thermal_speed

plasmapy.formulary.parameters.thermal_speed(T: Unit("K"), particle: plasmapy.particles.particle_class.Particle, method='most_probable', mass: Unit("kg") = <Quantity nan kg>, ndim=3) -> Unit("m / s")

Return the most probable speed for a particle within a Maxwellian distribution.

Aliases: vth_

Parameters

  • T (Quantity) – The particle temperature in either kelvin or energy per particle

  • particle (Particle) – Representation of the particle species (e.g., 'p' for protons, 'D+' for deuterium, or 'He-4 +1' for singly ionized helium-4). If no charge state information is provided, then the particles are assumed to be singly charged.

  • method (str, optional) – Method to be used for calculating the thermal speed. Options are 'most_probable' (default), 'rms', and 'mean_magnitude'.

  • mass (Quantity) – The particle’s mass override. Defaults to nan and if so, doesn’t do anything, but if set, overrides mass acquired from particle. Useful with relative velocities of particles.

  • ndim (int) – Dimensionality of space in which to calculate thermal velocity. Valid values are 1, 2, or 3.

Returns

V – The particle’s thermal speed.

Return type

Quantity

Raises

  • TypeError – The particle temperature is not a Quantity.

  • UnitConversionError – If the particle temperature is not in units of temperature or energy per particle.

  • ValueError – The particle temperature is invalid or particle cannot be used to identify an isotope or particle.

Warns

  • RelativityWarning – If the ion sound speed exceeds 5% of the speed of light.

  • UnitsWarning – If units are not provided, SI units are assumed.

Notes

The particle thermal speed is given by:

\[V_{th,i} = \sqrt{\frac{N k_B T_i}{m_i}}\]

where the value of \(N\) depends on the dimensionality and the definition of \(v_{th}\): most probable, root-mean-square (RMS), or mean magnitude. The value of \(N\) in each case is

Values of constant \(N\)

Dim.

Most-Probable

RMS

Mean-Magnitude

1D

0

1

\(2/π\)

2D

1

2

\(π/2\)

3D

2

3

\(8/π\)

The definition of thermal velocity varies by the square root of two depending on whether or not this velocity absorbs that factor in the expression for a Maxwellian distribution. In particular, the expression given in the NRL Plasma Formulary [1] is a square root of two smaller than the result from this function.

Examples

>>> from astropy import units as u
>>> thermal_speed(5*u.eV, 'p')
<Quantity 30949.6... m / s>
>>> thermal_speed(1e6*u.K, particle='p')
<Quantity 128486... m / s>
>>> thermal_speed(5*u.eV, particle='e-')
<Quantity 132620... m / s>
>>> thermal_speed(1e6*u.K, particle='e-')
<Quantity 550569... m / s>
>>> thermal_speed(1e6*u.K, "e-", method="rms")
<Quantity 674307... m / s>
>>> thermal_speed(1e6*u.K, "e-", method="mean_magnitude")
<Quantity 621251... m / s>