Plasmas are not often observed naturally on earth with the exception of lightning or the aurora borealis. Plasmas appear in space and astrophysics, laser-matter interaction, technology, fusion, etc. Technical plasmas, magnetic fusion plasmas and laser-generated plasmas are the main applications of plasma physics on earth .
In especially laser plasmas have extreme properties not found in more conventional plasmas (e.g., densities characteristic of solids). Fusion energy known as inertial confinement fusion is a major application of laser plasma physics, in this approach, high energy laser beams were focused towards to a small solid target to explode its until the densities and temperatures characteristic reaches to nuclear fusion. Another field of using laser plasma physics was in the particle accelerators, where a extremely strong electric fields was generated when a high intensity laser pulse passes through plasma to accelerate particles. High-energy physicists hope to reduce the size and cost of particle accelerators by using plasma acceleration techniques .
An interesting application in the field of laser induced plasma is Laser Induced breakdown spectroscopy (LIBS). LIBS is an atomic emission spectroscopy technique which utilizes a laser pulse with sufficient high energy as excitation source. LIBS can be used to analyze any state of matter (solid, liquid or gas). Almost all elements can be detected by LIBS because of thy emit light when excited to sufficiently high energies, depends only on the laser power as well as on the detector sensitivity of the dispersed radiation and wavelength range of the spectrometer . Atomic emission spectroscopy is a well-established technique for the measurement of materials. Excitation of atoms may be performed by raising the temperature of the sample using a flame , an inductively coupled plasma , a microwave induced plasma, an arc or spark between electrodes [6,7]. Emission measurements are preferred over absorption measurements when high sensitivity is required. An absorption signal is measured as the reduction in transmitted signal, “negative signal”, in which small absorptions involves detection of small signal decrease on a large transmitted background. However, emission signal is zero for no signal, then emission measurements can be performed with a better sensitivity than absorption measurements.
A large number of studies have been made on the laser induced
plasma on solid, liquid surfaces and in a gas. The plasma
is produced by focusing a laser beam in test sample.
The aim of our study is to study the physical properties
of the laser induced plasma in water. One of application of
this study is to improve the quality of water by resolving the
environmental pollutants .
Optical Emission Spectroscopy
Optical Emission Spectroscopy (OES) is broadly used
technique, mainly because it is easy to implement and non-intrusive.
It also relies on the study of the shape and width of
spectral lines that can be altered by numerous mechanisms.
The electron temperature of plasma was calculated
using Boltzmann relation (1):
where λmn, Imn, gm, and Amn are the wavelength, intensity,
statistical weight, and transition probability between the transition
states of upper level (m) and lower level (n), respectively,
and C is constant .
The electron density was calculated using Saha-Boltzmann
equation (2) for atom and ion spectral lines emitted
from the plasma:
Where is the ionization
energy of the species in ionization stage z in eV, Tz is the
line intensity for transition from upper level-j to lower level-
i, λji,zis the corresponding wavelength of transition from
level-j to level-i, is the statistical weight of transition from
level-j,Aji,z is the transition probability from level-j to level-i
and T is the electron temperature .
Other fundamental plasma parameter such as Debye
length (λD) , plasma frequency of electron (Pω
and Debye number (ND) can be calculate by the following equations:
Where is permittivity of free space,κβ the Boltzmann
constant, e the electron charge, ηe
is electron density and me is electron mass.
Nd: YAG laser (Hua Fei Tong Da Technology-Diamond
-288 Pattern EPLS) with different peak powers in air
on target, was used in optical emission spectroscopy of laser
induced plasma, with following characteristics:
• Laser model: Q-switched Nd: YAG laser first harmonic generation.
• Laser peak power: (6- 36) MW.
• Laser wavelength: (1064) nm.
• Repetition rate: (1- 10) Hz.
• Cooling method: inner circulation water cooling.
• Input Ac power supply: (220) Volt.
The laser spot diameter can be changed by changing
the distance between the laser lens and the target. Pulse duration
(9 ns) with 6 Hz repetition frequency. The distance fixed
during the measurements for system accuracy and precision.
In this work, we used 10 cm focal length lens. A shorter focal
length lens can produce a small beam waist, and therefore,
stronger breakdown, but it also has a smaller depth of focus,
Figure 1 shows a schematic diagram for the LIBS setup (1).
The spectrometer analysis was done using the light
emitted from sample bombarded by the pulse laser. The spectrometer
with short response time from Ocean Optics (HR
4000 CG-UV-NIR) was used in the setup to analyze emitted
The light produced by the ablated plasma was collected
by the optical fiber which was set at angle of about 45 degree
to axes of the laser beam to avoid splashing and then guided to
the entrance slit of the spectrometer. The spectrometer has a
high resolution depending on grating used in it, and responds
to a wavelength between (200-900) nm with 3648 pixels. Nd:
YAG laser at 1064 nm is tightly focused on the target to produce
In order to insure exposing a fresh surface after every
train of shots the target surface was rotated manually. The laser
pulse energy was varied from 300 to 800 mJ, each spectrum
was obtained over a (200-700) nm wavelength range.
Finally the results were analyzed and compared with
National Institute of Standards and Technology data (NIST)
 and evaluate the plasma parameters such as electron density
(ne), electron temperature (Te), plasma frequency (fp),
Debye length (λD) and number of particles in Debye sphere
(ND) at different laser energy.
Results and Discussion
Light emitted from the plasma is detected and the resulting
spectrum distribution plotted as intensity in arbitrary
units against wavelength (nm).
The optical emission spectra of Si: Al alloy target plasma
which confined in atmosphere was recorded using (OES)
technique. Figure 2 shows the emission spectra of laser induced
on Si: Al alloy target in air in the spectral range 225-640
It is clear from this Figure that the intensity of the
spectral lines increase with the increase in the laser peak power
from 300 to 800 mJ, this can interpreted as follows:
The increase of laser power leads to mass ablation rate
from the target, means more excited atoms and hence increasing
in the height of the spectral line intensities. The increase in
laser power will increase its absorption in the plasma resulting
in more ablation as well as plasma emission.
The prominent Silicon-Aluminum (Si: Al) alloy target
in air in spectral lines observed in Al I (237.3124, 257.5393,
309.27099 and 396.152) nm. Through the use of these wavelengths
plasma parameters were calculated.
The electron temperature (Te) was determine from the slope of
the best linear fit in Boltzmann plot (against upper level energy
(for corresponding transition) Boltzmann plot requires peaks
for the same atomic species and the same ionization state (Al
I in this part). Figure 3 shows the best linear fit in Boltzmann
equation for different laser energies.
The obtained results show that increases laser
pulse energy, leads to increase in the electron temperature,
Debye length and Debye number, while the electron density
and plasma frequency were decreased as shown in Figure 4.
Table 1 display the calculated electron density (ne),
electron temperature (Te ), plasma frequency (fp ), Debye
length (λD),) and Debye number (ND) for Si: Al target at different
laser pulse energies. All calculated plasma parameters
(λD, fp and ND) were satisfied the criteria for the plasma. It
shows that fp decrease with laser energy because it is proportional
with ne, while λD and Nd increase with it such as in Ref
1. The spectral lines intensities of the laser induced plasma
emission exhibited a strong dependence on the ambient conditions.
It is found that the intensities at different laser peak
powers increase with increasing laser peak power and then decreases
when the power continues to increase.
2. Plasma parameters such as electron temperature, electron
density, Debye length, number of particles in Debye sphere and
plasma frequency are found to be strongly influenced by the
3. The results showed that the values of Te ,ND and λD were
increased in case of laser induced plasma in air environment
while the values of ne and fp were decreased in laser induced
plasma air environment.