Ultrashort Electron Beam Generation andDiagnosis by Linear Accelerator
Mitsuru Uesaka, Takahiro Watanabe, and Nasr Hafz

Abstract

1. Introduction

A 240-fs 18-MeV electron single bunch
was generated and synchronized with a 100fs 0.3-TW Ti:sapphire laser with the time resolution of 330 fs. We have upgraded and tested
four diagnostic tools/methods: a femtosecond
streak camera, a coherent transition radiation( C T R ) i n t e r f e r o m e t e r, a f a r- i n f r a re dpolychromator, and a fluctuation method. The
subpicosecond synchronization system is used
to study radiation chemistry. We are close toexperimentally demonstrating that a 10-fs
tens-of-megaelectronvolt electron single
bunch can be produced by laser plasma acceleration using a 12-TW 50-fs laser. This will
enable a pump-and-probe analysis system witha time resolution of tens of femtoseconds to
explore the boundary between physics andchemistry.

Ultrashort-beam science and technology
began with the experimental verification of
50-fs (rms) 10-MeV electron multi-bunches1)
and of a 700-fs 35-MeV single bunch.2) The
S-band (2.856 GHz) laser-photocathode RF
electron gun has generated high-quality
femtosecond beams with low emittance (<10mm mrad),3,4) and fourth-generation synchrotron light sources are going to be constructed5,6). They will produce ≈100-fs intense
x-ray free electron laser via self-amplified spontaneous emission (SASE) and will make possible dynamic microscopic structural analysis.
Several compact inverse Compton hard-x-ray(≈10-keV) sources are also under development,7) and picosecond and subpicosecond
time-resolved synchronization systems using
electron linear accelerators and femtosecond
lasers have already been used to study ultrafast
radiation chemistry. 8-10) Furthermore, ul-

λ Shifter

Optical Delay Line3ω

Analyzing System
X-ray Diffractometer
X-ray Photon Spectrometer
Fourier Far-infrared Spectrometer10fs Electrons
Ions
X-rays

18 MeV Linac

RF
Gun

Accelerating Tube
Chicane-type
Compressor

Thermionic
Gun

Plasma Cathode35 MeV Linac
SHB

OpticalCompressor

ACC II

ACC I

Achromatic-arc
type Compressor

Diode Amp.

Power Amplifier 22.856GHz

Power Amplifier 1
100 fs Laser

Klystron
15MW
MasterOscillator
79.3MHz

Miltipass Amplifier
Regnerative Amplifier

Timing Stabilizer

x36

Ti:Sapphire Oscillator
with Kerr Lens Mode Locker
Regenerative
AmplifierStretcherMultipass
AmplifierFemtosecond S-band Twin Linac System

TimingStabilizer

Ti:Sapphire Laser
Oscillator with
Chirped Mirror Mode Locker

12TW 50fs Table-Top Laser

trashort beams of electrons, x rays, and ionscan be generated by means of laser plasma
acceleration using table-top multiterawatt
Ti:sapphire lasers. Ultrafast multibeam pumpand-probe analysis using those lasers andfemtosecond electron linear accelerators may
be able to detect and visualize the movementof atoms in laser-irradiated GaAs monocrystals. Confirming these time resolutions will
require the use of ultrashort-electron-pulse
diagnostic tools and methodologies, and here
we introduce four: a femtosecond streak camera, a coherent-radiation (CR) interferometer,a far-infrared polychromator, and a fluctuation method. Their resolution and reliability
have been crosschecked in detail and confirmed.

2. F e m t o s e c o n d U l t r a f a s t
Quantum Phenomena Research Facility
As shown in Fig. 1, the Femtosecond
Ultrafast Quantum Phenomena Research Facility at the University of Tokyo’s Nuclear Engineering Research Laboratory consists of two
S-band electron linear accelerators (linacs), a
0.3-TW 100-fs Ti:sapphire laser, a 12-TW 50fs Ti:sapphire laser, and equipment for analysis. The linacs are synchronized with the 0.3TW laser with the time resolution of 330 fs,11)
and an S-band laser photocathode RF gun
developed at Brookhaven National Laboratory(USA)3) is installed at the 18 MeV linac. The
best beam parameters are an energy of 3 MeV,an emittance of 6 πmm mrad, a pulse widthof 10 ps, and a charge per bunch of 7 nC.12,13)
Using the 12-TW laser, we are performing experiments to generate femtosecond and picosecond pulses of electrons, x rays, ions, andneutrons by means of laser plasma acceleration.

3. Magnetic pulse compression
As shown in Fig. 2, the magnetic pulse

Fig.1 Femtosecond Ultrafast Quantum Phenomena Research Facility.

Nuclear Engineering Research Laboratory, University of Tokyo

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5000
Glass block

4000

Compressed pulse
without chirping

dn/dGrating

Chirping

Beam intensity(a.u.)

downward

upward

3000
13 ps
2000
1000
0

(i) Chirped pulse compression for femtosecond laser
-1000
0

50

100

150

200

Time(ps)(a) without compression

Achromatic-arc
RF travelingwavetpulse shape

4000

Focusing magnet
E

Bending magnet
Energy modulation of electronsin accelerating tube

5000

E<E 0
E=E 0
E>E 0

Magnetic pulse compressor

E

Chicane

t

E 0<E
E 0=E
E 0>E

t

Beam intensity(a.u.)

E

3000

440ps

2000
1000
0
-1000
0

Bending magnet

5(ii) Magnetic pulse compression for femtosecond electron beam

10

15

20

Time(ps)(b) with compression

Fig.2 Magnetic pulse compression for electron pulse and chirped pulse compression for
laser pulse.

compression used to generate femtosecond
electron pulses is analogous to the chirped
pulse compression (CPA) in femtosecond lasers. CPA consists of chirping (longitudinalenergy modulation), stretching, amplification,and compression. Generally a set of gratings
is used for chirping and compression. Higher/
lower-energy photons are located in an earlier/later part of laser pulse for downward/
upward chirping. If we replace photons withelectrons, chirping and compression become
magnetic pulse compression. Downward andupward chirping are respectively done by putting the electron beam on an increasing or
decreasing RF phase of the accelerating traveling wave in an accelerating tube. Then the
beam passes through a magnet assembly consisting of bending and focusing magnets. The
chirping is transformed to path-length modulation because the bending radius is inversely
linear to the electron energy. Therefore thelater electrons can catch up with the earlier
electrons and the beam is compressed. At the
35-MeV linac we have an achromatic-are-type
compressor for downward chirping, and at the
18-MeV linac we have a chicane-type compressor for upward chirping.

Fig.3 Pulse shapes without and with compression measured by the femtosecond streak
camera.

Bunch shapes obtained with and withoutbunch compression are shown in Fig. 3. The
13-ps bunch is compressed to 440 fs (FWHM).
The best pulse compression was done aroundthe phase of the best emittance. After thismeasurement, we calibrated the streak camera by using an 86-fs (FWHM) Ti:sapphire laser (Spectra-Physics, Tsunami). The 86-fs laser pulse was elongated to 390 fs. Thus, according to the error propagation law, the error at the camera was estimated to be 370 fs(FWHM). We found that this error was mainly
due to the degradation of the microchannel
plate (MCP) in the streak tube. If we assume
independent Gaussian distributions for boththe electron bunch and the error, the error
propagation law gives a bunch length of 240
fs (FWHM) after the error reduction. When
we do not assume that the distributions are
Gaussian and use the error propagation function evaluated from all measured data, thebunch length becomes 290 fs (FWHM). After
this experiment, we replaced the degraded
MCP with a new one, the error of which has
been evaluated by the same procedure to be
240 fs.12)

4. Pulse shape diagnosis
a. Femtosecond streak camera
We have used a commercial femtosecond
streak camera (FESCA200, Hamamatsu
Photonics, Japan) with a time resolution of 200
fs (FWHM). We measure electron pulse shape
via Cherenkov radiation in air and Xe gas formore and less than 20 MeV electrons, respectively. If a focusing lens is used, the optical dispersion and wide spectrum of the Cherenkov
radiation yield serious pulse elongation. We
therefore instead use a narrow(about 10 nm)
bandpass filter. But this reduces the radiation
intensity at the camera so much that the signal-to-noise ratio becomes poor.14) To solve thisproblem we constructed a non-dispersive optics using only reflective mirrors to guide theradiation. This improved the signal-to-noise
ratio remarkably, maintaining the time resolution even when a 20-nm filter was used.15)

b. Coherent radiation interferometer
When an electron bunch passes throughthe interface of two media of different dielectric constants, it emits transition radiation (TR)JSAP International No.5 (January 2002)

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of the radiation emitted by one electron difIncoherent Radiation (Bunch length>>Wave length)
fers from that of the radiation emitted by another electron. And as
shown in Fig. 4, when
the wavelength islonger than the bunch
Bunch length
Wave length
length, the radiation is
Coherent Radiation (Bunch length<<Wave length)
almost coherent because the phase of theradiation by any electron is almost as that of
the radiation emitted
by the others. This reFig.4 Incoherent / Coherent radiation
sults in the temporal
coherence of radiation.
that carries the information of the bunch disCoherent radiation yields an interferogram
tribution. Similarly, when an electron bunchwhen we use an interferometer such as thepasses through an aperture in a dielectric maMichelson interferometer, and the information
terial or near the edge of a dielectric material,in the electron bunch can be inferred fromdiffraction radiation (DR) is emitted from thethe interferogram. Another important featureedge. The relation between the transition raof coherent radiation is the dependence of the
diation Ie,TR and the diffraction radiation Ie,DR ispower on the number of electrons in thegiven by the following equation:16)
bunch. The following equations show thatthe power of incoherent radiation increases(1)linearly with the number of electrons while
wherethat of coherent radiation increases linearlywith the square of the number of electrons.
(2)
When two coherent radiation pulses interfere,
Here J0 is the 0th-order Bessel function, D
the longitudinal bunch distribution can be deis the diameter of an aperture, K1 is the 1st
rived from the interferogram of the light inmodified Bessel function, v is the wave numtensity by the following procedure.
ber, θ is the radiation angle, and βγ are theWhen the cross section of the beam isrelativistic factor of the electron. The diffracsmall and the observation point is far from thetion radiation due to an infinitesimal hole (Dsource, the intensity of the transition radia≈ 0) corresponds to the transition radiation.
tion is expressed - by analogy to the intenWhen the wavelength of the radiation issity of coherent synchrotron radiation shorter than the length of the electron bunch,
as16,17,18)
the radiation is incoherent because the phase

Intensity (a.u.)

Signal (V)FWHM=0.23mm

0.05
0.04
0.03

Imax
=1.94
I0

0.02

0.16

6

0.12

4
0.08
2
0

0
1

2

3

4

5

6

7

0

-2

0.01
0

0.04

Water
absorption

8

5

10

15

20

25

30

35

40

JSAPI̲Vol5-new.p65(5)The form factor f (v) can be divided into
two parts, the longitudinal bunch form factorfL (v) and the transverse bunch form factorfT (v), as follows:18)
(6)
where
(7)
(8)
Here h(z) and g(ρ) are respectively the longitudinal and transverse distribution functionsof the electron bunch, and we assume thatthe transverse beam distribution is circular.
The transverse bunch form factor is obtainedby measuring the transverse distribution of theelectron bunch. When we observe the transi-

1

Wavenumber (cm-1)

0.5

Experiment600 fs (FWHM)
650 fs (FWHM)
700 fs (FWHM)
0.1
5

6 7 8 9 10

20

Wavenumber (cm-1)(b) spectrum and beam splitter efficiency |RT|2(c) longitudinal bunch form factor

JSAP International No.5 (January 2002)

Page 16(4)

where n is the unit vector directed from thecenter of the bunch to the observation point→
and x is the position vector of the electron
relative to the bunch center. Since N>>1,

Fig.5 Interferogram, spectrum and longitudinal bunch form factor of subpicosecond electron pulse by CTR interferometry.

16

→

→

Optical Path Difference (mm)(a) interferogram→ →→Beam splitter efficiency

8

0.06

where N is the number of electrons in thebunch, v is the wave number (the inverse of
the wavelength of the transition radiation) andIe (v) is the intensity of the transition radiation
emitted by a single electron. The first term ofEq. (3) represents the incoherent transition radiation and the second term represents thecoherent transition radiation. The quantity f(v)is the bunch form factor, which is given by
the Fourier transform of the distribution func→tion, S( x ), of electrons in the bunch:Longitudinalbunch form factor

0.07

(3)

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2.5 10-11

3
2.5
2
1.5
12

14

16

18

20

22

24

26

28

2 10-11
1.5 10-11
1 10-11
5 10-12
12

Wave number (cm-1)

14

16

18

20

22

24

26

Bunch form factor

Reconstructed pulse shape1

Density distribution (a.u.)

3.5

Longitudinal bunch form factor

Calibrated power spectrum
3 10-11

Intensity (W.m)

Intensity (mV)Direct voltage output
4

0.1

0.01

bff
900femto
1.0pico1.1pico0.001

0.0001

Wave number (cm-1)

5

6 7 8 910

20

30

3500
3000
2500
2000
1500
1000
500
0
-500
-2

-1.5 -1 -0.5 0

0.5

1

1.5

2

Distance (mm)Wave number (cm-1)

Fig.6 Reconstruction of subpicosecond electron pulse by
the polychromatortion radiation from an on-axis or nearly onaxis direction (i.e., θ 2>>1), cos θ and sin θ canrespectively be unity and zero. Equations (7)and (8) can then be rewritten approximately:(9)

StreakCameraAl foil
LINAC
CTR
Si
Bolometer

10 ch InSb
Detectors

Cryostat(Liquid He)

and
(10)
where FL(v) is the Fourier transform of the longitudinal bunch distribution h(z), g(ρ) is thetransverse bunch distribution, and J0(0) is the
0th-order Bessel function.
The interferogram of the light intensity of
two interfering coherent transition radiation
pulses as a function of the position of the
movable mirror of the Michelson interferometer is obtained as shown in Fig. 5(a). Theinterferogram can be written(11)
where S(δ) is the intensity of the recombined
radiation intensity at the detector (expressedin the time domain with an additional timedelay δ /c for the movable mirror) minus theintensity at δ→ ±∞, Ẽ(v) is the Fourier transform of the electrical field of the transition
radiation, and R and T are the coefficients ofreflection and transmission at the beam splitter.
Solving for | Ẽ(v)|2 yields

Michelson InterferometerFig.7 Measurement setup for the femtosecond streak camera, CTR interferometer and polychromator.

bers. This effect is compensated by using thetheoretical curve.
Using Eq. (5) and the relation Itotal(v)=| Ẽ(v)|2,
we can obtain the following equation for thebunch form factor:(13)
Then, as is seen in Fig. 5(c), the longitudinal bunch form factor is obtained using Eqs.
(6) and (13). Finally, the Kramers-Kronig relations and inverse Fourier transform give thelongitudinal bunch distribution h(z) from thelongitudinal bunch form factor as follows:16,18)
(14)

(12)
The spectrum deduced from the interferogramis shown in Fig. 5(b), where the theoreticalbeam splitter efficiency is also drawn in orderto show non-uniform sensitivity for wave num-

Polychromator

(15)
(16)

c. Far-infrared polychromator
By using the polychromator, the spectrumof the radiation can be obtained directly fromthe frequency measurement bins of the output voltage as shown in Fig. 6.15,19) Then thebunch distribution can be deduced by thesame procedure used in interferometry. One
can skip the procedure to get the spectrum
described as Eq. (12). This simplification of
the analysis is one of the advantages of thepolychromator method. Another important
advantage is that it can be used to diagnose
the electron bunch by a single shot. The spectral information obtained in an experiment,
however, is limited by the number of detectors. The typical output voltage of thepolychromator is shown on the left-hand sideof Fig. 6. The response function of the optics
and the sensitivity of the detector in eachchannel of the polychromator were calibrated

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d. Performance
The experimental setup is shown in Fig.
7. The incoherent Cherenkov radiation emitted in air was measured by the streak camera,
and the CTR in the far-infrared region wasmeasured by the Michelson interferometer andthe polychromator. At the Michelson interferometer the radiation was split into twobunches at a beam splitter and, after one
bunch was delayed by the linear stage, thetwo bunches were recombined at the same
beam splitter and the resulting signal was detected by a Si bolometer cooled in liquid helium. The radiation, which is transported tothe polychromator, is deflected by the grating
and then resolved by the 10-channel-detector array. The polychromator, which was constructed by Y. Kondo et al., 19) covers wave
numbers from 12.2 to 26.2 cm-1. When the
mirror mounted on the motorized linear stage
was moved to the beam path at the linac exit,
Cherenkov radiation emitted in air was transported to the streak camera. The major beam
parameters were as follows: the electron energy was 34 MeV, and the electron charge perbunch was controlled between 10 and 100
pC to keep the detectors from saturating. The

bunch length was adStreak camera (FWHM=1.0ps)
justed to be about 900
Interferometer (FWHM=1.2ps)
Polychrometer (FWHM=1.0ps)
fs (FWHM, or 380 fs
35rms) and 1.6 ps
30(FWHM, 680 fs in rms)
at the linac by using
20
the streak camera.
Consequently, the10longitudinal bunch distribution was inferred0from information provided by the streak
-1
-0.5
0
0.5
1
camera, the Michelson
Longitudinal axis (mm)interferometer, and thepolychromator (Fig. 8).
Fig.8 Measured electron pulse shape by the femtosecond streak camera, CTR
interferometer and polychromator.
This is the first timeagreement among thethree methods has been shown for a roughlyof independent parts in a bunch, Nindep. One
1-ps electron bunch.
can write

Density distribution(a.u.)using the mercury arc as well as the interferometer. Furthermore, the discreteness of theexperimental data, which depends on the grating pitch and the configuration of the measurement bins installed in the polychromator,
is interpolated in order to obtain a continuous spectrum.

(18)

e. Fluctuation method
When the wavelength of the radiation isshorter than the length of the electron bunch,
the random phase of the radiation emitted
from a single electron,E0(t) , causes the total
electric field E(t) in a bunch to fluctuate randomly: 20)(17)
A photodiode can measure only the timeintegrated intensity ∫|E(t)|2dt, and shot-by-shot
fluctuations in the radiation are eventually obtained in the experiment. As pointed out in
Ref. 20, the fluctuation of the time-integrated
intensity gives information about the number(b) without band pass filter

intensity[a.u.]4

Al foil

LINAC

Electron pulse

2

0
-800

-400

0

400

800

time[fs]

Band pass filter(BPF) ∆νTransition
radiation(c) with band pass filter
2.5

CCD
or
Photodiode(a) experimental setup2

intensity[a.u.]

Lens

1.5
1
0.5
0
-1000

-500

0
time[fs]

Fig.9 Measurement setup and numerical results of pulse structure for the fluctuation method.

18

JSAPI̲Vol5-new.p65

500

1000

where F is a form factor that is computed fromthe Fourier transform of the bunch distribution, and τcoh and τb are respectively the durations of the independent part and the bunch.
The duration of the independent part, τcoh,which is associated to the coherent time, results from the finite length of the envelope ofthe electric field emitted by a single electron
and it can therefore be calculated from the
bandwidth of the radiation.
We first measured the time-integrated intensity of the incoherent radiation in several
bandwidths. The experimental setup is shownin Fig. 9(a). Cherenkov radiation emitted in
air was measured with optical transition radiation (OTR). OTR, however, is much less intense than Cherenkov radiation, so the influence of OTR can be ignored. In the experiment the spectrum was limited by the
bandpass filter so that the coherent time
would be a fraction of the bunch duration.
The calculated intensities of electric field of
incoherent radiation with and without the
bandpass filter are shown in Figs. 9(b) and 9(c).
As is seen from the figures, by introducing theband pass filter, the electric field is made ofabout ten independent spikes, each of which
has a random amplitude. Figure 10 shows
the fluctuation of time-integrated intensity,where the measured pulse width was 1.0 ps
by the streak camera.21) However, by the measured fluctuation the pulse width was estimated to be 4.5 ps. The discrepancy between
the above results was due to the transverse
beam size of the bunch, since so far we have

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Table 1 Overall comparison of the four diagnostic methods

60

Fluctuations [%,rms]

1D simulation
50

Radiation

Single shot

Time resolution

Influence ofother parameters

Dynamic range

Timing jitter
evaluation

Incoherent
Low intensity

available

200fs

small

wide

available

not available

theoreticallyunlimited

large

relatively narrow

not available

available

theoreticallyunlimited

large

relatively narrow

not available

relatively small

relatively wide

not available

D = 0.01mm
D = 0.05mm

40

D = 0.1mm

Streak camera

30
Interferometer20
10

Polychromator

Coherent
High intensity
Coherent
High intensity

0
0

20

40

60

80

100

Fluctuation

Incoherent
Low intensity

theoreticallyavailable

unlimited

Bandwidth [nm]Fig.10 Fluctuation of radiation power at several bundpass filters.

assumed that the transverse beam size is infinitesimal. It is worth noting that an infinitesimal transverse beam diameter is assumed
in the above theory, whereas the transverse
beam diameter in our experiment was morethan 1 mm and cannot be ignored. To increase the precision of our work, we are now
using two-dimensional numerical analysis totake into account the effect of the transverse
beam size on the fluctuation. The transverse
beam size at the radiation sources was measured to be 3 mm (FWHM of Gaussian distribution) for a 1.0-ps pulse width.
The spectrum shows fluctuation shot byshot. It is known that observation of the spectrum provides more information than does
observation of the time-integrated power.22)
The longitudinal bunch distribution can beinferred by analyzing the variance of the Fourier transform of the spectrum, and information about the transverse beam size can also
be extracted from the spectrum. It is therefore expected that the pulse information will
be obtained in the frequency-domain measurements that are under way.

summarized in Table. 1.

5. Laser plasma cathode fortens-of-femtosecond electron
single bunch generationa. Pump-and-probe analysis using
femtosecond beamsb. Laser wakefield accelerator

Here we introduce the generation of ultrashort-bunched electron beams fromunderdense plasma driven by ultrashort andultra-intense laser pulses. For this purpose,computer simulations and experiments basedon the 12-terawatt 50-fs laser system havebeen conducted.
The target short electron bunch alongwith the short laser pulses already available
could be used to make a pump-and-probe system for studying, for example, the initial phaseOf particular interest in the laser plasma
acceleration schemes is the laser wakefield
accelerator (LWFA),23) in which an intense laser pulse propagates through underdense
plasma and generates large-amplitude plasma
waves (wakefields) by the effect of theponderomotive force associated with the laser pulse envelope. A plasma density no of
1018 cm-3 can support a field E0 of 100 GV/m.
The plasma wavelength of interest is of the

Detector

SupersonicGas jet
PlasmaWave

PlasmaElectrons
Laser Pulse

f. Overall Evaluation
For pulse widths up to 200 fs, it is obvious that the streak camera is the best. As forless than 200 fs, upgrade, combination andcrosscheck among the other methods areneeded. Shot-by-shot measurement is available for the streak camera and polychromatorand is important for evaluating the beam stability. The timing jitter between two pulses,
which determines the time resolution of
pump-and-probe analysis, can be evaluatedonly by the streak camera. The overall evaluation of our instruments and methodology isof radiation-induced chemical reactions. A
proposed configuration for such applicationis shown in Fig. 11. A beam splitter would be
used to split the 12-TW 50-fs laser beam into
an intense beam (≈11.5 TW) used for electron beam generation and a less intense beam(≈0.5 TW) used as a probe.

2mm
Electron pulse

12TW 50fs
Laser System
Wave
Breaking
Beam
Splitter

Plasma Channel
Plasma density

5µm

Water cell

12fs

0.5 TW

50fs

33fs
Optical Delay

Fig.11 Schematic diagram of a pulse radiolysis system based on femtosecond bunched electron beam generated from anultraintense ultrashort laser-plasma interaction.

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e3-field(envelope)

e1-field(average)

Time = 3360.00[1/ωn]Time = 3360.00[1/ωn]

5

2000

2000

0.4

4

2

x2 [c /ωn]

1000

0.2

500

1000
0.0

E1 [mecωne-1]

1500

3

E3 [mecωne-1]x2 [c /ωn]

1500

500

1
-0.2

0

0
0

200

400

600

0
0

800

x1 [c / ωn]

200

400

600

800

x1 [c /ωn]Fig.12 The 2D-PIC computer simulations (a) The contour plot of the laser pulse as it propagates through underdense
plasma. The laser is self-focused (b) The plasma wave generated behind the laser pulse.

200

250
ct = 0.5 mm
ω 0 t = 3984

ct = 1.5 mm
ω 0 t = 11760

200

P1(mc)

P1(mc)

150

100

150

100

50
50

0
4000

4200

4400

4600

4800

X1(c /ω 0 )01.17 1041.19 104

1.21 104

1.23 1041.25 104

X1(c /ω 0 )(19)

Fig.13 The 2D-PIC computer simulations. The longitudinal phase space of the trapped and accelerated plasma electrons
after the laser pulse has propagated (a) 0.5 mm through the plasma. (b) 1.5 mm through the plasma.

order of 10-100 µm, so the longitudinal lengthof the accelerated and/or injected (for additional acceleration) electron bunches should
be within this range. Furthermore, the electron bunch should be injected in the correct
phase in order to get trapped and accelerated
by the wave. Two schemes based on lasertriggered electron trapping in the plasma wave
has been proposed for generating ultrashortelectron beams.24) For this purpose, several
laser pulses should be synchronized in space
and time more precisely than currently possible. We have therefore proposed25) a system based on the use of a single ultra-intenselaser pulse focused in the plasma (Fig. 11).
The basic idea for generating the electron
bunch is to trap and accelerate some of theplasma electrons in a nonlinear plasma wave,
which is forced to ‘break’ in the plasma. At

20

JSAPI̲Vol5-new.p65

wave electric field generated behind the laser
pulse. The laser intensity was originally 1.1×
1019 W/cm2 and reached 14.2 × 1019 W/cm2
because of the self-focusing effect. The maximum amplitude of the electric field of theplasma wave,Ez, reached 0.7 TV/m. The selffocusing of the laser and the generation ofsuper-high electric field in the plasma wave
are essential in the process of electron trapping and acceleration as will be made clear
below. Figure 13 shows the longitudinal
phase-space of plasma electrons trapped andaccelerated to 110 MeV by the plasma waves.
A plasma-wavebreaking mechanism for producing for a sharp density gradient when electrons are injected into the plasma waves has
been proposed, 27) but for a laser pulse of relativistic intensity (a = eEL / mωLc >1), the electron injection (regardless of how sharp thedensity gradients are) is based on the breaking of waves produced by the relativistic acceleration of electrons into wakefield waves.
The relativistic force, which acts along the laser propagation direction, grows as a2.28) As
shown in Fig. 13(a), the relativistic force creates waves with wavelengths of the order ofthe laser wavelength propagating with thelaser pulse. The maximum energy of electrons
in the waves is determined by the longitudinal component of the electron momentum:

the wave-breaking region, many ultrashortbunched electrons are accelerated and leave
the plasma with relativistic energies.

c. Simulations
Two-dimensional (2D) simulations weredone using the recently developed fully relativistic particle-in-cell code OSIRIS.26) The simulations for the gas jet 1.5 mm in diameter wererun for a time of 12600 ω L-1( ωL is the laser
frequency), that is 4.6ps. We set the maximum plasma density to 5 × 1019/cm3 and thelaser pulse as 12-TW 50-fs with spot size of
10 µm.
Figure 12 shows part of the 2D-PIC computer simulation results. Figure 12 (a) shows
the contour plot of the laser pulse as it propagates through the plasma; the laser is self-focused. Figure 12(b) shows the contour plasma

These electrons constitute the injection source
for the following relativistic wave so that electrons acquire the maximum energy after
propagating 0.5 mm as shown in Fig. 13(a) or
after propagating 1.5 mm as shown in Fig.
13(b).
We can see in Fig. 13(b) that the averageenergy of all electrons is 22.5 MeV, but anenergy spread of 100 % was produced. Using a magnetic chicane, it is possible to filter
this beam in order to get a beam with a smaller
energy spread. In the energy range from 82
MeV to 110 MeV, the bunch length was 3.5µm(equivalent to ≈12 fs), the rms emittance was≈0.7 mm mrad, and the total charge of accelerated electrons was about 580 pC.

d. Experiment
The experiment was performed with the
12-TW laser system that delivers 600-mJ 50fs pulses at 790 nm. The laser beam was focused with an f/2 off-axis parabolic mirror toa spot about 20 µm in diameter. The peakintensity in the focus point is expected to ex-

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ceed 10 19 W/cm2. The focus was placed at
the edge of a free-expansion helium gas jet
generated by a high-pressure gas nozzle witha circular orifice 2 mm in diameter. The density profile has been measured by MachZehnder interferometry. The transverse density profile in the interaction region 2 mmabove the nozzle tip corresponds to a Gaussian
profile. The electron density in the tunnelionized underdense plasma created by the footof the laser pulse is, up to a maximum valueof 5 × 1020 /cm3, linearly proportional to thebacking pressure.
The transverse profile image of the electron beam observed when the laser power andplasma density were 4 TW and 2.7 × 10 20 /cm 3 is shown in Figure 14. Such beam has
been observed at a distance of 20 cm after
the gas jet inside the experimental vacuum
chamber. A thin foil of aluminum was inserted
after the gas jet to prevent the laser light fromreaching the electron beam detection system.
The angle of divergence of the electron beam
was about 120 mrad. Since the diameter ofthe beam was about 10 microns at the focus
point, the total beam emittance was about 4
π mm mrad. The charge of the electron beam
was measured by using a Faraday cup outside
the vacuum chamber. The total charge measured within an angle of 70 mrad was 14 pC.
It should be noted that this measurement underestimates the beam charge because of thelimitation due to the limited collection angle.
Further measurements to verify about 10fs
electron pulse are planned.

Conclusion

Fig.14 The transverse profile of the electron beamobserved in the intense laser plasma interaction. The laser
power was 4 TW and the plasma density was 2.7 × 1020/
cm3.

We introduced sophisticated technologies
for generating and diagnosing ultrashort electron beams, which enables a pump-and-probe
analysis to investigate ultrafast microscopic
dynamics in matters. These technologies areunder development at the University of Tokyo’s
Nuclear Engineering Research Laboratory,
where we are leaders in the race to “dynamic
nanotechnology.” We hope these technologies will help establish a major new scientificfield in the 21st century.

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