Fundamentals of Plasma Physics Basic concepts

Transcription

1 Fundamentals of Plasma Physics Basic concepts APPLAuSE Instituto Superior Técnico Instituto de Plasmas e Fusão Nuclear Vasco Guerra

2 Since the dawn of Mankind men has tried to understand plasma physics...

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7 Plasmas: The As basic perguntas o questions início fundamentais What is plasma? How is plasma created? Why is plasma interesting? What is plasma good for?

8 O What que As is é perguntas plasma? um plasma? fundamentais Ionized gas Quasi-neutral Exhibits collective behavior The potential energy of a typical particle is much smaller than its kinetic energy

9 Para que serve um plasma? Plasmas in nature 99% of the (visible) matter in the universe! Stars Nebulae Solar wind Ionosphere van Allen belts Lightning Aurora

10 The origin of the word plasma The word was introduced in 1928 by Irving Langmuir Langmuir, Oscillations in ionized gases, Proc. Nat. Acad. Sci. U.S. (1928) From the Greek πλάσμα (to mould), since the brightness of Crookes discharge tubes moulds to the shape of the tube Analogy with blood plasma... we have observed some phenomena of remarkable beauty which may prove to be of theoretical interest. Langmuir, Science (1924) 60 p 392

11 Como se forma um plasma? How is plasma created?

12 Quatro estados da matéria... e não Four As states... perguntas or four fundamentais elements??? quatro elementos!

13 Para que serve um plasma? How to produce a plasma? Increase the temperature Use non equilibrium to locally increase ne -photoionisation -electric discharges

14 Why is plasma interesting? Light electrons, tend to have high temperatures, while the ions, heavier, tend to have low temperatures (non equilibrium!) Reactions that normally can occur only at high temperatures can take place at low temperatures! Plasma emits UV and visible radiation Plasmas are useful for nearly everything!

15 Para What As que is perguntas plasma serve um good fundamentais plasma? for? For nearly everything!

16 Para que serve um plasma? Plasma applications Thermonuclear fusion Plasma propulsion Gas lasers Micro and nanoelectronics Depollution Biology and medicine Processing of materials Particle acceleration

17 Nuclear fusion Lawson s criterion: nτ>10 20 m -3 s

18 Magnetic confinement fusion

19 Inertial fusion

20 Fundamentals Debye shielding Plasma frequency Plasma parameter Ionization degree

21 Debye shielding Test charge: positive ion, infinite mass Attracts the electrons and repels the other positive ions The electron density in the vicinity increases A negative charge cloud forms - shielding - which tends to cancel the test charge

22 Debye shielding Electrostatic potential (r) = 1 Q T 4 0 r Debye potential ( adiabatic response ) (r) = 1 Q T 4 0 r exp rd

23 Debye shielding D = 0kT n 0 e 2 1/2 Quasi-neutrality does not hold inside a Debye sphere

24 Debye shielding The phenomenon is known as Debye shielding and is the first exemple of a plasma collective behavior The hypothesis of the typical particle s potential energy being much smaller than its kinetic energy is used in the linearization of Poisson s equation

25 Debye shielding Debye s potential due to a charge in a plasma decays much faster than in vacuum For r λd the shielding cancels qt and φ(r) 0 qt does not have to be a special particle: each charged particle in the plasma tends to establish its own shielding cloud Plasma condition: λd L (typical dimension of the system) quasi-neutral

26 Debye shielding Some typical values

27 Plasma parameter Distance between neighbouring particles ~ n 1/3 0 Potencial energy due to the closest neighbour ~ 1 4 " 0 e 2 r 1 " 0 n 1/3 0 e 2 Typical kinetic energy ~ 1 2 m v2 = 3 2 kt e Plasma condition: = n 0 3 D 1 Plasma: number of electrons in a Debye cube >>1

28 Ionization degree Plasma can coexist with another state In the ionosphere there are regions where 99% of the gas is neutral and only 1% is ionized We have then a partially ionized plasma The plasma parameter Λ is calculated only with the ionized component (and we have Λ>>1 and λd L) Usually there is a continuous exchange of charge between the particles of the neutral gas and the ones of the ionized plasm Medium is characterized by the ionization degree, ne/(ne+n0)

29 Plasma frequency E Oscillatory motion of angular frequency ωpe=

30 Plasma frequency It is one more example of collective behavior We can define a ionic plasma frequency and a total plasma frequency, ωp 2= ωpi 2 +ωpe 2 To have a plasma, the collective behavior must dominate over the individual collisions Plasma condition: ωpe νc (electron-neutral collision frequency), or ωpeτ 1

31 How to study plasmas? Single particle motion Kinetic equations Fluid equations

32 The wonderful world of plasmas!

33 Voyager I

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