The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Performance of Feed-Forward Circuit for the JAERI FEL Superconducting Linac
Masaru SAWAMURA, Nobuhiro KIKUZAWA, Ryoji NAGAI,.Nobuyuki NISHIMORI, Masayoshi SUGIMOTO, and Eisuke J.MINEHARAFree Electron Laser Laboratory, Japan Atomic Energy Research Institute (JAERI)
Tokai, Ibaraki 319-11, Japan
Abstract
The RF system of the JAERI FEL
superconducting linac is operated with the pulse
length of 3 ms and the repetition rate of 10 Hz. The
electron beam can be accelerated with the pulse
length of 1 ms or less. The beam load of the longpulse increased the beam energy dispersion, andresulted in some beam loss after the 180-deg
bending magnet system. The RF system with along time constant could not catch up with a fastchange of the beam load. A newly introduced feed
forward circuit compensated residual parts of theamplitude and phase feedback.
This circuit
successfully improved the beam transport.
1.

Introduction

The JAERI FEL superconducting linac isoperated with a pulse mode because of radiation
shields and cooling capability of He refrigerators.
The pulse length is 3 ms and the repetition rate 10Hz. As the superconducting cavity has high loaded
Q-factor of 1 X 106 [1], the pulse length of 2 ms is

Constant
Voltage
Supplyused to rise the accelerating field. Therefore, theelectron beam can be accelerated with the pulse
length of about 1 ms or less. Though the beamtransport was adjusted with short pulse length andthe pulse length was extended, the energy
dispersion got bad for the long pulse. This was due
to a long time constant of the RF system including
the superconducting cavity. When the perturbationsuch as a beam load occurred, the long time constantcaused the feedback delay and the feedback system
could not catch up with the changes of theperturbation. To compensate the delay of thefeedback system, a feed-forward circuit wasinstalled to the RF system. With this system theacceleration test was done.
In this paper we
describe the feed-forward system and the results ofthe beam test.
2.

Feed-Forward Circuit

The RF system with the feed-forward circuit
was shown in figure 1. The feed-forward processes
were as the following.
(1) The feedback circuit was equipped in the RFArbitrary
Waveform
Generator

RF Reference

Summing Amp

Control Level

Amplitude

Feedback

Fig.1 The Block diagram offeed-forward circuit-200-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

(2)
(3)
(4)

(5)
(6)

system.
The feedback levels of amplitude and phasewere controlled by external constant voltage
sources.
Waveforms of the amplitude and phase in thecavity were measured with beam acceleration.
The waveforms were transferred to a computer
to search and optimize the control signals ..
According to the calculated data, an arbitrary
waveform generator (AWG) generated theadditional control signals.
The both signals of the constant voltage supply
and the AWG were summed with the summing
amplifier. The summed signal was used as thereference signal of the RF control.

Fig.2

The beam current at the exit of the first
60-degree bending magnet without thefeed-forward circuit(7) The same processes from (3) to (6) wererepeated until the levels of the amplitude andphase in the cavity got flat.
3.

BeamLoad

The validity of the feed-forward circuit wasexamined by comparing the beam current with andwithout the circuit. The beam was acceleratedthrough the superconducting linac and wastransported through the three 60-degree bending
magnet system and an undulator. As the large
energy dispersion resulted in the beam loss at thebending system, the beam current was measured at
the exit of the first 60-degree bending magnet. A
core monitor was used for measurement. The beampulse length was 600 J.I.S.
First, the measured beam current without thefeed-forward circuit is shown in figure 2. The beam
current decreased after 50 J.I.S. This means that theenergy dispersion increased after 50 J.I.S.
Second, the second cavity of the four
superconducting ones was controlled with the feedforward circuit. The beam current was shown infigure 3. The beam current was almost constant allover the pulse length except for the beginning of thepulse.
The change of the amplitude in the cavity wasshown in figure 4. The amplitude decreased about
6 % at maximum during the acceleration without
the feed-forward circuit. While the change of theamplitude with the feed-forward circuit was within±2%.
The energy dispersions caused by theseamplitude changes were calculated with thePARMELA code. The calculations were done by
fixing the all phases and by changing the amplitude
0.205 ---,----,--,---r---y---r---,----r---r----r---,----r--·---,----r---T---,----,--,---

::~

0.2

.....

.

.

:

-------- --;------ --- :-\·--------~-:

'\

....

~ 0.195 -- ---------- - :. ----- -------- '----\~~------ t ______________ __[ ______________ ••0i~ o.a [m :m I + ~~~-··~r=·......

0 • 18 5 '---'----'----'--'---'----'---'--'---'----'----'--'---'----'----'--'---'----'-----'------'
-200
-400
200
0600
600'l'illle (us)

Fig.4
Fig.3

The beam current at the exit of the first
60-degree bending magnet with the feedforward circuit-201-

The amplitude in the second cavity withand without the feed-forward circuit.

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997120

the bending magnet.
The control signal from the AWG was generated
by a simple line patterns of the pulse shape.
Therefore, the fine optimization of the control signal
will decrease the change of the amplitude in thecavity..

-j- •'j'--2%:---0%" ~culOO -'.
.-4Ill....0t: 80tIH

4.

60

Conclusion

0&40---------·------ ----,-- --------'-

20

100 200 300

400AW (keV)

Fig.5

Energy distributions calculated with thePARMELA code. The field decreased 0%,2% and 6%.

of the second cell and fixing the others. The
calculated energy dispersions are shown in figure 5.
'1\vo-percent drop of the amplitude indicates 1.4
times increase of the energy dispersion. However,
six-percent drop indicates 3.6 times increase, which
would be too large to accept the transport throughThe feed-forward circuit made the beamtransport through the bending magnet good evenwhen the pulse length was long. This means thatthe energy dispersion got small in the long pulse
mode operation. We have been developing theprogram for optimizing the signal of the feedforward circuit. This will improve the energy
dispersion further.
Reference[1] M.Sawamura et al., Nucl Instr. Methodes inPhys. Res., A341, 391 (1994)-202-