Plasma parameters explained

Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged and neutral particles of various species (electrons and ions) that responds collectively to electromagnetic forces.^[1] Such particle systems can be studied statistically, i.e., their behaviour can be described based on a limited number of global parameters instead of tracking each particle separately.^[2]

Fundamental

The fundamental plasma parameters in a steady state are

n _{s} of each particle species s present in the plasma,T _{s} of each species,m _{s} of each species,q _{s} of each species,B .Using these parameters and physical constants, other plasma parameters can be derived.^[3]

Other

All quantities are in Gaussian (cgs) units except energy and temperature which are in electronvolts. For the sake of simplicity, a single ionic species is assumed. The ion mass is expressed in units of the proton mass,

\mu = m _{i/m} _{p} and the ion charge in units of the elementary charge e , Z = q _{i/e} (in the case of a fully ionized atom, Z equals to the respective atomic number). The other physical quantities used are the Boltzmann constant (k ), speed of light (c ), and the Coulomb logarithm (lnΛ ).

Dimensionless

• number of particles in a Debye sphere $$\backslash left(\backslash frac\backslash right)n\backslash lambda\_D^3\; \backslash approx\; 1.72\; \backslash times\; 10^9\backslash ,\backslash left(\backslash frac\backslash right)^\backslash frac$$
• Alfvén speed to speed of light ratio $$\backslash frac\; \backslash approx\; 7.28\backslash ,\backslash frac$$
• electron plasma frequency to gyrofrequency ratio $$\backslash frac\; \backslash approx\; 3.21\; \backslash times\; 10^\backslash ,\backslash frac$$
• ion plasma frequency to gyrofrequency ratio $$\backslash frac\; \backslash approx\; 0.137\backslash ,\backslash frac$$
• thermal pressure to magnetic pressure ratio, or beta, β $$\backslash beta\; =\; \backslash frac\; \backslash approx\; 4.03\; \backslash times\; 10^\backslash ,\backslash frac$$
• magnetic field energy to ion rest energy ratio $$\backslash frac\; \backslash approx\; 26.5\backslash ,\backslash frac$$

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

\nu ^{*} is defined as^{[4] [5]} $$\backslash nu^*\; =\; \backslash nu\_\backslash mathrm\backslash ,\backslash sqrt\backslash ,\backslash frac\; \backslash ,\; qR,$$where \nu _{ei} denotes the electron-ion collision frequency, R is the major radius of the plasma, \epsilon is the inverse aspect-ratio, and q is the safety factor. The plasma parameters m _{i} and T _{i} denote, respectively, the mass and temperature of the ions, and k _{B} is the Boltzmann constant.

Electron temperature

Temperature is a statistical quantity whose formal definition is $$T\; =\; \backslash left(\backslash frac\backslash right)\_,$$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, $\backslash langle\; E\; \backslash rangle\; =\; \backslash frac\; 3\; 2\; \backslash ,\; k\_\backslash text\; T$.

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 ; this factor is the ratio of the Boltzmann constant to the elementary charge.^[6] Each eV is equivalent to 11,605 kelvins, which can be calculated by the relation

\langle E \rangle = k _{B} T .

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.

See also

• Ball-pen probe
• Langmuir probe

References

• NRL Plasma Formulary – Naval Research Laboratory (2018)

Notes and References

1. Peratt, Anthony, Physics of the Plasma Universe (1992);
2. Parks, George K., Physics of Space Plasmas (2004, 2nd Ed.)
3. Book: Bellan . Paul Murray . Fundamentals of plasma physics . 2006 . Cambridge University Press . Cambridge . 0521528003.
4. Nucl. Fusion, Vol. 39, No. 12 (1999)
5. Wenzel, K and Sigmar, D.. Nucl. Fusion 30, 1117 (1990)
6. Web site: CODATA Energy conversion factor: Factor x for relating K to eV . Mohr . Peter J. . Newell . David B. . Taylor . Barry N. . Tiesenga . E. . 20 May 2019 . The NIST Reference on Constants, Units, and Uncertainty . National Institute of Standards and Technology . 11 November 2019 .