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The interaction of laser plasma with respect to a surface is reported in the process of laser plasma interaction .The dynamic explanation of the plasma was visualized by using CCD video camera. Before we discuss about laser plasma interaction we must be familiar with the term laser so firstly we will discuss laser.
LASER AND LASER LIGHT-
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It is a special type of emission that involves either atoms or molecules .Since the discovery of laser in 1960 it has been used in numerous system designed to operate in the gas, liquid and solid state. These systems emit radiation s with wavelength ranging from infrared visible and ultraviolet. The advent of laser was truly revolutionized science medicine and technology.
Laser is a process by which we can get a light beam which is coherent, highly monochromatic or it has precisely known wavelength, and hence energy, and almost perfectly parallel or very much intense. The word laser is also used for the light obtained by this process. All the photons in the light beam, emitted by different atoms at different instants are in phase. The spread change in wavelength is very small, it is of the order of 106 nm, which is about 103 times smaller than the spread in the usual Kr light.
A beam of laser can go to the moon and return to the earth without much loss in the intensity. This shows that laser may be obtained as an almost perfectly parallel beam.
To understand the process involved in laser , we have to first discuss stimulated emission.
STIMULATED EMISSION -
Let us consider an atom which is in the higher energy state E2.If we just leave the atom there,it will eventually having energy E2-E1.This process is called spontaneous emission. Typically an atom stays for 10ns in an exited state. The average time for which an atom stays in an exited state is called the lifetime of that state. There are atoms which are having certain exited state having a lifetime of the order of a millisecond. That is about 105 times longer than the usual lifetime. Such states are called metastable states.
Finally suppose the atom is in the higher energy state E2 and a photon having energy E2-E1 is incident on it. The incident photon interacts with the atom and may cause the atom to come down to the lower energy state. A fresh photon is emitted in the process. This process is different from spontaneous emission in which the atom jumps to the lower energy on its own. In the present case the incident photon has stimulated the atom to make the jump.
When an atom emits a photon due to its interaction with a photon incident on it, the process is called stimulated emission. The emitted photon has exactly the same energy, phase and direction as the incident photon.
PROPERTIES OF LASER-
(I)LASER LIGHT IS HIGHLY MONOCHROMATIC-
Laser from an ordinary incandescent monochromatic. The radiation from a fluorescent neon sign is monochromatic, true, to about 1 part in 106, but the sharpness of definition of laser light can be many times greater, as much as 1 part in 1015.
(II)THE LASER LIGHT IS HIGHLY COHERENT-
Individual long waves for laser light can be several hundred kilometer long. When two separated beams that have traveled such distance over separate paths are recombined, they remember their common origin and are able form a pattern of interference fringes. The corresponding coherence length for wave trains emitted by a light bulb is typically less than a meter.
(III)LASER LIGHT IS HIGHLY DIRECTIONAL -
A laser beam spreads very little; it departs from strict parallelism only because of diffraction at the exit aperture of the laser. For example, a laser pulse used to measure the distance to the Moon generates a spot on the Moon's surface with a diameter of only a few meters. Light from an ordinary bulb can be made into an approximately parallel beam by a lens, but a beam divergence is much greater than for a leser light. Each point from a light bulbs filament forms its own separate beam, and the angular divergence of the overall composite beam is set by the size of the filament.
(IV)LASER LIGHT CAN BE SHARPLY FOCUSED -
If two light beams transport the same amount of energy, the beam can be focused to the smaller spot will have the greater intensity at that spot. For laser light, the focused spot can be so small that the intensity at that spot. For laser light, the focused spot can be so small that the intensity of 1017 W/cm2 is readily obtained. An oxyacetylene flame, by contrast, has an intensity of only about 103W/cm2.
TYPES OF LASER-
(I)THE RUBY LASER
(II)THE HELIUM NEON LASER
(III)THE LEVEL SOLID STATE LASER
(IV)THE CARBON DIOXIDE LASER
The first laser to be operated successfully was the ruby laser which was fabricated by MAIMAN in 1960. It consists of a single crystal of ruby whose ends are flat and on of it is completely silvered and the other partially silvered ; the two ends thus form a resonant cavity. A flash lamp is used to excite the chromium atoms to a higher energy level. The excited atoms are unstable, so at a given instant some of them will return to the ground state by emitting a photon in the red region of the spectrum . The photon bounces back and fourth many times between mirrors at opposite ends of the laser tube. This photon can stimulate the emission of photons of exactly the same wavelength from other excited chromium atoms; these photons in turn can stimulate the emission of more photons, and so on. because the light waves are in phase -that is their maxima and minima coincide-the photons enhance one another, increasing the power with each passage between the mirrors. One of the mirror is partially reflecting, so that when the light reaches the certain intensity it emerges from the mirror as a laser beam. Depending on the mode of operation, the laser light may be emitted in pulses or in continuous waves.
HELIUM -NEON LASER-
The helium neon laser was the first gas laser to be operated successfully; the helium neon laser consist of a long discharge tube filled with about 1 torr helium and about 0.1 torr of neon . The gas mixture helium and neon forms the lasing medium and this mixture is enclosed between the set of mirrors forming a resonant cavity; one of the mirror is completely reflecting and the other is partially reflecting so as to couple out the laser beam.
The light from a gas laser as compared to that from solid state laser is found to be more directional and much more monochromatic. This is due to the various imperfections present in the solids and also the heating caused by the flash lamp. The gas laser are also capable to supply a continuous laser without the need for elaborate cooling arrangements.
The disadvantage of using internal mirrors is that the mirrors are usually eroded by the gas discharge and have to be replaced.Having the resonator mirrors external to the laser cavity also provides for greater flexibility. However, when the external mirrors are used, the ends of a discharge tube also cause a loss due to reflaction, so polarize light passes through the plane without reflection loss.when such windows are used, the output laser beam is polarized.
FOUR LAVEL SOLID STATE LASER-It consists of neodymium ions . neodymium ions are exited to energy levels above metastable energy level with a lifetime of about 0.25msecs.Laser action is obtained between the metastable energy level and ground energy level.
Typical threshold of a few hundred watts continuous wave power inputs are required for attainment of population inversion. Output powers of several hundred watts have been obtained by using several laser rods in cascade.
THE CARBON DIOXIDE LASER-
Carbon dioxide laser is much more efficient than other gas lasers. In this laser one makes the transition between a rotational sublevel of a vibrational level and rotational sublevel. The addition of N2 gas to the carbon dioxide gas is seen to increase the efficiency of the carbon dioxide laser. Thus the nitrogen molecules are excited to the upper vibrational layer and it is seen that the energy difference between this layer and vibrational ground level of the nitrogen molecule matches very well with the energy required to excite the carbon dioxide molecule to the upper laser level. Thus there is very efficient transfer of energy between a nitrogen molecule and a carbon dioxide molecule.
The wavelength at which CO2 lasers lase falls in the band where atmospheric attenuation is very little. Hence carbon dioxide laser should also find applications in open layer communications system. It also be useful in optical radar system.
DYE LASERS-Dyes used in lasers are organic substances which absorb in the near ultraviolet, visible, or the near infrared regions of the electromagnetic spectrum. Because of adsorption of light, dye molecules get exited from the ground state S0 to higher vibrational rotational levels of the next electronic state S1. Because of thermal redistribution in the levels in the level S1 (which takes place in times of about 10-11sec), most of the dye molecules drop down to the lowest vibrational level to any higher lying vibrational sublevels of S0. This is termed as fluorescence. The lifetime for S1 is about 10-9sec.hence the peak wavelength of the emitted fluorescence spectrum is higher than that of the absorption spectrum.
Molecules from state S1 can also make a nonradiative relaxation to the triplet level T1; this is referred to as inter system crossing. Firstly this process leads to a population of level S1 which is the upper laser level. Secondly the absorption spectrum of T1-T2 usually overlaps the emission spectrum of S1-S2, thus leading to the loss at the wavelength corresponding to the laser emission. In fact this loss can be so strong as to even prevent laser oscillation.
If we form a junction between p-type and n- type semiconductor then the excess electron in the n type region tends to flow across the junction into the p-type region and excess holes in the p region tends to flow to the n region. This creates an electric field opposing this flow and the flow stops.
If we connect the p type semiconductor to the positive terminal of the direct current source and the n type to the negative terminal then one produces a flow of electrons and holes into the junction region
.these electrons and holes may recombine to produce radiation. This amounts to the deexcitation of an electron from the conduction band to the valence band. If the current is large enough one may have laser actions.
PLASMA-plasma is the fourth state of matter. It is the most common form of matter. Plasma which is in the stars and in the tenuous space between them makes up
the visible universe and perhaps most of that is not visible.
On the earth island upon which we live is made up of ordinary matter. Generally we find the three different states of matter which are solid liquid and gas. We learnt to work , play and rest using these familiar states of matter. Sir William Crookes an English physicist identified a fourth state of matter, now called plasma in 1879.plasma temperatures range from relatively cool to very hot, and densities range from tenuous(like aurora) to very dense (like the central core of star).ordinary solids liquids and gasses are both electrically neutral and cool or dense to be in a plasma state.
The word plasma was first used to ionize gas by Dr Irvin Langmuir, an American chemist and physicist in 1929.
Plasma consists of a collection of free moving electrons and ions, atoms that have lost electrons. Energy is needed to strip electrons from atoms to make plasma. The energy can be of various origins, thermal electrical or light (ultraviolet light or intense visible light from a laser) with insufficient sustaining power plasma recombine into neutral gas.
Plasma can be accelerated or steered by magnetic or electric fields, Which allows it to be controlled and applied. Plasma research has yield a greater understanding of the universe. It provides many practical uses. New manufacturing techniques, consumer products. The prospects of abundant energy , more efficient lightening, surface cleaning, waste removal and many more application topics.
LASER PLASMA INTERACTION-
The process of interaction of laser plasma requires very high energy. This is due to the fact that unless the nuclei have very high kinetic energies, the coulomb repulsion will not allow them to come sufficiently close for interaction to occur. The temperature required are usually ~100 million 0K,and at such a high temperature the matter is in a fully ionized state and its confinement causes laser plasma interaction.
The formation and evolution of plasma produced during the interaction of a high power beam with solid dielectrics are topics of practical interns.
The experiment was carried out at the Rutherford Appleton Laboratory, employing the 100 TW Nd Glass Vulcan laser operating in the chirp pulse amplification mode. The duel CPA configuration was employed, providing two CPA pulse with adjustable relative relay sat PS precision . Each of the output beams delivered approximately 50J in 1.2 ps(FWHM) duration. By using off axis parabolas the beam were focused down, on different targets to spot of beams interacted with the He gas a supersonic nozzle driven at 50bar pressure. The other CPA beam was employed to generate the probe proton beams by irradiating it onto a 10 micro m Au foil. The detector of 2-3 cm from the gas jet. In the condition of the experiment this provides a multi-frame temporal scan of the interaction for upto 50 ps in a single shot. The time of arrival of the proton of a given energy at the interaction pint was controlled by the relative time of arrival of the split CPA beams on their respective destination.
RESULTS AND DISCUSSION-
The typical results were obtained from plasma glass interaction. The frames are increased in ascending order of the laser energy.
The plasma was appeared in contact with glass surface. The plasma was expanding supersonically towards the laser beam.
The entry of the short pulse into the field of view and its propagation through the plasma were observed due to the generation of an instantaneous electron depleted ion channel under the action of the strong radial ponder motive force of the laser. The radial electric field present in the positively charged ion channel detected the probe proton outward from the laser axis, creating a negative shadow of the channel over the RCF. The proton projection channels are a bullet shaped channel with sharp boundary, and a white channel with a dark line on the axis.
The image shown is the snapshot of the plasma taken during the propagation of the laser pulse through the propagation of the laser pulse through the gas pre ionized by the prepulse. The features mentioned above are imprinted in the proton probe cross section due to the effect of the radial field surrounding the propagation axis at various stages of plasma evolution during or immediately after the pulse propagation. At even later times the development of quasi- periodic modulations inside the channel was observed. These structures evolved into circular structures which were observed to decay on hydrodynamic time scales.