The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan1993

THE RF SYSTEM OF FELl
Y.Morii, Y.Miyauchi, A.Koga, H.Abe, T.Keishi, I.Bessho, and T.Tomimasu
Free Electron Laser Research Institute, Inc.(FELI)
2-7-4 Kyoumachibori Nishi-ku Osaka city 550

Abstract
FEU (Free Electron Laser Research Institute, Inc.) isconstructing a Free Electron Laser facility covering from 20 J1. m(infi:a red region) to 0.35 J1. m (ultra violet region}, using an
S-band linac. The building will be completed in November
1993 and installation of the linac will start in December 1993.
The linac consists of a thermoionic 0.5ns-pulse triggered
gun, a 714MHz SHB (subharmonic buncher), a 2856MHz
standing wave type buncher, and 7 ETL (Electrotechnical.Laboratory) type accelerating sections.
An RF system of the linac for FELs is required of longpulse duration and high stability. S-band klystrons (TOSHIBAE3729) are operated in three pulse operation modes (pulse widthand peak RF power); 24 f..L s-24MW, 12.5 p. s-34MW, 0.5 p.
s-70MW. Each klystron modulator has the PFN consisting of 4parallel networks of 24 capacitors and 24 inductors, and it has aline switch of an optical thyristor stack. These equipments aremanufactured now, and an S-band klystron and modulator will
be combined to test their performance at the works of NISSINELECfRIC Co. in October 1993.

where R denotes shunt impedance, P denotes RF power, Ldenotes the length of the accelerating structure, and r denotes
an attenuation parameter. Second term at the right side is beamloading effect and somewhat small. So the stability of energy
gain is determined by the stability of RF power P. To stabilize
the energy within 0.1 %, the stability of P should be better than0.2%.
The klystron output power stability l:.P/P is related to thestability of cathode voltage 6. VN as follows,

6P

5t:.v

p2v.

(2}

So the stability of klystron. modulator voltage should be better
than 0.08%[2).
200
1!!0r

Cal

100
<!
50
0Introduction

"'""

-50

The electron beam of linac for high quality FEL should
satisfy following requirements.
(1) Long pulse duration(2) High peak current(3) Low energy spread(4) Low emmitance(5) Period of micro pulse should be highly stabilized
So the RF system of linac for FEL is required of longpulse duration, high stability in RF power, RF phase.
A laser performance data at FELIX[1) illustrated in Fig. 1
is very instructive. It shows that it takes about 3 J1. s fromelectron beam rising to laser's saturation at the wave length of
35 p. m. It should take longer at shorter wave length laser
because gain of undulator decreases. On the other hand thetypical pulse duration of most of S-band linacs in Japan are afew micro seconds. So it is difficult to saturate the FEL in sucha short pulse linac. The pulse duration of FEU is 24 f..L s for
shorter wave length laser (visible or ultra violet}, 12.5 f..L s forinfra red region laser, and 0.5 J1. s for injection mode to a storage
ring with an FEL resonator.
The beam energy gain T is(1)__: 175-.
~?;-2i!!

10t(fU)

Fig. 1 Beam current and laser intensity in FELIX[1](a) is beam current, and (b) is laser intensity.
The layout of RF system are shown in Fig. 2. Mainsignal generator oscillates 22.3125MHz, and the frequency ismultiplied to 178.5MHz, 714MHz and 2856MHz. The pulser
for thermoinic gun uses 178.5MHz for 5.6ns micropulse
repetition or 22.3125MHz for 44.8ns one. The micropulse
width from the gun is 0.5ns, which corresponds to 130• for714MHz. So the SHB uses a 714MHz. As an SHB cavity ismade of stainless steel, the shunt impedance of cavity is about
0. 7M Q . Therefore the SHB cavity needs comparatively large
RF power 15kW.
The first main klystron of S-band supplies RF power to abuncher and three accelerating sections, and the second mainklystron supplies RF power to four accelerating sections.

FREQUENCY UP CONVERTOR---.,:+- TIIC

22. 312511z
t30dllGRID
POLSER I:+-PULSE IIDTI-- -----,:.,_TRIG

l't- PULSE IIDTB'''Fig. 2 The layout of RF system of linac

S-band klystron
As the klystron of our linac for FEL should be availablefor long pulse operation, we chose the klystron E3729(TOSHffiA) which was modified from the type of E3712 (4.U s-80MW, 50pps). In long pulse operation (24 .U s-24MW,lOpps) it is no problem for the cathode and collector of E3712
in the thermal point. So E3729 uses same body of E3712, but
the tuning of cavity is changed so as to improve the efficiency
at long pulse and low power operation. But the efficiency atshort pulse and high power operation is sacrificed a little. The
properties of E3729 are shown in Table 1.
a0.5
12.5
24Pulse width( .U s)
390304285
Cathode voltage(kV)477333308Cathode current(A)347024
RF output power(kw)Table 1 Characteristic properties of klystron(E3729)

Klystron Modulator

The inductors are equipped with a n;totor drive plunger tuner tovary the inductance. A computed wave form of output voltage
in 24 .U s operation is shown in Fig. 4.
.
A charging power supply consists of not IVR and De-Q
circuit but an inverter and converter for high stability. It candeliver 3A at 50kV with an output voltage stability within ±0.01% with input voltage stabilized within ± 1% by a motor
generator.
The line switch consists of not thyratron but 30 opticalthyristors of type SLG1500GX22 (TOSHmA) in series. An
saturable inductor is placed in series in order to protect the eachthyristor from overvoltage because of time lag of each thyristor's
tum on.
The voltage drop across thyratron lies 100-250V, andthis voltage varies about 10%. If the output voltage of PFN is20kV, this variation of voltage means an uncertainty of 0.1%.
The stability of final out voltage should be within 0.08%, sothyratron is not acceptable. For this reason, we choose
thyristor. Though the pulse rise time of our modulator iscomparatively small (about 2.U s), the specification of operationof thyristor's stack (42kV, 10000A) is difficult. So the life time
test of thyristor was done when we chose thyristor.

·In order to satisfy the requirement of 24 .U s pulse durationand 0.08% output voltage stability, our klystron modulator has
following measures. The block diagram of klystron modulator
is shown in Fig. 3 and its properties are shown in Table 2.
PFN (pulse forming network) consists of 24 sections 4parallels in 24 .u s operation, 14 sections 4 parallels in 12.5 .u s
operation, and 3 sections 8 parallels in 0.5 .U s operation.. Eachsection contains a inductor (11.4 .U H) and a capacitor (37nF).

-176-Flat top duration ( .u s)
Output voltage (kV)
Output current (A)Output stability (%)
Repetition frequency (Hz)

24285
308
0.08
10

12.5
304
333
0.08
100.5
390
477
1.5
10

Table 2 Characteristic properties of klystron modulator

S-band driver amplifier

Acknowledgment

An S-band driver amplifier has a klystron Tif2436(IHOMSON). Its peak power is 3kW and the RF power isdelivered to three klystrons. Two of them are klystrons E3729
and one is klystron for an RF Gun. This modulator uses acommercially available MOS-FET modules (BEHLKE),switching l.lA at lOkV.
A preamplifier is a class C type transistor amplifier with apin diode switch. This switch is used for deciding input RF

The authors would like to express thanks to H. Yonezawa
of TOSHffiA Co. for the klystrons of E3729 and 1AV88R, Y.
Miyai, H. Ooshita, I. Ito of NISSIN ELECTRIC Co. for theS-band modulator, S. Miyake of DAIHEN Co. and H.
Matsumoto of NIHON KOSHUHA Co. for the S-band driver
amplifier, M. Yuura of PULSE ELECTRIC ENGINEERING
CO. for the SHB modulator, and T. Suzuki, S. Nishihara ofMITSUBISIHI ELECTRIC Co. for the injector and accelerator

pulse width and protecting for these klystrons.

tubes.

SHB amplifier

References

The amplifier for SHB contains a klystron lAV88R(TOSHffiA), which is widely used for UHF TV. Its maximum
output power is 15kW (714MHz). The modulator uses MOSFET modules (PULSE ELECTRIC ENGINEERING) switching
2.1A at 16kV.
A preamplifier is a class C type transistor amplifier with a[1] R. J. Bakker, "Free electrons as a versatile souce of coherent
radiation. Doctorial Thesis in FELIX 1993.
[2] B. J. H. MEDDENS "A 50MW Pulse forming network witha voltage stability within 0.03%." The FELIX project report1992.

pin diode switch.

Klystron
CON!NV3¢60HzEOL

DCG

PFN

DC50kV

Bias coil Healer power supply

440V±l%Charging
sectionControl
section

Pulse fanning
sectionFig. 3 The Block diagram of S-band klystron modulator88.69

no88.6S
80iJ

88.61

so~~

0.08~

8S.S7

2088.531020~lls)

30

40

soo·

1020

30

40~"''

Fig. 4 A cumputed wave form of output voltage of PFN
Right side view is expanded view of flat top of left side.

-177-