plasma_frequency

plasmapy.physics.parameters.plasma_frequency(n, particle='e-', z_mean=None)

Calculate the particle plasma frequency.

Parameters:
  • n (Quantity) – Particle number density in units convertible to per cubic meter
  • particle (str, optional) – Representation of the particle species (e.g., ‘p’ for protons, ‘D+’ for deuterium, or ‘He-4 +1’ for singly ionized helium-4), which defaults to electrons. If no charge state information is provided, then the particles are assumed to be singly charged.
  • z_mean (Quantity, optional) – The average ionization (arithmetic mean) for a plasma where the a macroscopic description is valid. If this quantity is not given then the atomic charge state (int) of the ion is used. This is effectively an average plasma frequency for the plasma where multiple charge states are present.
Returns:

omega_p – The particle plasma frequency in radians per second.
Return type:

Quantity
Raises:
  • TypeError – If n_i is not a Quantity or particle is not of an appropriate type.
  • UnitConversionError – If n_i is not in correct units
  • ValueError – If n_i contains invalid values or particle cannot be used to identify an particle or isotope.
Warns:

~astropy.units.UnitsWarning – If units are not provided, SI units are assumed

Notes

The particle plasma frequency is

\[\omega_{pi} = Z e \sqrt{\frac{n_i}{\epsilon_0 m_i}}\]

At present, astropy.units does not allow direct conversions from radians/second for angular frequency to 1/second or Hz for frequency. The dimensionless_angles equivalency allows that conversion, but does not account for the factor of 2*pi. The alternatives are to convert to cycle/second or to do the conversion manually, as shown in the examples.

Example

>>> from astropy import units as u
>>> plasma_frequency(1e19*u.m**-3, particle='p')
<Quantity 4.16329453e+09 rad / s>
>>> plasma_frequency(1e19*u.m**-3, particle='D+')
<Quantity 2.94462452e+09 rad / s>
>>> plasma_frequency(1e19*u.m**-3)
<Quantity 1.78398636e+11 rad / s>