Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

RF PROPERTIES OF THE JAERI ERL-FEL FOR LONG-PULSEOPERATION
M. Sawamura, R. Nagai, N. Kikuzawa, N. Nishimori, R. Hajima, E. Minehara
JAERI, Ibaraki, Tokai, 319-1195 Japan
Abstract
After the success of energy recovery linac (ERL) for
the superconducting free-electron laser (FEL) in the Japan
Atomic Energy Research Institute (JAERI), the JAERIERL-FEL has been upgrading for high-power and longpulse operation. The properties of the superconducting
linac required for long-pulse operation were measured
such as pressure in the cryostat, vibration of cavity
frequency and piezo tuner response.

INTRODUCTION
JAERI has been developing a high-power FEL with asuperconducting linac (SCA). After the initial goal ofkilowatt FEL lasing was achieved in 2000 [1], the linac
has been modified into an ERL [2]. Energy recovery is
the process by which the energy invested in accelerating abeam is returned to the RF cavities by decelerating thebeam. Energy recovery of an FEL beam driven by a SCA
is a possible way of greatly increasing the efficiency ofthe laser since most of the beam energy remains afterlasing occurs. This energy-recovery technology with aSCA is the most promising for the next stage of 10kW
FEL lasing owing to increasing the beam current without
additional RF power sources.
Aiming for the higher FEL power the new grid pulser
for the thermoionic cathode gun is under development inorder to increase the beam current by multiplying therepetition rate of 10 MHz to 20 MHz. The new RF
sources of CW mode for higher power to the non-energyrecovery parts have been installed and tested.
In spite of using the SCA the JAERI ERL-FEL has
been operated in pulse mode because of the followingreasons.
1) The JAERI FEL linac uses closed-loop refrigerator
systems directly attached to the SCA module. The
cooling power of this refrigerator system is not
sufficient enough to operate in CW mode.
2) The shield of the building for the linac is not thickenough for the radiation protection
The linac has been operated in pulse mode of 1 msec
macro pulse length and 10 Hz repetition rate. There are
increasing many requirements for operation mode such as
higher duty or long pulse from the view of various
applications.
In the present paper we will describe the properties ofthe JAERI ERL-FEL at high power or long pulse
operation.

JAERI ERL-FEL CONFIGURATIONThe JAERI ERL-FEL SCA consists of an injector, aninjector merger, two main modules of 499.8 MHz 5-cellsuperconducting accelerators, two 180-degree bendingarcs, an undulator and a half-chicane before the undulator.
The injector consists of a 230 kV thermoionic electron
gun driven by a grid pulser, an 83.3 MHz normal
conducting subharmonic buncher (SHB) and two modulesof 499.8 MHz single-cell superconducting accelerators [2].
Electron micro bunches with a charge of 0.5 nC atrepetition of 10.4125 MHz are produced and accelerated
to 230 keV in a DC electron gun. The average current
corresponds to 5 mA. The bunches are compressed by theSHB, pass though the two single-cell modules and areaccelerated to 2.5 MeV. The output beam is injected into
the two 5-cell main modules where it is accelerated up to17 MeV. The beam then passes through the first arc, thehalf-chicane and the undulator. Afterward it isrecirculated through the second arc, returned into themain modules in the decelerating RF phase and dumped
at the injection energy of 2.5 MeV.

RF SYSTEM FOR SCA
RF power sources for SCA are all-solid state amplifiers,
whose powers are 6 kW for the single-cell cavity and 50
kW for the 5-cell cavity. These RF power sources areoperated in pulse mode of macro pulse width of 2 msec,
repetition of 10 Hz. In order to increase the beam power
and the macro pulse width, IOT (Inductive Output Tube)
amplifier systems of 50 kW CW operation are going to be
replaced with, and two IOT systems have been installed
already for the two single-cell cavities, which require
more power to increase the beam power because of thenon-recovery parts. The other two power sources for the5-cell cavities are going to be installed this year.

Pressure Rise in SCA cryostat
The JAERI ERL-FEL SCA has been using a uniquerefrigerator system that consists of a liquid Herecondensor and a shield cooler using close-loop He gas
refrigerators attached directly to the cryostat [3]. He gas
evaporated due to the standby loss and the RF wall loss isrecondensed by a heat exchanger of the 4K refrigerator
inserted into the liquid helium container. The coolingpower of the refrigerator is about 8W and the standby loss
is estimated to be about 4W. A heater within the cryostat
is used to keep pressurizing the cryostat by dissipating theresidual cooling power. Though the pressure in thecryostat can be kept constant by controlling the heater
power, the pressure cannot be kept stable in long-pulse
operation in the order of seconds. Figure 1 shows thepressure in the cryostat of the single-cell for the singlepulse operation of 5-second length. From the start of thepulse the pressure increased linearly and after the pulse

305Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Fig.3 Spectrum of resonant frequency vibration for
low coupling and CW mode.

Fig.1 Pressure rise and decay in the cryostat for 5sec
pulse operation.
the pressure decreased exponentially. According to thefalling time constant of this operation was about 9seconds. This indicates that the single-cell cavity can beoperated at 10% duty without adding another helium
refrigerator.

Vibration of SCA
The He refrigerators use a piston action to compress
and expand the helium gas, whose repetition rate is about
1 Hz, The refrigerators are attached to the SCA module
insulated by using rubber insulators and bellows so that
this action should not vibrate the cavity. Though thisvibration is so slow and small that the accelerating field

can be controlled by the feedback loop system, it might
affect the cavity in the high Q-value coupling operation
for the high efficient ERL operation. The influence of therefrigerator to the cavity was measured in two cases; oneis high coupling and pulse mode (same as before) and low
coupling and CW operation (for the future operation). The
RF power from the pickup coupler of the cavity and theforward power of the RF power source were acquired bysampling the amplitudes at the same point of every pulse.
These time domain data were transformed into thefrequency domain by FFT (Fast Fourier Transform) as
shown in Fig.2. While the pickup power spectrum has nopeak below 5 Hz because of the good control of thefeedback system, the forward power spectrum has several
peaks of multiple of 1 Hz. The maximum peak around 1
Hz suggests that there is resonant frequency vibration ofabout 60 Hz. Another measurement for the low coupling
and CW operation was done by controlling the oscillator
frequency with a phase lock loop (PLL) and measuring
the control voltage for frequency modulation. Figure 3
shows the result of FM control voltage in frequency
domains. Compared to the pulse mode operation, thevibration frequencies are almost same but the amplitudeof the resonant frequency vibration of the cavity is verysmall. This is thought to be due to the main coupler
position and/or the effect of Lorentz force detuning ofpulse operation.

Piezo Tuner Response

Fig.2 Spectrums of forward power (upper) andpickup power (lower) for high coupling and pulse
mode.

The resonant frequency of the SCA cavity is tuned byadjusting the cavity length with two tuners; one is acourse tuner using a stepping motor and another is a fine
one using piezo elements which is mainly used during theoperation. As the response of the piezo tuner is so slowthat it is impossible to tune the frequency during theduration of a macro pulse, the phase error signal isdetected during the macro pulse and the piezo tuner
moves in the intervals between the macro pulses, whichmeans that the piezo tuner is operated below 10 Hz. In thecase of CW operation it is important to know the piezo
tuner response not to induce an unexpected instabilities
caused by the tuner action. The piezo tuner responses
were measured by adding the sinusoidal piezo control
306

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Fig.4 cavity frequency oscillation as a function of thefrequency of the piezo control signal.

Fig.5 Measured frequency shift as a function of duty
factor of the RF power.

signal, changing the voltage and frequency and measuring
the FM control voltage of PLL. Figure 4 shows the cavity
frequency oscillation as a function of the frequency of thepiezo control signal. The cavity frequency was stably
controlled below 40Hz but above 50Hz there are someresonance.

RF SYSTEM FOR SHB
While the main part of the JAERI ERL-FEL consists ofthe superconducting cavities, the SHB is normal
conducting. The RF power source for the SHB also uses
an all-solid state amplifier of 4kW pulse operation. Thisamplifier is also replaced with the two-stage triode
amplifier of 10kW CW operation. As the SHB wasdesigned for the low duty operation, the possibility for
CW or high duty operation of the SHB was investigated.

Frequency Shift due to High Duty Operation
The high duty operation of the SHB causes thetemperature rise due to the increase of the RF power
dissipation, which induces the frequency shift of thecavity. Figure 5 shows the measured frequency shift. The
resonant frequency decreases with increase of the duty at
the ratio of -13.4 kHz per 1% duty. As the upper
frequency margin is about 47 kHz, the duty is limited
below 3.5%. This result is compared to calculation of heatand frequency shift problem with SUPERFISH andMIFIA. The frequency shift due to the temperature rise isabout -1.3 kHz/K, measured by changing the coolingwater temperature for the SHB without RF power. This
corresponds well to the calculation from the SUPERFISHresults. Figure 6 shows the calculated temperature
distributions of the SHB, from which the calculated
frequency shift is -72 Hz per 1% duty. This is muchsmaller than the measurement. This is thought to beFig.6 Calculated SHB temperature distribution of 3%duty.
deformation of the tuner that is not cooled enough withthe result of much higher temperature than the SHB cavity.
When the tuner can be cooled properly, it is expected touse the SHB for the CW or long pulse operation as it nowstands.

CONCLUSION
The properties for the JAERI ERL-FEL were measured.
The single-cell cavity can be operated in the duty of morethan 10%. The frequency vibration caused by therefrigerators was detected around 2 Hz at the resonant
frequency vibration amplitude from 2 to 60 Hz for thedifferent operation conditions. We are going to investigate
what the differences come from: coupling or operating
mode.

REFERENCES[1] N.Nishimori et al., Nucl. Instr. Meth. A 475 (2001)
266-269[2] R.Hajima et al., Nucl. Instr. Meth. A 507 (2003) 115119[3] N.Kikuzawa et al., Nucl. Instr. Meth. A 331 (1993)
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