Generation of Relativistic Electrons via
Interaction between Ultra-Short Laser Pulses
and Supersonic Gas Jet
Tomonao Hosokai \ Kenichi Kinoshita2, Takahiro Watanabe \ Koji
Yoshii!, Toru Ueda1, Alexei Zhidkov2, Mitsuru Uesaka^Hideyuki
Kotaki3, Masaki Kando3, Kazuhisa Nakajima4
Nuclear Engineering Research Laboratory School of Engineering, University of Tokyo,
22-2 Shirane-shirakata, Tokai, Naka, Ibaraki, Japan
2
National Institute of Radiological Sciences, 4-9-1 Anagawa Inage Chiba, Japan
3
Advanced Photon Research Center Kansai Research Establishment Japan Atomic Energy Research
Institute, 8-1 Umemi-dai Kizu Soraku Kyoto, Japan
4
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, 305-0801 Japan
Abstract. We have generated relativistic electrons by interaction between a high intensity ultrashort laser pulse (Ti:Sapphire, max 12TW, ^ = 790nm , 50fs, lOHz) and a supersonic gas-jet.
Shock-wave free supersonic nozzles were designed, to perform well-defined laser plasma
interaction experiments. The energetic electrons (up to 40MeV) ejected from the gas-jet with a
narrow cone angle were observed under the conditions of the relativistic laser intensity of 1.0 x
1019 W/cm2 and the helium gas jet density range of 1.4 - 3.0 x!019cm~3. It was found that
generation of the narrow-cone energetic electrons depended strongly on the laser pre-pulse
duration and that the proper pre-pulse was essential for the generation.
INTRODUCTION
Recent rapid progress in intense ultra-short pulse lasers has opened high-field
sciences such as particle acceleration via laser-plasma interactions, which seem to be
quite promising [1]. The laser-plasma accelerators [2], and electron beam generation
[3,4] have been studied in underdense plasma region. In these experiments an ultraintense laser pulse is focused on a gas jet, which is sufficiently strong to generate a
plasma inside the jet and to excite a high-amplitude plasma wakefield. Subsequently,
electrons in the plasma are trapped and accelerated in this field. We have been
studying generation of relativistic electrons in a plasma wake wave-breaking scheme
[5] aiming at a compact ultra-short pulse relativistic electron accelerator / injector
which we called laser-plasma cathode [6]. It is a crucial issue for the studies to
produce a high-density, low-vacuum contaminative and spatially well-defined gas jet.
As a first step of the study we have designed a conical Laval supersonic nozzles which
can produce a gas jet in the density range of 1018- 1020cm~3 to perform well-defined
laser-plasma interaction studies for energetic electron generation. With these nozzles
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
628
we have started a relativistic electron generation experiment. In this report we show
the recent results of the experiment at NERL of Univ.of .Tokyo.
SUPERSONIC GAS JET TARGET
Until recently the gas injection into vacuum have been often used as a target owing
to its easy handling and high repetition rate [7, 8]. In order to form a well-defined,
localized gas column with sharp boundary, i.e., to suppress the transverse expansion
due to the thermal and fluid motion of the injected gas, the temperature and pressure
of the nozzle exit flow should be as low as possible. On the other hand, the number
density of the gas must be higher than a certain value for the laser plasma interaction.
Therefore a supersonic gas jet produced by well-designed nozzle is supposed to
provide a high-density shock-free flow. We have designed shock-wave-free nozzle
contours with minimum length [9] and Me = 4.2, for He (y= 1.660) by a method of
characteristics [10]. Fig.2 shows the calculated density distribution near the exit of the
nozzle. As shown in the figure, the nozzle exit condition is maintained until the arrival
of expansion waves and profiles are fairly flat with sharp boundary compared with a
case of sonic injection. The gas density at nozzle exit are expected 1.1 x 1020cm"3for
He, at Po = 80 atm and Me = 4.2. The desired gas density and its distribution are
reached by choosing the stagnation pressure, Mach number at the exit, and the radius
of the nozzle exit.
The pulsed supersonic gas injection device consists of a nozzle block designed by
us and a solenoid fast pulse valve. The high pressure gas is regulated into a reservoir
of the nozzle block through an aperture of the valve, which can control the pulsed
supersonic injection. In case of the pulsed supersonic injection, area ratio of an
aperture of the valve to a throat of the nozzle (Aa / A*) must be greater than 1, where
area of the aperture and the throat are given as Aa and A*, respectively. In our nozzle
the A* is 0.5 mm2, while the Aa is supposed to vary from 0 to n mm2 as a function of
FIGURE 1 The density profiles of the supersonic jet near the exit of the nozzle for Me=4.2 of helium
obtained by the method of characteristics. Density is normalized by stagnation density at the reservoir.
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the valve motion. The temporal and spatial density distributions of the gas jet from the
nozzle were experimentally measured by interferograms with a fast gate CCD camera
and they were corroborated with results of the calculation. The temporal density
structure of the jet due to slow response of the valve was observed. The required high
pressure gas was regulated into the reservoir of the nozzle for only 50 us, though the
valve was driven for more than 5 ms. According to experimental results, the pulse
valve with larger effective aperture area and fast opening and closing motion is
essential for production of low vacuum contaminative supersonic gas injection.
Therefore, we have started development of new fast pulse valves with a larger
effective aperture, and would like to report it in the next article.
RELATIVISTIC ELECTRONS GENERATION EXPERIMENT
The typical experimental setup is shown in Figure 2. The nozzle with the pulse
valve mentioned in the previous section was fixed inside the vacuum chamber. The
pulse valve was driven for 5 ms a shot at a repetition rate of 0.2 Hz. The stagnation
pressure of the valve is varied from 5.0 to 20.0 atm. With these pressures the density
at the exit of the nozzle ranged from 7 x 1018 to 3 x 1019cm"3. The background pressure
of the vacuum chamber was kept lower than 1.0 x 10"4 Torr during the jet operation.
The 12 TW TiiSapphire laser system based on CPA technique gives up to 600 ml, 50
fs laser pulses at a wavelength of 790 nm at the repetition rate of 10 Hz. In this study,
available laser power at the target in the vacuum chamber was up to 5 TW. As shown
in Fig.4 the p-polarized laser pulse with diameter of 50mm was delivered into the
vacuum chamber and focused on the front edge of the helium gas jet column at the
height of 1.3 mm from nozzle exit with any/3.5 off-axis parabolic mirror. Figure 3
shows a typical image of a laser focal spot at the target position, which was imaged
with a 20 x objective lens on to 14-bit CCD camera. The size of the spot was
calibrated with a wire of 10 um in diameter. The spot size was approximately 5um full
width at half maximum (FWHM) in diameter. The maximum laser intensity on the
TiiSapphire Laser
(SOOnm 12TW-50fs)
e-Deflect or
(Magnet)
Imaging
Plate
\
\tt
Supersonic GAP
6asjet
90degree
M=4.2
FIGURE 2 Experimental setup.
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FIGURE 3 Typical focal spot image at the target position and corresponding transverse profile.
target was estimated to be approximately 1.0 x 1019W/cm2. The electron emission
from the gas jet was directly measured by a cup-shaped detector consist of imaging
plates (I.P.) (FUJI FILM BAS-SR). They were laminated with an aluminum foil in 15
um thickness on the surface to avoid exposure to X-rays or laser pulses. The electron
signals were accumulated for 300 shots. The electron energy spectra of the generated
beam were obtained by a compact magnetic electron spectrometer set on the laser axis
behind the jet. With this setup, the energy distribution of the generated electrons up to
40MeV can be detected. The visible light emission of the plasma was observed from
side direction by a time gated CCD camera with blue-pass filter. In order to
investigate correlation between the electron generation and laser pre-pulse, ns-order
laser pre-pulse was produced before the main pulse by de-tuning of the pockels cell of
the TiiSapphire laser system. The shape of the laser pulse including the pre-pulse was
monitored by a photo-diode just before laser pulse compressor.
EXPERIMENTAL RESULTS
Typical image of electron signals on I.P. s, the side and bottom plates of the cupshaped detector, were shown in Figure 4. The transverse profile of the generated beam
on the bottom plate was also shown in Fig.4 (up-right) As shown in the bottom plate,
energetic electrons were observed in the forward direction with a narrow cone angle of
approximately 2.5 degree at gas-density of 2.8 x 1019cm"3. The energy of the electrons
was ranged up to 40 MeV, which was upper limit of the detection. The lower energy
electrons less than 500 keV with a wide cone angle of larger than 30 degree from the
laser axis were observed on the side-plate. According to these results, an overview of
the electron generation is illustrated in Figure 5. Furthermore it was found the beam
generation depended strongly on the laser pr-pulse. Figure 6(a)-(c) shows typical
images of beam spot on the bottom plate. In case of a laser pulse with Ins duration of
the pre-pulse just before the main pulse there was no electron beam generation as
shown in (a). In the second case of a 2.5ns pre-pulse, the narrow cone energetic beam
was generated and an enhanced spot was observed as shown in (b). In the third case of
a 5ns pre-pulse, the beam was exploded to pieces and smaller spots were observed as
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shown in (c). In addition strong correlation between the energetic electron beam
generation and plasma dynamics measured by the CCD camera were observed. These
results will be reported in details in the next article.
Narrow coned Beam
Energetic electrons
upto~40MeV
N-2.8xlOl9cm-3
8.1mm
(FWHM)
Thermal electrons from
Plasma(E<500keV)
25,0
50,0
[mm]
75,0
100.0
Transverse profile of e-beam
Generated,
f y
Electrons \ /
Laser Pulse
FIGURE 4 Typical image of the generated electrons inside the cup-shaped I.P. detector. The electrons
were generated by 4.8TW laser pulses with a gas density of 2.8xl019cm~3 of helium jet.
Lower energy
electrons
E<500keV
I. P.detector
Ti: Sapphire
Laser
Energetic
Gas jet ~2mm
He N~2.8xlOl9Cm-3
electron beam
8 ~2.5deg.
E: up to ~40MeV
FIGURE 5 Overview of the results of the electron generation experiment.
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(a)
"1ns pre-pulse
(b)
(c)
"2.5nspre-pulse ~5ns pre-pulse
125mm
FIGURE 6 Typical images of the bottom plate of the cup-shaped I.P. detector. A laser pulse with Ins
duration of the pre-pulse just before the main pulse(a), 2.5ns (b) and 5ns (c).
SUMMARY
We have started the relativistic electrons generation experiment. The supersonic,
rapid expansion conical Laval nozzle was designed using the method of characteristics
to make a well-defined gas jet target. The ultra-short laser pulse was focused on the
supersonic helium jet in the gas density range from 7 x 1018 to 3 x 1019 cm"3 with the
relativistic intensity of approximately 1.0 x 1019W/cm2. The energetic electron beams
(up to 40MeV) were observed in the forward direction. It was found a proper laser
pre-pulse duration was essential for generation of the narrow-cone energetic electrons.
REFERENCES
1. D.Umstadter, Phys. Plasmas., 8, 1774 (2001)
2. K. Nakajima, Phys. Plasmas., 3, 2169 (1996)
3. C. Gahn, G. D. Tsakiris, A. Pukov, J. Meyer-ter-Vehen, G. Pretzler, P. Thirolf, D. Habs, and K. J. Witte, Phys.
Rev. Lett, 83, 4772 (1999)
4. V. Malka, J. Faure, J. R. Marques, F. Amiranoff, J. P. Rousseau, S.Ranc, J. P. Chambaret, Z. Najmudin, B.
Walton, P. Mora, and A. Solodov, Phys. Plasmas., 8, 2605 (2001)
5. S. Bulanov, N. Naumova, F. Pegoraro, and J.Sakai, Phys. Rev. E, 58, R5257 (1998)
6. N. Hafz, M.Uesaka, J.Koga, and K.Nakajima, NIM. A, 455,148 (2000)
7. S. Semushin and V. Malka, Rev. Sci. Instrum., 72, 2961 (2001)
8. V. Malka, C. Couland, J. P. Geindre, V. Lopez, Z. Najmudin, D. Neely, and F. Amiranoff, Rev. Sci. Instrum., 71,
2329(2000)
9. J. D. Anderson, Jr, Modrrn Compressible Flow with Histrical Perspective 2nd Edition, (Me Graw Hill, 1989),
Chap.5,Chap.ll
10. R. A. Greenberg, A. M. Schneiderman, D. R. Ahouse and E. M. Paramentier,AIAAJ., 10, 1494 (1972)
633