alfven_current = 4 * pi * eps0 * m_e * c**3 / e = IA = I_A
classical_electron_radius = 1/(4 * pi * eps0) * e**2 / (m_e * c**2) = r_e
proton_mass = 1.67262192e-27 * kg = m_p
[number_density] = 1 / [length]**3
[emittance] = [length] * [angle]
[electric_field] = [electric_potential] / [length]
[charge_density] = [charge] / [length]**3
[linear_charge_density] = [charge] / [length]
[surface_charge_density] = [charge] / [length]**2