JAERI-Conf 95-021

2a4-4
Development of Long Pulse, High...; Flatness Pulse Modulator
for an S-hand Klystron
E Oshita, S. Okuma, K. Wakita and T. Tomimasu

Free Electron Laser Research Institute, Inc. (FEU): 4547-44, Tsuda, Hirakata, Osaka 573._01, japan
I. Ito, Y. Miyai, K. Nakata and M. Hakota

Nissin Electric, Co., Ltd.: 47, Umezu-Takase-cho, Ukyo-ku, Kyoto 615, japan
Table 1 Parameters of Klystron Modulator

Abstract
Two infrared free electron lasers (PELs) of theFELl are operating using an S-hand 80-MeV linac
with a thermionic gun and a 165-MeV linac andUV-FEL facility are in the commissioning stage.
Since an RF system for linac-based FELs is requiredof long pulse duration and high quality, improved
pulse klystrons (Toshiba E3729) have been operated in
24-MW, 24- f.1 s pulse mode for the FELl linac. Our
klystron modulator developed by the FELl and NISSINElectric Corp. has an inverter-converter charging
system. The line switch is consisted of 30 lightA
triggered thyristors (Toshiba SL1500GX22).
30
saturable reactor is used in series to protect
thyristors from overvoltage caused by a delay ofthyristor's turn-on time. The flatness of modulator
pulses is 0.08%P-P at 24-MW, 24-f.ls pulse
operation. The 24- f.1 s stable RF pulses can increase aconversion efficiency from electron beam power toFEL power for IR- and UV-FELs.

1. Introduction
The FELl is now operating two IR-FELs using an80-MeV linac with a thermionic gun and is testing a165-MeV linac and an UV-FEL facility. It isessentially necessary for linac-based FELs using
pulsed rf sources to get a stable and long rf pulse froma klystron. A stable and long rf pulse sources enables
to yield a stable and saturated FEL pulse source. TheFELIX group has succeeded in keeping a pulsed rf
source stable to accelerate a 22.5-MeV, 10- f.1 s beamwith an energy spread of 0.5% [1 ]. For this purpuse,
we have developed a 24-MW, 24- f.1 s pulse modulator
for an S-hand klystron (Toshiba E3729) at threeoperation modes shown in Table 1. Mode 1 andMode 2 are for FEL generation and Mode 3 is forinjection to a storage ring.

2. Klystron Modulator
Figure 1 shows the circuit diagram of the klystron
modulator. This modulator is consisted of the charging

Mode
Output voltage(kV)
Output current(A)Pulse width( f.1 s)
Flat-top <P-Pl (%)
Stability (%)
Repetition(pps)
Rise time( f.1 s)
Fall time( f.1 s)Mode 1285
280
240.080.081023Mode 2304
305
12.5
0.08
0.08
1023

Mode 3
390
477
0.5
1.5
1.5
102
3.5

*Rise and fall time is measured from 10-90% of theoutput pulse.
section using the converter-inverter, the pulse forming
network(PFN) section, the main switch section using
light triggered thyristors, and the mounting tank for theklystron to supply high voltage by a pulse transformer.
The output voltage is measured at the secondary sideof the pulse transformer by a capacitive divider.
Details of these sections are as follows.
(1) Charging Section
We use a converter-inverter charging circuit
instead of IVR"'De-Qing one, because De-Qing one
has the following problems.
i) A charging voltage of the PFN involves about 0.2%
fluctuation due to an input voltage.
ii) Pulse repetition rate is low (lOpps). Therefore, thescale of charging section becomes bigger due to a large
time of constant at resonant charging.
Therefore, our inverter is consisted of 5 cascades at
15kHz and we have acheived a high stability within ±1
x 10- 4 of charging voltage.
(2) Pulse Forming Section
The output pulse width of the PFN are 24 f.1 s, 12.5/.1 s and 0.5 /.1 s at mode 1, 2 and 3, respectively. Eachreactor of the PFN is adjustable by means of a remotecontrol system using a motor driving plunger. The
minimum adjustable amout of the PFN reactor is0.005% and the maximum adjustable span is 45%.

-215-JAERI-Conf

Converter I nvcrter DCG~OLPFN
Clipper (Pulse forming Network)

95-021

Earthing
SwitchSaturable
ReactorCrowbarSwitchCharging

Pulse forming

Main switch

sectionsectionsection

Klystron mounting
tankFig. 1 Circuit diagram of klystron modulator
Therefore, the adjustment of output waveform has been
easily performed, and a 0.08% flat-top of output pulse
waveform has been achieved. If the klystron has
break down during a high voltage is applied, the PFN
capacitors suffer from large reverse voltage. To
reduce this damage, an EOL (End of line) clipper
circuit is set and the reliability of the modulator isimproved.
(3) Main Switch Section
We have used the light triggered thyristor(Toshiba
SL1500GX22, 30series) stack as the main switch forkeeping the output voltage stable. Generally speaking,a thyratron is suitable for switching of high voltage
and large current. But in this case, the thyratron is notsuitable because a change of its resistance becomelarge during the conduction time, if the pressure in thetube changes. Therefore, we decided not to use thethyratron in order to achieve the 0.08% stability.
However, in order to use the light triggered
thyristors, we had to solve the following problems.
i) The value of dl/dt is more ten times ("'3000N 11 s)
than the thyristor's specification.
ii) It is necessary to trigger 30 thyristors
simultaneously as a switch.
Before adopting the light triggered thyristor, we havetested dl!dt of the same device. The results was thatthe thyristor was breaked down at about 1700A/ 11 s.
The damage occmred at the only part of near the center
gate. The reason is that the light triggered thyristor
could not have the conduction space by the high dl!dt.
Therefore, we have set a saturable reactor in series
with the light triggered thyristors to secure theconduction space before large and high dl/dt maincurrent follows and to keep a counter-measure to adelay of each thyristor's turn on. The use of thesaturable reactor enables us its running under a hardcondition of dl!dt "'3000N 11 s. On the other hand, theuse of the light triggered thyristors makes it is easy toinsultate the gate drive circuit and to withstand to ahigh reverse voltage.
3. Performance(1) Light Triggered Thyristors
Figure 2 shows the time response of the resistanceof light triggered thyristors at the mode 1. After themain current reached the peak, the resistance is about
0.6-0.3Q/30devices, that is, 20mQ -10mQ/1 device.
The resistance at the whole conduction is about 0.5m Q (at 4kA). Therefore, the conduction space of thisthyristor is about 1/40-1/20 of the whole conduction atthe mode 1. Though we have already tried 4 x 10 7shots under this condition, there are no any troubles atall.

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triggered thyristor-216-0:'-'JAERI-Conf 95-021

It is easily understood from Fig. 2 that theresistance of the light triggered thyristor decreases in
micro-second order, so we can adjust the waveform soas to cancel this effect.
:::-.."" f~ r r..r.· ~c'-r;s~\=m:l-.JIII - !~:1 .,, 1 • ~:'w~,;(2) Klystron Modulator and KlystronTable 2 shows the characteristics of the output atthree modes and Figs. 3 and 4 show the waveforms ofthe output voltage at the mode 1.

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::JModeOutput voltage(kV)
Output current(A)Pulse width( 11 s)
Flat-top <P-Pl (%)
Stability (%)
Repetition(pps)
Rise time( 11 s)
Fall time( 11 s)
RF output(MW)from E3729

Mode 1285
280
23.2
0.08
0.07
102.0
4.5
24

Mode 2304
305
12.0
0.08
0.06
10
2.5
6.5
34

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II

J++!

0I

-- -ttime(S 11 s/div)

Fig. 3 Waveform of output voltage at the Mode 1~-Till -~~ ·.' 1.

*Rise and fall time is measured- from 10-90% of theoutput pulse.

(3) Modulator Pulse and 24- /1 s rf Pulse_
Figure 5 shows the waveforms of the modulator
current pulse and 24- 11 s rf pulse. Tiny ripples seenon the waveforms are due to white noise of thesampling oscilloscope (TDS460-Tecktronics).

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The rise time of the output is about 2-3/1 s. Thisshows that the light triggered thyristors can withstandfor the · rise of 2-3 11 s because of the effect ofsaturable reactor. Fig. 4 shows the flatness of themodulator pulse is kept within 0.08%P-P at 24-MW,
24- 11 s pulse operation./I
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tg>
.....

Mode 3390477
0.5
0.3
0.15
10
2.0
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Table 2 Performance of Klystron Modulator andKlystron E3729

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Fig. 4 Expanded view of top part of the waveformshown in Fig. 34. Conclusions

We have succeeded in the development of the longpusle (24!1s), and high-flatness (0.08%) klystron
modulator using the light triggered thyristor as a main
switch and the remote controll systems for variable
reactors of the PFN. This modulator have contributed
needless to say much to the FEl oscillations[2].

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::J0References[1] D. A. Jaroszynski, et al., Nucl. Instr. Meth., A331,(1993) 52[2] T. Tomimasu, et al., Proc. PAC'95 (DALLAS, May1-5, 1995) FAA30-217-time(S 11 s/div)
Fig. 5 Modulator pulse and 24- 11 s rf pulse