Introductionsas2.elte.hu/flf/szemin/A_plazma/Helsinki_lectures/Introduction.pdf · Plasma and radio waves • radio wave propagation in and through the Earth’s plasma environment
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Introduction
• Plasma state• On the history of plasma physics• What is plasma physics• Basic plasma concepts
Plasma state
Plasma is quasi-neutral ionized gas containing enough free charges to make collective electromagnetic effectsimportant for its physical behaviour.
– ionization• 0.1% clear plasma properties• 1% almost perfect conductivity
– fourth state of matter: solid liquid gas plasma• gradual, no phase transition
– production: heating, ionizing radiation, collisional ionization, electric discharges
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Some 99.9…% of baryonic matter in theUniverse is in plasma state
Dynamical natural plasmas near us
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and a little further away
Some history• ”plasma”: studies of Tonks and Langmuir on gas discharges in 1929
– Crookes (1879): the fourth state of matter• ”aurora borealis”: Galileo (1619)
– Celsius and Hjorter (1790): auroras disturbthe magnetic needle
– Gauss (1832): invention of magnetometer
?Celsius
Gauss
Galileo
Langmuir
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100 years ago• Birkeland’s Terella 1896• Birkeland and Størmer early 20th
century– auroral light comes from altitudes
100 – 500 km
• Solar physics early 20th century – Hale (1908): magnetic field on
sunspots– Kelvin: known solar energy sufficient
for 20 million years only!
Birkeland and his Terella experiment
Modernmagnetogramof the Sun
White: B out ofthe surface
Black: B into the surface
1920s – 1930s
• 1920s: explanation of radio wave propagation via the ionosphere
– start of fluid description of plasma MHD
• Chapman and Ferraro 1932 –1933:
– first theories of magnetic storms in terms of plasma clouds from the Sun
• Atkinson and Houtermans (1929)– energy from fusion
• Bethe and others (1938) – explanation of fusion reactions
Sydney Chapman lecturing on space plasma physics
Hans Bethe explainingthe energy productionin stars.
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Hannes Alfvén (1908 – 1995): Father of plasma physics
• Developed ”cosmic electrodynamics” from the 1940s
– MHD waves 1942 (known as Alfvén waves)
– guiding centre approximation– magnetic field aligned currents
(i.e., J || B)proposed by Birkeland in 1913
– critical ionization velocity– etc., etc.– Nobel prize in physics,1970.
Plasma and radio waves• radio wave propagation in and through
the Earth’s plasma environment– useful: VLF communications,
plasma diagnostics– problems: perturb HF communications
and GPS signals• radioastronomy (Jansky in 1930s):
– bremsstrahlung and cyclotron/synchrotron emissions of charged particles
Dish and dome of theMetsähovi Radio Observatoryin Kirkkonummi
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Solar wind
• Biermann 1951: – details of cometary tails cannot be explained by radiation pressure only
there must be a continuous solar plasma outflow• Alfvén 1957:
– solar wind must be magnetized• Parker 1958:
– theory of plasma escape from the Sun• Formation of the magnetospheres
Eugene Parker
radiation pressure tail
solar windinduced tail
Classic Parker solutionsvSW = v(r)
Controlled thermonuclear fusion
• In the Sun:
• In fusion devicesd + t (He++ ; 3.5 MeV) + n (14.1 MeV)requires:
density of 1020 m–3
a few times 108 K temperature (a fewtens of keV)confinement time of a few seconds; in this time a 10-keV electron mustmove about half a million km withouthitting the walls of the device!
• The next big step: ITER is being built in Cadarache (near Marseille)
– http://www.iter.org/
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Plasma applications
Pulsed plasma arcdischarge techniquefor diamond coating
Knee and hip implants withdiamond surface
Plasma assisted etching
What kind of physics is involved
Electrodynamics Statistical physics
Radiation
Single particlemotion
Basic plasma equations
Vlasov
MHD
Wave motion
Stability
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Plasma oscillation
( u0 = 0 electrons are assumed cold )0
0
Assume: n0 fixed ions (+) & n0 moving electrons (–)
Apply a small electric field E1
electrons move:
Electron continuity equation:
Linearized continuity equation (1st order terms only): !!Force:
1st Maxwell:
plasma frequency
0 2nd order
Debye screening+
++
++ +
++
Coulomb potential of each charge:
Assume thermal equilibrium (Boltzmann distribution)
labels the particle populations (e.g., e, p)
Home exercise: ;
Debye length: Plasma parameter:
Number of particles in a Debye sphere:
”Definition of plasma”
L is the sizeof the system
Introduce a test charge qT. What will be its potential?
qT
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Collisions
Cross section: (m2) Mean free path:Collision frequency:
Weakly ionized plasmas• charge neutral
Fully ionized plasmas• Coulomb collisions• small-angle collisions dominate, i.e., long-range force (exerc)
Coulomb logarithme.g.
If T large and/or n small, then large ; 1/ small
Plasma becomes ”collisionless” and the effects of Coulomb collisions areare included through average electromagnetic fields E and B
+
+
Useful to remember
Plasma frequency (angular frequency)
Debye length Note the units !(1 eV 1.16 104 K)
Gyromotion in the magnetic field
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