A laser standing forÂ Light Amplification by Stimulated Emission of Radiation is a device which produces electromagnetic radiation often visible light, using the process ofÂ optical amplificationÂ based on the stimulated emissionÂ of photons within a so-called gain medium. The emittedÂ laser lightÂ is notable for its high degree of spatial and temporal coherence, unattainable using other technologies. Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. This is in contrast to thermal or incoherent light emitted by ordinary sources of light whose instantaneous amplitude and phaseÂ varys randomly with respect to time and position. Although temporal coherence implies monochromatic emission, there are lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously. Most so-called "single wavelength" lasers actually produce radiation in severalÂ modesÂ having slightly different frequencies (wavelengths), often not in a single polarization. There are some lasers which are not single spatial mode and consequently their light beams divergeÂ more than required by the diffractionÂ limit. However all such devices are classified as "lasers" based on their method of producing that light and are generally employed in applications where light of similar characteristics could not be produced using simpler technologies.
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A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
Laser pumping energy
TYPES OF LASER
According to their sources:
According to the nature of emission:
According to their wavelength:
CHARACTERISTIC OF LASER
* Laser ray is highly pure beam of light with respect to the wavelength and the frequency of the photons forming it.
* laser beam is highly intense and very narrow beam this is because its divergence is very small.
* Laser beam transfers in straight lines approximately parallel to each other.
* The laser photons are coherent,in phase and have the same direction.
USES OF LASERS
Medical lasers can be used as a scalpel. Since the laser can be controlled and can have such a small contact area it is ideal for fine cutting and depth control. Medical lasers can also be used to reattach retinas and can be used in conjunction with fiber optics to place the laser beem where it needs to be. Medical lasers can also be used to stitch up incisions after surgery, by fusing together skin. (LFI)
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Laser shows are quite popular and the special effects are amazing. These use lasers that are in the visible spectrum along with vibrating mirrors to paint images in the air. Here is an example of a dance with lasers in the background:
You might noticed the fog in the background, that is what allows the laser light to reflect and you to view it. Another example of laser entertainment is the use of laser signs at trade shows. Here is an example of a laser Microsoft sign:
Computers and Music:
One popular use of lasers is the reading of CD. CD's function by having a reflective aluminum layer that has very small pits put in the aluminum. The pits and the lack of are translated into binary by the computer and then are used for information. Another use of lasers is in the use of fiber optics. Since lasers travel very fast they make an ideal way to communicate. The laser is shot down a fiberglass tube to a receiver. These wires can be very long with no loss of signal quality. Also modern multiplexing of the line lets two lasers of different frequencies share the same line. (Serway)
Lasers very accurate point and solid state construction make it ideal or industrial production. Lasers allow better cuts on metals and the welding of dissimilar metals with out the use of a flux. Also lasers can be mounted on robotic arms and used in factors. This is safer then oxygen and acetylene, or arc welding. (Impulse)
In physics and chemistry, plasma 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 electrons.
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 it "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
TYPES OF PLASMA:
1 Pseudo-plasmas vs real plasmas
2 Cold, warm and hot plasmas
2.1 Hot plasma (thermal plasma)
2.2 Warm plasma
2.3 Cold plasma (non-thermal plasma)
2.4 Ultracold plasma
3 Plasma ionization
3.1 Fully ionized plasma
3.2 Partially ionized plasma (weakly ionized gas)
4 Collisional plasmas
4.1 Collisional plasma
4.2 Non-collisional plasma
5 Neutral plasmas
5.1 Neutral plasma
5.2 Non-neutral plasma
6 Plasmas densities
6.1 High density plasma
6.2 Medium density plasma
6.3 Low density plasma
7 Magnetic plasmas
7.1 Magnetic plasma
7.2 Non-magnetic plasma
8 Complex plasmas
8.1 Dusty plasmas and grain plasmas
8.2 Colloidal plasmas, Liquid plasmas and Plasma
9 Active and passive plasmas
9.1 Passive plasma
9.2 Active plasma
Uses of Plasma
Magnetic fusion energy (MFE) - tokamak, stellarator, reversed field pinch, magnetic mirror, dense plasma focus
Inertial fusion energy (IFE) (also Inertial confinement fusion - ICF)
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Food processing (nonthermal plasma, aka "cold plasma")
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WHAT IS INTERACTION
Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect. A closely related term is interconnectivity, which deals with the interactions of interactions within systems: combinations of many simple interactions can lead to surprising emergent phenomena.
Self-focusing is a non-linear optical process induced by the change in refractive index of materials exposed to intense electromagnetic radiation.A medium whose refractive index increases with the electric field intensity acts as a focusing lens for an electromagnetic wave characterised by an initial transverse intensity gradient, as in a laser beam. The peak intensity of the self-focused region keeps increasing as the wave travels through the medium, until defocusing effects or medium damage interrupt this process.
Self-focusing is often observed when radiation generated by femtosecond lasers propagates through many solids, liquids and gases. Depending on the type of material and on the intensity of the radiation, several mechanisms produce variations in the refractive index which result in self-focusing: the main cases are Kerr-induced self-focusing and plasma self-focusing.
PLASMA SELF FOCUSING
Advances in laser technology have recently enabled the observation of self-focusing in the interaction of intense laser pulses with plasmas.Self-focusing in plasma can occur through thermal, relativistic and ponderomotive effects.
THERMAL SELF FOCUSING: Thermal self-focusing is due to collisional heating of a plasma exposed to electromagnetic radiation: the rise in temperature induces a hydrodynamic expansion which leads to an increase of the index of refraction and further heating.
RELATIVISTIC SELF FOCUSING: Relativistic self-focusing is caused by the mass increase of electrons travelling at speed approaching theÂ speed of light, which modifies the plasma refractive indexÂ nrelÂ according to the equation
where Ï‰ is the radiationÂ angular frequency and Ï‰pÂ the relativistically correctedÂ plasma frequency
PONDEROMOTIVE SELF FOCUSING:
Ponderomotive self-focusing is caused by theÂ ponderomotive force, which pushes electrons away from the region where the laser beam is more intense, therefore increasing the refractive index and inducing a focusing effect.
The evaluation of the contribution and interplay of these processes is a complex task,Â but a reference threshold for plasma self-focusing is the relativistic critical power
whereÂ meÂ is theÂ electron mass,Â cÂ the speed of light, Ï‰ the radiation angular frequency,Â eÂ the electron charge and Ï‰pÂ the plasma frequency. For an electron density of 1019Â cm-3Â and radiation at the wavelength of 800Â nm, the critical power is about 3 TW. Such values are realisable with modern lasers, which can exceed PW powers. For example, a laser delivering 50 fs pulses with an energy of 1 J has a peak power of 20 TW.
Self-focusing in a plasma can balance the natural diffraction and channel a laser beam. Such effect is beneficial for many applications, since it helps increasing the length of the interaction between laser and medium. This is crucial, for example, in laser-driven particle acceleration,Â laser-fusion schemesÂ and high harmonic generation.
LASER PLASMA INTERACTION
Relativistic Non-Linear Optics
The MPQÂ ATLASÂ laserÂ produces focussed laser intensities up to few times 1019Â W/cm2Â on target material, which ionizes and turns into plasma.Â At intensities above 1018Â W/cm2, the laser light accelerates target electrons almost to the velocity of lightÂ such that their masses increase by the relativistic factor Î³ = (1- v2/cÂ 2Â )-1/2Â . In ATLAS experiments we encounter electrons which are 10 - 100 times heavier than electrons at rest. This strongly changesÂ laser plasma interaction.Â
Â Light of frequency Ï‰ propagates in plasma according to the dispersion relation Ï‰2=Ï‰p2/<Î³>+kÂ 2c2Â , which is plotted here. It depends on the plasma frequency Ï‰p2=4Ï€e2neÂ /mÂ Â and the average <Î³>-factor. In dense plasmaÂ withÂ Ï‰ <Â Ï‰Â pÂ , light cannot propagate and is reflected from the surface. However, for relativistic intensities generating large <Î³> -factors, the plasma becomes transparent. We call this induced transparency.
Due to the transverse intensity profile of the light beam, the relativistic effects are strongest on the axis and modulate the index of refractionÂ n=(1-(Ï‰p2/<Î³>)/Ï‰Â 2)1/2Â accordingly.Â An initially planar wavefront is deformed in a plasma as shown in the figure. Since the phase velocityÂ vph=c/nÂ is smaller on the axis, the plasma acts like a positive lens and leads to self-foccusing for laser powers beyond a critical level.
Another important effect is the steepening of pulse envelopes propagating with group velocityÂ vgr=cn. The peak region with high intensity runs faster than those with low intensity at the pulse head, and this leads to optical shock formation. Pulse shapes with steeply rising fronts are interesting for studying high intensity effects in matter
Relativistic channeling and electron beam generation
Beyond a critical powerÂ PcritÂ = 17.4 nc/nÂ eÂ GW, a laser pulse propagating Â in plasma undergoes self-focussing as it is seen in the figure below. HereÂ neÂ /ncÂ is the electronÂ Â density normalized to the critical densityÂ nc. In three-dimensional space the laser beam self-focusses to a super-channel just 1-2 wavelengths in diameter. An outstanding feature is the relativistic electron beam accelerated in the channel in the direction of laser propagation. With a density of orderÂ nÂ cÂ ~ 1021Â cm-3Â , it produces a current density of orderÂ 1012Â A/cm2Â and total currents of someÂ 10Â kA, which generate a quasi-stationary magnetic field in the order ofÂ 100Â MegaGauss.Â The pinching effect of the magnetic field adds to the self-focussing.
Electron and ion spectra
The energy spectra of the electrons show a characteristic exponential decay and the corresponding effective temperatures scaleÂ according toÂ TeffÂ ~ 1.5Â I1/2MeVÂ with intensityÂ IÂ in units ofÂ 1018Â W/cm2. This is in agreement with measured spectra. Since electrons are expelled from the channel, a radial electric field is created which accelerates ions in radial direction. Depending on laser intensity, multi-MeV ion energies are found inÂ simulation as well as experiment. In deuterium plasma, these energetic ions cause fusion reactions, and the correspondingÂ 2.45Â MeV neutrons have been detected experimentally. Â
Â Â Â Â Â Â Â Â
3D-PIC simulation compared to experiment
Self-focussing and electron beam generation have been observed in MPQ experiments, using gas jet targets and a 150 fs laser pulse with focussed intensity 6 x 10Â 19Â W/cm2Â . The measured electron Â spectra were found to be in excellent agreement with the corresponding 3D-PIC simulation. This opened the possibility to investigate the electron acceleration mechanism in more detail. The electron phase space is shown below on the right-hand side as a snapshotÂ after 300 fs propagation time. The pulse propagates from left to right. The longitudinalÂ EÂ zÂ field reveals some self-modulated laser wakefield excitation near the laser head and wakefield acceleration in the Î³-plot, but apparently the plasma wave breaks after a few oscillations. Nevertheless, strong electron acceleration withÂ Î³ ~ Â 40 - 50 is visible in the broken-wave region, and the question arises what is the acceleration mechanism here. Zooming the phase space in the regionÂ z/Î»Â ~Â 270 - 280, one finds that it is modulated with the laser periodÂ and shows large transverse momentaÂ px, indicating that direct laser acceleration takes place.
How do the electrons gain energy?
Electrons can gain energy only from the electric field, either the transverse component mainly originating from the laser pulse or fromÂ the longitudinal component mainly originating from plasma waves. To find out which mechanism dominates, we have determined the total longitudinal and transverse gain for each electron and show the result in the figure. Surprisingly, most electrons in this simulation gained their final energy from the transverse laser field, and the longitudinal field had rather a decelerating than an accelerating effect. The deceleration can be attributed to the negative longitudinal component of the laser field occuring in narrow channels.
Relativistic channels as Inverse Free Electron Lasers
The result obtained above can be understood in terms of an Inverse Free Electron Laser model. The azimuthal magnetic and the radial electric field of the self-focussed channel acts like the wiggler of a free electron laser (FEL), causing transverse oscillations of relativistic electrons with betatron frequency Ï‰Î²2= Ï‰p 2/(2Î³) when moving along the channel axis. This is exactly the configuration of an FEL. At resonance when the Doppler-shifted laser frequency coincides with Ï‰Î² = Ï‰L ( 1 - v|| / v ph ) , the electron can experience acceleration from the laser field over many laser periods, and this explains the large transverse momenta. It is then the magnetic laser field which turns the transverse motion into longitudinal motion without adding further energy.
Laser hole boring into overdense plasma
In the case of overdense plasma (here ne/nc = 10), the laser light cannot penetrate into the plasma initially, but the light pressure starts to bore a hole into the overdense region. This is observed in the ion density plot at 330 fs and 660 fs. Matter is pushed to the side and forms a conical shock. Electrons are accelerated in the hole region and corresponding strong currents are seen in the magnetic field pattern. At the surface of the hole the current is directed outwards, while in the inner regions of the hole it is directed into the plasma. A particular interesting feature is seen in the overdense part of the plasma which has not yet been reached by the hole boring and into which only the electron current can penetrate. Here the electron current is seen to disintegrate into current filaments at 330 fs, but these filaments have apparently reunited in a single thick current filament at 660 fs. Filamentation is due to Weibel instability.
Current filamentation and filament coalescence
We have also studied current filamentation by 2D PIC simulation in the plane transverse to the current. At time Ï‰pt=0, a uniform relativistic electron current is assumed having 10% of the plasma density. Initially it is completely compensated by a uniform return current. This two stream configuration quickly decays into many filaments, which, in a later phase, coalesce and form a few thick filaments. The process of coalescence is found to be highly dissipative leading to strong anomalous stopping of the initial beam. These features may be relevant to the concept of fast ignition of fusion targets.
Confined electron-positron plasma
Electron-positron and Î³-photon production by high-intensity laser pulses has been investigated for a special target geometry, in which two pulses irradiate a very thin foil (10-100 nm < skin depth) with same intensity from opposite sides. A stationary solution is derived describing foil compression between the two pulses. Circular polarization is chosen such that all electrons and positrons rotate in same direction in the plane of the foil. We discuss the laser and target parameters required in order to optimize the Î³-photon and pair production rate. We find a Î³ -photon intensity of 7 x 10 27/ (sr sec) and a positron density of 5 x 1022 cm -3 when using two 330 fs , 7 x 1021 W/cm2 laser pulses.
The spectacular increase in brightness and decrease in pulse duration of X-ray and particle beams will revolutionize the way researchers investigate matter. Fundamental events in biology, chemistry and solid-state physics can be recorded with ångström space resolution to capture electronic, atomic or molecular transient dynamics. The X-ray pulse requirements will depend on each experiment. Basic applications have been carried out with 103Â photons/pulse/0.1% Â BW at 2Â keV; 1010Â photons/pulse/0.1% BW at a few tens of kiloelectronvolts are required to obtain a Laue diffraction pattern in complex molecules and in a single shot. Achievement of high X-ray intensities will extend nonlinear optics to the X-ray spectral range and enable the creation of new states of matter such as plasmas of astrophysical or geophysical interest. Source compactness, broad spectral range and perfect synchronization of particle and radiation bursts are unique properties that could extend the breadth of applications. As we have seen, the high peak current of laser-plasma electron beams could lead to compact XFEL facilities, on a size affordable by small-scale laboratories. High dissemination towards multidisciplinary users is then foreseen in fundamental science, but also in more societal fields. As an example, implementation in existing hospitals of phase-contrast imaging techniques developed at synchrotrons to provide high-resolution images with micrometre resolution could enable significant advancement in clinical diagnostics. Finally, time-resolved experiments, where the particle burst or the X-ray flash can act indifferently as a pump or a probe, would significantly extend the field of investigation in the dynamics of matter, compared with currently available techniques using a visible pump and X-ray or visible probes. Below, we discuss some of these novel application opportunities in medicine, radiation biology andÂ physics.
Up to now, X-rays with energies of a few megaelectronvolts represent the vast majority of ionizing radiations used for cancer radiotherapy of several million patients throughout the world. X-rays are commonly used because they are produced using flexible, compact and affordable machines. Higher quality, more energetic electron beams, such as those produced by laser-plasma accelerators, could be used for radiotherapy and provide better clinical results. It was shown that such beams are well suited for delivering a high dose peaked on the propagation axis, a sharp and narrow transverse penumbra, combined with a deep penetration. Comparison of dose deposition for 250Â MeV laser-accelerated electrons with that of 6Â MeV X-rays showed significant improvement for a clinically approved prostate treatment plan (T. FuchsÂ etÂ al., manuscript in preparation). Target coverage was computed to be the same or even slightly better for electrons, and dose sparing of sensitive structures was improved (up to 19% ). These findings are consistent with previous results regarding very high-energy electrons as a treatment modality. The lack of compact and cost-efficient electron accelerators could be overcome by laser-plasma systems using existing commercial systems delivering tens of femtoseconds, 1Â J laser pulses, and operating at 10Â Hz repetition rate to deliver the required clinical electron beam dose in a few minutes.
With more than 30,000 patients worldwide with successful clinical results, proton and hadron therapies are still emerging, but represent promising methods for the specific treatment of deep tumours and radio-resistant cancers. However, although this treatment is expanding considerably (more than 20 new projects are under consideration worldwide), its use is still strongly limited owing to the size and cost of the infrastructure, which exceedsÂ . The infrastructure requirements, which include accelerator, beam lines, massive gantries of more than 100 tons and building, are not accessible to the majority of radiotherapy centres. With the outstanding progress in laser physics and fast development of high-power laser systems, several laser-based projects have emerged with the goal of reducing the cost of proton therapy treatment. These costs could be cut, not only by changing the accelerator itself (commercial accelerators that deliver stable and reliable 200Â MeV protons beams cost aboutÂ ), but mainly because the building footprint would be strongly reduced, and the gantry could be replaced by a smaller and lighter structure. Several severe conditions have to be met before considering such an approach for medical applications. It is necessary to
(1)Â increase the proton energy up to 200Â MeV, for which petawatt class lasers will probably be require
(2)Â have enough protons at this energy to treat patients in sessions of a few minutes, for which high repetition rates (10Â Hz) could be needed,
(3)Â have a reliable and stable laser-plasma accelerator. The dose requirement and dose profile could be achieved with particle selectors or structured targets. This promising application is also extremely challenging, as it requires the development of high-contrast, petawatt lasers operating at 10Â Hz, as well as dedicated research activities in target design and high-intensity interaction. In a related field, lower-energy protons of several megaelectronvolts delivered with compact cyclotron machines of a fewÂ Â are used to produce radio-isotopes for medical diagnostics. A laser-based alternative has been considered, but does not seem economically competitive because higher repetition rate lasers would be required with a cost in excess of existing accelerators.
Progress in conventional and conformational radiotherapies is highly dependent on innovative developments of radiation source quality, physics and engineering. Concerning radiation biology, a crucial domain for cancer therapy, it is commonly admitted that the early spatial distribution of energy deposition following ionizing radiation interactions with biomolecular architectures is decisive for the prediction and control of damage at cellular and tissular levels. The complex link existing between radiation physics and biomedical applications concerns the complete understanding of spatio-temporal events triggered by an initial energy deposition in confined spaces called spurs. Microscopic radiation effects on integrated biological targets such as water, 'the solvent of life', nucleic acids or proteins cannot be satisfactorily described by an absorbed dose profile or a linear energy transfer. As primary radiation damage on biological targets is dependent on the survival probability of secondary electrons and radicals inside nanometric clusters of ionization, a thorough knowledge of these processes requires real-time probing of early events on the submicrometric scale. In the temporal range of 10- 15-10- 10Â s, this domain concerns low- and high-energy radiation femtochemistry.
The course of ultrafast elementary ionizing events occurring in spurs is largely unknown because of the long duration of contemporary radiation sources used to probe it. In this context, laser-plasma accelerators providing shorter particle bunches open exciting opportunities for real-time probing of high-energy radiation physical chemistry and biology. Femtolysis experiments (from femtosecond radiolysis) of aqueous targets carried out with ultrashort, few-megaelectronvolt electron bunches produced by laser-plasma acceleratorsÂ have given new insights into the early behaviour of secondary electrons in the prethermal regime of nascent ionization clusters . Pioneering femtolysis studies emphasized that the early hydrated electron yield atÂ t5Â ps is higher than predicted by calculations using classical stochastic modelling of irradiated water molecules, and underlined the pre-eminence of quantum effects during the ultrafast relaxation of secondaryÂ electrons.
Figure :Â Time-space relationship characterizing energy deposition during the interaction of a relativistic electron beam (MeV) with liquid water.
In less than 10- 16Â s, energy quanta of 200 and 20Â eV are delivered in primary nanometric tracks and spurs, respectively. The early behaviour of secondary electrons produced in neoformed clusters of ionization events is dependent on the excess energy relaxation occurring in the temporal windows 10- 14-10- 12Â s. Within this prethermal regime, a quantum excited state of the secondary electron (p-like excited state) follows a non-adiabatic transition towards anÂ s-like ground state of the hydrated electron. Beyond 10- 12Â s, fully relaxed excess electrons in liquid water exhibit submicrometric dispersive diffusion processes.
As the early spatial distribution of ionization clusters is a major factor for the biological effectiveness of radiations, spatio-temporal radiation biology would also benefit from the ability of laser-plasma accelerators to generate perfectly synchronized and jitter-free relativistic particle bunches. In the 2.5-15Â MeV range, femtosecond electron beams may enable real-time observation of disulphide molecule reduction by quantum states of secondary electrons. Hence, the effective reaction radius of a molecule for a direct subpicosecondÂ p-like electron attachment would be around 10Â ÅÂ . Such data provide information on spatial radiation processes in track structures. The new domain of radiation femtochemistry would provide guidance for further developments in nanodosimetryÂ for which a typical target areal mass of about 110- 6Â gÂ cm- 2Â corresponds to 100Â ÅÂ at a density of 1Â gÂ cm- 3.
The real-time investigation of relativistic particle interactions with biomolecular targets opens exciting opportunities for the sensitization of confined environments (aqueous groove of DNA, protein pockets) to ionizing radiation. However, compared with classical dose rate delivery in radiotherapy,Â Â 1Â GyÂ min- 1, the very high dose rate delivered with laser-plasma accelerators,Â 1013Â GyÂ s- 1, may challenge our understanding of biomolecular repair, as ultrafast radiation perturbations may be triggered on the timescale of molecular motions, ångström or sub-ångström displacements. With short relativistic particle bunches, high-energy radiation femtochemistry would foreshadow the development of new applications for spatio-temporal radiation biology, anticancer radiotherapy and radioprotection including multiple low-dose effects with nanometric spatial accuracy, predictive consequences of very high dose delivery in cellular environments and selective activation of prodrug in cancerous cells. Indeed, potential advances in cell biology are expected in the next decade, mixing the characteristics of pulsed monochromatic particle beams with those of X-ray generation: development of a charged particle micro-beam for irradiation of living targets, three-dimensional imaging of collective cellular responses (such as the so-called bystander effect) andÂ in vivoÂ X-ray microfluorescence of trace elements in living tissues subject to degenerative processes.
Material and plasma science
Fundamental phenomena of condensed matter and plasma dynamics can also be probed with these unique particle beams. Vigorous research is underway to use laser-accelerated beams to heat matter at solid density on a timescale shorter than that for hydrodynamic expansion. Controlled production of plasmas in these 'warm dense matter' thermodynamic conditions is a key to progress in their theoretical description. Alternatively, energetic, low-emittance proton beams are a powerful probe for quasi-static magnetic and electric fields that develop in laser-produced plasmasÂ and are also good candidates for injection into conventional accelerators. Proof-of-principle experimentsÂ have also demonstrated the applicability of proton-based radiography to the probing of dense materials opaque to conventional photon sources, for shock measurement or inertial confinement fusion sciences. Key beam properties are put to use for these applications: short duration at the source, small virtual source size and ability to focus the beam down to micrometre spot size.
Electron beams produced in laser-plasma accelerators can be used to generate secondary radiation sources. The electron beam energy is efficiently converted into multi-megaelectronvolt Bremsstrahlung photons when it interacts with a solid target of high atomic number, providing a submillimetre pulsedÂ -ray source that is significantly smallerÂ (450Â m) and of shorter duration (in the picosecond range) than other sources available today. UltrashortÂ -ray sources are interesting for several applications, including imaging material compression to high density. A train of short laser pulses may enable recording of movies of dense objects under fragmentation, or of the damage evolution of structures with a spatial resolution of 100Â m. Light and flexible devices for non-destructive material inspection would also be interesting, with potential applications in motor engineering, aircraft inspection andÂ security.
The ultrashort duration of these particle and radiation beams will provide unprecedented time-resolved measurements down to the motion of electrons on atomic scales, and a zooming onto the two fundamental molecular building blocks, the electron and the atom. It will enable exposure of atoms and molecules to relativistic intensities before their disintegration. Coherent diffraction on single molecules will then become accessible, opening an entire new field of research. Time-resolved absorption spectroscopy and Thomson scattering of high-density plasmas require penetrating radiation such as X-rays and an ultrafast time resolution to reveal the properties of the warm dense matter produced in a laser-plasma experiment. Time-dependent measurements of plasma temperature and density will provide a valuable contribution to the understanding of degeneracy and coupling, as well as long- and short-range interactions between charged particles in dense plasmas. Finally, the simultaneous use of particles and radiation as probe or pump beams offers unique opportunities. As an example with societal issues, the study of the ultrafast kinetics following matter excitation by high-energy particles is a major subject in radiation physics and in nuclear technology, with implications on nuclear reactor lifetime. At present, the physical effects of intense particle energy deposition can only be accessed through modelling. There is a crucial need to look at vacancy dynamics that occur in the few-hundred-femtosecond timescale, by ultrafast X-ray or visible probing. Novel laser-based sources will provide the necessary tools.
laser plasma interaction with glass
Laser processing of optical materials is becoming increasingly important and may be considered a genuine alternative to standard mechanical and ultrasonic techniques for dicing or drilling. The formation and evaluation of plasma produced during the interaction of a high-power laser beam with solid dielectrics are topics of practical interest. In the early stages of pulsed laser interaction with dielectrics, a number of processes contribute to the formation of free electrons: multiphoton absorption, absorption at lattice and surface defects, nonlinear absorption due to phase changes and chemical decomposition. If sufficient laser energy is provided avalanche process and thermal ionization lead to the formation of a dense plasma at the surface; the higher the laser intensity, the less in depth effects are stated.
Recently, considerable interest has arisen in laserinduced surface microstructure of dielectric material due to its potential application in optoelectronic. Optical breakdown of dielectrics is rapid ionization and formation of plasma when the material is exposed to electric fields exceeding some critical value. High peak intensities associated with ultrashort laser pulses provide large photon fluxes necessary to initiate nonlinear absorption process (multiphoton initiated avalanche ionization). Ablation takes place when the density of the free conduction band electrons reaches a critical density.
This happens above a certain laser fluence threshold at which point the electrostatic forces are high enough to breakdown the material and to eject the ionized nuclei. The minimum laser fluence below which ablation cannot be initiated is defined as the ablation threshold or optical breakdown threshold. This shows that laser plasma interaction occurs at the surface.
Optical breakdown of various dielectric materials such as fused silica, calcium fluoride (Ca[F]) and barium aluminum borosilicate glass when exposed to ultrashort laser pulses had been reported. In the short-pulse regime, the optical breakdown is a nonthermal process and various nonlinear ionization mechanisms (multiphoton, avalanche, tunneling) become important. An atmospheric pressure and gas condition at which the material is ablated has an effect on damage threshold. Glass is so common that most of us take it completely for granted. The transformation of the solid material into dense plasma is also interesting from a fundamental physics point of view.
The laser-induced damage process is believed to be associated with localized formation of plasma, heating of the material leading to melting and transient stresses that instigate mechanical damage. A damage site on the surface of an optical material generated using nanosecond laser sources usually appears as a crater with rough surfaces that strongly scatter the in coming laser beam. Most often cracks originating at the bottom of the damage crater are visible. It has been recently shown that damage growth is due to re-ignition of the damage process due to absorption of the laser light by the defect population formed at damage sites as well as a result to field intensification arising from cracks.
Study of laser induced plasma is an interesting field since it is important for both the laser-material interaction and many practical applications, for example, in laser fusion research. Laser matter interaction process involves in many applications including laser drilling, micro-matching and depth resolved chemical analysis. The laser induced plasma from a deep down in the body of material has larger electron number density and to produce higher temperature compared to plasma formed on the surface. The deeper the depth, the higher the plasma temperature and more electron number density inside the cavity. Other criteria have been used to define damage threshold such as volume of material ablated and in situ scattering light technique to detect the surface damage.
In laser fusion research glass bubble was used to enclose fusion target. Fundamental study on glass plasma interaction will offer better understanding on fusion phenomenon. The motivation of this work arises from the need to detect and characterize laser induces defects in optical components to achieve fusion ignition in the laboratory. The purpose of this paper is to devise the expansion of laser generated plasmas into surrounding air at normal condition with pulses of comparable energy density in nanosecond (pulse duration =8 ns) that can provide high resolution images of material modification including defect populations and subsurface cracks. The plasma expansion and its effect of interaction in terms of damaged area and penetration depth of glass are investigated