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-Plasma can be produced when a laser ionizes gas molecules in a medium
-Normally, ordinary gases are transparent to electromagnetic radiation. Why then is it possible
for lasers to produce plasma out of thin air?
-Several parameters effect the ability of a laser to form plasma, including the pressure of the
medium gas, the intensity of the laser itself, what type of gas is within the medium, The electron
density within the medium, the length of the pulsed laser, the frequency of the laser and
frequency of pulsation, as well as the diameter of the beam.
-Lower pressures as well as the ability for the material to conduct electricity help with the
plasma forming process, so metals such as Tin are often used within a vacuum, for its conductive
properties and low vapor pressure. The extremely low pressure can be maintained while the
metal is turned into a vapor.
-The threshold of the plasma tends to decrease with an increase in either the duration of the
laser pulse as well as an increase in laser frequency. A longer duration of the laser light as well
as a higher frequency of the light itself allows for more energy to be dumped into the medium,
causing more ionization.
-Scientists would also focus the beam down to a point to increase the intensity per unit area
passed a point called ignition intensity; when plasma formation can take place. Ignition intensity
must be surpassed or no interaction shall take place.
-Inverse free-free absorption. Where the electrons will absorb the laser radiation and turn that
energy into kinetic energy. This process can create hot plasma, where the system is in thermal
equilibrium due to electrons colliding with ions and transferring energy to them.
-When the electric field produced by the laser becomes large enough, the field will decrease the
potential barrier that is keeping the electron within the atom enough that the electron can
escape. This process is called Tunnel Ionization.
-When the electron absorbs numerous photons, it can surpass the ionization energy level
needed in a process called Multi-Photon Ionization. This of course depends on the material and
the wavelength used.
-After the plasma is created, electrons will transfer their thermal energy into kinetic energy. This
will cause the plasma to expand and thus cool. This expansion will create a shockwave that
travels at supersonic speeds.
-The E field and B field within the plasma is a combination of the electric field and magnetic field
the laser and other outside sources would impose as well as the resulting electric and magnetic
field from the motions of the electrons after excitation.
-Non-optimal laser coupling with the medium will result in an incomplete ionization process and
the production of aerosol made from the target called “Debris”. The debris is scattered by the
shock wave of the laser hitting the target as well as during evaporation and can coat the mirrors
within the machine and reduce reflectivity.
-To reduce the production of debris, liquid sources can be used however the absorption
coefficient is density dependant and with the lower density, the laser must be more intense to
surpass the ignition intensity.
-Misalignments of the laser system can cause the laser to miss the target, leading to no
ionization and the possibility to cause damage to the components which leads to lost time while
components are ordered and swapped out.
-The Plasma will expand and cool rapidly, thus the ionization process will quickly dissipate
through the recombination process unless the system is constantly pumped with energy.
-Ultra Violet Lithography: This is a process where the plasma, which is produced by lasers, is
harnessed and used to etch into silicon wafers to make processors. A process called Extreme
Ultra Violet Lithography is currently under way which would allow smaller etches to be made
and therefore more powerful processors.
-Laser produced plasma is a key element in Inertial Confinement Fusion, or the process of
initiating nuclear fusion through heating and compressing a target. The surface of the target is
heated into a plasma which then radiates X-rays to heat the rest of the target. The shockwave
created by the plasma fuel the compression process of the target.
-Laser Induced Breakdown Spectroscopy is a technique commonly used to analyze the presence
of heavy metals within soil. The laser heats the metals within the dirt into a plasma and the
emission from the plasma is measured. This emission is characteristic of the elements and can
be used to find the composition of the ground.
http://cdn.intechopen.com/pdfs-wm/12569.pdf
https://www.cymer.com/files/pdfs/Technology/2012/Laser_Produced_Plasma_EUV_Sources_fo
r_Device_Development_and_HVM.pdf
http://www.hindawi.com/journals/physri/2012/249495/
http://www.euvlitho.com/2011/P6.pdf
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