An important concept in plasma physics is the Debye length, which describes the screening of a charge's electrostatic potential due to the net effect of the interactions it undergoes with the other mobile charges (electrons and ions) in the system. It can be shown that, given a set of reasonable assumptions about the behaviour of charges in the plasma, the electric potential due to a "test charge", $q_\mathrm{T}$ is given by
$$
\phi = \frac{q_\mathrm{T}}{4\pi\epsilon_0 r}\exp\left(-\frac{r}{\lambda_\mathrm{D}}\right),
$$
where the electron Debye length,
$$
\lambda_\mathrm{D} = \sqrt{\frac{\epsilon_0 T_e}{e^2n_0}},
$$
for an electron temperature $T_e$ expressed as an energy (i.e. $T_e = k_\mathrm{B}T_e'$ where $T_e'$ is in K) and number density $n_0$. Rigorous derivations, starting from Gauss' Law and solving the resulting Poisson equation with a Green's function are given elsewhere (e.g. Section 7.2.2. in J. P. Freidberg, *Plasma Physics and Fusion Energy*, CUP (2008)).

The following Python code plots the shielded and unshielded Coulomb potential due to a point test charge $q_\mathrm{T} = +e$, assuming an electron temperature and density typical of a tokamak magnetic confinement nuclear fusion device.

```
import numpy as np
from scipy.constants import k as kB, epsilon_0, e
from matplotlib import rc
import matplotlib.pyplot as plt
rc('font', **{'family': 'serif', 'serif': ['Computer Modern'], 'size': 16})
rc('text', usetex=True)
# We need the following so that the legend labels are vertically centred on
# their indicator lines.
rc('text.latex', preview=True)
def calc_debye_length(Te, n0):
"""Return the Debye length for a plasma characterised by Te, n0.
The electron temperature Te should be given in eV and density, n0
in cm-3. The debye length is returned in m.
"""
return np.sqrt(epsilon_0 * Te / e / n0 / 1.e-6)
def calc_unscreened_potential(r, qT):
return qT * e / 4 / np.pi / epsilon_0 / r
def calc_e_potential(r, lam_De, qT):
return calc_unscreened_potential(r, qT) * np.exp(-r / lam_De)
# plasma electron temperature (eV) and density (cm-3) for a typical tokamak.
Te, n0 = 1.e8 * kB / e, 1.e26
lam_De = calc_debye_length(Te, n0)
print(lam_De)
# range of distances to plot phi for, in m.
rmin = lam_De / 10
rmax = lam_De * 5
r = np.linspace(rmin, rmax, 100)
qT = 1
phi_unscreened = calc_unscreened_potential(r, qT)
phi = calc_e_potential(r, lam_De, qT)
# Plot the figure. Apologies for the ugly and repetitive unit conversions from
# m to µm and from V to mV.
fig, ax = plt.subplots()
ax.plot(r*1.e6, phi_unscreened * 1000,
label=r'Unscreened: $\phi = \frac{e}{4\pi\epsilon_0 r}$')
ax.plot(r*1.e6, phi * 1000,
label=r'Screened: $\phi = \frac{e}{4\pi\epsilon_0 r}'
r'e^{-r/\lambda_\mathrm{D}}$')
ax.axvline(lam_De*1.e6, ls='--', c='k')
ax.annotate(xy=(lam_De*1.1*1.e6, max(phi_unscreened)/2 * 1000),
s=r'$\lambda_\mathrm{D} = %.1f \mathrm{\mu m}$' % (lam_De*1.e6))
ax.legend()
ax.set_xlabel(r'$r/\mathrm{\mu m}$')
ax.set_ylabel(r'$\phi/\mathrm{mV}$')
plt.savefig('debye_length.png')
plt.show()
```

## Comments

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## Dominik Stańczak 4 years, 9 months ago

If you'd like to avoid doing the annoying unit conversions, Python has a bunch of packages like astropy.units (which I ten to use, http://docs.astropy.org/en/stable/units/ ) or unyt (which I've heard good things about) :)

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