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Laser cooling and fusion

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Laser cooling and fusion

  1. 1. 6/6/2019 1 LASER Cooling & LASER for Fusion
  2. 2. 6/6/2019 2
  3. 3. Contents  Introduction  LASER Cooling  Why LASER is used rather than the ordinary light?  Mechanism by which LASER cooling works (Doppler cooling)  LASER beams arrangement  Applications 6/6/2019 3
  4. 4. Introduction  About 50yrs ago, LASERs have been invented.  They are able to heat, vaporize, cut & weld materials.  When a beam of light shines on a body, it gets hotter (because it absorbs the light & transforms to heat).  LASERs can also used to cool atoms, so how could it be used for cooling?  An exciting thing that happened during the past 10yrs, Two Nobel prizes (in 1997 & 2001) in physics were related to the use of LASER for cooling. 6/6/2019 4
  5. 5.  LASER beams are very narrow, very bright, can be focused into a very tiny spots  It achieves extremely low temperatures (10-9 K)  In this type of application, we need light of certain frequency (wavelength) and also need a control over frequency of the light.  Choose the type & wavelength of LASER so that it suits for cooling atoms Why LASER is used rather than the ordinary light? Laser cooling • LASER cooling is a technique that uses light to cool down atoms to a very low temp ( near absolute zero) so that atom’s velocity get decreased. 6/6/2019 5
  6. 6. Mechanism by which laser cooling works • Common method is doppler cooling • Doppler effect is the change in frequency (and wavelength) of light (made of photons & momentum (zero mass)) • A stationary atom sees the LASER neither red- nor blue- shifted and does not absorb the photon • An atom moving away from the LASER sees it red- shifted and does not absorb the photon • An atom moving towards the LASER sees it blue-shifted and absorbs the photon, slowing the atom 6/6/2019 6
  7. 7. : • An atom is moving towards any of the LASER, that particular beam would appear at a slightly higher frequency (Doppler effect) photon frequency actual frequency • The LASER light is tuned to a frequency with an energy just below the energy of an electronic transition of an atom to excite the atoms to higher state. • The atoms will always scatter more photons from the LASER beam pointing opposite to their direction of motion. • This small increase in frequency makes the frequency of exactly equal to the needed one for atomic excitation • The photon thus absorbed and excites the atom, moving an electron to a higher quantum state6/6/2019 7
  8. 8. 6/6/2019 8 • The atoms recoils back due to momentum provided by the photons and re-emits a new photon in random direction coming back to its ground state • So many ‘absorption-emission’ cycles repeated and thus atoms slows down and thus kinetic energy decreases => temperature(random motion) decreases=> velocity too decreases. Therefore, slow atoms are cold atoms Low v=> Low Temp High v=> High Temp. • We can cool atoms to temperature that is lower than the Doppler temperature by using (a) Zeeman effect (Zeeman tuned cooling) (b) Dye laser beam (chirped cooling) (c) Evaporative cooling
  9. 9. Laser beam arrangement • Whatever we seen its in 1 Direction, so only 2 LASER beam needed. • We know Atoms are always moves in random motion & doesn’t move in a certain direction • Consider 3 Direction (X,Y,Z axis) where an atom is at centre so that we need 6 LASER beam • This way atoms moving in any direction may absorb photons from a LASER beam • This simplest arrangement of LASER cooling known by name ‘optical molasses” 6/6/2019 9
  10. 10. Applications  Construction of atomic clocks in satellites ,they are laser cooled very accurate otherwise our phone or GPS wont work  Atomic interferometers and atom lasers  In the development of instruments for atom optics and atomic lithography.  Used in quantum-mechanical behavior of atoms as matter waves  Observation of a Bose-Einstein condensation in a dilute atomic gas 6/6/2019 10
  11. 11. 6/6/2019 11
  12. 12. Contents  Introduction  Fusion Reaction  Requirements for fusion  LASER energy requirements  How can we compress pellet? 2 methods: Direct and Indirect drive LASER fusion  Advantages  LASER driven fusion system problem  Conclusion  References 6/6/2019 12
  13. 13. Introduction  The idea of fusion is not a new term for all of us.  Its similar to a fission.  In fission, we broke out the nuclear force & electric energy comes out.  In fusion, we do the opposite that we take the advantage of strong nuclear forces  So, its the interplay btw electric repulsion and nuclear force that holds things together.  Fusion is the reaction in which two nuclei of hydrogen combine to form a helium nuclei  Fusion process powers h2 bombs & the sun and other stars 6/6/2019 13
  14. 14. Fusion Reaction 6/6/2019 14 • Fuel  D-T • Both are heavy isotope of hydrogen 1 N, 1 P 2 N, 1 P Deuterium • Found naturally in water • Nonradioactive Tritium •Rare, • Made by bombarding Lithium (readily available in earth’s crust) + neutrons • Weakly radioactive as it emits very low energy radiation) After combining D & T, they lose a bit of their mass or the difference in mass is released as kinetic energy acc to E=mc^2; E= Energy m= Mass; c= Speed of light in vacuum (17.6MeV) E converted  heat (He) (alpha particle) . • Law of conservation of energy is conserved (as Masses of D + T >> Mass of He (heavier nuclei) More mass=> More gravitational pressure =>higher density, Temp=> faster reactions/ time => more energy produced. So, these reactions occur mostly in high density, high temp. environment
  15. 15. Requirements for fusion  Goal of fusion – to confine fusion ions at High Temperature (order 100millionºC = 10keV) & High Pressure for a long enough time to fuse 6/6/2019 15 Structure of an atom Binding forceElectrostatic force  Nuclear Force is always stronger than Electrostatic Repulsion  Charge independent, small distance ( cm), beyond this short range force, forces are of Coloumb type 
  16. 16. 6/6/2019 16 • Due to presence of 2 protons (of same +ve charge), there exist coloumb force.To overcome this repulsion, heat given,so nuclei moves faster enough and gets closer & touch each other => energy is released. So needed high temperatures. • For fusion to occur, 2 nuclei of hydrogen atoms (D-T) must come closer; otherwise fusion wont happenAt such high temp, atoms of hydrogen fuel are in a fully ionized state known as plasma • Plasma isn't common on earth & when used in fusion energy, its difficult to heat it in 100 millions ºC to compress & it cant even hold it using a solid container because it get melts
  17. 17. Laser energy requirements 6/6/2019 17  The temperature and pressure required for any particular fuel to fuse is given by Lawson criterion nτ>=1014 cm-3 where τ = confinement time (it measures the rate of energy loss of plasma) n= plasma density [for n=1015ions/cm3, τ must be ≥ 0.1s]  For laser induced fusion systems (instead of nτ), we use where F= Fractional burn up of fuel ʃ= Fuel density (g/ cm3) R= Fuel radius (cm) • 2 ways to fulfill ‘Lawson’s criteria’ • In magnetic fusion, ‘ʃ’ is limited by material property so burn efficiency increase by extending the duration of ‘τ’ •In Inertial fusion, Newton’s law & thermal velocity limit ‘τ’, so fuel compressed to higher densities i.e. increase ‘ʃ’ of fusion plasma
  18. 18. How do we try to do fusion?  There are many ways to do fusion but imp to confine these hot gases : 1. Gravitational confinement 6/6/2019 18 • Because of their large masses, the sun and other stars are able to hold hot gases together. • Energy of sun is due to thermonuclear reactions • Plasma has a temp of 10keV(-100MILLION K),its confinement is due to gravitational force
  19. 19. 6/6/2019 19 • The idea of fusion using laser pulses that involves compressing, heating & confining thermo nuclear material by inertial forces • LASER pulse interacts with thermonuclear material (that’s in form of solid pellet), for such confinement, its not necessary to have magnetic field . here, plasma confined by the inertia of its own mass instead of magnetic field— so the term Inertial Confinement Fusion (ICF) • Lawson criteria is satisfied under a condition while using magnetic confinement with a Russian device (tokmark - a donut shaped reactor that helps to keep plasma in place with magnets and heat it up). • Here plasma not been confined for such long time 2. Magnetic confinement 3. Inertial confinement
  20. 20. How can we compress pellet? •In this approach, LASER strikes directly the pellet (fuel target) •The rapid heating caused by the LASER makes the outer layer of the target explode. •In keeping with Isaac Newton’s Third Law (“For every action there is an equal and opposite reaction”), the remaining portion of the target is driven inwards causing compression of target=> shock wave formed => further heats the fuel in the very center => results in a self-sustaining burn. 6/6/2019 20 millimeter sized pellet having D-T mix LASER (of high- energy beams) Direct drive LASER fusion
  21. 21.  LASERs focused onto the inner walls of hologram & heating it up at 100million (100,000,000)˚C. to a superhot plasma  Around pellet , forms a plasma envelope that radiates mostly in X-rays  The X-rays (from this plasma) are then absorbed by the pellet’s surface, imploding it in the same way as if it had been hit with the lasers directly. (i.e. he heat & radiation converts pellet into plasma & compress it until fusion occurs  During the final part of the implosion, the fuel core reaches 20 times the density of lead and ignites at 100million ˚C.  Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy so, If heat output > heat input, the reaction is self sustaining 6/6/2019 21 Indirect drive laser fusion Pellet hologram = burning chambers, a cavity whose walls are radiative LASER (of high- energy beams)
  22. 22. Advantages  Abundant fuel supply (fuel used D,T)  Safe (no uncontrolled energy release unlike fission=> less radiation than the natural background radiation we live within our daily lives)  Clean (as ash = helium(by product-small, its an inert gas=> no air pollution )  Less nuclear waste (Unlike fission, what goes is Hydrogen and some lithium n what comes out is helium (that one filled in baloons) &some neutrons  Ensures the use of high-quality raw materials, product reliability and functionality in a fully controlled environment. if some this goes wrong in reactor, the fusion process stops 6/6/2019 22
  23. 23. LASER driven fusion system problem  High capital cost reactors necessary  Building of high lasers to a stage when output energy from s/s >> input energy  Technically complex as Design of complex targets & reliable production of such targets with extremely good surface finish (if there any surface irregularities > 1% of the thickness of wall => very unstable compression of thick shells) 6/6/2019 23
  24. 24. Conclusion  What ever we see are some applications of laser  Lasers not only used for cutting, welding or heating but we seen how a set of intersecting lasers can be used to cool atoms  Still research going on ICF for our future use.. 6/6/2019 24
  25. 25. Reference  Thyagarajan. K. & Ghatak A K Lasers, Theory and Applications Macmillan, 1991 6/6/2019 25
  26. 26. THANK YOU!!!! 6/6/2019 26

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