The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

STATUS OF JLC X-BAND MAIN LINAC DEVELOPMENT
Y. H. Chin and JLC X-Band Main Linac Group, KEK, Tsukuba, Japan
Abstract
In this paper, we summarise the status of the X-band
main linac R&D efforts for the JLC (Japan e•e· LinearCollider) project including the PPM klystron, the IGBT
solid state modulator, the DLDS pulse distribution
system, and the RF structure.

Table l: Design parameters and actual performance of thePPM-1 and PPM-1.5 klystrons.
Design>50>50
1.5
0.850

Peak power (MW)
Efficiency (%)
Pulse length (J!s)
Micro-perveance
Repetition rate (pps)

1 X-BAND MAIN LINAC

Achieved
68
49.6
1.5 (at 56MW)
0.79
5 (25Hz oossible)

1.1 KlystronThe 0.5-TeV JLC project [1] requires about 1800(/1inac) klystrons operating at 75 MW output power with
1.5 J!S pulse length. Periodic Permanent Magnet (PPM)
klystrons are being developed to eliminate the expense andpower requirements of the focusing solenoids. KEK has
begun a two-year project with Toshiba to produce two
PPM klystrons in two stages[2]. The goal is to produce
50MW output power with efficiency>50% at 1.5 J!S pulse
length at the frrst klystron and then to advance to 75MWwith efficiency=55% at the second one. Figure 1 shows
photos of the PPM-1 (right) and PPM-2 (left) klystrons.
The first PPM klystron and its revised version (PPM-1.5)
have been tested, and they achieved 56MW power with
50% efficiency at the standard 1.5 J!S 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 performances of
PPM-1 and PPM-1.5 are tabulated in Table 1.

The second PPM klystron is currently under highpower testing. It has improved water cooling system of
PPM circuit and the output cavity for 150Hz operation.
The RF system was also revised for a higher efficiency.
Up to date (June 6, 2001), the PPM-2 klystron produced
73.2MW at 1.4 J!S pulse length and 70MW at 1.5 J!Spulse length with the efficiency of 54.5%. The
oscilloscope traces are shown in Fig. 2. The maximum
efficiency reached 56% at the specified cathode voltage.
The performance ofPPM-2 klystron is tabulated in Table
2. figure 3 shows the output power and the efficiency a'l
a function of the cathode voltage. More details of
measurement results are found in Ref. 3. The high-power
testing will be continued to attain 75MW output powerwith the standard 1.5 J!S pulse.
()()-100-20'- -200-400~·~

0> -300-60~

a~-400i......~~.§-80-soo L._._._._.~=~=:o;::~~.J00.5

J1.522.53

3.5Time(JlS)

Figure 2: Oscilloscope traces ofPPM-2 output pulses.
Table 2: Latest measurement results of performance of thePPM-2 klystron (June 6, 2001)

Figure 1: Photos of the PPM-I (right) and PPM-2 (left)
klystrons.

Peak power (MW)
Efficiency (%)
Pulse length (J!s)

Design75
55
1.5

Micro-perveance
Repetition rate (pps)0.8
150-122-

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

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Cathode Voltage (kV)
Figure 3: Output power and efficiency vs. cathode voltage
at PPM-1, PPM-1.5 and PPM-2 klystrons

1.2 DLDS Pulse Distribution Systemwaveguide 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[3]. Its scheme is illustrated in Fig.5. It consists ofalmost identical dual mode DLDS systems with long ani
short waveguide. The two propagation modes were chosen
to be TE01 and TE 02 modes from the results of the joint
experiments with SLAC and BINP performed at KEK on
a 55m long delay line assembled in the ATF linac tunnel[4]. In these experiments we measured the stability ofvarious modes and their transmission losses over the 55mlong pipe. The TE01 and TE 02 modes were found to havethe lowest transmission losses and they are lesssensitivity to the pipe imperfection (no electric field at thesurface of the pipe). This allows a looser tolerance for thepipe fabrication and the insertion of expansion joints toabsorb the thermal expansion of the pipe without
significantly increasing the transmission loss. KEK is
now pursuing the TE0 /TE02 2x2 DLDS scheme and wecompleted the design of all RF components. Their coldmodels will be tested soon.

In the simplest DLDS (Delay Line Distribution
System), illustrated in Fig.4, the RF power from two
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 thespeed of light upstream of the klystrons; the other port isconnected to a local feed The frrst half of the input RFpulse of duration equal to the sum of the structure fills
time and the bunch train time, is sent to the upstream reedthrough the delay. The second half of the RF pulse is :ted
into the linac close to the klystrons, without delay.

X-Band Main Linac Unit
TEOlmoJ<:

fiOi.lMW

extractor'\

4(1()M2\..'I!)MW'f5MV!m

JL55-m

SSm55mFigure 5: Schematic view ofTE0 /fE 02 2x2 DLDS

1.3 Modulator

structures

Figure 4: A schematic diagram of the simplest :factor-2DLDS.
The largest drawback of the original (single mode)
DLDS is that it requires long waveguides: the maximumof 27 waveguides run 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
waveguide. Thus, the sub-pulses in the distribution

To improve the reliability of the modulator, KEK isnow developing a new IGBT (Insulated Gate BipolarTransistor) modulator as a two-year project. The main
specifications of this modulator are tabulated in Table 3.
The IGBT modulator diagram is illustrated in Fig. 6. It
consists of a DC power supply, four modules ofswitching unit at 25kV, a pulse transformer with a ratioof 1:5, and a waveform compensation circuit. Each
module contains 13 stacks of energy storage capacitors,
IGBT switches and IGBT gate drivers, and it produces
25kV voltage in total. The four modules are connected ina series-parallel arrangement and turns on into the primaryof a pulse transformer to produce 500kV-output pulse at
530A with a flattop width of l.5!ls. The individual IGBT
has its own gate driver, which can control the output-123-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

waveform to an arbitrary form. The diode network allows
the isolation of faulted klystron load and protects theIGBT 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 successfully. The
entire system at full specifications with the waveformcompensation circuit and the over-current protection
circuit will be built and tested in spring 2002.
Table 3: The main specifications of this modulator.
Number of klystrons per modulator
Peak klystron voltage
Total peak current
Pulse width
Pulse top flatness
Energy efilciency
Repetition rate

2500kV530A
1.5 j.lS
2%
70%
100Hz

examination uncovered significant electrical damage in theupstream cells. This has sharp contrast with earlier testingof the structures with lower group velocity (less than 5%)
and shorter length, which showed no sign of degradation
at the gradient of more than 80MV.
In the hope that a lower group velocity and a shorter
length will help to reduce the damage, a series ofstructures with different lengths and group velocity
profiles are being built and tested. All of the low groupvelocity structures (5%-3%) have reached gradient of morethan 70MV/m and show minimum shift in phase advance.
The processing voltage history is shown in Fig. 6. Basedon these results, we are changing the design for JLC/NLC
structures to have a peak group velocity of 3-5% and alength of 0.9m. This, however, demands the average iris
size too small to meet short-range wakefield requirements.
To increase the iris size, while keeping the group velocity
low, the phase advance per cell should be increased to 150
degree from 120 degree.

Modulator Unit

Processing History of Several Accelerator Structure· (X-band)
2650A

70MV/m

L.....1---1..----1.---1........J......J :f oo~-~-,""
ci40 •. ·i;

120

JGBT Module

Time with RF On at 60 Hz (hours)
1.8 m long with 12% v~

Figure 6: Diagram of the IGBT modulator.

1.4 RF Structure
The main requirement for JLC RF structures is toefficiently provide a 70MV/m-accelerating gradient to abeam, while controlling the short and long-range
wakefields to preserve the very low emittance of themulti-bunch beams. In the past years, the various typesof RF structures have been designed, built and tested incollaboration with SLAC. The latest design is called
RDDS (Rounded Damped Detuned Structure) which not
only detunes higher-harmonic modes out, but also
aggressively damps them. Its first prototype was
successfully built in 1999 and was tested for wakefieldcharacteristics at the ASSET facility at SLAC in 2000.
At KEK, much of attentions have been paid to thefabrication issues such as high-accuracy diamond cutting
and the diffusion bonding of cells.
These structures are 1.8m long and the group velocity
varies from 12% at the input end to 3% at the other end
During recent tests at high power, it was discovered thatthe net RF phase advance through the prototype structures
had increased by roughly 20 degrees per I 000 hours ofoperation at gradient as low as 50MV/m. The microscopic

0.5 m long with 5% vi

1.0 m long wtth 5% v110.5 m long with 3% v~

Figure 7: Processing voltage history for (a) 1.8m
structure with the peak group velocity of 12%, (b) 0.5m
structure with the peak group velocity of 5%, (c) 1m
structure with the peak group velocity of 5%, (d) 0.5m
structure with the peak group velocity of 3%. (Courtesy
D. Burke, 2001)

2 CONCLUSIONSThe X-band R&D efforts are making steady andsignificant progresses in the last few years. As the criticalstep to the next stage of R&D, we are working for thethree-year project to build a basic main linac unit (or itshalf:.scaled version) by 2004 at KEK for testing of its
functionality as system at high power.

3 REFERENCES[ 1] "JLC Design Report Study", KEK Report 97-l, 1997.
[2] Y. H. Chin, et. al., " X-Band PPM KlystronDevelopment For JLC", in Proc. ofPAC2001.
[3] J.Q. Wang, Y.H. Chin, et. al., " Study of RF
Components for JLC 2x2 DLDS" in Proc. of
PAC99, 1999, P.l058.
[4] S. Tantawi, et. al., "Evaluation of the TE(l2) Modein Circular Waveguide for Low-Loss, High-PowerRF Transmission", Phys. Rev. ST Accel. Beams 3:
082001,
2000.

-124-