JAERI-Conf 95-021

2a1-3
GENERATI ON OF SUBPICOS ECOND ELECTRON SINGLE PULSE
Takahiro KO'ZAWA, Toshiaki KOBAYASHI,
Torn UEDA, Mitsuru UESAKA, and Kenzo MIYA
Nuclear Engineering Research lAboratory, Faculty of Engineering,
The University of Tokyo,2-22 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki 319-11, Japanthe time resolution of pulse radiolysis remains a few
couple of ten picoseconds. Recently, generation ofsubpicosecond pulse (900 fs, 150 pC/pulse, 7.1 mm X11 mm) was succeeded at the university of Tokyo[4].
However, the charge was too little to detect radiationinduced reactions. We attempted to generate highintensity subpicosecond pulse.

Abstract
Generation of subpicosecond single electron pulse withthe charge of 1 nC have been sua:eeded at the S-band(2856 MHz) linear accelerator of the university of·Tokyo. 34.8 MeV electron pulses with the pulse widthof 10 ps have been compressed by the achromatic
magnetic pulse compression system. 890 fs (FWHM)
single pulse have been generated with the charge of 1nC through a 3 cfl slit. No satellite of the main pulse
could be observed.

2. Experimental Setup
2-1. Achromatic pulse compression system
The achromatic pulse compression experiment wascarried out at the 35 MeV S-band linac of the utnl twin
linacs. This linac has two accelerating tubes {ACCIand ACCll). The experimental setup is shown in Fig.
1. The achromatic pulse compressor consists of two45• sector magnets and four quadrupole magnets. The
upstream sector magnet was also used as an energy
analyzer magnet. The longitudinal distribution ofelectron pulse was modulated for pulse compression byadjusting RF phase of ACCII.

1. Introduction
The development of short pulse electron accelerator
enables direct observation of ultrafast dynamics ofelectron-matter interaction~ In 1977, a 10 ps electron
single pulse was first generated at the 35 MeV S-band
linear accelerator of the university of Tokyo[1 l, andthen the pulse radiolysis system was established[2, 3].
Since then, ultrafast phenomena such as excitation,
ionization and relaxation of atoms and molecules havebeen investigated in picosecond regime with the time
resolution of a·few couple of ten picoseconds[2, 3]. Inthe case of photoinduced reactions, investigation on
ultrafast phenomena in femtosecond region have been
started by using femtosecond laser. On the other hand,SITTube

2-2. Induction system
The injector consists of an thermionic electron gun (Y796) and a d-e biased grid-cathode pulse generator
placed on a -90 kV high potential deck, a 476 MHz(116 of the main accelerating microwave frequency ofg

Trajectory ofElectron Beam

Bending Magnet

Streak Camera ~Optical Filter --01

Convex Lens
f=150mm-~ Quadrupole Magnet

I

Mirrorlll

MirrorllTrajectory of Cherenkov Radiation

Fig. 1. Magnetic pulse compressor and Cherenkov
radiation measurement system.
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95-0212856 MHz) subharmonic buncher (SHB), a 2856 MHz
traveling Wave type 6 cell prebuncher and· focusing
system. The accelerating potential of the electron gun
is provided by 90 kV pulses of 8 J.I.S duration. The
duration of flat top is 4 J.I.S. The velocity of theelectron beam injected from the electron gun ismodulated by the electric field at a gap of the SHB. fuorder to generate an isolated pulse without satellite, afast rise-time , low jitter trigger pulse synchronizedwith the accelerating RF waves is required. Up to now,a single pulse have been generated by the grid pulser,
whose voltage is 300 V with the pulse width of 1 ns.
A higher voltage pulser was purchased by Kentechcorporation. The output voltage of the new pulser is 1
kV and the pulse width is 1 ns. We attempted toincrease the charge of emission from the Y796 electron
gun using this pulser.

3. Results and Discussion
3-1. Magnetic pulse compression at achromatic systemfu the magnetic pulse compression experiment, the RFphase in ACCI was tuned so as to minimize the energy
spectrum of the pulse. Its energy and energy spread are
19.1 MeV and 0.26 %, respectively. The RF phase inACCII was tuned so as to accelerate electrons in theearly phase of the pulse more than those in the later
phase of the pulse. The best RF phase in ACCII wasadjusted so as to make the shortest pulse monitoring
its width by using streak camera. The peak electricfield in ACCII was 10 MV/m. The charge passing
through a 3 cjl slit at the straight beam port was 700
pC/pulse. A typical measured . pulse shape ofcompressed pulse riding on the phase of 72" is shownin Fig. 2. The pulse width is 850 fs and the horizontal

2-3. Measurement of pulse shape
Pulse shape of emission from electron gun with theenergy of 90 keV was measured by co-axial beam
catcher at a distance of 2.5 em from the anode. The
time-resolution of co-axial beam catcher is less than 50
ps. The charge of emission was measured by faradaycup.
Pulse width of relativistic beam was evaluatedby measuring Cherenkov radiation emitted by therelativistic electrons in air at the beam ports.
Cherenkov radiation was measured by using afemtosecond streak camera which has a time resolutionof 200 fs (HAMAMATSU). The optical measurement
system is shown in Fig. 1. All data were acquired bysingle shot measurement to avoid effects of jitter byaccumulation. Beam sizes were measured by using
phosphor screens (Desmarquest AF995R) at the beam
ports.

8000-~ 6000~~--+-~--~-4--~--~-1::J::f~}-f~~~t~~~=Ill+JI\Lo14000~ 2000
05>
~ 40......

.....,>.

~ 39 1-----+-1---t---t----l~

38 ._._._............_........_._._.__._._._._..............,-10 -5 05 10-r">.

tf 39Time (ps)0I>.

510~-10 -50

510At the downstream exitof ACCII

Eo .......,
E-5 1-----+____:.-+'-'--+--1-10 u................~......_._.........J.................,~-10 -5 0 5 10 ('fi"B) Al!SU~lUT

II I t 2.~=: ~C&cubtion

jlt3.3mm ~ Experimental~§-10 -5c:c:~-~--__T_i_m_e_(p_~--~~ j -

4.8mm
5.5mm""

x(mm)

=;_:

~5 1-----+__,~---+--1~--t+-----1

Time (ps)- :"' I YDS.
%:.
~i I.

0.-----,-,---, ,...

!

)

38
-10 -5

10 .------,----,------,-----.,

I

:

~

20

Beam sizes41>~ 40::E"-'

15Fig. 2. Measured pulse shape of compressed single
pulse generated by using the 300 V grid pulser.

Longitudinal phase space distribution and pulse width
41 ,..-----,--,.--,------,

10Time (ps)05Time (ps)10:::::

0 5c:~ -10 -5x(mm)-10

At the Beam PottFig. 3. Calculated longitudinal phase space distributions and beam sizes after energy modulation and after pulse
compression.
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and vertical beam sizes of the compressed pulse were
3.3 mm and 5.8 mm, respectively. The charge was 208
pC/pulse. Also, 900 fs single pulse with the beam sizeof 3.3 mm X 3.0 mm was measured at the RF phaseof 61.5". Figure 3 shows the results of simulation.
The simulation parameters are 100 nmm mrad as 90 %normalized emittance, 19.1 MeV as the energy and0.26% as the energy spread at the exit of ACCI. Boththe pulse width and the beam sizes are agreed with theexperimental results.

10QQr-~,-~-r~~--.----~~~

r. . . . . . . .

~800 ..................................................................

~

600 ...........................................................

!'. . . . . . . ..

. .............. ..

_.;;- 400 ................. ................. .............
(/)

c:Q)

E 20Q~--~---4~~Y*

3-2. Change of Grid Pulser
By increasing applied voltage to the grid up to 900Vfrom 80V, the emission from Y-796 electron gun wasincreased up to llA with the pulse width of 1 ns. With
the increase of the emission, the SHB power wasincreased up to 2.8 kW from 2.0 kW so as to form asingle pulse. The electric field of SHB cavity wasincreased up to 0.029 MV/cm from 0.024 MV/cm.
Figure 4 shows the pulse shape at the beam port in thestraight direction. The satellite of the main pulse could
not been observed within 10 ns before and after themain pulse. The charge of the original pulse passing
through 3 ell slit was 2 nC/pulse. The charge wasincreased more than twice. This pulse was compressedby achromatic pulse compression system. Figure 5
shows the typical pulse shape of subpicosecond single
pulse with the energy of 34.8 MeV passing through 3ell slit. The charge was 1.04 nC. The charge of thecompressed pulse passing through 5 ell slit was 1.36
nC. About 20 percent of the initial charge before
compression is lost by the energy modulation.
Furthermore, a part of the charge is lost at the slit of
the beam port.

4. Summaryo~~~~.n~~n..u0102030

4050

600

800

1000

Time (ps)-.........

·::Jc:

400.0,._ctl>-"(ijc:200Q)

c:

200

400

Time (ps)Fig. 4. Measured pulse shape of original single pulse
before compression generated by using the 1 kV grid
pulser.

34.8 MeV electron pulses with the pulse width of 10
ps have been compressed by the achromatic magnetic
pulse compression system at the university of Tokyo.
890 fs single pulse have been generated with the chargeof 1 nC through 3 ell slit. The satellite of the main
pulse could not been observed. Subpicosecond pulse
radiolysis system is under construction to investigate
radiation chemistry and physics in a subpicosecond
time domain.

.........

·c:::J€ctl... 750fs-References[1] Y. Tabata et al., J. Fac. Eng. Univ. Tokyo, 34B(1978) 619.
[2] S. Tagawa et al., Chern. Phys. Lett. 64 (1977)
258.
[3] Y. Katsumura et al., J. Phys. Chern., 84 (1980)
833.
[4] M. Uesaka, T. Kozawa et al., Physical Review ESO(1994) 3068.

.II0nl

LJ ~ . ~. l.h J.].. •• tA.

10_.u..... l~

20

Time (ps)Fig. 5. Measured pulse shape of compressed single
pulse with the charge of 1 nC.

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