aula 1 09 - ulisboa 1 09.pdfthe origin of the word “plasma”as&perguntas&fundamentais...
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Plasma science and technology Basic concepts
ATHENS 2015Instituto Superior Técnico
Instituto de Plasmas e Fusão Nuclear
Vasco Guerra
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Since the dawn of Mankind men has tried to understand plasma
physics...
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As perguntas fundamentais
•What is plasma?
•How is plasma created?
•Why is plasma interesting?
•What is plasma good for?
Plasmas: o inícioThe basic questions
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As perguntas fundamentaisComo se forma um plasma?How is plasma created?
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As perguntas fundamentaisQuatro estados da matéria... e não quatro elementos!Four states... or four elements???
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As perguntas fundamentaisO que é um plasma?
•Ionized gas
•Quasi-neutral
•Exhibits collective behavior
The potential energy of a typical particle is much smaller than its kinetic energy
What is plasma?
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As perguntas fundamentaisThe 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) 14 628
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
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As perguntas fundamentais
For nearly everything!
Para que serve um plasma?What is plasma good for?
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As perguntas fundamentaisWhat 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!
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As perguntas fundamentaisFundamentals
•Debye shielding
•Plasma frequency
•Plasma parameter
•Ionization degree
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As perguntas fundamentaisDebye 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
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⇥(r) =1
4�⇤0
QT
r
As perguntas fundamentaisDebye shielding
•Electrostatic potential
⇤(r) =1
4⇥⌅0
QT
rexp
�� r
�D
⇥•Debye potential
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�D =�
⇥0kT
n0e2
⇥1/2
As perguntas fundamentaisDebye shielding
Quasi-neutrality does not hold inside a Debye sphere
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As perguntas fundamentaisDebye 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
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As perguntas fundamentaisDebye 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”
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As perguntas fundamentaisDebye shielding
Some typical values
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As perguntas fundamentaisPlasma parameter
•Distance between particles ~ n�1/30
•Potencial energy due to the closest neighbor ~
•Typical kinetic energy ~12m�v2⇥ =
32kTe
Plasma: number of electrons in a Debye cube >>1
� = n0�3D � 1➡ Plasma condition:
⇠ 1
4⇡"0
e2
r⇠ 1
"0n1/30 e2
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As perguntas fundamentaisIonization 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)
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As perguntas fundamentaisPlasma frequency
E
Oscillatory motion of angular frequency ωpe=
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As perguntas fundamentaisPlasma frequency
•It is one more example of collective behavior
•We can define a “ionic plasma frequency” and a total plasma frequency,
ωp2=ωpi2+ωpe2
•To have a plasma, the collective behavior must dominate over the individual collisions
➡Plasma condition: ωpe ≫ νc (electron-neutral collision frequency), or ωpeτ ≫ 1
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As perguntas fundamentaisHow to study plasmas?
•Single particle motion
•Kinetic equations
•Fluid equations
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As perguntas fundamentaisThe wonderful world of plasmas!
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As perguntas fundamentaisVoyager I
http://www.jpl.nasa.gov/interstellarvoyager/
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