The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Electron Jet Generation by Laser Plasma
Hideki DEW A, *Takako FUKUDA, Noboru HASEGAWA, Tomonao HOSOKAI, Masaki KANDO,*Yasuteru KODAKA, Shuji KONDO, Hideyuki KOTAKI, **Nasr A. MOHAMED HAFZ,
Kazuhisa NAKAJIMA, *Kimio NIW A and **Mitsuru UESAKA.
Japan Atomic Energy Research Institute (JAPAN)
8-1, Umemidai, Kizu-chou, Soraku-gun, Kyoto-fu 619-0215, Japan*Nagoya University
I, Furouchou, Chigusa-ku, Nagoya-si, Aichi-ken 464-8602, Japan**The University of Tokyo2-22, Shirakatashirane, Tokai-mura, Naka-gun, Ibaraki-ken 319-1106, JapanAbstract

The generation of high energy electrons by high intenselaser was examined. The laser pulses with 2 TW, 100 fs
were used for generating plasma. They were focused innitrogen gas, which generated thin plasma and electron jets
transversely. We measured the radiation of the jets withimaging plates and emulsion track detectors. It wasconfirmed the radiation is transversely symmetrical x-ray
and the mean energy of the x-ray is around 5 keY. The xrays may be generated by bremsstrahlung of electrons ofjets. The measurement of ~he radiation of the jets withimaging plate and emulsion track detector is discussed.

Ti:Sa laser system with a chirped pulse amplification at I0
Hz. The laser pulses were injected into a vacuum chamber
through a glass window and were focused by an off-axis
parabolic mirror with a focal length of 480 mm. The radiusof the beam waist in vacuum was estimated as 12 J.lm. The
laser pulses ionized the gas at the focus and then generated
thin plasma there. The images of plasma were taken througha window at the side of the chamber by a CCD camera
synchronized with the laser system. In order to measureelectrons or x-rays from the plasma, we set the several
imaging plates on upper and lower sides or right and left
sides of the focus. Their distances from the focus are around
23 mm long.
Imaging plates or emulsion track detectors

1 Introduction

We have already carried out an experiment of the laser
acceleration of injected electrons[ I]. In the process of theexperiment, we found the generation of jets from plasmathat spread transversely. The jets were visible to the nakedeyes through a glass window. These jets were thought to beelectron flows because they were bent in the external
magnetic field. The length of the jets was from I0 to 20 mm
long. Computer simulations also got results of thegeneration of jets from the laser plasma [2].
In order to measure the jets, we used some imagingplates(IP) and emulsion track detectors. The IP is a twodimensional detector to measure radiation. Though it is usedfor measuring the two-dimensional distribution of radiation,
it can be also used to estimate the energy of radiation withcovering thin AI foils. The mean energy of x-rays isdetermined by the absorption coefficient in aluminum foil,
while the energy of the electrons is estimated by the energy
decrease of electrons in the foil.
The emulsion track detector is used to measure thetracks of high energy particles. Because the high energy
electrons leave tracks in the emulsion while x-ray leavesmall dots or crumpled tracks of compton scatterings, it ispossible to separate the electrons and x-rays by track shapes.
2 Experiments

The experimental setup for the measurement of thegeneration of laser plasma electrons is shown in fig. I. The
plasma was generated in a gas chamber filled with Nitrogengas by focusing laser pulses with a wavelength of 790 nm, amean pulse energy of 200 mJ, a pulse length of 100 fs and adiameter of 40 mm. The laser pulses were generated by aTi:Sa 790 nm laser
200 mJ, 100 fs
40 mm diameter

Fig. I Experimental setup for the generation of laser
plasma electron.

2.1/maging plate
The IP is very useful for the measurement of x-rays andparticles, because of its high resolution, wide dynamic range,
and the possibility of computational analysis. It is very
similar to the x-ray film, but it does not need to bedeveloped. After the irradiation on IP, we can read it by an
IP scanner. The profile of the IP is shown in Fig.2. The
luminescence layer is made of BaFBr:Eu 2+ fine crystals of3-5 J.lm which is photo-stimulated luminescent material.
The BaFBr:Eu 2+ crystals emit light of the wavelength
400nm after the stimulation of He-Ne laser light from IP
reader if they have been excited by x-ray or high energy
electrons. The IP scanner can detects the photo-stimulated
luminescence from the crystals with a photo multiplier tube.
As the signals are taken by scanning the laser position andare converted to 8 bit or 16 bit digital data, the numericalradiation image information can be obtained. For theprotection of the surface, there is a protection layer of II
J.lm polyethylene terephthalate (PET) on the luminescent-510-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

layer. As the IPs lose their information by even faint light,
they must be covered with an envelope of black paper.

shell absorption at 1.56 MeV.

Aluminum foilenvelope of black paper

180 jlm BaFBr:Eu2+luminescent layer
PET 250 jlm base layer

Fig. 2 Profile of Imaging Plate.

Io·'L.._l.....JL~~:::~....J0In order to estimate the energy of the electrons or x-rays,
we put some sheets of aluminum foil on the IP. The
ionization energy loss of electrons in materials is given bydE=dxcDEFGHMaterial
23 mm path length in N2 gas at 40 Torr
A + Surface protection layer of IP
B + a sheet of black paper
C + 15 Jlm aluminumC + 30 Jlm aluminumC + 45 Jlm aluminumC + 60 Jlm aluminumC + 75 Jlm aluminum

Fig. 3 Minimum
materials.

penetrationenergy

20

40

5060

Fig. 4 The x-ray attenuation ratio for several x-ray
energies as a function of the thickness of aluminum.

(I)

Some images of the laser plasma for several gas
pressures from 22 mTorr to 760 Torr were taken by a CCD
camera as shown in Fig. 5. The plasma was the thinnest at22 mTorr, but it got wider as the gas pressure was increased.
The plasma was the widest at 50 Torr and generate jets
transversely. The direction of the jet was not exactly
transverse but somewhat tilted to forward. The jets weresymmetrical but not stable and changed the tilt angle atevery pulse. They seemed to have two modes of jet
generation as they had two kinds of tilt angles. We thought
jets were high energy electron's flow, because they werebent by magnetic field.

---------

117
128
139
149of1 Torr. 245 mJ

electron~mSO Torr, 245m.!

------

2.3 Attenuation ofx-ray intensity in aluminum foil

We can also estimate the x-ray energy from the intensity
attenuation on an IP with several sheets of aluminum foil.
The intensity of radiation depending on the thickness ofaluminum foil is given by the following formula,

1=10 exp(- 11 x)
p30

2. 4 measurements ofelectron jets

4Cmec2 ln[j32mec2 _ _!_ln2+_!_]/3 22/22

where C=nr/ZNIA=0.150ZIA, Z is atomic number, A is
mass number, r, is classical radius of electron, N is
Avogadro number, I is the mean ionization energy, m, ismass of electron, c is the speed of light, f3 is the ratio of thespeed of the electron to the speed of light c. The minimum
energy of an electron that can penetrate in nitrogen gas,some sheets of aluminum foil, a paper envelope and asurface protection layer was calculated by eq. (I). Minimum
penetration energy in materials are shown in Fig. 3.

AB

10

Thickness of ahninum [Jlm]2.2 Energy loss ofelectron in material(2)

where I, ~~· Jllp, x are radiation intensity, an incident
intensity, mass attenuation coefficient and mass thickness ofaluminum foil respectively. As Jl/p is a function of energyof x-ray, we can estimate the energy of x-ray by Jl/p. The
function Jllp was calculated by the NIST x-ray· massattenuation database [3]. The calculated ratio I/I0 is shownin Fig. 4. The strong attenuation at 2 MeV is due to the K-

400 Torr ,245 m.J----f-10 Torr, 245 mJ

100Torr, 24SmJ~760 Torr .245 mJFig. 5 The CCD images of laser plasma taken through aside window. The jets appeared from 10 torr to 100Torr.
The radiation images of the jets were measured by IP at thebottom and side of plasma at 20 Torr. As the both images
had almost same radiation intensity; the bottom one had 8.6
PSL (Photo-Stimulated luminescence per Square) and theside one had 7.8 PSL, the radiation from the jet wassymmetric and did not depend on the direction of the laser
polarization.
Then we measured the intensity on IP from 6 Torr to150 Torr of nitrogen gas pressures. Any aluminum foil was-511-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

not used in this measurement. The intensity at the maximum
point for each gas pressure is shown in Fig.6. The intensity
was the strongest at 40 Torr. Because the jet was the largest
at 50 Torr as described above, the intensity of radiation andthe generation of jets are closely related.

position but also the direction of the track.
Aluminum foil of 6 or 15 11m thickenvelope for shading light

Fig. 8 Profile of an emulsion track detector. In order toreduce the energy decrease of electrons in material,
a window with thin aluminum foil is on a hole of anenvelope.
A image on the emulsion is shown in Fig.9. We could
get only small dots and no connected chain-like tracks. It
implies that high energy electron more than 100 keV did not
go through the emulsion.

0 .':-'-~~,._.-L.....,L..-c--L~.L-...__t_,_.._.j02040fiO

XO

I(K)

140

12016()

Gas pressure ITorr I

Fig. 6 Gas pressure dependence of the radiation from theplasma.
2.5 energy measurement of the radiation
We also measured the radiation from jet with IP covered
with aluminum foil. The pressure of the nitrogen gas wasfixed at 40 Torr and the number of the sheets of aluminumwas changed from zero to four. The radiation images on IPs
are shown in Fig. 7. There are radiation images except (e).
If the radiation is electron, the energy is estimated above
139 keY, considering the range of electrons in the materials
from the plasma to IP (see fig. 3). If they are x-rays, theenergy of x-ray is estimated as much as 5 keV by eq.(2). We
could not identify the radiation by the measurements only.

(a)(b)

(c)

(d)

(e)

Fig. 7 The radiation intensity distribution on IP with andwithout aluminum foil. (a) reference measurement
without aluminum foil, (b) with I sheet of l5Jlm
aluminum foil, (c) 2 sheets, (d) 3 sheets (e) 4 sheets.
The intensity on (e) is almost background level. The
maximum intensities for each are 22, 6.5 4.5, 2.5
and 2.0 PSL respectively.
In order to identify the radiation, we measured theradiation with emulsion track detectors. The profile of anemulsion track detector is shown in Fig. 8. The emulsion
was covered with an envelope made of aluminum andshading paper, because faint light could expose theemulsion. A window of thin aluminum foil was on arectangular hole of the envelope. The emulsion detector has
to be developed after the irradiation is finished. The track
images can be obtained with a microscope operated
automatically. The emulsion can measure not only theFig. 9 Image of emulsion with radiation from jets of laser
plasma. Dots are intrinsic fogs in emulsion or tracksof x-rays from jets.
Discussion
The electron jet of laser plasma was measured by threeways: CCD images, radiation images on IPs and emulsion
track measurements. We could not measure the electron
directly but the x-ray from the electron jet, because theelectron energy from the jet was not high enough topenetrate nitrogen gases, an envelope and a protection layerof the IP or a sheet of AI foil for emulsion track detector.
We think all of the electrons from the jet stopped in thematerials. They should have emitted bremsstrahlung x-rays
with the energy around 5 keV. As the gas jet image and thex-ray image on IP are symmetrical, the electron emission
may be also symmetrical. The mean energy of electrons isestimated as about 10 keV nearly twice of the x-ray energy
5 keV considering an energy distribution of bremsstrahlung.
The process of the generation of electron jet is not stillclear. Symmetrical radiation suggests it is due to transverse
laser wake field. This interpretation may also supported bya large acceleration gradient of laser wake field acceleration
at the gas pressure of helium larger than 20 Toor [I].
References[I] H. Dewa et al. Nucl. lnstr. and Meth. in Phys. Res. A
410,p357, 1998
[2] Private communication[3] J. H. Hubbell and S. M. Seltzer. http://physics.nist.gov/-512-