24F09
Highly directional sub-MeV electron emission form long wires irradiated byintense laser pulses
高強度レーザー照射長尺ワイヤーから放射される高指向性sub-MeV電子ビーム
Shigeki Tokita, Akihiro Nakajima, Kazuto Otani, Shunsuke Inoue, Masaki Hashida and Shuji Sakabe

時田 茂樹，中島 宏章，大谷 一人，井上 峻介，橋田 昌樹，阪部 周二
Advanced Research Center for Beam Science, Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011, Japanand Department of Physics, Graduate School of Science, Kyoto University,
Kitashirakawa, Sakyo, Kyoto 606-7501, Japan
京都大学 化学研究所 先端ビームナノ科学センター 〒661-0011 京都府宇治市五ヶ庄
京都大学 理学研究科物理学・宇宙物理学専攻 〒606-8502 京都市左京区白川追分町
We have experimentally demonstrated that fast electrons emitted from a metallic wire irradiated by a 5 ×
1018 W/cm2 laser pulse can be collimated along the wire, and that their intensity is significantly enhancedin the axial direction of the wire. As wire length is increased up to 30 mm from the laser focal spot, theangular divergence of the emitted electrons decreases to 65 mrad.

1. Introduction
Fast electron generation and transport via theinteraction of relativistic-intensity laser pulses withsolid-density targets have been extensively studied
because of their potential applications, such as
electron diffraction [1–3], particle acceleration [4],and fast ignition fusion [5]. Many efforts have been
made to improve the intensity, directivity, andmonochromaticity of electron beams.
Collimated beam generation has been demonstrated experimentally using various targets. A shortthin wire attached to a cone-shape target irradiatedwith a 0.3 PW laser pulse has been used to guideand to collimate electrons in the MeV energy range[6]. A small metal target with a resistivity boundary
has been used to collimate electrons by irradiation
with a 1020 W/cm2 laser pulse [7]. Using planar
targets, collimated beam generation has also been
demonstrated [8–11]. In studies on relativistic
laser–solid interactions, such as the abovementioned works, particle-in-cell (PIC) plasma
simulations for a small space (hundreds ofmicrometers at most) near the laser irradiation spot
have been successfully carried out to explain theexperimental results. That is to say, such phenomena have been explained by effects in small-scalemodels.
Here, we experimentally demonstrate collimatedelectron-beam generation using long metallic wire
targets with lengths of the order of centimeters. The
characteristics of the electron emission strongly
depend on the length of the wire: collimation
becomes stronger with increasing wire length.

2. Experimental Setup
A Ti:sapphire chirped-pulse amplification system
operating with a central wavelength of 800 nm anda pulse duration of 150 fs is used as the laser source.
Laser pulses with a pulse energy of 140 mJ are
focused with an f / 3.5 off-axis parabolic mirror to aspot size of 3 μm × 4 μm, resulting in a peakintensity of 5 × 1018 W/cm2. Amplified spontaneousemission is measured to be less than 10−7 of thepeak intensity of the laser pulse. The laser pulse isp-polarized and irradiates a tungsten wire withdiameter of 300 μm at an incidence angle of 45°.
Imaging plates (IPs) are used to detect fast electrons.
The IPs have high sensitivity in the energy range
from 40 to 1000 keV and are most sensitive ataround 200 keV. The whole setup is placed in avacuum chamber with pressure of 0.1 Pa.
Figure 1 shows the experimental setup formeasuring the angular distribution of electronemission. Wire length L between the laser focal spot
and the end of the wire is varied from 2.5 to 30 mm.
Double-layer stacked IPs are placed at a distance ofz = 150 mm from the laser focal spot and arecovered with aluminum foil of 11 μm in thickness
in order to prevent exposure of the IPs to light.

Fig. 1. Experimental setup [12].

Fig. 2. Single-shot images detected by IPs for L = 2.5, 5, 7.5, 10, 15, 20, 25 and 30 mm. The color scale isset independently for each image for maximum contrast. The actual dimensions of the images are 75 mm
× 80 mm. The center of each image roughly corresponds to the wire axis.
3. Results
Figure 2 shows typical single-shot images detected with the IPs for L = 2.5 to 30 mm. On thesecond-layer IP, electrons with energies greater thanabout 400 keV can be detected. The electrons form
distinctive ring-shaped patterns on the IPs. The
patterns for each wire length are highly reproducible; however, the center of the ring-shaped patternshows slight shot-to-shot fluctuation within ±30mrad because its position is sensitive to the axial
direction of the wire. The overall size of the patternis drastically reduced as the wire length L isincreased. The size of the hole in the ring decreases
from 65 to 6 mm in height as L is increased from
2.5 to 30 mm. The full angle at half-maximum of
the electron emission in the horizontal and verticaldirection is as narrow as 20 and 65 mrad, respectively, at L = 30 mm. In contrast, the IP signal
intensity increases with increasing the wire length:
the signal intensity at the brightest spot for L = 30mm is 7-fold higher than that for L = 2.5 mm.
Assuming a typical electron energy to be 300 keV,
the total number of detected electrons is estimatedto be of the order of 3 × 109 at L = 30 mm, resulting
in laser-to-electron energy conversion efficiency ashigh as 0.1%.
4. Conclusion
We have demonstrated collimated fast electron
emission through the interaction of intense
femtosecond laser pulses with long metallic wire
targets with lengths of the order of centimeters.
Emission of electrons with high angular density ofnearly 1012 sr−1 and energies of hundreds of keV
was observed. From the viewpoint of potential
applications, since the generated electron beam has
a broad velocity distribution and is energy chirped
in time, it would be particularly useful as a novel
electron source for single-shot time-resolved

measurements of ultrafast phenomena, for instance,
an electron streak camera [1] or ultrafast electron
diffraction with pulse compression [3]. In regard toanother aspect of intense laser interactions, theseexperimental results proved that large regions of ametallic target can affect electron emission. It canbe considered that the present phenomenon isstrongly related not to the acceleration process, butrather to the target shape.
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