Multi-bunch generation by thermionic gun
M. Kuriki, H. Hayano, T. Naito, KEK, Tsukuba, Ibaraki, JapanK. Hasegawa, Scientific university of Tokyo, Noda, Chiba, Japan
2Abstract
KEK-ATF is studying the low-emittance multi-bunch electron beam for the future linear collider. In ATF, thermionic
gun is used to generate 20 bunches electron beam with thebunch spacing of 2.8 ns. Due to a distortion of the gunemission and the beam loading effect in the bunching system, the intensity for each bunch is not uniform by up to40 % at the end of the injector. We have developed a system to correct the gun emission by precisely controlling thecathode voltage with a function generator. For the beamloading effect, we have introduced RF amplitude modulation on Sub Harmonic Buncher, SHB. By these technique,bunch intensity uniformity was improved and beam transmission for later bunches was recovered from 67% to 91%,
but intensity for first five bunches is still lower than others.

MULTI-BUNCH BEAM GENERATION

The gun assembly consists from a thermionic gun, Gridpulser, and a high voltage gun pulser.
The thermionic gun, is a triode type, EIMAC Y796. Theelectron current is controlled by Grid bias.
To make a multi-bunch electron beam with a bunch spacing of 2.8 ns, 357 MHz RF signal is applied to the GUN
cathode. 357MHz ECL level RF signal is amplified by apower amplifier. This output has a pulse height of 400 V
peak-to-peak, but the amplitude is gradually changing at
the rise and fall edge as shown in FIG. 1.

357MHz RFCombined pulse

1 INTRODUCTION
KEK-ATF is a test facility to develop the low emittancemulti-bunch beam and beam instrumentation technique forthe future linear collider. That consists from 1.5 GeV Sband linac, a beam transport line, a damping ring, and adiagnostic extraction line.
In the linac, the electron beam is generated by athermionic electron gun. Typical intensity is 1:0  10 10electron/bunch. The bunch length is compressed from 1
ns to 10 ps by passing a couple of sub-harmonic bunchers,
a TW buncher, and the first S-band accelerating structure.
This area is called as injector part. After the injector part,electron energy becomes 80 MeV.
The electron beam is then accelerated up to 1.3 GeV by 8of the S-band regular accelerating section. One section has
two accelerating structures driven by a klystron-modulator.
Klystron is Toshiba E3712 generating 80 MW with a pulse
duration of 4:5s RF. A peak power of 400 MW with apulse duration of 1:0s is obtained by SLED cavity andmakes a high gradient accelerating field, 30 MeV/m.
20 of bunches separated by 2.8 ns are accelerated by one
RF pulse. This multi-bunch method is one of the key technique in the linear collider.
In April 2000, we achieved horizontal emittance 1:3 10 9rad:m, vertical emittance1:7  10 11rad:m (both for
2:0  109electron=bunch, single bunch mode )[1] which
are almost our target.
In November 2000, we have started the multi-bunch operation. The commissioning was successfully done. Dueto lack of the instrumentation device for the multi-bunch
diagnostic, emittance for each bunch is not measured yet.

Grid bias
Rectangular signal

Figure 1: To omit the rise and fall edge of 357 MHz RF
signal, a rectangular signal is combined. Grid bias is determined to clip uniform multi-bunch beam.
If the RF signal is directly applied to Gun cathode, bunch
intensity becomes not flat. To get uniform bunches, an additional rectangular pulse is combined as shown in FIG 1.
The grid bias is determined that the rectangular pulse clips
out the flat part of the RF signal. Finally, only the flat partof the RF pulse is obtained as real beam current.

3EMISSION CORRECTION

FIG. 2 shows the multi-bunch beam generated bythermionic gun. The vertical and horizontal axes show timein ns and the beam current in A respectively. The grid bias
was set to 240 V. The left side is early bunch. The beam
current is measured by a current transformer which is set
right after the gun exit. The current transformer measures
the beam current as the induction voltage, so the output decays with a time constant.
Intensity for the first three bunches is still increasing.
This behavior is due to the rounded rising edge of the clipping rectangular pulse.
In addition, several bunches around 13th and 14th havelower intensity than others. A study for the gun emission[2] demonstrated that the gun response to the rectangular40-0.1
-0.2
-0.3
-0.4
-0.5

-80

-60

-40-20(ns)Figure 2: Multi-bunch beam measured by a current transformer. Vertical axis shows the beam current in A. Grid
bias was 240 V.

(A)pulse reproduced this dip, but the reason was not fully understood. This is not any problem on the electrical circuit
such as reflection signal because any dip was not observedin direct measurement of the rectangular pulse applied tothe cathode.
0.1
0-0.1
-0.2
-0.3
-0.4
-0.5

-80

SHB AMPLITUDE MODULATIONElectron beam generated by the thermionic gun has approximately 1 ns bunch length which is larger than acceptanceof S-band acceleration. A couple of 357 MHz standing
wave Sub-harmonic bunchers, and a traveling wave S-bandbuncher are placed to gather electrons into the S-band acceptance, 10 20ps.
In multi-bunch operation, the bunching field is decreased
by beam induced field, i.e. wake field. This is thebeam loading effect. Beam loading effect is larger forlater bunch, so the condition becomes worse for the laterbunches.

-60-40-20(ns)Figure 3: Multi-bunch beam applying the correction signal.
Grid bias was 240 V.
To correct this dip, an additional signal source was introduced. The correction signal is produced by a function
generator, Tektronix AWG 510 which can make an arbitrary waveform with 1 GHz clock speed. The signal istransfered to the gun high voltage station through an optical cable , amplified 20 W RF amplifier, and combinedwith the main signal through a resistive power combiner.
Typical amplitude of the correction signal is 30 V which isroughly 10% of the main signal applied to the gun cathode.
FIG. 3 shows the gun output by applying the correction signal. The grid bias was set to 240 V. The first threebunches have still current lower than others, but the large
dip on 12-15th bunches in FIG. 2 was well compensated.

Cavity amplitude

(A)

0.1

Amplitude modulation
Actualcavityamplitude
Beam loading
timeFigure 4: In amplitude modulation method, amplitude ofinput RF is changed synchronously with the beam timing.
Cavity RF amplitude is then gradually increased with thefilling time as shown by the dashed line. On the other hand,
RF amplitude is decreased by the beam loading effect asshown by the dotted line. Totally, cavity RF amplitude iskept flat.
To compensate the beam loading effect, we have introduced amplitude modulation on pulsed RF for SHBs.
In amplitude modulation, the amplitude of pulsed RF ischanged synchronously with the beam timing. FIG. 4
shows the beam loading compensation by amplitude modulation schematically. Cavity RF amplitude is then gradually increased with filling time as shown by the dashed line.
On the other hand, RF amplitude is decreased by the beamloading effect as shown by the dotted line. Totally, cavity
RF amplitude is kept flat. The bunching quality becomesequal for all bunches.
The beam loading effect always decelerates the followedbunches. In ATF, the first SHB, SHB1 is operated in deceleration mode and the second, SHB2 is in acceleration
mode. The beam loading effectively increases RF amplitude in SHB1 and decreases in SHB2 . Modulation sign isthen negative for SHB1 and positive for SHB2.
Optimization for the amplitude modulation has been
done by looking beam transmission at the end of injector
part. A wall current monitor is placed at the exit of theinjector part to observe the beam current. FIG. 5 shows
the response of the wall current monitor to the multi-bunch
beam. The dotted and solid curves indicate those obtainedwith the conventional pulsed RF and the amplitudely modulated pulsed RF on SHBs respectively. Transmission forWC signal

151010550-5020406080
nsFigure 5: Multi-bunch beam profile by wall current monitor. Horizontal axis shows time in ns. Vertical axis shows
wall current monitor response in V. The dotted and solid
curves were obtained without and with amplitude modulation on SHB RF.
the later bunches was recovered by the amplitude modulation.
The beam loading effect affects the transmission for thelater bunches, then we should investigate the bunch transmission to examine the beam loading effect. Since the absolute transmission for each bunch is hard to measure exactly, the intensity ratio of the early bunch and later bunch
can be used instead of the absolute transmission.
Intensity of the last bunch is much lower than others dueto the less sharpness of the clipping rectangular pulse. Because of that, effect of the amplitude modulation should
be examined by the last second bunch rather than the lastbunch.
The transmission ratio of the second last bunch was 0.67
for the conventional SHB RF and 0.91 for the amplitudely
modulated SHB RF respectively. The most intense bunch
was used as the reference. Improvement of the transmission by the amplitude modulation was 24%.
FIG. 6 shows distributions of bunch intensity for 6th andlater bunches. The peak voltage of wall current monitor
is here used instead of the real beam current. The solidand hatched histograms are those with the amplitude modulation and the conventional pulsed RF on SHB respectively. With the amplitude modulation, most bunches aredistributed more than 18 V, but with the conventional RF,bunches are spread widely from 12 V to 20 V. The amplitude modulation improved the flatness of intensity for theselater bunches.
FIG. 6 does not include the first five bunches. The lower
intensity of these bunches is due to the rounded rising edgeof the clipping pulse. That will be one of the main issue on
the multi-bunch operation.

5SUMMARYIn KEK-ATF, multi-bunch beam was successfully generated by a thermionic electron gun with bunch spacing of05

10

1520

WC output (V)Figure 6: The horizontal axis shows bunch intensity measured by wall current monitor. The vertical axis is numberof bunches per 1.0 V. The solid and hatched histograms
show those with the amplitude modulation and the conventional RF on SHB respectively.
2.8 ns. The beam already reached to the extraction line,
but the emittance was not measured yet due to lack of theinstrumentation for multi-bunch beam.
Intensity for each bunch is not uniform because of ; 1)
gun emission un-uniformity; 2) beam loading effect.
For gun emission problem, we have applied a correction signal generated by an arbitrary function generator toGun cathode. Bunch intensity flatness was significantly improved by this emission correction. However, Gun emission for first five bunches is still lower than others. That
will be one of the main issue in future.
For beam loading effect, we have introduced amplitude
modulation on SHB RF. The amplitude modulation compensated the beam loading effect and recovered the beamtransmission from 67% to 91%.

6REFERENCES[1] http://lcdev.kek.jp/ATF/[2] M. Kuriki et al., ’Multi-bunch beam generation by
Thermionic Electron Gun’,24th Linac meeting at Sapporo,1999