Plasma parameters

Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds "collectively" to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or charged ion beams, but may also include dust and grains. The behaviour of such particle systems can be studied statistically.
Fundamental plasma parameters.
All quantities are in Gaussian (cgs) units except energy and temperature expressed in eV and ion mass expressed in units of the proton mass formula_1; formula_2 is charge state; formula_3 is Boltzmann's constant; formula_4 is wavenumber; formula_5 is the Coulomb logarithm.
Frequencies.
\ /\mbox{s}</math>
Lengths.
 \approx 6.919 \times 10^{-8}\,\frac{1}\ \mbox{cm}</math>
where formula_14 is an average velocity of the electron (ion) and formula_15 is the electron or ion collision rate.
Velocities.
 \approx 2.18 \times 10^{11}\,\frac{B}{\left(\mu n_i\right)^\frac{1}{2}}\ \mbox{cm/s}</math> in cgs units,
Collisionality.
In the study of tokamaks, collisionality is a dimensionless parameter which expresses the ratio of the electron-ion collision frequency to the banana orbit frequency.
The plasma collisionality formula_23 is defined as
where formula_25 denotes the electron-ion collision frequency, formula_26 is the major radius of the plasma, formula_27 is the inverse aspect-ratio, and formula_28 is the safety factor. The plasma parameters formula_29 and formula_30 denote, respectively, the mass and temperature of the ions, and formula_31 is the Boltzmann constant.
Electron temperature.
Temperature is a statistical quantity whose formal definition is 
or the change in internal energy with respect to entropy, holding volume and particle number constant. A practical definition comes from the fact that the atoms, molecules, or whatever particles in a system have an average kinetic energy. The average means to average over the kinetic energy of all the particles in a system.
If the velocities of a group of electrons, e.g., in a plasma, follow a Maxwell–Boltzmann distribution, then the electron temperature is defined as the temperature of that distribution. For other distributions, not assumed to be in equilibrium or have a temperature, two-thirds of the average energy is often referred to as the temperature, since for a Maxwell–Boltzmann distribution with three degrees of freedom, formula_33.
The SI unit of temperature is the kelvin (K), but using the above relation the electron temperature is often expressed in terms of the energy unit electronvolt (eV). Each kelvin (1 K) corresponds to 8.617 333 262...×10−5 eV; this factor is the ratio of the Boltzmann constant to the elementary charge. Each eV is equivalent to 11,605 kelvins, which can be calculated by the relation formula_34.
The electron temperature of a plasma can be several orders of magnitude higher than the temperature of the neutral species or of the ions. This is a result of two facts. Firstly, many plasma sources heat the electrons more strongly than the ions. Secondly, atoms and ions are much heavier than electrons, and energy transfer in a two-body collision is much more efficient if the masses are similar. Therefore, equilibration of the temperature happens very slowly, and is not achieved during the time range of the observation.