STATUS OF X-BAND POWER SOURCE DEVELOPMENT FOR JLC
Y. H. Chin, High Energy Accelerator Research Organisation (KEK), Tsukuba, Japan
Abstract
The main X-band linac consists of klystrons,
modulators, DLDS pulse compression/delivery systems,
and RF accelerating structures. The periodic permanent
magnet (PPM) klystrons are under development in the
two-year/two-stage project with Toshiba. The goal is to
produce 50MW output power at 1.5 µs pulse length at thefirst klystron and then to advance to 75MW at the second
one. The first PPM klystron has been tested, and it
achieved 56MW power with 50% efficiency at 1.5 µs
pulse length. The second PPM klystron is currently under
testing. Up to date (June 6,2001), the klystron alreadyproduced 73.2MW at 500 kV at the 1.4 µs pulse length.
The maximum efficiency reached 56% at the specified
cathode voltage. As for the modulator, KEK is now
developing a new IGBT (Insulated Gate Bipolar
Transistor) modulator as a two-year project. Two
prototypes, with three and ten stages of modules for 6kV
and for 20kV output pulse each, were built and tested
successfully. KEK is developing the multi-mode 2x2
DLDS (Delay Line Distribution System) power
distribution system. For the study the stability of
propagation modes on a delay line and for themeasurement of their transmission losses, joint
experiments with SLAC and BINP were performed at
KEK on a 55 m long waveguide assembled in the ATF
linac. The experiments showed the best combination of
two modes to be TE01 and TE02 modes. The design of all
RF components for the TE01/TE02 2x2 DLDS wascompleted and their cold models will be tested soon.
Details of these developments and measurement results arepresented.

beam size is tuneable between 2.3 mm and 3.3 mm bychanging a combination of the bucking coil and matchingcoil currents. A critical issue for klystron performance isthe actual dimension of the gun at operating temperature(so-called hot dimension). The PPM-1 hot dimension was
estimated by the thermal code ANSYS (see Fig. 1). An
actual measurement was conducted using a anode-like test
fixture and a laser beam, showing a good agreement with
the ANSYS calculation within 100µm. Prototyping of
the 4.5 period (13.5cm long) PPM circuit and thefabrication of this PPM circuit established the quality
control method on the magnet pieces of 0.5%.
Table1: Main parameters of the KEK PPM-1 and PPM-2
klystrons.

Peak power (MW)Beam voltage (kV)
Micro-perveance
Efficiency (%)
Pulse length (µs)
Repetition rate (pps)
Bandwidth (MHz)
Cooling of PPM

PPM-1
>50
480 - 500
0.8
>50
1.5
50
80 at -1 dB
Air

PPM-2
75
480 - 500
0.8
55
1.5
150
80 at -1 dB
Water

1 PPM KLYSTRON
The 1-TeV JLC (Japan e+e- Linear Collider) project [1]requires about 3200 (/linac) klystrons operating at 75 MW
output power with 1.5 µs pulse length. Periodic
Permanent Magnet (PPM) klystrons are being developed
to eliminate the expense and power requirements of the
focusing solenoids. KEK has begun a two-year projectwith Toshiba to produce two PPM klystrons in twostages [2]. The design parameters of those klystrons areshown in Table 1. The main emphasis of the KEK PPM1 klystron is to test a new gun design and to study thedesign and manufacturing of the PPM circuit. The
damping structure of parasitic modes was adapted by using
the stainless steel beam pipes and the Monel cavities, andby tilting the output couplers slightly to break thesymmetry in the output cavity to damp TE01 modes. The

Figure 1: The thermal calculation of KEK PPM-1 gun by
ANSYS.
Figure 2 shows the KEK PPM-1 klystron before
mounted on the test bench. The high power testing of thePPM-1 klystron was started in July 2000. Figure 3 shows
the output power and the efficiency as a function of thecathode voltage. It achieved the output power of 68MW at
514kV cathode voltage with the corresponding efficiencyof 47%. It also produced 56MW power with standard 1.5

µs pulse length. Neither oscillation of parasitic mode nor
gun oscillation was observed. The particle transmission
was found to be 100% when no RF signal is applied. The
perveance agrees well with the design within 1.5 %.

8070Output Power (MW)
Efficiency (%)

60
50
40
30
20
10

Output Power of PPM-1
Efficiency of PPM-1
Output Power of PPM-1.5
Efficiency of PPM-1.5
Output Power of PPM-2
Efficiency of PPM-2

0
360 380 400 420 440 460 480 500 520
Cathode Voltage (kV)experience and knowledge gained from the design and
Figure 3: Output power and efficiency vs. cathode voltage
at PPM-1, PPM-1.5 and PPM-2 klystrons

Figure 2: Photo of KEK PPM-1 klystron.
The gain cavities were mistakenly fabricated at wrong
frequencies by the manufacture. It was expected to cost 23% on the efficiency. PPM-1 klystron was sent back to
the manufacture to apply the following corrections; (1)replacement of the gain cavities by the ones with correct
frequencies (2) fine tuning of the output cavity
fundamental frequency to a value closer to the design (3)replacement of the RF windows by the more robust
Kazakov windows for a better high-power capability.
PPM-1 was returned to KEK as PPM-1.5 and testing was
resumed from February 2001 and is still under way. Now,
the efficiency was increased to near 50% at 470-500kV
region as shown in Fig.3. The performances of PPM-1
and PPM-1.5 are tabulated in Table 2.
Table 2: Performances of the PPM-1 and PPM-1.5
klystrons.

Peak power tested (MW)
Efficiency (%)
Pulse length (µs)
Micro-perveance

PPM-1
PPM-1.5
68
67.5
47%
49.6
1.5 (at 56MW)
0.79

The second PPM klystron has been delivered and is
now under high-power testing. This klystron incorporates

testing of PPM-1. The main emphases of the PPM-2
klystron are full satisfaction of JLC specifications andrefinement of the design and manufacturing process for
future mass production. To meet these goals, the PPM-2
introduced the water-cooling system of the PPM klystronbody and the PPM circuit. At the same time, the watercooling system of the output cavity was improved for ahigher repetition rate. Some revisions of resonant
frequency of the penultimate cavity and design of theoutput coupler were also applied for a higher efficiency.
Up to date (June 6,2001), the klystron already produced
73.2MW at 500 kV at the 1.4 µs pulse length.
Oscilloscope traces are shown in Fig. 4. The maximum
efficiency reached 56% at the specified cathode voltage.
The performance of PPM-2 klystron up to date istabulated in Table 3. The high-power testing will be
continued to attain 75MW output power with the standard
1.5 µs pulse.
Table 3: Latest measurement results of performance of the
PPM-2 klystron (June 6, 2001)

Peak power (MW)
Efficiency (%)
Pulse length (µs)

Design75
55
1.5

Micro-perveance
Repetition rate (pps)

0.8
150

Achieved
75.1
56
1.5 (@70MW)
1.4 (@73.2MW)
0.79
25

Following the success of those two klystrons, thedevelopment plan is now expanded to the coming two
years. In FY 2001, we will build the PPM-3 klystron;
this klystron has an even higher efficiency of 60% by

0

0

-100

-20

-200

-40

-300

klystrons with independent phasing is combined through a
3-dB coupler. One output port of the 3-dB hybrid isconnected, through low-loss waveguide, to a linac feed
about one half of the compressed pulse width times the
speed of light upstream of the klystrons; the other port isconnected to a local feed. The first half of the input RF
pulse, of duration equal to the sum of the structure fills
time and the bunch train time, is sent to the upstream feedthrough the delay. The second half of the RF pulse is fed
into the linac close to the klystrons, without delay [3].

Ouput Power (MW)Beam Voltage (kV)installing two 2nd-harmonic cavities to increase the beam
bunching. A small revision of the output cavity shaperealises a lower (70MV/m) surface electric field there for
an improved reliability. The PPM-4 klystron to be built
in FY 2002 is a mass production version. For this end,
the design of a clamp-on magnet system is under way.

-60

Reference line
Phase switch
Modulator

Klystron#2

Klystron#1
3dB coupler-400

Output Power (MW)-80

Delay line

Beam Voltage (kV)

-500
0

0.5

1

1.5 2 2.5
Time (µs )

3

3.5

4

-100
structures

structures

Beam direction

Figure 5: A schematic diagram of the simplest factor-2
DLDS.

Figure 4: Oscilloscope traces of the output power andbeam voltage of PPM-2 klystron for 73.2MW output
power at the 1.4 µs pulse length with 54% efficiency.

The largest drawback of the original (single mode)
DLDS is that it requires long waveguides: the maximumof 17 waveguides runs together inside the linac tunnel. A
conceptual improvement was proposed by SLAC tofurther reduce the length of waveguide system bymultiplexing several low-loss RF modes in a same

2 DLDS PULSE COMPRESSION AND
DISTRIBUTION SYSTEM
In the simplest DLDS (Delay Line Distribution
System), illustrated in Fig.5, the RF power from two

Modulator
500kV
1.5µs
100-150Hz

Klystrons
75MW
1.5µs
100-150Hz

X-Band Main Linac Unit

Wall plug
100V
50Hz

Load
TE02 mode
extractor
Waveguide (φ4.75inch)

600MW
400ns

Super
hybrid

1/2 tap-off

200MW
75MV/m

1/3 tap-off

55 m

DLDS
TE01-TE02
mode launcher
600MW
800ns each

RF structure

55 m

55 m

Figure 6: Schematic view of TE01/TE02 2x2 DLDS

waveguide. Thus, the sub-pulses in the distribution
waveguide are carried by different waveguide modes so thatthey can be extracted at designated locations according totheir mode patterns. Taking advantage of both the single
mode and the multi-mode DLDS, a 2x2 DLDS wasproposed at KEK to deliver RF power to four RF clusters[4]. Its scheme is illustrated in Fig.6. It consists ofalmost identical dual mode DLDS systems with long andshort waveguide. Only two modes (TE01 and TE02 /or
TE12) are used in each waveguide to minimise thecomplication caused by handling of multi-modes in one
waveguide, while still providing some of the benefit ofmulti-mode operation by considerably reducing the total
length of waveguide. The advantages of 2x2 DLDS can besummarised as (1) Cost saving: It reduces the total lengthof waveguide to 2/3 of single mode DLDS. (2) Highefficiency: The power loss is about 30% smaller than the
SLAC 3+1 DLDS. (3) High expandability: It can beeasily expanded to feed up to 5-7 RF clusters. The lastadvantage is truly beneficial for cutting the construction
cost of LC if a long RF pulse (2-3 ms) is available fromklystrons in the future.
To demonstrate the fundamental principle of the 2x2
DLDS, a cold model of the dual mode DLDS basic unit
(almost identical to the system with the shorter waveguidemanufactured and tested at KEK. The testing results
proved that it was plausible to deliver power at different
places using different modes with good efficiency (better
than 95%) [5].
In the initial design of the 2x2 DLDS, the TE01 andTE12 modes were chosen to be the propagation modes dueto their theoretically small surface losses (smallest among
all modes) on a round pipe. The critical issues in thisscheme were the stability of linearly polarised TE12 modeand the actual loss in a long waveguide against theimperfection of the pipe shape and the surface condition
due to the fabrication error, pumping slots, flanges, etc.
Meantime, the TE02 mode was conceived as an alternative
choice against TE12 mode due to its small loss (3rdsmallest) and less sensitivity to the pipe imperfection (noelectric field at the surface of the pipe). This allows alooser tolerance for the pipe fabrication and the insertionof expansion joints (to absorb the thermal expansion of
the pipe) without increasing the transmission loss toomuch. Joint experiments with SLAC and BINP were
performed at KEK on a delay line assembled in the ATF
linac tunnel [6]. The typical set-up is illustrated in Fig. 7.
The transport line in these experiments was composedof eleven sections of a circular waveguide with a diameterof 12.065 cm. Each section was five meters long, for a
total length of 55 meters. The sections were connectedusing either a choked flange or conflat flange. The mode
analyser was installed at the end of the line to measuremode conversion due to this highly over-moded waveguide(102 modes can propagate in this guide.). The main
findings of the experiments are;

1.

2.
3.

The rotation of the TE12 mode is smaller than 1
degree. Furthermore, the level of the circularly
polarised component in both cases is very small.
Hence one can conclude that there is no significant
mode cross polarisation mixing.
The mode contamination in all modes is well below 20 dB before and after the transport line.
The power transmission losses over the 55m longpipe are approximately
TE01
TE02
TE12

2% (theory: 1.7%)
4% (theory: 3.2%)
6% (theory: 2.5%)

As for the discrepancies between the theoretical losses
and the measured ones, one can only conjecture that there
are additional losses due to conversion to some TM
modes, which were not measured by our mode analyser.
From these experiments, one can conclude that all themodes are well stable and their transmission losses arewithin tolerances. But, TE01 and TE02 modes are the best
choices in terms of low loss, less sensitivity to the pipe
imperfection and the simplicity of the system. KEK is
now pursuing the TE01/TE02 2x2 DLDS scheme and wecompleted the design of all RF components. Their coldmodels will be tested soon.

Mode La unche r

Tra ns port Line , 55 me te r of circula r
wa ve guide tha t ha s a dia me te r of 12.065 cm
dia me te r

Mode Ana lyze r
Multi-mode Loa d

Low Nois e
Amplifie r

54 me te r of WR90 Re cta ngula r wa ve guideHP 8510C
Dis pla y/P roce s s or

All Conne ctions a re ma de with a pha s e
a nd a mplitude s ta ble ca ble s
HP 8510C IF
De te ctor
8514A S -P a ra me te r
Te s t S e t

HP 8510S ys te m Bus
S we e p In
S top S we e p

Te s t S e t RF Input
HP 8350 S we e p
Os cilla tor
20-dB Dire ctiona l Couple r 1-Wa tt Amplifie rThis P C controls both the ne twork a na lyze r a nd
the mode a na lyze r. It is a ls o us e d for da ta
a cquis ition

GP IB
P C (P e ntium
ba s e d)Figure 7: Experimental setup for the mode transmission
experiments.

3 MODULATOR
To improve the reliability of the modulator, KEK is
now developing a new IGBT (Insulated Gate Bipolar
Transistor) modulator as a two-year project. The main
specifications of this modulator are tabulated in Table 4.
The IGBT is another solid-state switch and is mainly usedin trains. The IGBT typically operates at 2.5kV andswitches up to 3000A. Thus, it cannot replace a thyratron
as a single device and needs to be stacked. The IGBT
modulator diagram is illustrated in Fig. 8. It consists of a
DC power supply, four modules of switching unit at
25kV, a pulse transformer with a ratio of 1:5, and aNumber of klystrons per modulator
Peak klystron voltage
Total peak current
Pulse width
Pulse top flatness
Energy efficiency
Repetition rate2
500 kV530 A
1.5 µs2%
70 %100 Hz7

Output Voltage

70

6

60

5

50

4

40
Gate Voltage(IGBT, 1st stage)

3

30

2

20

1

10

0

0

-1
-1000

0

1000
2000
Time(ns)

3000

Gate Volatge(V)

Table 4: The main specifications of this modulator.

Figure 9: The three-stage prototype of switching module.

Output Voltage(kV)waveform compensation circuit. Each module contains 10
stacks of energy storage capacitors, IGBT switches and
IGBT gate drivers, and it produces 25kV voltage in total.
The four modules are connected in a series-parallel
arrangement and turns on into the primary of a pulse
transformer to produce 500kV-output pulse at 530A witha flattop width of 1.5µs. The individual IGBT has its
own gate driver, which can control the output waveform
to an arbitrary form. The diode network allows theisolation of faulted klystron load and protects the IGBT
circuits from an over-current. Two prototypes of themodules, with three and ten stages for 6kV and 20kV
output pulse each, were built and tested. The three-stage
prototype of switching module is shown in Fig. 9. Figure
10 shows the measured voltage waveform. The waveform
can be made flatter by using a waveform compensation
circuit. The entire system at full specifications with the
waveform compensation circuit and the over-current
protection circuit will be built and tested in spring 2001.

-10
4000

Figure 10: Output voltage waveform of the three-stage
prototype module.
25kV
Switch unit

50kV
2650A

25kV
Switch unit

Two 75 MW PPM Klystrons

Klystron Output Pulse

Step-upInverter
25kV
Switch unit

2.5kV
IGBT Switch

2.5kV
IGBT Switch

Compensation
Circuit

500 kV530 A
1.5 µs Flat-top

50kV
2650A
Pulse Transformer
1:5

Main Output Pulse
Waveform
Compensation
Output Pulse
Waveform

Figure 8: Diagram of the IGBT modulator.

REFERENCES[1] "JLC Design Report Study", KEK Report 97-1, 1997.
[2] Y. H. Chin et. al., "Development of the X-Band RFPower Source for JLC", in Proc. of PAC99, 1999,
p.3414.
[3] "International Study Group Progress Report on Linear
Collider Development", KEK Report 2000-7, SLAC
R-559, 2000.
[4] Y. H. Chin et. al., "Status of the X-Band RF Power
Source Development for JLC", in Proc. ofEPAC2000, 2000, p.456.
[5] J.Q. Wang, Y.H. Chin , et. al., " Study of RF
Components for JLC 2x2 DLDS" in Proc. of
PAC99, 1999, P.1058.
[6] S. Tantawi, et. al., "Evaluation of the TE(12) Modein Circular Waveguide for Low-Loss, High-PowerRF Transmission", Phys. Rev. ST Accel. Beams 3:
082001, 2000.