LOVELY PROFESSIONAL UNIVERSITY

TERM PAPER

LASER PLASMA INTERACTION

Subject: Physics (PHY 102)

DOA- 25-8-10

DOS- 10-11-10

Submitted to- Submitted by-

Shreekanth Reddy Debakshi Dutta

Department of Physics Roll No- RE6001B52

Reg. No:- 11005922

Class:- Ist sem

Acknowledgement

I owe a deep sense of reverence to shreekanth Reddy my immediate instructor, who at every step guided me with sincere efforts and enriched me with their profound knowledge of laser plasma interaction. I thank them for their inspirational guidance and frequent stimulation despite their busy schedules.

Words elude me in expressing my profound gratitude to my whole them their pains taking guidance, constant, encouragement, constructive suggestions, thought provoking discussion and giving useful opportunity to practically handle the whole project.

I would also like to thank the technical staff who helped me a lot in understanding the complex details of the machinery.

Last but not the least I am thankful to my parents who helped me to reach the position I am today at. It is a result of their constant support and guidance that inspired me to go for these studies

CONTENTS

1. Introduction2. Plasma Property and parameter3. Definition

Plasma approximation Bulk interaction Plasma Frequency

4. Ranges Of Plasma parameter5. Degree of ionization6. Temperature7. Magnetization8. Comparison of Plasma and gas phases9. Laser plasma interaction10.Interaction of matter with ultra high peak power11.Motivation12.Future application13.Laser based accelerator

Fast ignitator fusion Non Linear Quantum Electrodynamics Future increase in Laser energy

14.References

Introduction

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. The basic premise is that heating a gas dissociates its molecular bonds, rendering it into its constituent atoms. Further heating leads to ionization (a loss of electrons), turning it into a plasma: containing charged particles, positive ions and negative electron]

The presence of a non-negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma, therefore, has properties quite unlike those of solids, liquids, or gases and is considered to be a distinct state of matter. Like gas, plasma does not have a definite shape or a definite

volume unless enclosed in a container; unlike gas, under the influence of a magnetic field, it may form structures such as filaments, beams and double layers. Some common plasmas are stars and neon signs.

Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called physics "radiant matter"). The nature of the Crookes tube "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897, and dubbed "plasma" by Irving Langmuir in 1928, perhaps because it reminded him of a blood plasma. Langmuir wrote:

Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.

Fig1

Plasma properties and parameters

Fig 2Artist's rendition of the Earth's "plasma fountain", showing oxygen, helium, and hydrogen ions which gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the aurora borealis, where plasma energy pours back into the atmosphere

Definition of a plasma

Plasma is loosely described as an electrically neutral medium of positive and negative particles (i.e. the overall charge of a plasma is roughly zero). It is important to note that although they are unbound, these particles are not ‘free’. When the charges move they generate electrical currents with magnetic fields, and as a result, they are affected by each other’s fields. This governs their collective behavior with many degrees of freedom. A definition can have three criteria:

1.The plasma approximation: Charged particles must be close enough together that each particle influences many nearby charged particles, rather than just interacting with the closest particle (these collective effects are a distinguishing feature of a plasma). The plasma approximation is valid when the number of charge carriers within the sphere of influence (called the Debye sphere whose radius is the Debye screening length) of a particular particle are higher than unity to provide collective behavior of the charged particles. The average number of particles in the Debye sphere is given by the plasma parameter, "Λ" (the Greek letter Lambda).

2.Bulk interactions: The Debye screening length (defined above) is short compared to the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral.

3.Plasma frequency: The electron plasma frequency (measuring plasma oscillations of the electrons) is large compared to the electron-neutral collision frequency (measuring frequency of collisions between

electrons and neutral particles). When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics.

Ranges of plasma parameters

Plasma parameters can take on values varying by many orders of magnitude, but the properties of plasmas with apparently disparate parameters may be very similar (see plasma scaling). The following chart considers only conventional atomic plasmas and not exotic phenomena like quark gluon plasmas:

Fig 3

Range of plasmas. Density increases upwards, temperature increases towards the right. The free electrons in a metal may be considered an electron plasma.

Typical ranges of plasma parameters: orders of magnitude

Characteristic

Terrestrial plasmas

Cosmic plasmas

Sizein meters

10−6 m (lab plasmas) to102 m (lightning) (~8 OOM)

10−6 m (spacecraft sheath) to1025 m (intergalactic nebula) (~31 OOM)

Lifetimein seconds

10−12 s (laser-produced plasma) to107 s (fluorescent lights) (~19 OOM)

101 s (solar flares) to1017 s (intergalactic plasma) (~16 OOM)

Densityin particles percubic meter

107 m−3 to1032 m−3 (inertial confinement plasma)

1 m−3 (intergalactic medium) to1030 m−3 (stellar core)

Temperaturein kelvins

~0 K (crystalline non-neutral plasma[) to108 K (magnetic fusion plasma)

102 K (aurora) to107 K (solar core)

Magnetic fieldsin teslas

10−4 T (lab plasma) to103 T (pulsed-power plasma)

10−12 T (intergalactic medium) to1011 T (near neutron stars)

Degree of ionization

For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the

number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms which have lost (or gained) electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e., response to magnetic fields and high electrical conductivity). The degree of ionization, α is defined as α = ni/(ni + na) where ni is the number density of ions and na

is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through ne = <Z> ni where ne is the number density of electrons.

Temperatures

The kinetic energy of a plasma particle is considerably higher than its potential, where charged particles travel at high speeds. If the potential were greater than the kinetic, then the plasma state would be destroyed as the ions and electrons would want to clump together into bound states—atoms.[13] This is why plasmas typically arise at very high temperatures.

Plasma temperature is commonly measured in kelvins or electronvolts and is an informal measure of the thermal kinetic energy per particle. In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example, due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium amongst themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason, the "ion temperature" may be very different from (usually lower than) the "electron

temperature". This is especially common in weakly ionized technological plasmas, where the ions are often near the ambient temperature.

Based on the relative temperatures of the electrons, ions and neutrals, plasmas are classified as "thermal" or "non-thermal". Thermal plasmas have electrons and the heavy particles at the same temperature, i.e., they are in thermal equilibrium with each other. Non-thermal plasmas on the other hand have the ions and neutrals at a much lower temperature, (normally room temperature), whereas electrons are much "hotter".

Temperature controls the degree of plasma ionization. In particular, plasma ionization is determined by the "electron temperature" relative to the ionization energy, (and more weakly by the density), in a relationship called the Saha equation. A plasma is sometimes referred to as being "hot" if it is nearly fully ionized, or "cold" if only a small fraction, (for example 1%), of the gas molecules are ionized, but other definitions of the terms "hot plasma" and "cold plasma" are common. Even in a "cold" plasma, the electron temperature is still typically several thousand degrees Celsius. Plasmas utilized in "plasma technology" ("technological plasmas") are usually cold in this sense.

Potentials

Fig 4Lightning is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, X-rays and even gamma rays.[14] Plasma temperatures in lightning can approach ~28,000 kelvin and electron densities may exceed 1024 m−3.

Since plasmas are very good conductors, electric potentials play an important role. The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The good electrical conductivity of plasmas causes their electric fields to be very small. This results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma (ne = <Z>ni), but on the scale of the Debye length there can be charge imbalance. In the special case that double layers are formed, the charge separation can extend some tens of Debye lengths.

The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the "Boltzmann relation":

.

Differentiating this relation provides a means to calculate the electric field from the density:

.

It is possible to produce a plasma which is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise it will be dissipated by the repulsive electrostatic force.

In astrophysical plasmas, Debye screening prevents electric fields from directly affecting the plasma over large distances, i.e., greater than the Debye length. But the existence of charged particles causes the plasma to generate and can be affected by magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object which separates charge over a few tens of Debye lengths. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magneto hydrodynamics.

Magnetization

Plasma in which the magnetic field is strong enough to influence the motion of the charged particles is said to be magnetized. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic field before making a collision, i.e., ωce/νcoll > 1, where ωce is the "electron gyrofrequency" and νcoll

is the "electron collision rate". It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are anisotropic, meaning that their properties in the direction parallel to the magnetic field are different from those perpendicular to it. While electric fields in plasmas are usually small due to the high conductivity, the electric field associated with a plasma moving in a magnetic field is given by E =

−v x B (where E is the electric field, v is the velocity, and B is the magnetic field), and is not affected by Debye shielding.[15]

Comparison of plasma and gas phases

Plasma is often called the fourth state of matter. It is distinct from other lower-energy states of matter; most commonly solid, liquid, and gas. Although it is closely related to the gas phase in that it also has no definite form or volume, it differs in a number of Plasma properties and parameters

Fig5

Artist's rendition of the Earth's "plasma fountain", showing oxygen, helium, and hydrogen ions which gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the aurora borealis, where plasma energy pours back into the atmosphere.

LASER PLASMA INTERACTION

Over the past decade fem to second lasers have found much utility. Fem to second lasers produced plasmas are violent objects with extreme conditions of temperature and pressure and so it is bright source of electromagnetic radiation and high charge state ions with energies extending from an eV to MeV. Unusual electron temperatures are as high as few KeVs. These properties of this type of laser can be utilized for the generation, optimization and applications of fem to second x-ray source. The physics underlying the efficient laser plasma coupling schemes, plasma heating and x-ray generation can be investigated with a number of low z and high z targets. It can be used to provide a predictable x ray source for applications in time resolved experiments.

Extremely short pulse can be made use of to study the lattice dynamics and transient chemical reaction dynamics.

Through high harmonic generation fem to second or sub fem to second coherent pulses are generated in the VUV to soft x-ray region but phase matching of the generated fields with the fundamentals one is one of the challenges toward staining maximum efficiency.

Interaction of matter with ultra high peak powersWhen an intense laser interacts with matter, the laser electromagnetic field can far exceed the coulomb field that binds electrons to atoms, stripping off the electrons (at 1019 W/cm2, the laser electric field is close to 1011 V/ cm2 which is 20 times the field binding the ground state electrons in the hydrogen atom.

In high fields, electrons can actually move close to c. This is predicted to have several

important consequences, due to the action of component of Lorentz force, resulting in efficient harmonic generation by non linear Thomson scattering and the electron mass should increase significantly, modifying the index of refraction of the medium. Large index changes will in turn modify the propagation of the laser which will further modify the medium and so on.Hence non linear effects such as laser self focusing or self modulation should arise.

Motivation

Studying of interaction of a high power pulsed laser with atomic clusters and molecules which provide a form of matter intermediate between molecules and bulk solids. From a practical point of view the hot dense plasma created by the irradiation of atomic clusters by ultra short high power laser pulses form a compact source of x-rays, which has applications in extreme ultraviolet lithography, x-ray microscopy and x-ray tomography.

When an intense laser interacts with matter, the laser electromagnetic field can far exceed the coulomb field that binds electrons to atoms, stripping off the electrons (at 1019 W/cm2, the laser electric field is close to 1011 V/ cm2whichis 20 times the field binding the ground state electrons in the hydrogen atom.

Plasmas produced from solids using intense ultra short laser pulses are found to be rich sources of hard and soft x-rays, MeV electrons and high energy ions.

Such sources can be explored for their use in time resolved x-rays diffraction and imaging.

The exact characterization of these plasmas is required before it can be utilized as potential x-ray source.

The source would be characterized in terms of absolute x-ray yield, temporal duration (ion and electron energies etc.)

Future applications

Laser based acceleratorLaser-plasma accelerated electron beams (and the x-ray radiation that they can make) may be useful for either medical or industrial radiology. Besides their compactness, laser accelerators have the advantage of small electron size, 10times smaller than conventional sources. This permits a much higher spatial resolution (micron-size scale) of small features in radiological application.

Fast- ignitor fusionEvidence suggest that laser induced burst of hot electrons or protons could be used as a spark plug to ignite a thermonuclear reaction with inertial confinement fusion. With solid-density targets, due to the combination of the very large radiation pressure –far exceeding the thermal pressure- combined with a relativistic decrease of plasma frequency, a pulse incident on an over dens plasma will penetrate over several wavelengths. This effect referred to as hole boring has led to the concept of fast ignitor. The fast ignitor concept offers the possibility of high target gain at reduced total drive energy, compared with conventional inertial confinement fusion.

Non linear quantum electrodynamicsElectron positron pair production directly from laser requires an intensity of the order of 1030 W/cm2, corresponding to a laser field of 1016 V/cm, which is about four to five orders of magnitude above the available laser. This enormous gap is filled using 50 GeV electron beam corresponding to a relativistic factor of 105.

Further increase in laser energyAs laser intensities increase further to the order of peta watt and exa watt ? and laser accelerated protons become relativistic, exotic plasmas such as dense electron positron plasmas which are of astrophysical interest can be created in laboratory.

REFERENCES

1. Sturrock, Peter A. (1994). Plasma Physics: An Introduction to the Theory of Astrophysical, Geophysical & Laboratory Plasmas.. Cambridge University Press.

2. Crookes presented a lecture to the British Association for the Advancement of Science, in Sheffield, on Friday, 22 August 1879

3. Announced in his evening lecture to the Royal Institution on Friday, 30th April 1897, and published in Philosophical Magazine 44: 293. 1897. http://web.lemoyne.edu/~GIUNTA/thomson1897.html.

4. I. Langmuir (1928). "Oscillations in ionized gases". Proc. Nat. Acad. Sci. U.S. 14: 628. doi:10.1073/pnas.14.8.627.

5. IPPEX Glossary of Fusion Terms 6. Plasma fountain Source, press release: Solar Wind Squeezes Some of Earth's Atmosphere

into Space 7. Hazeltine, R.D.; Waelbroeck, F.L. (2004). The Framework of Plasma Physics. Westview

Press.. 8. R. O. Dendy (1990). Plasma Dynamics. Oxford University Press. ISBN 0198520417.

http://books.google.com/?id=S1C6-4OBOeYC. 9. Daniel Hastings, Henry Garrett (2000). Spacecraft-Environment Interactions. Cambridge

University Press. ISBN 0521471281. 10. Peratt, A. L. (1966). "Advances in Numerical Modeling of Astrophysical and Space

Plasmas". Astrophysics and Space Science 242: 93–163. doi:10.1007/BF00645112. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996Ap&SS.242...93P.