Status of JLC Accelerators Development

Yong Ho Chin, Toshiyasu Higo, and Junji Urakawa

High Energy Accelerator Research Organization (KEK)

**Abstract.** In this paper, we summarize the status of the JLC X-band
linac development and ATF (Accelerator Test Facility) activity. The
report on the C-band RF system development can be found in Ref.1.

# X-band main linac

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 the first klystron and then to
advance to 75MW at the second one. The first PPM klystron has been
tested since July 2000, and it outperforms its goal and all other
(solenoid-focussed) X-band klystrons built at KEK. As for the modulator,
the three-stage IGBT modulator module was tested successfully and
produced 6kV-2kA output pulse. KEK is developing the multi-mode 2x2 DLDS
(Delay Line Distribution System) power distribution system, in which the
stability of linearly polarized TE~12~ mode in a long (up to 160m)
waveguide is the key for low loss power transport. The experiment was
carried out successfully in the ATF linac tunnel in collaboration with
SLAC using a specially installed 55m long waveguide. Details of these
developments and measurement results are presented.

## Klystrons

The 1-TeV JLC (Japan e^+^e^-^ Linear Collider) project \[2\] 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 project with Toshiba to
produce two PPM klystrons in two stages. The design parameters of those
klystrons are shown in Table 1. The main emphasis of the Toshiba PPM-1
klystron is to test a new gun design and to study the design and
manufacturing of the PPM circuit. The damping structure of parasitic
modes was adapted by using the stainless steel and the Monel as the tube
and cavity materials and by tilting the output couplers slightly to
break the symmetry in the output cavity. Figure 1 shows the Toshiba
PPM-1 klystron before mounted on the test bench. The high power testing
of the Toshiba PPM-1 klystron was started in July 2000 and is still
under way. Figure 2 shows the output power as a function of the cathode
voltage. Up to the date (November 9,2000), it achieved the maximum
output power of 68MW at 514kV beam voltage (the corresponding efficiency
is 47%).

No oscillation of parasitic mode was observed. The particle transmission
was found to be 100% when no RF signal is applied. At the design pulse
length of 1.5 μs, the output power of 56MW was achieved so far. But,
this is not the limit. Further conditioning of the windows would
increase the maximum output power at 1.5 μs. From the measurement of the
power gain, it was found that Toshiba fabricated the two gain cavities
at wrong frequencies. This results in the loss of efficiency by about
3%. The re-testing of the PPM-1 klystron is currently under way after
the wrongly made cavities were corrected and more robust Kazakov windows
\[3\] were installed.

**TABLE1:** Main parameters of the Toshiba PPM-1 and PPM-2 klystrons.

  ----------------------------------------------------- --------------------- ---------------------
                                                        **Toshiba PPM-1**     **Toshiba PPM-2**

  **Peak power (MW)**                                   \>50                  75

  **Beam voltage (kV)**                                 480 - 500             480 - 500

  **Micro-perveance**                                   0.8                   0.8

  **Efficiency (%)**                                    \>50                  55

  **Pulse length (μs)**                                 1.5                   1.5

  **Repetition rate (pps)**                             50                    150

  **Bandwidth (MHz)**                                   80 at -1 dB           80 at -1 dB

  ![](media/image1.jpeg){width="2.7715277777777776in"   Air                   Water
  height="3.6958333333333333in"}**Cooling of PPM**                            
  ----------------------------------------------------- --------------------- ---------------------

**FIGURE 1**: Toshiba PPM-1 klystron before mounted on the test bench.

![](media/image2.pct){width="3.7006944444444443in"
height="3.6798611111111112in"}**FIGURE 2:** Output power vs. cathode
voltage at Toshiba PPM-1 klystron

The second PPM klystron is now in manufacturing. This klystron
incorporates experience and knowledge gained from the design and testing
of Toshiba PPM-1. The main goals of the Toshiba PPM-2 are to manufacture
a PPM klystron that meets the specifications of the JLC project and to
refine the design and manufacturing process for future mass production.
To meet these goals, the PPM-2 has the water-cooling to allow a higher
repetition. The RF system was also improved for a higher efficiency. The
high power testing is to start in April 2001.

## DLDS Pulse Compression and Distribution System

In the simplest DLDS (Delay Line Distribution System), illustrated in
Fig.3, the RF power from two klystrons with independent phasing is
combined through a 3-dB coupler. One output port of the 3-dB hybrid is
connected, 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 is connected 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 feed
through the delay. The second half of the RF pulse is fed into the linac
close to the klystrons, without delay \[4\].

The largest drawback of the original (single mode) DLDS is that it
requires long waveguides: a maximum of 17 waveguides run together inside
the linac tunnel. A conceptual improvement was proposed by SLAC to
further reduce the length of waveguide system by multiplexing several
low-loss RF modes in a same waveguide. Thus, the sub-pulses in the
distribution waveguide are carried by different waveguide modes so that
they can be extracted at designated locations according to their mode
patterns. Taking advantage of both the single mode and the multi-mode
DLDS, a 2x2 DLDS was proposed at KEK to deliver RF power to four RF
clusters \[5\]. Its scheme is illustrated in Fig.4. It consists of
almost identical dual mode DLDS systems with long and short waveguide.
Only two modes (TE~01~ and TE~02~ or TE~12~) are used in each waveguide
to minimize the complication caused by handling of multi-modes in one
waveguide, while still providing some of the benefit of multi-mode
operation by considerably reducing the total length of waveguide. The
advantages of 2x2 DLDS can be summarized as (1) Cost saving: It reduces
the total length of waveguide to 2/3 of single mode DLDS. (2) High
efficiency: The power loss is about 30% smaller than the SLAC 3+1 DLDS.
(3) High expandability: It can be easily expanded to feed up to 5-7 RF
clusters. The last advantage is truly beneficial for cutting the
construction cost of LC if a long RF pulse (2-3 μs) is available from
klystrons in the future.

![](media/image3.pct){width="4.665277777777778in"
height="2.410416666666667in"}

**FIGURE 3:** A schematic diagram of the simplest factor-2 DLDS.

![](media/image4.pct){width="4.507638888888889in"
height="3.2131944444444445in"}

**FIGURE 4:** Schematic view of 2x2 DLDS.

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 waveguide manufactured 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%)
\[4\].

The critical issue in this scheme is the stability of linearly polarized
TE~12~ mode in a long waveguide (that of theTE~01~ mode has been well
investigated at SLED-II). Joint experiments with SLAC were performed at
KEK on a delay line assembled in the ATF linac tunnel \[6\]. The typical
setup is illustrated in Fig. 5. The transport line in our experiment was
composed of eleven sections of a circular waveguide with a diameter of
12.065 cm. Each section was five meters long, for a total length of 55
meters. The sections were connected using a choked flange. This
connection was designed to operate well for both the TE~01~ mode and the
TE~12~ mode. The mode analyzer was installed at the end of the line to
measure mode conversion due to this highly over-moded waveguide (102
modes can propagate in this guide.). The experimental findings can be
summarized as follows:

1.  The rotation of the TE~12~ mode is smaller than 1 degree.
    Furthermore, the level of the circularly polarized component in both
    cases is very small. Hence one can conclude that there is no
    significant mode cross polarization mixing.

2.  The mode contamination in all modes is well below -20 dB before and
    after the transport line.

The overall level of the signal received is -0.37 dB, which indicates
that approximately 8.5% of the input power has been lost. This cannot be
accounted for by the losses in the two mode transducers and the
arc-tapers (4.5%), the transmission line losses (theoretically 2.8%),
plus mode conversion to theTE~14~ mode (0.5%). One can only conjecture
that there are additional losses due to conversion to some TM modes,
which were not measured by our mode analyzer.

From this experiment, one can conclude that the TE~12~ mode can be used
as one of the dual modes in the 2x2 DLDS. Recently, the difference
scheme of the 2x2 DLDS was proposed at KEK in which theTE~02~ mode is
used instead of the TE~12~ mode. The TE~02~ mode has no electric field
at the surface of the pipe, and thus it should be less sensitive to the
imperfection of the pipe shape and the surface condition than the TE~12~
mode. This allows a looser tolerance for the pipe fabrication and the
insertion of expansion joints (to absorb the thermal expansion of the
pipe) without increasing the transmission loss too much. The experiment
of TE~02~ mode stability and the measurement of loss in the 55 m long
pipe were done recently at KEK and it shows that the TE~02~ mode is
indeed quite stable and has a 30% smaller loss than the TE~12~ mode. The
RF components for the TE01/TE02 dual DLDS were all designed and their
cold models will be tested soon.

![](media/image5.pct){width="4.177083333333333in"
height="2.652083333333333in"}

**FIGURE 5:** Experimental setup for TE~12~ mode transmission
experiment.

## Modulator

To improve the reliability of the modulator, KEK has developed a
solid-state switch to replace the thyratron tubes \[7\]. The Static
Induction SI-thyristor is suitable for the switch device because of its
high-power handling and fast turn-on capability. KEK has investigated
the NGK RT103N 4kV reverse conducting SI-thyristor (including
freewheeling diode with press-pack ceramic housing. To evaluate the
performance of this device, the fast turn-on characteristics of five
stacked SI-thyristors connected in series were studied using a very
low-inductance circuit. By using a coaxial structure, the residual
inductance was successfully reduced to less than 136 nH. When an anode
voltage of 15 kV was applied, a maximum peak current of 10 kA, dI/dt of
110 kA/μs, and switching time of 128ns were obtained. The switching time
is the time required for the anode voltage to decrease to 10% of its
maximum value. It was confirmed that the turn-on characteristics of the
SI-thyristors are comparable to thyratrons.

Next, a 45kV solid-state switch was designed and built using the same
devices. The switch uses a stack of 15 circuit card assemblies in
series. Each circuit card assembly consists of a SI-thyristor, a
resistor capacitor network and a gate-driving circuit. The trigger and
DC power for each card were isolated from high voltage through ferrite
core transformers. A photograph of the stack assembly is given in Fig.
6. The stacked devices are housed in a single cylindrical tank with a
diameter of 300mm and a height of 550mm. The tank is filled with oil to
insulate and cool the internal devices.

The solid-state switch performance was investigated with a line-type
5045 klystron modulator at the KEK Accelerator Test Facility, ATF. The
switch was successfully operated at 45kV and 6000A with a 6μs
pulse-width at 25 Hz. The switch losses were measured by calorimeter and
found to be 41 J/pulse at 45kV. This value corresponds to 5% of the
total PFN stored energy, with about 90% of this loss dissipated in the
devices themselves. Therefore, these switches still need to be improved
to further reduce losses. However, it has been confirmed that the 45 kV
Si thrystor-switch has a switching capability comparable to a thyratron.

![](media/image6.pct){width="2.7756944444444445in"
height="4.163194444444445in"}

**FIGURE 6:** 45kV solid state switch.

KEK is now developing a new IGBT (Insulated Gate Bipolar Transistor)
modulator as a two year project. The IGBT is another solid-state switch
and is mainly used in trains. The IGBT typically operates at 2.5kV and
switches 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. 7. 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
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 with a flat-top width
of 1.5μs. A prototype of the module with only three stages for 6kV-2kA
output pulse was built and tested successfully (Fig. 8). Figure 9 shows
the measured voltage waveform. The waveform can be made flatter by using
a waveform compensation circuit to be built in 2001.

![](media/image7.pct){width="5.342361111111111in"
height="3.3881944444444443in"}

**FIGURE 7:** Diagram of the IGBT modulator.

![](media/image8.jpeg){width="5.083333333333333in"
height="3.8402777777777777in"}

**FIGURE 8:** The three-stage prototype of switching module.

![](media/image9.pct){width="3.9131944444444446in"
height="2.5729166666666665in"}

**FIGURE 9:** Output voltage waveform of the three-stage prototype
module.

## RF Structure

The required features of the main linac structures are the good
alignment of the constituent cells and the realization of the design
frequency distribution of the dipole modes in addition to the
accelerating mode frequency tolerance \[4\]. These features have been
realized by utilizing the present-day high-accuracy diamond turning
technology followed by the diffusion bonding process. This technique has
been applied in Damped Detuned Structures, DDS. Recently, a Rounded
Damped Detuned Structure, RDDS, was designed to increase the shunt
impedance by shaping the inner shape of each DDS accelerating cell.

In order to keep the acceleration efficiency high under random frequency
errors in accelerating mode, the random error tolerance was set at 3 MHz
RMS. On the other hand, we set the tolerance of 5 degrees in the
integrated phase advance error at any point along the structure. This
requirement is equivalent to the systematic frequency tolerance of 0.1
MHz if the error is uniform over a structure. Even though this
requirement seems very tight, it can be met with applying a feed-forward
in some dimension of the cells of the later fabrication during the
continuous fabrication of disks.

An important requirement on the dipole-mode frequency distribution is
its smoothness of its distribution. The random error in the higher-mode
frequencies was 3 MHZ RMS. On the other hand, a systematic error as an
offset over the whole structure does not affect the wake field at all.
However, higher components of the error distributions should be within a
few MHz.

We specified that the contour shape of the cell stayed within ±1 μm.
This amount of error limits the frequency error to be 3 MHz at maximum,
even in the extremely unlucky error distribution both in the
accelerating mode and in the first dipole mode. Therefore, under this
tolerance, the requirement on the random error is satisfied.

The most severe requirement is the systematic frequency tolerance of the
accelerating mode. If the contour shape error is within the same
tolerance, the accelerating mode frequency deviates within a MHz because
such dimensions as beam hole radius, \"a\", and accelerating cell
radius, \"b\", are very well correlated by the help of NC
characteristics so that the frequency perturbations due to \"a\" and
\"b\" cancel even partly to reduce the frequency sensitivity on
dimensional error considerably. In this situation with a moderate
controllability of frequency, we can make a feed-forward correction to
the dimension of \"b\" in the later fabrication of disks based on the
average frequency information of the early-made disks.

Figure 10 shows the scattering of the π-mode frequencies of the dipole
mode of each disk for RDDS1 structure. This frequency smoothness is
essential to controlling the dipole wake field. We found that the
deviation of 0.6 MHz RMS is much less than the requirement of 3 MHz.

![](media/image10.pct){width="3.6145833333333335in"
height="2.1868055555555554in"}

**FIGURE 10:** Each-disk frequency deviation from smooth fit.

![](media/image11.pct){width="3.0902777777777777in"
height="2.234722222222222in"}In Fig. 11 are shown the measured
accelerating mode frequencies, the associated integrated phase slip
along the structure and the feed-forwarded values in \"2b\". It is seen
from the figure that the integrated phase slip was kept within 5 degrees
showing that the feed-forward method worked well. It is to be noted that
this feed-forward mechanism can compensate not only the machining error
of precision lathe but also the errors in 3D geometries made by milling
machine which is usually less accurate than the precision lathe.

**FIGURE 11:** Accelerating mode frequency.

From this fabrication of the disks for the first prototype RDDS
structure, we proved that the disks which satisfy the frequency
requirements of both accelerating mode and dipole mode can be realized
through a high-accuracy diamond turning machining with the help of a
careful feed-forward technique.

# ATF

The ATF \[8\] has been designed to investigate the feasibility of the LC
operation scheme and to develop beam-control techniques for the LC. The
purpose of the ATF is to develop accelerator technology that can stably
supply to the main linear accelerator an extremely flat \"multi-bunch
beam\" that can be squeezed down to a few nanometers at the collision
point.

The KEK/ATF consists of an S-band high gradient linac (Linac), a
beam-transport line (BT), the damping ring (DR) and an extraction line
(EXT). The pre-injector was completed in Aug. \'93, when the development
of multi-bunch beam-diagnostics started. In Nov. \'95, we completed the
high-gradient linac so that experiments on the acceleration of a
multi-bunch beam and on the compensation of multi-bunch beam loading
could be performed. After installation of the main hardware components,
in Jan. \'97 we started beam commissioning in the damping ring. In
November 1997, we completed the extraction line for precise beam
diagnostics. Presently, we are refining the beam-tuning techniques and
are stabilizing the key machine components to supply the extremely small
emittance beam stably into the extraction line. Since many beam
instrumentation devices in the ATF turned out not to be sufficient for
precise beam tuning and measurements, we are also upgrading each of the
systems and are developing new diagnostics. For example, a laser wire
and skew quadrupole magnets were installed near the end of JFY \'99 to
measure the tiny beam size in the ring and to control the tilt of the
beam at the extraction line. Table 2 summarizes the achieved accelerator
performance of the ATF.

The purpose of the ATF is to generate beams with very small transverse
and longitudinal emittances as required for future linear colliders. So
far, a horizontal emittance of 1.4 ± 0.3 nm was measured with
wire-scanners in the extraction line and with the horizontal spatial
coherence using a SR interferometer. In addition, a vertical emittance
of 15 ± 2.5 pm was measured with 5 wire scanners in the extraction line.
Still much effort is needed in order to stably produce a beam with 10 pm
vertical emittance at 1.3 GeV. A large number of beam studies conducted
in the ATF Linac since JFY 1995 proved the soundness of the ±∆F
multi-bunch beam loading compensation scheme. The experiment
demonstrating this scheme was conducted with 6×10^9^ electrons/bunch and
20 bunches/train. The original multi-bunch energy spread of 5% from head
to tail was corrected to within 0.6%. Since JFY 1994 various new
instrumentation has been developed for multi-bunch high-precision beam
diagnostics, for example, synchrotron and optical transition radiation
(SR and OTR) monitors with 3 nsec fast gate, a micron beam size monitor
using the extraction line wire scanners for emittance measurements, an
SR interferometer, and a laser wire beam size monitor in the ring. All
of these are still being improved in order to establish beam control
techniques adequate for the micron world \[9\].

**TABLE 2:** Achieved and design parameters at ATF

  ------------------------------ -------------------- -------------------
  **Item**                       **Achieved values**  **Design**

  **ATF Linac Status**                                

  **Maximum Beam Energy**        1.42GeV              & 1.54GeV

  **Maximum Gradient with Beam** 28.7MeV/m            30MeV/m

  **Single Bunch Population**    1.7 × 10^10^         2 × 10^10^

  **20 Multi-bunch Population**  7.6 × 10^10^         40 × 10^10^

  **Bunch Spacing**              2.8 ns               2.8 ns

  **Repetition Rate**            12.5Hz               25Hz

  **Energy Spread (Full Width)** \< 2.0% (90% beam)   \<1% (90% beam)

  **Damping Ring Status**                             

  **Maximum Beam Energy**        1.28GeV              1.54GeV

  **Circumference**              138.6 ± 0.003m       138.6m

  **Momentum Compaction**        0.00214              0.00214

  **Single Bunch Population**    1.2 × 10^10^         2 × 10^10^

  **COD(peak to peak)**          x≈2mm, y≈1mm         1mm

  **Bunch Length**               ≈6mm                 5mm

  **Energy Spread**              0.06%                0.08%

  **Horizontal Emittance**       (1.4±0.3) ×10^-9^ m  1.4 ×10^-9^ m

  **Vertical Emittance**         (1.5±0.25) ×10^-11^  1.0 ×10^-11^ m
                                 m                    
  ------------------------------ -------------------- -------------------

# Acknowledgments

The authors would like to thank all members of the JLC X-band and C-band
Main Linca Groups and the ATF for preparation of this document.

# references

1.  H. Matsumoto, T. Shintake, et. al., \"R&D status of the C-band
    RF-system development for e+e- Linear collider in Japan\", KEK
    Preprint 99-83, 1999.

2.  \"JLC Design Report Study\", KEK Report 97-1, 1997.

3.  S. Y. Kazakov, KEK Report 98-120, 1998.

4.  \"International Study Group Progress Report on Linear Collider
    Development\", KEK Report 2000-7, SLAC R-559, 2000.

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) Mode in Circular
    Waveguide for Low-Loss, High-Power RF Transmission\", Phys. Rev. ST
    Accel. Beams 3: 082001, 2000.

7.  M. Akemoto, et.al., \"Development of a High-Power Klystron Modulator
    with SI-thyristor Switch\", SSID-00-5, 2000.

8.  Edited by E. Hinode, et. al., KEK Internal 95-4, 1995.

9.  J. Urakawa, \"Experimental results and technical research and
    development at ATF (KEK)\", KEK Preprint 2000-67, 2000.