied pulse was 0.7 µs.
We fitted a quadratic function to the experimental
results. According to the obtained function, toaccomplish a beam energy jitter of less than 0.1 %requires that the timing jitters be less than 7 ns. The newtrigger system will adopt RF synchronized timing
circuits, so that the timing jitters will be basically muchsmaller. The delay time generated by the circuit,however, must be controlled with a time step of less than5 ns.

4 RF MEASUREMENT SYSTEM
A signal-analyzing system will be newly installed, which
will continuously take monitor signals from themodulator and SLED into its computer. The computer
will analyze the data and diagnose any problem involving
the RF source, when it occurs. This system will also
provide automatic execution of the RF conditioning of
the high-power RF units including the accelerator guides.
A prototype system based on a VXI system has beendeveloped from 1995 (figure 6). This VXI systemcomprises a controller, a wave-form digitizer, an ADC
and a digital I/O; also, along with the RF measurement
equipment (a peak power meter, a phase detector, etc.), anRF monitor station for observing two high-power RFunits has been developed. All of the monitor stations
will be networked, and a few servers will manage the VXI
controllers and databases. Measured and analyzed data orwave forms can be monitored on a remote X terminal
anywhere. The wave form of a SLED RF pulse isRFMonitor

RFMonitor

1

SLED Detuner
Controller

2

Figure 6: Block diagram of the prototype RF monitor
station.

5 REFERENCES[1] Design Report on PF Injector Linac Upgrade for
KEKB (in Japanese), KEK Report 95-18, 1996.
[2] A. Enomoto, et al., "Reformation of the PF 2.5 GeV
Linac to 8 GeV", Proceedings of the 1994 International Linac Conference (Tsukuba), p 184.
[3] H. Honma, et al., "Upgrade of the PF 2.5-GeV Linac
Klystron Modulator for the KEK B-Factory Project",
Proceedings of the 1994 International Linac
Conference (Tsukuba), p 436.
[4] S. Fukuda, et al., "Development of the B-Factory
Linac 50-MW pulsed klystron", Proceedings of the1994 International Linac Conference (Tsukuba),p 427.
[5] H. Hanaki, et al., "Use of SLEDs for High-GradientAcceleration", Proceedings of the 1994 InternationalLinac Conference (Tsukuba), p 430.
[6] H. Katagiri, et al., "An RF Monitor System for
KEKB Linac"(in Japanese), Proceedings of the 20thLinear Accelerator Meeting in Japan (Osaka, 1995),
p 296.