24F11
Long-distance Guidance of Fast Electrons with Fine Wires
Irradiated by Intense Femtosecond Laser Pulses
高強度短パルスレーザー照射極細ワイヤーによる高速電子の長距離誘導
Hiroaki Nakajima, Shigeki Tokita, Kazuto Otani, Shunsuke Inoue,
Masaki Hashida and Shuji Sakabe中島宏章，時田茂樹，大谷一人，井上峻介，橋田昌樹，阪部周二
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan andDepartment of Physics, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-7501, Japan

京都大学化学研究所 〒611-0011
京都大学大学院理学研究科 〒606-7501

京都府宇治市五ヶ庄
京都府京都市左京区北白川

We have studied the characteristics of fast electrons emitted from fine (10-μm diameter) wires irradiated bya femtosecond laser pulse with intensity of approximately 1.0 × 1019 W/cm2. We have found the property
that the electrons emitted from a metal wire are self-guided to the axial direction of the wire, and in thispaper we demosntrated 150-mm length guidance of fast electrons from the laser focal spot to the wire end.
400-keV electrons are guided along the wire in space of the diameter of 3.5 mm. Furthermore, we haveconfirmed that the emitted electrons have directionality over 100 mm even after leaving the wire end.

1. Introduction
Fast electrons can be generated and accelerated
by the interaction of intense short pulse lasers withsolid-density targets and have potential for fastelectron beam sources, which have been extensively
studied for applications such as electron diffraction[1,2], particle acceleration [3,4], and fast ignition
fusion [5]. To develop high-intensity and directional
electron beams, laser generated electrons affectedby the characteristics of the laser and target havebeen intensively studied.
Many experiments have been performed togenerate collimated beams using various targets.
For planar target experiments [6,7], for instance, anelectron beam emission along the surface of ametal foil target has been observed. This directional
beam emission is interpreted by the static electricand magnetic fields generated on the target surface[6].
We have been studying characteristics of fastelectrons emitted from metal wires irradiated byintense femtosecond laser pulses. We havedemonstrated that the electrons emitted from a wire
are self-guided along the wire direction, usingtungsten wires with diameter of 300 μm [8]. To
investigate the physical mechanism of self-guiding
along the wire in more detail and the feasibility ofhigher directionality, we have performedexperiments of electron transport along a rather
long and fine wire (tungsten wires with diameter of10 μm).

2. Experiment
Laser pulses used are delivered from aTi:sapphire chirped-pulse amplification system isused. The central wavelength is 800 nm, the pulse
duration is 160 fs and the pulse energy is 250 mJ.
The laser pulses are focused with an f/3.5 off-axis
parabolic mirror to a spot of 3 μm × 4 μm, resulting
in the peak intensity of approximately 1.0 × 1019
W/cm2. The laser pulse is p-polarized and isirradiated on a tungsten wire with diameter of 10μm at an incidence angle of 45°. Imaging plates(IPs, Fujifilm FDL-UR-V) are used to detect fastelectrons. The IPs have high sensitivity in theenergy range from 40 to 1000 keV. The
experiments are performed in a vacuum chamberwith pressure of 0.02 Pa.
(a)Aramid
Fiber
Laser

Tungsten
Wire

IP stack

150 mmL(b)LeadShielding

Laser

TungstenWire
150 mmAramidFiberIPElectron
Trajectory

DipoleMagnet30 mm

Fig.1 (a) Experimental setup to observe emitted electrons
using stacked IPs. (b) Experimental setup to observe theenergy distribution of electron beam along the wire andthrough a 0.5-mm aperture.

Figure 1(a) shows the experimental setup toobserve the emitted electrons. L is the distance fromthe laser focal spot to double-layer stacked IPs. An
aramid fiber of diameter 14μm is connected to theend of the tungsten wire with adhesive. When L is
150 mm or less, a hole with diameter of 1 mm is
made in stacked IPs to pass the wire through it.
Figure 1(b) shows the setup to observe the energy
distribution of the electron beam, using aspectrometer composed of a 0.1-T dipole magnet
and an IP. The diameter of the incidence aperture of
the spectrometer is 0.5 mm.
3. Results
Figures 2(a),(b) are typical single-shot images ofthe second layer IP with L = 150 mm, L = 250 mm.
On the second layer, electrons with energies higher
than about 400 keV can be detected. The FWHM ofcentral part intensities of the images in Fig. 2(a) and(b) are 3.5 mm × 3.5 mm and 2.0 mm × 2.0 mm,respectively. The number of electrons per unit solid
angle through the aperture is about 2.8 × 1011 /sr at
L = 30 mm. This value is ten times higher than thatobtained by a planar target with a 2.0 × 1018 W/cm2
laser [6]. The electron beam is clearly guided to theFig.2 (a) and (b) Typical single-shot images of electron
emission obtained on the second layer IP with L = 150mm, L = 250 mm, respectively. The color scale is set
independently for images for maximum contrast. The
actual dimension of the images are 35 mm × 35 mm.
Center black spot seen in (a) is a hole with diameter of 1
mm is made in the IP. (c) Typical energy distribution ofelectrons.

wire direction. A ring with diameter of 28mm can beseen in Fig. 2(b). This ring may be due to theinfluence of adhesive at the end of tungsten wire. A
typical energy distribution of the emitted electrons
is shown in Fig. 2(c). The distribution has a peakaround 50–100 keV.
Tokita et al. has reported the interpretation forthe directivity of the emitted electrons as follows[8]. The electric field in outward radial direction isgenerated as a result of the interaction of an intenselaser pulse with the wire. The field vanishes withintens of picoseconds. The field decelerates theelectrons emitted from the wire, consequently someelectrons are pulled back to the wire or are guided
along the wire-axial direction. The guided electrons
propagate in spiral trajectories along the wire withthe balance between the centrifugal force by thespiral motion and the electrical force by the laser
generated field. However, it seems that the electricfield lives in rather long time as much as tens ofpicoseconds because the emitted electrons are
guided over a few hundred millimeters.
4. Conclusion
We have demonstrated that fast electrons emitted
from a fine wire irradiated by intense femtosecond
laser pulses are guided along the wire over a few
hundred millimeters.
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