Plasma Physics and Numerical Simulations

Plasma is a partially or fully ionized gas. Ionized gas contains free electrons and ions which carry electric charges. Thus, plasma is subject to electromagnetic forces and can react to external electric and magnetic fields. Moreover, since plasma contains free charge carriers, their relative motion can also set internal electric and magnetic fields, which in turn also influence their dynamics. In addition, plasma is subject to other forces typical for gases, such as gravity or pressure gradient. Thus, the plasma dynamics is complex and gives rise to a large variety of dynamic phenomena. The gas gets ionized when the temperature is high enough to allow for ionization. Furthermore, to consider the plasma particle to be a free charge carrier, its interaction energy with other plasma particles should be smaller than its thermal energy. Thus most of plasmas are very hot, with thermal energies above electronvolts, to provide ionizaton and prevent recombination 1 . An important property of plasma is quasineutrality. Quasineutrality means that at large scales there is the same number of negative and positive charges and the net charge is zero, and that small-scale deviations from neutrality will be quickly restored by plasma. The plasma particles (i.e., electrons and positive and negative ions) are mobile, and their relative motion gives rise to charge separation setting up electric fields which can set other plasma particles into motion. Even though there can be a local charge imbalance resulting in, for instance, plasma waves and oscillations, at large scales the system remains neutral. Quasineutrality is related to another basic property of plasma which is the Debye shielding. The electric charge of individual plasma particle is collectively screened by other plasma particles. A positive ion will attract electrons as well as negative ions which will form a ”cloud” around it, so that the electric field due to the positive ion is reduced as compared to the vacuum. Similarly, the negative charges will be shielded by positive ions (and will repel other electrons). This collective behavior is one of the fundamental properties of plasma. The characteristic screening length is called the Debye length and is given for electrons by: λDe = √ 0kTe/en0, where 0 is the permittivity of vacuum, k Boltzmann constant, Te electron temperature, e electron charge, n0 plasma density. The Debye length refers