# What Debye length should be used in plasma?

### In short

Which expression should be used: (1) or (2)?

### At length

Definition of the Debye length $$r_D$$ is following:

$$r_D=\left( \sum_{j}\frac{4\pi q_j^2 n_j}{kT_j} \right)^{-\frac{1}{2}}$$

It is calculated from the Poisson's equation in electrostatics with help of the Boltzmann distribution for every $$j$$-th charge species.

Let's consider two component plasma, which is consisted of electrons with charge $$e$$ and ions with charge $$Ze$$. Then the Debye screening radius looks like so:

$$r_D = \left[ \frac{4\pi e^2}{k} \left( \frac{n_e}{T_e} + \frac{Z^2 n_i}{T_i} \right) \right]^{-\frac{1}{2}} = \left[ \frac{k T_e}{4\pi e^2 n_e \left( 1 + Z \frac{T_e}{T_i} \right)} \right]^{\frac{1}{2}}$$

In experiments we often have that electrons temeprature $$T_e$$ is significantly larger than ions temperature $$T_i$$. It means that the Debye distance is determined only by ions:

$$r_D = \left( \frac{k T_i}{4\pi e^2 n_e Z} \right)^{\frac{1}{2}} = \left( \frac{k T_i}{4\pi (Ze)^2 n_i} \right)^{\frac{1}{2}} \tag{1}$$

But nevertheless people use another form in their theoretical and experimental investigations:

$$r_D = \left( \frac{k T_e}{4\pi e^2 n_e} \right)^{\frac{1}{2}} \tag{2}$$

despite $$T_e \gg T_i$$. And that means that electrons play crucial role in screening, and ions don't. I understand this fact only qualitative: electrons are lighter and more mobile than ions, and only they can quickly compensate any charge excess. But I don't understand how to prove that using (2) is accurate and using (1) is not.

Wikipedia says the following:

Even in quasineutral cold plasma, where ion contribution virtually seems to be larger due to lower ion temperature, the ion term is actually often dropped, although this is only valid when the mobility of ions is negligible compared to the process's timescale.

and gives the link on the Hutchinson's book. But I can't find there any proof of the statement.

• Are you looking for something like the following: ui.adsabs.harvard.edu/abs/2019PhPl...26e0701L/abstract Commented Jun 11, 2019 at 15:25
• As an aside, the general rule is that the mobility of electrons are so much higher than ions that there is no such thing as an ion Debye length. It really should be thought of in terms of thermal speeds and plasma frequencies of each given species. However, as you can see in the link above there are caveats and nuances that require the inclusion of ion terms. Commented Jun 11, 2019 at 15:32
• @honeste_vivere, thank you for the article! Could you please also indicate what page or section I should look at? It's quite difficult to understand the whole paper at the first time. And it seems that the author uses the other concentration distribution, but the mentioned problem appears with using the Boltzmann distribution. Or is this fact the key to the solution? Commented Jun 11, 2019 at 17:46
• There will almost never be a space plasma measured with in situ spacecraft that observes a Maxwellian velocity distribution. All relevant processes actually drive them away from isotropic Maxwellians. The inclusion of a kappa distribution is to illustrate that it alters the nominal form of the Debye length. If you are new to the field or a student, do not feel bad if a refereed article seems opaque and difficult at first. It takes a few times through to really get the point sometimes. I still re-read papers as I often find things I missed the first few times through... Commented Jun 11, 2019 at 18:29
• @honeste_vivere, I went through the article and the book (from the first link in the article) and didn't find an answer. Using kappa distribution just slightly changes the Debye length: $\lambda^{kappa}_D = \lambda^{Boltzmann}_D\sqrt{\frac{\kappa_0}{\kappa_0+1}}$. It doesn't change the Poisson's equation form: $\Delta \Phi=\lambda_D^{-2}\Phi$. So the summation of inverse temperatures doesn't disappear. That means the Debye length is still determined by ions: $\lambda^{kappa}_D = \sqrt{\frac{kT_i}{4\pi(Ze)^2n_i}\cdot\frac{\kappa_0}{\kappa_0+1}}$ Commented Aug 6, 2019 at 6:01

I looked into a few plasma physics text books [1,2], and according to them the Debye length is defined as $$\lambda_{D,j} = \left( \frac{\epsilon_0 k_B T_j}{n_j e^2} \right)^{\frac{1}{2}},$$ where the index $$j$$ denotes either electrons or ions. We thus have an electron Debye length, and an ion Debye length.

In addition, the total Debye length is defined as $$\frac{1}{\lambda_D^2} = \frac{1}{\lambda_{D,e}^2} + \frac{1}{\lambda_{D,i}^2},$$ which is the same as you have written it, just arranged in a different way.

Let's calculate a few examples assuming $$n_e=n_i$$: $$\begin{array}{ccc|ccc} \hline T_e \mbox{ in eV} & T_i \mbox{ in eV} & n_{e,i} \mbox{ in}\ \mathrm{m}^{-3} & \lambda_{D,e} \mbox{ in m} & \lambda_{D,i} \mbox{ in m} & \lambda_{D} \mbox{ in m}\\ \hline 1 & 1 & 10^{17} & 2.3\cdot10^{-5} & 2.3\cdot10^{-5} & 1.6\cdot10^{-5}\\ 10 & 10 & 10^{17} & 7.2\cdot10^{-5} & 7.2\cdot10^{-5} & 5.1\cdot10^{-5}\\ 10 & 1 & 10^{17} & 7.2\cdot10^{-5} & 2.3\cdot10^{-5} & 2.2\cdot10^{-5}\\ \hline \end{array}$$

As you have said in your question, for $$T_e \gg T_i$$ the ion Debye length will dominate or, in other words, the total Debye length will be closer to the ion Debye length.

In the laboratory, however, $$T_e$$ is often easier to measure than $$T_i$$. This is of course not generally true, but using for example Langmuir probes is a simple diagnostic to get the electron temperature. My answer is therefore that the electron Debye length is often used because the electron Temperature is relatively easy to measure (compared to the ion temperature).

If the plasma density is higher, like in fusion plasmas, coupling between ions and electrons is strong and one can assume $$T_i\approx T_e$$, for which the total Debye length is close to the electron (or ion) Debye length. For such cases it is a valid approximation to use the electron Debye length instead of the total Debye length.

In summary, the most important thing is to state explicitly what you are using (electron/ion/total Debye length). Since the difference between these quantities is usually not too large (especially when you just want to compare the Debye length to the plasma dimension to check on the validity of the plasma definition), often the electron Debye length is used.

[1] A. Dinklage et al.: Plasma Physics: Confinement, Transport and Collective Effects (Springer, 2005)

[2] A. Piel: Plasma Physics: An Introduction to Laboratory, Space, and Fusion Plasmas (Springer, 2010)