What is plasma? Plasma is quasi-neutral ionized gas containing enough free charges to make collective electromagnetic effects important for its physical behaviour. –Ionization •0.1% clear plasma properties • 1% almost perfect conductivity temperature (eV) Degree of ionization from Saha’s Eq. 1 eV = 11,600 K ni nn 3/ 2 T 3 1027 e ni U /T Ionization energy Fourth state of matter: solid Enthalpy liquid gas plasma Plasma production: 1) heating 2) ionizing radiation Glowing plasma inside a tokamak Dayside ionosphere (solar UV) Collisional ionization auroral oval (precipitating ions and electrons) ~99% of the (baryonic) matter in the universe is in plasma state Orion Nebulae “ Plasma state: William Crookes, 1879 Nature of plasma: J.J. Thomson, 1897 Space is filled with plasma, 1913: Kristian Brikeland Term plasma, first experiments: Iriving Langmuir, Levy Tonks1927 • radio broadcasting ionosphere EM wave propagion • magnetohydrodynamics - magnetic reconnection - dynamo • Hydrogen bomb • space age fusion Van Allen Belts • astrophysical plasmas • Laser plasma physics plasma trapping/instabilities space plasma physics VASIMIR In-situ plasma studies • • • • Solar wind Magnetosphere Ionosphere Laboratóries What happens if plasma equilibrium is perturbed? Set-up: n0 immobile ions & n0 cold (T=0) electrons perturbation in electron density plasma frequency plasma starts to oscillate with 1.1 electrons displaced electric field attracts electrons and ions back together (restoring force) simple harmonic motion of electrons most fundamental frequency in plasma physics Note: also often called the plasma frequency Electric field affects also ions, but their oscillation much slower ( m-1/2 ) Finite electron temperature wave propagates (Langmuir wave) How does plasma respond to an extra charge? ions: repelled from qT electrons: attracted towards qT neutralizing ‘cloud’ potential reduced qT + Debye length: Shielding potential: Collective behaviour many particles in ‘Debye’s sphere’. Plasma parameter: Quasineutrality: (L = size of the system) How spacecraft affect surrounding plasma Electric sail. Debye length ~ 20 m http://www.electric-sailing.fi/ charge-neutral charge-charge (Coulumb) weakly ionized plasmas most space plasmas binary many particles short range long range (infinite) collision cross-section: (m2) known v=average speed electron-ion collisions: Sufficiently large ”collisionless plasma” typical magnetic field values Gyromotion in the magnetic field solar wind 10-9 T Earth surface 10-5 T sunspot 0.1 T pulsar 108 T GAS electrical conductivity Independently acting spcies velocity distribution interactions PLASMA low usually very high one several (different charge) Maxwellian (collisions) binary often Non-Maxwellian (external forcing modifies) collective long-range Useful to remember Plasma frequency (angular frequency) Debye length Plasma definition: 1. 2. D l D ne 1/ 3 Gyromotion in the magnetic field