Long plasma channels and high-voltage discharges
induced by strong picosecond laser pulses
Hua Yang (杨 华)*, Xiaosong Tang (唐小松), Xingwei Zhang (张兴卫), Li Zhang (张 黎),
Fuli Tan (谭福利), Jianheng Zhao (赵剑衡), and Chengwei Sun (孙承纬)**
Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
*Corresponding author: yangqst@yahoo.com; **corresponding author: sunchengwei@hotmail.com
Received August 15, 2015; accepted September 29, 2015; posted online October 21, 2015
The formation of long plasma channels and laser-induced high-voltage discharges are demonstrated by focusing
infrared picosecond laser pulses in air. Based on measurements of the channel conductivity, the maximum
electron density in excess of 10
14
cm
−3
is estimated. The plasma channels are good conductors, through which
long-air-gap high-voltage discharges are triggered. The breakdown voltages show large drops but the
discharging paths are not well guided: in this, the plasma spots distributed along the channel might play an
important role.
OCIS codes: 320.5390, 350.5400, 140.3440.
doi: 10.3788/COL201513.113201.
Intense laser pulses launched in air can form thin columns
of weakly ionized plasma, which can persist over long
distances if the laser intensities are properly maintained.
Laser-induced plasma channels hold great promise for a
number of advanced technologies, including remote sens-
ing
[1–3]
, microwave channeling
[4]
, and the remote manipula-
tion of high-voltage (HV) discharges
[5–9]
.
To date, extensive studies on laser-induced plasma
channels have been carried out using infrared femtosecond
lasers
[10]
. The low-energy femtosecond pulse focused in
air can collapse into a thin filament with the diameter
of about 100 μm and an electron density of about
10
15
− 10
16
cm
−3
. In general, the propagation of the fila-
ment is maintained by a dynamic balance between self-fo-
cusing due to the optical Kerr effect and beam defocusing
from free electrons
[11]
, which is often not robust enough for
long-range applications. Increasin g the laser energy will
eventually lead to multiple filaments
[12]
, which can benefit
the robust prop agation over long distances but may
introduce large complexity and instability due to multi-
filament competition. Moreover, due to complex nonlinear
effects, some important properties of the filaments, such as
electron density, cannot be easily manipulated.
Strong picosecond laser pulses can offer another ap-
proach to produce and apply the laser-induced plasma
channels. Provided with a relatively high power and high
energy, effective ionization in air can also be achieved;
thus, plasma channels can be effectively produced and
maintained over relatively long ranges. The formation
and application of long plasma channels using strong
infrared picosecond laser pulses has great potential that
is worth studying.
In this Letter, we present an experimental study on
long-range plasma channels and HV discharges induced
by strong infrared picosecond pulses focused in air. Long-
range plasma channels with low resistances were effec-
tively produced, along which plasma spots appeared
and traced out along the channel path. The electron
density depended on the laser energy, which might offer
a new knob in various applications. Through the plasma
channels, which are good conductors, laser-induced long-
air-gap discharges under HV direct-current (DC) fields
were triggered. The breakdown voltages showed large
drops but the discharge paths were not well guided: in this,
the plasma spots might play an important role.
The laser pulses were output from a picosecond
Nd:YAG/YVO amplifier system with a maximum pulse
energy of 100 mJ at 1064 nm, 10 Hz, and a pulse duration
of 30 ps. The output beam diameter was 11 mm and the
beam divergence was less than 0.5 mrad. To loosely focus
the laser beam, a telescope-type double-lens system was
employed. The effective focal distance could be controlled
by changing the distance between the two lenses
[13]
. The
focal lengths of the double lenses were 150 and −125 mm.
In our experiment, the effective focal distance was kept at
about 250 cm, where long plasma channels formed around
the focus.
The channel paths became discernible in our dark labo-
ratory due to small plasma spots, which distributed ran-
domly along the channels. These plasma spots might
derive from localized avalanche ionization
[14]
. Further
growth of the plasma spots might be limit by the short
pulse duration time; thus, the number and size of these
spots remained small in each laser shot, and the laser
propagation was not severely disturbed. As the laser
was operated at a low repetition rate of 10 Hz, a color
camera was employed to record the plasma spots over long
exposures. These plasma spots can intuitively indicate the
length and the intensity distribution of the plasma
channel, as illustrated in Fig.
1(a). The channel paths
persisting over long distances of more than 60 cm were
clearly revealed by 30 s exposures. The lengths of the
plasma channels could be controlled by the pulse energy.
Different laser energies also led to different plasma
COL 13(11), 113201(2015) CHINESE OPTICS LETTERS November 10, 2015
1671-7694/2015/113201(5) 113201-1 © 2015 Chinese Optics Letters