The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Properties of the electron beam generated by photo-cathode RF gun
M. Kuriki, J. Urakawa, K. Nakajima, H. Hayano, K. Hasegawa(a),T. Oshima(b), S. Kashiwagi(b), R. Kuroda(b), and M. Washio(b)High Energy Accelerator Research Organizationa) Scientific university of Tokyob) Waseda university
Abstract
KEK-ATF is a test accelerator to establish the lowemittance multi-bunch electron beam required by thefuture linear collider. In ATF, electron beam is generated by combination of a thermionic electron gun
and a bunching system. Electron beam generated
by this system, however, is unstable due to fluctuations of many components, e.g. buncher RF phaseetc. On the other hand, Photo-cathode RF Gun is thebest candidate in future high-current, short-pulse, andlow-emittance electron sources. These features meetthe requirements of the future linear collider. Photocathode RF gun might be a good replacement of thecurrent injector of ATF. To examine this possibility,
we have performed a test experiment that a RF gun
is set in ATF instead of the conventional thermionicgun. We have confirmed that the RF gun has enoughperformance as the electron source for ATF in single
bunch operation.

1 INTRODUCTION
KEK-ATF is a test accelerator to establish the lowemittance multi-bunch electron beam for the future
linear collider. ATF consists of 1.5 GeV S-hand linac, abeam transport line, a damping ring, and a diagnostic
extraction line.
In the conventional ATF configuration, electron
beam is generated by a thermionic gun. Bunch lengthis shrinked from 1 ns to around 15 ps to fit S-handacceleration by a couple of 357 MHz Sub HarmonicBunchers, and a TW buncher. Multi-bunch with 2.8
ns separation is implemented by applying 357 MHz RF
signal on the gun grid.
In ATF, the energy and orbit fluctuation of the electron beam caused by the bunching section(two SHB
cavities, and a TW buncher) has been observed. It
disturbs precise study for the beam property. In addition, the beam stability itself is an essential point torealize the linear collider. In the linear collider, multibunch method is assumed to get high-current beamkeeping the peak current low. In that case, the longrange wake field would be serious because of the short
aperture of the X-band accelerating structure that will
be used in the linear collider main linac. The longrange wake field makes the beam emittance growththat kills the luminosity. A special accelerating structure, DDS (Damped Detuned Structure) is employed
to control the wake field precisely to suppress the emit-tance growth. To work this method well, the bunch
intensity should be equal by less than 1.0%. Therefore, in a view of not only the study efficiency, butalso the linear collider realization, the beam stability
is an important issue.
Photo-cathode RF gun is the best candidate in future high-current, short-pulse, and low-emittance electron source. The bunch length made by the RF gun
is determined by the temporal structure of laser. Because 10 ps pulse laser is ordinary, we can easily obtain
15 ps length electron beam with the RF gun without any bunching devices. We might be free from thebunching instability and could improve the beam stability with the RF gun.

2 THE BEAM LINEFig. 1 shows a schematic view of ATF linac injector
modified for the RF gun test.
BN1 RF gun IV [1] is placed at the first end of
the beam line. Up to 15 MW of 62 MW S-hand RF
power provided by Toshiba E3712 klystron is fed to theRF gun through 6dB directional coupler. Rest of thepower is distributed to the first accelerating structure,
10.
The RF gun is followed by a solenoid magnet thatsuppress the emittance growth by the space charge
effect.
Faraday cup is set to measure the emitted current.
The Faraday cup is usually off the beam line and remotely loaded on it. In the same load system, a screen
monitor is also implemented. Other two screen monitors are set after 10 and in end of the injection system.
The last screen was used to measure the emittance.
Three Quadrapole magnets stand side by side to perform emittance measurement by the Q-scan method.
An integration current transformer, ICT is placed
after 10. Several ICTs are also set at downstream,e.g. the end of linac. By comparing these data, thebeam transmission of a certain part can be evaluated.
With the analyzer magnet, the beam momentum
can be measured. The beam energy was typically 78
MeV.
Three BPMs are distributed along the injector section, in downstream of the solenoid magnet, front of10, and rear of 10.
Optical Transition Radiation, OTR monitor is implemented to measure the bunch length. OTR light
generated by the interaction of the beam and the in--208-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Figure 1: Electrons induced bylaser are immediately accelerated
by RF field in the RF gun. RF
power from a klystron is distributed to the RF gun and theMany
accelerating structure.
instrumentation devices such as
screens, etc. are implemented.

Pre-inJector diagnosuc section
ToSu'Cak
Cameraserted SUS plate is observed by a streak camera thatcan measure the signal with sub-pico second resolution.
A wire scanner is introduced to obtain the beam profile. 50 J.Lm gold coated tungsten wire scans the beamline along vertical and horizontal axes. By integrating
the gamma ray emitted by the wire, the beam profile
is re-constructed. This monitor provides another way
to measure the emittance.
This injection system is followed by the regular accelerating sections. 16 structures accelerate the electron beam up to 1.3 GeV with the field of 25 MeV jm.
The electron beam is then transported and injected tothe damping ring.

3 LASER SYSTEM

4 BEAM PROPERTY
3 shows beam current measured by ICTFig.
at downstream of 10 as function of the gun RFphase. The current marks its maximum of 1.6 x
10 10electronjbunch around 20 °. It decreases down
to 0.5 x 10 10 electron/bunch at 70 °.

1.5Laser hutar

rather than a circle. In such case, the beam emittancewill be worse than that with a circle spot. A prism isintroduced to correct the spot shape on the cathodeto be a circle.
The final mirror to deflect the laser to the gun cavity is remotely movable to control the laser positionon the cathode in both axes, vertical and horizontal
respectively.
The laser power was 100 J.LJ at the output port ofthe laser module. In front of the RF gun view port, it
was 55 J.LJ.

Figure 2: It shows a schematic view of the laser system for the RF gun. The laser module is placed in thelaser hut constructed behind of the concrete shield.
The laser spot size and position can be controlled bymoving the final focus lens and the final deflecting mirror.
In RF gun, electrons are induced by the photoelectron effect on Cu cathode. As the photon source,
262nm UV light made by Nd:YLF laser was used. Fig.
2 shows the schematic view of the laser system. Laser
hut was constructed on the other side of the linac tunnel across the concrete shield. The laser hut temperature is controlled 21° C to keep the laser condition. Inthe laser hut, the laser is expanded to approximately
20mm diameter by combination of f=-35 and f=300
lenses. The laser is then introduced to the linac tunnel through a hole on the concrete shield.
In the tunnel, a lens (f=1000) focuses the laser on
the gun cathode plate. The lens is mounted on a movable stage to change the spot size of the laser on thegun cathode. The RF gun is operated usually at thespot size of 2.0 "' 3.0mm diameter.
The laser is injected to the gun cavity with angle of
67.5 degree from the cathode plane as shown in Fig. 2.
The spot on the cathode becomes therefore an ellipse

444J~•&41 20

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RF gun phase (deg)RF gun phase (deg)Figure 3: Beam current
by ICT at LO out is shown
as function of the gun RFphase.

Figure 4: Bunch length
measured by Streak camera is shown as functionof the gun RF phase.

Fig. 3 shows bunch length as function of the gun
RF phase. Bunch length is obtained by observing theoptical transition radiation light with a streak earnera. The bunch length is 30 ps at 0 ° RF phase anddecreased down to 11 ps at 70 °.
From these results, the operation condition of thegun RF phase was determined to be 40 ° where thebunch length of 17 ps fits to S-hand acceleration withenough current, 1.4 x 10 10 electron/bunch.
Fig. 5 and 6 show the emittance as function of thesolenoid magnet current and the gun RF phase respectively. The beam emittance was measured at downstream of 10. The emittance was evaluated by Q-scan-209-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

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'method. Horizontal and vertical beam profiles weretaken with the wire scanner.
Both emittance on horizontal and vertical are minimized at 92 A of the solenoid magnet current. By toggling gun RF phase at this condition, both emittanceat 60 ° are smaller than those at 40 °. These results,
1.2.4 1rmm.mrad and 14.0 1rmm.mrad on horizontal andvertical respectively, are the smallest emittance in ourstudy. The beam current was 5.3 x 109electron/bunch.

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.

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1.2
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RF Gun pbase(deg)Figure 6: Beam emittance measured by Wirescanner at 10 out is plotted as function of the gun
RF phase.

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2000 '-'--:':20:-'-..L-J....J40l:-'--'--'-60L...J..._J

Soleooid cummt (A)Figure 5: Beam emittance measured by Wirescanner at 10 out isshown as function of thesolenoid magnet current.

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Figure 7: This figure
shows distributions of thebeam current measured
by ICTs. The solid anddashed lines corresponds
to those at 10 out andBTend respectively.

Figure 8: Horizontal andvertical axes show thebeam current measured
at 10 out and BT end respectively. Area of theboxes is plotted as proportional to the numberof entries.

niagnet in the damping ring. The injection efficiency
was, however, almost 100%. Therefore, the injectedcurrent would be fully stored if the ring was operated
in the usual condition. With the RF gun a beam loss
free operation would be possible.

6 SUMMARY
5 INJECTION STABILITY
The electron beam generated by the RF gun is but also
fully accelerated by the linac up to 1.3 GeV and injected to the damping ring. Fig. 7 show the beam current measured by ICT at downstream of 10 (solid) andthe end of the beam transport line, BTend {dashed).
The peak position of BTend is slightly larger thanthat of 10 out, but it could be a systematic error ofthe ICTs. The transmission efficiency from 10 out toBTend is therefore 100% within the errors. The distributions, however, look too broad as compared with
the ICT resolution, typically several %. In fig. 8,
the same data are plotted in a two-dimensional plane..
The horizontal and vertical axes show ICT output at
10 out and BTend respectively. By looking this plot,
the beam current is highly correlated between theseICTs. It means that there is no any significant fluctuation between ICT at 10 out and BTend, but there issome fluctuation of the initial beam current.
There are several candidates to cause this fluctuation, jitter of laser intensity, laser timing, laser position, RF amplitude, RF phase, and solenoid current.
There was no correlation between RF amplitude andthe beam current fluctuation. RF phase is stabilized
by a feed-back system within 1% of S-hand. RF is thennot the criminal. It is hard to imagine the solenoid current moves so much. Some of laser instability is themost suspectable candidate at this moment.
The efficiency of the stored current in the damping
ring to the injected current was limited up to 50%.
The reason of this bad store is power supply fail of a

We 'performed a test experiment to examine the feasibility of the photo-cathode RF gun to KEK-ATF.
By using Nd:YLF UV laser with 10 ps pulse width,
12 "' 20ps length electron beam was obtained. The
required beam current, 1.0 x 1010 electronfbunch waseasily achieved.
We confirmed the horizontal and vertical beam emittance of 12.4 1rmm.mrad and 14.0 1rmm.mrad respectively at 5.3 x 109electron/bunch.
The electron beam generated by this RF gun system was injected to the damping ring. The beam wastransported and injected into the ring without any significant beam loss. The beam intensity jitter might becaused by some laser instability.
In contrast to the operation with the usual
thermionic gun, the RF gun system has many advantages on the beam emittance, the injection stability
etc. The beam intensity jitter could be stabilized bycareful tuning on the laser module.
Even these nice features, we can not switch to theRF gun. The multi-bunch beam can not be made with
the RF gun at this moment because 10 ps continuous
pulses separated by 2.8 ns laser light does not exist.
If such laser was realized, the photo-cathode RF gun
was a perfect replacement of the current thermionic
gun system.

7 REFERENCES[1] X.J. Wang et al., Proc PAC 1995 {1995) p890

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