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