1.1 Interaction of Lasers and Materials
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a high-temperature and high-density plasma. A plasma is a mass composed of large
quantities of charged particles (electrons and ions), atoms and molecules, which as a
whole is electrically neutral. There are three main mechanisms of plasma generation
by high power density laser: photoionization, thermal electric ionization and collision
ionization. Photoionization refers to the phenomenon that when electrons in atoms are
irradiated by laser and absorb sufficient photon energy, ionization occurs due to the
photoelectric effect or the multi-photon effect. Photoionization is mainly applicable
to the generation process of initial charge carriers in a relatively cold medium, while
the laser plasma is in a completely ionized state, and therefore photoionization is
not the main mechanism for its formation. Thermal electric ionization refers to that
when the temperature of the vapor under laser action is high enough, the atoms with
high thermal velocities at the high temperature collide with each other, making their
electrons excited, and the energy of some of the electrons exceeds the ionization
potential, which leads to the ionization of atoms. Collision ionization refers to that
charged particles in vapor accelerate under the action of electric field and collide
with neutral atoms, resulting in energy exchange, which enables the electrons in the
atom to acquire sufficient energy to ionize.
The ionization degree of vapor in the thermodynamic equilibrium state is
completely determined by the density and temperature of vapor. In a partially
ionized gas, the energy of the incident laser is absorbed by the thermally excited
atoms through the binding-free mechanism and by the particles through the inverse
bremsstrahlung. Vapor absorbs laser energy and heats up, leading to further increase
in the ionization degree and absorption coefficient. This kind of positive feedback
contributes to the formation of plasma in the vapor.
The plasma absorbs the energy of the laser beam propagating in it through
various mechanisms, which increases its temperature and ionization degree. Inverse
bremsstrahlung absorption is the main mechanism of plasma absorbing laser energy.
Inverse bremsstrahlung absorption refers to that the electrons in a laser electric field
are excited to oscillate at high frequency, and collide with particles with a certain
probability to transfer energy to heavier particles (ions and atoms), thus heating up
the plasma.
The vapor of a metallic material is a monatomic gas, which has a very high
evaporating temperature T v but a very low ionization potential, so the evaporating
rate of the material is not high. When the vapor pressure is only slightly higher
than the ambient pressure, there are also a large number of free electrons in the
weakly ionized vapor, which can lead to effective inverse bremsstrahlung absorption
of laser. In other words, free electrons in an ion Coulomb field absorb laser energy,
and accelerate or change the direction of motion, thus transferring the laser energy
to ions and heating up the vapor.
When the light intensity is high and the energy absorption rate of the vapor exceeds
its various losses, the vapor ionizes and the number of free electrons increases exponentially with time, making the vapor completely ionized and opaque to the laser. As
the size of the plasma increases by absorbing energy, the power density of the laser
reaching the surface of the material decreases, and therefore the evaporation effect
weakens, resulting in a drop in the density and temperature of the plasma, as well as
11
a high-temperature and high-density plasma. A plasma is a mass composed of large
quantities of charged particles (electrons and ions), atoms and molecules, which as a
whole is electrically neutral. There are three main mechanisms of plasma generation
by high power density laser: photoionization, thermal electric ionization and collision
ionization. Photoionization refers to the phenomenon that when electrons in atoms are
irradiated by laser and absorb sufficient photon energy, ionization occurs due to the
photoelectric effect or the multi-photon effect. Photoionization is mainly applicable
to the generation process of initial charge carriers in a relatively cold medium, while
the laser plasma is in a completely ionized state, and therefore photoionization is
not the main mechanism for its formation. Thermal electric ionization refers to that
when the temperature of the vapor under laser action is high enough, the atoms with
high thermal velocities at the high temperature collide with each other, making their
electrons excited, and the energy of some of the electrons exceeds the ionization
potential, which leads to the ionization of atoms. Collision ionization refers to that
charged particles in vapor accelerate under the action of electric field and collide
with neutral atoms, resulting in energy exchange, which enables the electrons in the
atom to acquire sufficient energy to ionize.
The ionization degree of vapor in the thermodynamic equilibrium state is
completely determined by the density and temperature of vapor. In a partially
ionized gas, the energy of the incident laser is absorbed by the thermally excited
atoms through the binding-free mechanism and by the particles through the inverse
bremsstrahlung. Vapor absorbs laser energy and heats up, leading to further increase
in the ionization degree and absorption coefficient. This kind of positive feedback
contributes to the formation of plasma in the vapor.
The plasma absorbs the energy of the laser beam propagating in it through
various mechanisms, which increases its temperature and ionization degree. Inverse
bremsstrahlung absorption is the main mechanism of plasma absorbing laser energy.
Inverse bremsstrahlung absorption refers to that the electrons in a laser electric field
are excited to oscillate at high frequency, and collide with particles with a certain
probability to transfer energy to heavier particles (ions and atoms), thus heating up
the plasma.
The vapor of a metallic material is a monatomic gas, which has a very high
evaporating temperature T v but a very low ionization potential, so the evaporating
rate of the material is not high. When the vapor pressure is only slightly higher
than the ambient pressure, there are also a large number of free electrons in the
weakly ionized vapor, which can lead to effective inverse bremsstrahlung absorption
of laser. In other words, free electrons in an ion Coulomb field absorb laser energy,
and accelerate or change the direction of motion, thus transferring the laser energy
to ions and heating up the vapor.
When the light intensity is high and the energy absorption rate of the vapor exceeds
its various losses, the vapor ionizes and the number of free electrons increases exponentially with time, making the vapor completely ionized and opaque to the laser. As
the size of the plasma increases by absorbing energy, the power density of the laser
reaching the surface of the material decreases, and therefore the evaporation effect
weakens, resulting in a drop in the density and temperature of the plasma, as well as