Plasma science and technology Basic concepts

Transcription

1 Plasma science and technology Basic concepts ATHENS 2015 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 basic As o perguntas início questions fundamentais What is plasma? How is plasma created? Why is plasma interesting? What is plasma good for?

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

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

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

11 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

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

13 What is plasma good for? 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!

14 Fundamentals Debye shielding Plasma frequency Plasma parameter Ionization degree

15 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

16 Debye shielding Electrostatic potential (r) = 1 Q T 4 0 r Debye potential (r) = Q T r exp rd

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

18 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

19 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

20 Debye shielding Some typical values

21 Plasma parameter Distance between particles ~ n 1/3 0 Potencial energy due to the closest neighbor ~ 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

22 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)

23 Plasma frequency E Oscillatory motion of angular frequency ωpe=

24 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

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

26 The wonderful world of plasmas!

27 Voyager I

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