Proceedings of LINAC 2002, Gyeongju, Korea

PERFORMANCE OF A 324-MHZ KLYSTRON FOR A HIGH-INTENSITYPROTON ACCEREATOR
S. Fukuda, S. Anami, Y. Fukui, M. Kawamura, C. Kubota, M. Ono, S. Yamaguchi, KEK, Tsukuba,
305-0035, Japan
S. Miyake, M. Sakamoto, K. Tetsuka, A. Yano, Toshiba, Ohtawara, 324-8550 Japan
Abstract
A 324-MHz high-power pulsed klystron with amodulating anode has been developed as an rf source forthe 200-MeV proton linear accelerator in the HighIntensity Proton-Accelerator Facility. A maximum powerof 3 MW and a working power of 2.5 MW are required tofeed the power to RFQ, DTL and SDTL structures. This
tube is pulse-operated with an rf pulse width of 600-650µsec and a 50-pps repetition rate. Seven klystrons weremanufactured up to April in 2002. At the first stage ofdevelopment, undesirable oscillations due to backstreaming electrons from the collector and someinstabilities were observed. After intense studies andmodifications of the critical parts, we could obtain
klystrons with the specified requirements. We havealready ordered 16 more klystrons for this project. At thesame time, a proton-beam test has been conducted at theKEK facility. This paper describes the development andperformance of the 324-MHz klystron.

modulators have been successfully operated in a test
facility of KEK shown in Fig. 1.
Detailed specifications of this klystron have alreadybeen reported [1]. Seven tubes were manufactured todevelop the full specification. The development ofprototype tubes is described in chapter 2, and the final
development is described in chapter 3 of this paper.

1 INTRODUCTION
The KEK/JAERI Joint Project, an accelerator complex,
comprises a 600-MeV proton linac, a 3-GeV rapidcycling synchrotron and a 50-GeV synchrotron [1]. The
600-MeV linac comprises a 200-MeV, 324-MHz low-β
section, a 972-MHz section from 200 MeV to 400 MeV
and a 972-MHz super-conducting section from 400 MeV
to 600 MeV. Recently, due to a budget shortness, theconstruction of the last two linac sections was postponedto the Phase II plan. The development of the 60-MeVlinac in KEK started as a low-energy front of thisKEK/JAERI project, comprising a negative hydrogen-ionsource, a 3-MeV RFQ linac, a 50-MeV DTL and a SDTL
[2]. The accelerating frequency is 324 MHz. High-power,
high-duty rf sources are required for these structures and a324-MHz klystron with a modulating anode has been
manufactured as the rf power source. Although themaximum rating of the klystron is 3-MW for 600-650µsec rf pulse duration and a 50-pps repetition rate, theactual working level of the klystron is 2 MW. Thus,
because working rf power must be well controlled both inamplitude and phase, the klystron is required to generate asaturated output power of 2.5 MW at the same duty andvoltage. It is operated therefore at a 2-MW output power
level by controlling the drive power. Since a klystron witha modulating-anode is used, modulating-anode power
supplies with a vacuum tube and a semi-conductor as theswitching device were developed and tested. Both718

Figure 1 Klystrons, waveguide systems and control
cabinets in 324-MHz klystron gallery in KEK.

2PROTOTYPE KLYSTRON

Strong spurious oscillations were observed in the firsttest of the prototype klystron. These occurred under highvoltage operation in the voltage range of 65-72 kV,higher than 90 kV without any drive input power. The
oscillation frequency was nearly the same as the operatingone. After experimental investigations, it was clarified
that the backstreaming electrons scattered in the collector
induced these strong oscillations. This was confirmed by
impeding the backstreaming electrons under a weak
deflecting magnetic field in the collector. Thisphenomenon was also analyzed by a computer simulation,
including the effect of scattered electrons at the collector
wall, calculated using the EGS4 code [3]. It was thenclarified that this phenomena was the same those widely
observed in the TWT and magnetron devices [4]. In theprototype klystron design, the diameter of the tube
determined by the frequency-scaling low was larger thanthe cathode diameter. Therefore, the drift-tube diameternear to the cathode was chosen to be smaller than it, andthen the diameter was expanded in order to reduce thefocus magnetic field strength. The amount of backstreaming electrons was shown to be determined by theaspect ratio of the drift-tube diameter to the collectordiameter. A detailed analysis showed that the smaller wasProceedings of LINAC 2002, Gyeongju, Korea

Figure 2 Beam trajectories in the collector of #1A
klystron.
this aspect ratio, the greater were the number ofbackstreaming electrons. In Fig.2, the trajectory examplesof the prime beam and the backstreaming electrons in thecollector region are shown. From a simulation using theEGS4 code, it became clear that the backstreaming
electrons could be decreased by using a larger-diametercollector or a smaller drift-tube diameter [5]. Several
experiments using klystrons with different collector
shapes were performed to eliminate these oscillations aswell as associated unstable phenomena related to theinput drive power. The test results after changing thecollector diameter are summarized in Table 1. Clear
improvements were obtained by these changes of thecollector size. Finally, we obtained the performance, as
shown in Fig.3. Nearly a 3-MW output power and anefficiency of 52% were obtained when the anode voltage
was applied at a value 10% higher than the nominal
dividing ratio between the cathode and the modulating
anode voltage for a cathode voltage of 105.6 kV. These
proto-type klystrons are now being used for high-power
tests of the waveguide components and accelerator
structure in a new klystron gallery at KEK.
Table 1 Collector shapes and experiment results for theoscillation threshold beam voltage.
Tube Collector

Collector

Oscillation

radiuslengththreshold beam

(cm)

(cm)

voltage

Figure 3 Performances of the prototype klystron #2.
tube diameter to the collector-diameter was chosen to belarger than that in the prototype klystrons. In order toachieve this design, the whole focusing magnetic fieldstrength was increased from 200 to 350 Gauss. An
example of the modulating anode gun design [6]calculated using the EGUN code is shown in Fig. 4.
This new tube was manufactured and tested in 2001.
Though no spurious oscillation was observed in this newtube’s performance, a slight instability was observed in
the relation between the input and output characteristics.
Since this was thought to be undesirable when we
perform the amplitude and phase feedback control, anintense investigation was conducted. This instability
involved very high-frequency components at around 1500
MHz. A further experimental investigation, by changing
the focusing field and tuning each cavity, showed
evidence that stray electrons caused the instability at theintermediate klystron cavity region. By choosing theproper magnetic field at this region, we succeeded in tosuppress this instability. Partly, this instability may be(kV)
#1

6.5

62.4

63<V<71, 90<V

#1A

11.5

92.4

95<V

#2

11.5

122.4

104<V or nooscillations up to110kV

3 NEW DESIGNIn order to aim for more stable operation, a newly
designed klystron, based on a more simple structure witha constant drift-tube diameter, was developed at Toshiba
Corporation. In this design, the aspect ratio of the drift719

Figure 4 Beam trajectories in the gun region of
newly developed klystron.
related to the high gain characteristics of about 60 dB,
since the Q value of the first cavity was chosen to beProceedings of LINAC 2002, Gyeongju, Korea

larger than before. The final tube has an improved design
and tuning for the intermediate cavity. A maximum
output power of 3.04 MW was achieved at a cathodevoltage of 110 kV, and an anode voltage of 95.8 kV.
Practical operation outputting 2.5-MW power, which
requires two focusing coil power supplies and thestandard setting of the modulating-anode and cathodevoltages giving the specified beam perveance, wasachieved at a beam voltage of about 104 kV and anefficiency of about 56%. These data are shown in Fig. 5and the waveforms are shown in Fig. 6.

3

Efficiency(%)

50

Efficiency

240

1.5

30

120

Saturated
Output Power0.5060The development of a 324-MHz high-power klystronwas successfully conducted and all tubes required for the200-MeV proton linac were ordered. RF performances
including the high-power klystrons were also successfully
obtained in the klystron gallery of the test facility of theJoint Project at the KEK. A commissioning study of thefront accelerators of the 60-MeV proton linac is now
being conducted. From the fall of 2004, the installation ofklystrons in the newly constructed building at the Tokaisite is scheduled.

60708090100

Beam Voltage(kV)

100110

Output Power(MW)Output Power(MW)32.5

4 SUMMARY

2.5
2V=110kV

1.5
105

1015

20

Input Power(W)2530

Figure 5 Characteristics of the newly developed klystron.
Since in this operation mode, the working point ischosen to be in a region of 10-20% lower than thesaturation level in the range of the input drive power, thisobtained result was satisfactory. We had tested 7
klystrons, and so far had no serious troubles, such asarcing, which the crowbar circuit started to act on. Duringhigh-power evaluation tests, troubles with the dummyload were experienced at a 3-MW output power with a620-µsec pulse width and a 50-pps repetition rate. It was
thus necessary to improve the high-power coaxial dummyload. The klystron window’s performance was operated
without any serious troubles. The original design adopted
forced air-cooling to the window. We recently changed itto water-cooling after considering the maintenance. Froma practical usage point of view, it was necessary toreinforce the X-ray shield of the klystron, though it isequipped with a lead shield at the region from the gun tothe drift-tube and the collector region. Measuring the Xrays from the klystron at the full rating was performed
and used to develop a compact X-ray shield. Anadditional shield was set in the region from thepenultimate cavity to the output cavity. Therefore, acomplete full model suitable for installing in the klystrongallery of the 200-MeV proton linac was established.
An rf feedback study and high-power component tests
given in reference [7] were performed using this klystronin the gallery. High-power tests of the RFQ, DTL andSDLT structures [8] were also performed using these 324MHz klystrons.

Figure 6 Waveforms of pulsed current from the cathode(top), output RF power (middle) and input RF power(bottom). One division for the horizontal scale is 200 µs.

5 REFERENCES[1] Accelerator Technical Design Report for HighIntensity Proton Accelerator Facility, to be published.
[2] F. Naito and Joint Project Team (JAERI/KEK),"JAERI/KEK Joint Project and its Performance",
Proc. of Linac 2002, Pohan, Korea (2002).
[3] W. R. Nelson, H. Hirayama and D. W. O. Pogers,"The EGS-4 code system", SLAC report 265 (1985).
[4] M. E. Reads, et al., "Depressed Collector for Highpower Gyrotrons", IEEE Trans. Electron Devices,
vol. ED-37, pp. 1579-1589 (1990).
[5] Z. Fang and S. Fukuda, "Instability Caused byBackstreaming Electrons in Klystron", Proc. ofAPAC2001, Beijing, China, (2001).
[6] A. Yano et. al., "Development of the Klystrons for
the High Intensity Proton Accelerator Facility", Proc.
of APAC2001, Beijing, China, (2001).
[7] S. Yamaguchi et al., "An Overview of the Rf System
for the JAERI/KEK High Intensity Proton Linac",Proc. of Linac2002, Pohan, Korea (2002).
[8] See references sited in reference[2].

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