Source

Source

The source operator in GUERNICA is an optional additive term that injects neutrals into the distribution function. It is enabled by including source under operators in [components]. In the time integration, it is added directly into the full right-hand side alongside advection and any other active operators.

At present, the implemented source is a Maxwellian source. The source operator constructs a fixed distribution in velocity space of the form

\[ S(v_x,v_y,v_z)=\frac{n}{(2\pi T)^{3/2}} \exp\left(-\frac{v_x^2+v_y^2+v_z^2}{2T}\right), \]

where \(n\) is the source density and \(T\) is the source temperature. This source is precomputed once and then added to the right-hand side during each operator application. It does not depend on the current solution \(f_n\), so it acts as a prescribed inflow of neutrals rather than a collision-like term.

In the current implementation, the source is uniform in configuration space. After the Maxwellian is built in velocity space, the same value is assigned to every spatial degree of freedom for a given velocity point. That means the present source operator represents a spatially uniform neutral source with a Maxwellian velocity distribution.

To turn the source on in the input file, include it in [components] and add a [source] section. A typical setup looks like this:

[components]
species = n
operators = source

[source]
type = maxwellian
neutral_species = n
density = source_density
temperature = source_temperature
ux = default_u
uy = default_u
uz = default_u

[profile:source_density]
type = constant
value = 1.0

[profile:source_temperature]
type = constant
value = 1.0

A useful implementation detail is that, although ux, uy, and uz appear in the input file, the current MaxwellianSourceOperator only uses density and temperature when building the source. The present source is therefore a zero-drift Maxwellian centered at \((0,0,0)\) in velocity space.

Similarly, the current source builder extracts the source density and temperature using constant-profile values before constructing the operator. So in practice, the current source setup is best thought of as a spatially uniform, constant Maxwellian particle source for neutrals.