# Handling two-fluid five-moment and ten-moment source terms¶

The two-fluid system treats a plasma as a mixture of electron and ion fluids, coupled via the electromagnetic field and collisions. In the ideal two-fluid system collisions and heat-flux are neglected, leading to a closed system of coupled PDEs: one set of fluid equations for each of the fluids, and Maxwell equations for the electromagnetic field. Non-neutral effects, electron interia as well as displacement currents are retained. Further, the fluid pressures can be treated as either a scalar (five-moment model) or a symmetric $$3\times 3$$ tensor (ten-moment model), or a combination.

While solving the two-fluid system there are two distinct solves: the hyperbolic update, which can be performed for each equation system separately and the source update, which couples the fluids and the fields together. In this note I only focus on the source updates, for both the five as well as the ten-moment equations. When written in non-conservation law form, the only sources in the system are the Lorentz force in the momentum equation, the plasma currents in the electric field equation and, in the ten-moment model, the rotation of the pressure tensor due to the magnetic field. The latter equation is uncoupled from the source update for the momentum and the electric field, and can be treated separately.

The equations for the five-moment source updates are

$\begin{split}\frac{d \mathbf{v}_s}{dt} &= \frac{q_s}{m_s} \left( \mathbf{E} + \mathbf{v}_s \times \mathbf{B} \right) \\ \epsilon_0\frac{d \mathbf{E}}{dt} &= -\sum_s q_s n_s \mathbf{v}_s\end{split}$

where, for the plasma species $$s$$, $$n_s$$ is the number density, $$\mathbf{v}_s$$ is the velocity, $$q_s$$ and $$m_s$$ are the charge and mass respectively. Further, $$\mathbf{E}$$ is the electric field, and $$\epsilon_0$$ is permittivity of free space. In these equations the magnetic field and number density are constants, as there are no source terms for these quantities.

It is more convenient to work in terms of the plasma current $$\mathbf{J}_s \equiv q_s n_s \mathbf{v}_s$$, which leads to the coupled system

$\begin{split}\frac{d \mathbf{J}_s}{dt} &= \omega_s^2\epsilon_0\mathbf{E} + \mathbf{J}_s \times \mathbf{\Omega}_s \\ \epsilon_0\frac{d \mathbf{E}}{dt} &= -\sum_s \mathbf{J}_s\end{split}$

where $$\mathbf{\Omega}_s \equiv q_s\mathbf{B}/m_s$$ and $$\omega_s \equiv \sqrt{q_s^2 n_s/\epsilon_0 m_s}$$ is the species plasma frequency. This is a system of linear, constant-coefficient ODEs for the $$3s+3$$ unknowns $$\mathbf{J}_s$$ and $$\mathbf{E}$$.

### Implicit solution¶

To solve the system of ODEs we replace the time-derivatives with time-centered differences. This leads to the discrete equations

$\begin{split}\frac{\mathbf{J}_s^{n+1/2}-\mathbf{J}_s^n}{\Delta t/2} &= \omega_s^2\epsilon_0\mathbf{E}^{n+1/2} + \mathbf{J}_s^{n+1/2} \times \mathbf{\Omega}_s \\ \epsilon_0\frac{\mathbf{E}^{n+1/2}-\mathbf{E}^n}{\Delta t/2} &= -\sum_s \mathbf{J}_s^{n+1/2}\end{split}$

where $$\mathbf{J}_s^{n+1/2} = (\mathbf{J}_s^{n+1}+\mathbf{J}_s^{n})/2$$ and $$\mathbf{E}^{n+1/2} = (\mathbf{E}^{n+1}+\mathbf{E}^n)/2$$. This is a system of $$3p+3$$ system of linear equations for the $$3p+3$$ unknowns $$\mathbf{J}_s^{n+1/2}$$, $$s=1,\ldots,p$$ and $$\mathbf{E}_s^{n+1/2}$$ and can be solved with any linear algebra routine.

$\begin{split}\frac{d}{dt} \left[ \begin{matrix} P_{xx} \\ P_{xy} \\ P_{xz} \\ P_{yy} \\ P_{yz} \\ P_{zz} \end{matrix} \right] = \frac{q}{m}\pmatrix{0&2\,B_{z}&-2\,B_{y}&0&0&0\cr -B_{z}&0&B_{x}&B_{z}&-B_{y}& 0\cr B_{y}&-B_{x}&0&0&B_{z}&-B_{y}\cr 0&-2\,B_{z}&0&0&2\,B_{x}&0\cr 0&B_{y}&-B_{z}&-B_{x}&0&B_{x}\cr 0&0&2\,B_{y}&0&-2\,B_{x}&0\cr } \left[ \begin{matrix} P_{xx} \\ P_{xy} \\ P_{xz} \\ P_{yy} \\ P_{yz} \\ P_{zz} \end{matrix} \right].\end{split}$