Swedish Institute of Space Physics, IRF

Plasma Physics for beginners

There are a lot of parameters defining a plasma and here are the two basic ones.
Two different kinds of simulations of the same one dimensional non magnetized plasmacan be made .

Plasmafrequency


By setting at time zero an amplitude for a space dependent impulse on all the electrons a plasma- oscillation will start. The dependence in space is sinusoidal with one full period along the entire plasma.
With some buttons one can change the chargedensity and the temperature for the plasma. With help of the stopWatch one can measure the plasmaperiod, which should be found to be inversly proportional to the squareroot of the chargedensity.
Simulation demonstration of plasma frequency

The plasma frequency parameter will in the following be a parameter which define the strength of the "induced" electric current by and this currents modification of the magnetic field in the radiowaves. It will mostly be electric currents and there associated magnetic fields which will oscillate and be coupled to the radiowaves.

Debyelength - statistical("thermodynamic") length parameter


The debyelength is a length related to screening of the electric field from a charge particle in a plasma.
The used simulation plasma is a time reversible physical system and as such it has no relaxation mechanism to thermodynamic equlibrium. When putting a testcharge in the middle of such a plasma it will responde in a time reversible way; the plasma will start to oscillate. But if one recursively do thermalization by "hand" it is possible to relax the plasma to thermodynamical equlibrium and experience the screening.
With the stopWatch one can study the relations between the thermal velocity, the debyelength and the plasmafrequency, which can be taken from the simulation done above.
It will be found that the debye length is close to the length a thermal particle moves during one fifth of a plasmaperiod.
Simulation demonstration of Debye length
The debye lenght will not be used in the following. It can normally be taken as zero in the ionosphere when radiowaves are studied.

An extensive parameter for a Plasma



Above only intensive parameters, parameters related to the medium, were discussed. Here is an example with an extensive parameter, a parameter related to the whole system. The plasma in this example consists of two beams, one moving to the right and the other to the left. The parameter is simply the difference between the velocities of the beams. An instability exists in this plasma and it is "local", absolute, which means that a local disturbance will develope locally.
Simulation demonstration of Two beams Instabillity

Mars 2002 christer juren