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As documented in the "controlling NUM from UI" HowTo the background density is not yet in use (see last column "in use" of the arrays of this HowTo).

It was defined in UI so as to be user later as a user-defined background density, probabibly of neutrals to compute charge exchange. You want to use it as a charge density, why not. In any case I am happy to see somebody willing to enter into the code.

So, if you what to consider this backround density as a background charge density, the general scheme is the following:

• Retrieve the corresponding object in SPIS/NUM, a ScalVolField:

ScalVolField backgroundDensity = (ScalVolField) LocalParameter.extractLocalParam(LocalParameter.backGroundDens).getValue();

certainly in SimulationFromUIParams.init() (look at many other parameters extraction examples in that routine).

• Pass it to the object that will handle it, i.e. your plasma, an MmfPlasma (add an extra object "ScalVolField backgroundDensity" to MmfPlasma , and define it through a setter, don't modifier the constructor, it would be too heavy)
• Add this density at each total plasma density computation in MmfPlasma:

add(ScalVolField otherFieldToAdd)
 (you get that information preferably through auto-completion in Eclipse (ctrl-space), or in the API documentation (Vol/ScalVolField)

NB: as you see, nothing is to be changed in SPIS/UI for that

Good luck and don't hesitate to ask again When you're done, the next step in complexity is to generate ions following that given density (CEX e.g.), you can ask for the same kind of guidance.

JF