PERFORMANCE OF THE RF-SOURCE FOR THE KEKB LINAC
S. Fukuda, S. Michizono, Y. Saito, K. Nakao and S. Anami
National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba, Ibaraki 305, JapanAbstract
The KEKB project, which requires an energy upgrade of theKEK linac from 2.5 GeV to 8.0 GeV, started in 1994, and has
been progressing. About fifty 50-MW high-power klystrons(including equivalent 40-MW tubes) have been produced andtested. Twenty-eight of them have already been installed in theklystron gallery. We also obtained more than 60-MW rf peak
power with a reasonable efficiency from this 50-MW tube. The
klystron assemblies, including the magnets and pulse
transformers, have been operated with no problems. In order tooperate the SLEDs, we have also developed a sub-booster
klystron, a driver klystron which produces more than 60-kW
peak power for 8 high-power klystrons. Two of them have beensuccessfully operated in the klystron gallery. We have started
SLED operation at 2 sectors (sectors #4 and #5) and areaccumulating data concerning the SLED and accelerated beams.
We now describe this performance of the rf-source for theKEKB linac.
Introduction
An upgrade of the PF linac in order to increase theacceleration energy from 2.5 to 8 GeV, by using a combinationof 59 klystrons having an average output power of 41 MW(max. 46 MW) and SLAC-type rf compressors (SLED) is now
in progress[1]. We have achieved progress concerning the subbooster klystron, the driver klystron, and feeding to 8 highpower klystrons, since it is necessary to change the driving
scheme (shown in Fig. 1) in order to use the SLED operation
with proper timing. In this modification, more than 40 kW ofoutput power is required from the sub-booster klystron bytaking account of the transmission losses. We had manufactured
5 tubes, including a prototype; 2 tubes have already been set in
the gallery in order to test the SLED operation. Installation of
the high-power tubes is proceeding on schedule. Twenty-eighttubes have already installed to the gallery, and 12 tubes arebeing operated under the SLED mode. Those tubes are operated
during an injection to the PF ring and AR (Accumulating ring)for the SOR experiment users. Some of the high-power tubes
are being operated at 335 kV to 350 kV, and have succeeded inoutputting more than 60 MW. They are expected to be used at
the unit just after the positron target position, where highgradient acceleration is desirable.
Sub-booster Klystron Development
For the KEKB energy upgrade project, new 50-MW
klystrons have been developed. In order to feed a drive power tothese 8 high-power tubes, which are operated with SLED
cavities, a 60-kW sub-booster klystron (SBK) is required. Since
there are no commercial tubes which satisfy our specifications,this tube has been designed at KEK, and manufactured under
the collaboration of KEK and MHI (Mitsubishi Heavy IndustryCo.)[2]. The specifications for the sub-booster klystron areFig.1-a
Old Systemx6
10kW
Klystron
TH-2436

10kW
Klystron
TH-2436
IφA

IφA

IφA

IφA

IφA

IφA

IφA

IφA

IφA

IφA

IφA

30MW
Klystron
PV3030A3

IφA

1W

O/E

Fig.1-b
New System

60kW
SB Klystron
IφA

IφA

IφA

IφA

IφA50MW
KlystronFig. 1. Comparison between the old and new driving systems.
the collaboration of KEK and MHI (Mitsubishi Heavy IndustryCo.)[2]. The specifications for the sub-booster klystron aregiven in the table 1.
We have been using two 10-kW tubes of Thomson CSF(TH2436) in parallel in each sector, as shown in Fig. 1a[3]. The
different points between the old TH2436 tube and the new
SBK-tube are as follows: (1) Electromagnet focusing has been
adopted instead of permanent magnet focusing. (2) The tuning
frequencies for the each klystron cavity are fixed, since ourpurpose of this tube is limited. The new tube has 6 cavities,
while the old tube has 4. These modifications enable us toobtain a high gain. (3) The new tube has an integrated ionpump. (4) Water cooling is adopted in order to stabilize theoperation performance. (5) The input power feeder is set
vertically to make the inside bore diameter of the magnet small.
(7) The output waveguide is a coaxial 39D-type waveguide andthe output waveguide flange type is BFX-39D, which is popularin Japan (Old type is EIA-39D standard.) (8) An Ir-coateddispenser cathode of 1 inch diameter is adopted instead of theoxide cathode based on a life consideration. (6) Our tube
configuration is partly based on the design of the SLAC subbooster klystron, especially concerning item (5). [4]The basic design of the tube is fulfilled in KEK and somemanufacturing processes: for instance, as the cathodeprocessing, baking-and-evacuation of the tube and pinching off

item
Peak pulse voltage
Peak pulse current
Microperv
Pulse width(rf)Pulse width(beam)
Repetition
Peak RF power
Average RF power
Efficiency
Gain
Input power
Total length
Electric gun
Focusing magnetCoolingIon Pump
Cavity numberOutput Waveguide
Output Flange

unitspecification
kV
25.0
A
7.91
3/2
PA/V
2.0
Psec
4.0
Psec
>6.0
pps50
kW>60
W12%
>30
dB57mW
120mm
about 690
BI CathodeElectromagnet
Water cooling
1 l /sec integrated ion pump
639D Coaxial Waveguide
BFX-39D standard

were first demonstrated at KEK. A proto-type tube wasmanufactured in FY94. In FY95 it was tested and a 60-kW
output power was obtained at a beam voltage of 25 kV, which
was supplied by the newly developed SBK-modulator using asemiconductor switching device. In FY95 three tubes wereordered and two were tested. These tubes were installed at theklystron gallery in order to evaluate the SLED operations, andhave operated satisfactorily. We need 8 sub-booster klystrons
for KEKB-project. Up to now, we could achieve an output
power of around 60 kW, while the efficiency is about 30%.
Status of High Power Klystron
Test and Installation in the Gallery
As previously reported[5, 6], we have been developing 2
types of high-power klystrons for this project: one is an
improved type of an old 30-MW tube by enlarging the highvoltage ceramic-seal; the other is an improved one by using alarger cathode and larger high-voltage seal. PV3030A3(MELCO; Mitsubishi Electric Company) and E3728 (Toshiba)
are the former types of tubes and PV3050 (MELCO) and E3730(Toshiba) are the latter types. Both types have abilities toproduce more than 50 MW of output power at 310-kV appliedvoltage. The focusing electromagnet has compatibility between
these two types with only a slight change at the gun region ofthe tube.
An output power of 50 MW and an efficiency of 45-46%
were achieved on the average in the 50-MW tubes. The
saturation point is located at 250-300 W at the input power on
an average ( a gain of around 53 dB is achieved). The typicalperformances of the 50-MW tubes are shown in Figs. 2 and 3.
Our tubes have a single window and cooling structures are seton the upper and lower waveguides of the windows. The
window material of our tube is high-density pure alumina of
99.7% (HA997: NGK) and has a very low tanG value[7]. The
evaluation after running in the gallery is satisfactory.

We have already purchased 50 tubes, including both types.
Performance tests of 27 tubes have been finished and the tubes
have been installed in the klystron gallery. During the first stageof tube development, some instabilities and arcing problems
were observed in the Toshiba 50-MW tubes, which werecompletely solved by changing the cathode processing of the
manufacturing process. Another type of instability and poor
gain problems were observed in the MELCO tubes. These weresolved by changing the structures inside the tube so as toprevent any distortion of the cavity during the manufacturing
process.
60-MW test using the 50-MW klystron
Performances at an applied voltage higher than 310 kV havebeen of interest since the FCI[8] calculation predicted an output
power of 70 MW at the 350-kV beam voltage. This test has
been attempted using a prototype tube ( PV3050#2 ) as atentative low-duty test; a 64-MW output power was observedwith an efficiency of about 42%; the performance has strongly
depended on the magnetic-field distribution near to the outputcavity region, as predicted by FCI. Furthermore, the E3730 tube
produced a 60-MW output power at 331 kV beam voltage and rf
pulse width of 2 Ps with an efficiency of 47% at a factory test.
These output power level are highest when using single output
windows. We have been planning that this tube operation mode
will be used at the #2-1 unit, which is located just after thepositron-conversion target, since high-gradient acceleration isrequired.

55
50
Output Power P 0 ( MW)

Table 1. Specification of SBK

4540
V 0 = 310kV

3530

FCI Prediction

25P V3050#22015E3730#20200 400 600 800 100012001400
Input Power( W)

Fig. 2. Input -output power characteristics predicted by the FCI
and those of two 50-MW tube performances.
Design and Status of the Socket Assembly
The final design of the pulse transformer is such that thestep-up ratio has been changed to be 1 : 13.56; there are 7
primary turns and 95 secondary turns. This is an bifiller autowinding type; a core reset bias is applied. More detail
descriptions are given in reference [6]. We have newlydeveloped corrugated high-voltage insulators made from epoxymaterial to support the klystron heater transformer, which is at ahigh-voltage potential. The heater transformer has beenredesigned, and the final thickness is half that of the old one.
Owing to these design changes, we can continue using the sameoil tanks and same configurations, including the waveguideports. A feeder section inside of the tank comprises a knifeswitch-type connector made by Multi-contact Co., whichenables it to be easily disconnected. The capacitive divider, used
as a voltage monitor, has been replaced from the Pearson-Inc.
type to the Stangenes-Inc. type for higher voltage applicationsof up to 350 kV. Two small-size current transformers are set ina tank circuit instead of the old home-made one; one is used forthe current monitor and the other for a dedicated application of
an interlock signal. So far, we have experienced no troubles up
to around 320 kV in full duty and 350 kV under in lower duty.
For the 350-kV application it is necessary to use a pulsetransformer with a step-up ratio of 1:15; this approach is being
prepared
Forty-nine electromagnets have been manufactured up toFY95, including 2 types of magnets. Both types can be easilychanged from one type to another by replacing the iron skirts
and coil part. Thirty-two pulse-transformer assemblies havebeen modified from the old type to a new type by rewinding thepulse-transformer windings and adding pulse-circuits
components. Since machine operation has been continued
during the construction periods, the reformation schedule for thepulse transformer is the most tightest part.
70
PV3050-64MW(42%)
Output Power P 0 ( MW)60
E3730-60MW(47%)
50

processing data as discharging and the time dependence of theprocessing etc. have been analyzed. The averaged energy gainand energy-multiplication factor of the developed units inoperation is 163 MeV/unit and 1.93, respectively[9].
The performances of the two sub-booster klystrons have sofar been satisfactory. However, the tube efficiency is around the30%, and the optimum focusing-magnetic field is quite different
from the design field. It was found that a weak parasite
oscillation exists under some conditions. It might be necessary
to check the magnetic field, especially near to the cathoderegion. So far an output power of about 60 kW is sufficient foreach high-power klystron to work at the saturation point, while
10 kW from the previous TH2436 tube was short for saturationpoint operation for some poor-gain tubes. It is not clear that thesome unstable operation of the SBK affects the SLED'soperation or not.
A long processing time was necessary for some special unit
up to the specified value. The main task for us will be toinvestigate what kind of the causes prevent full processing. Thissummer we will install another 2 sub-booster klystrons in thesectors #2 and #3, and we will also start operating in the SLED'smode there.
Conclusion
We are progressing satisfactorily regarding high-power
klystron testing and installation in the klystron gallery.
Purchasing the tubes and focusing electromagnets is onschedule. The final design of the pulse-transformer assembly
has been fixed, and also continuously modified. The sub-booster
klystrons, which are inevitable for our SLED's operation, arebeing developed and evaluated in the klystron gallery. Since
FY95, the useful processing data have been accumulated by theSLED's operation of the sectors #5 and #4. These kinds ofstudies will be continued after this summer shut-down; roughly
half of the construction will be completed.

40References

30P V3050#2

20E3730#2E3730

10P V3050

0220 240 260 280 300 320 340 360
Applied Beam Voltage ( kV)

Fig. 3. Output-power characteristics as a function of theapplied beam voltage
SLED Operation in the Gallery
Up to the FY95, 12 SLEDs have been installed in thegallery, and more 13 SLEDs are being installed during thesummer shut-down period of 1996. Two sub-booster klystrons
were installed in sectors #4 and #5; complete mode operation ofthe SLED has been carried out. In these sectors, conditioning ofthe upgraded high-power units is proceeding, and such

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Linac" Proc. of the 1981 Int. Linac Conference, Santa Fe, NM, USA,pp. 177-179 (1981)[4] Private communication[5] S. Fukuda et. al., "Development of the B-Factory Linac 50-MW
Pulse Klystron", 17th International Linac Conference, Tsukuba, ,Japan, pp.427-429 (1994) .
[6] S. Fukuda et al., "Design and evaluation of a compact 50 MW rf
source of the PF linac for the KEKB project", Nucl. Instrum. andMethod A368, pp.561-571 (1996)[7] S. Michizono, Ph. D. Thesis, Tokyo University (1994).
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