Neutral transport

TRANSP uses the FRANTIC neutral transport code to track the neutral population. FRANTIC is an extension of ANTIC [S. Tamor, J. Comput. Phys. 40 (1981) 104] to include multiple ion and neutral species (including Helium), interaction with fast ions, recombination, volume neutral sources from beam deposition, and to use updated cross-sections; it has also been extended to compute the effect of neutral transport on momentum balance.

As an advantage, FRANTIC is very fast and it provides a qualitatively correct description of neutral gas transport and penetration in the core plasma. Energy, momentum, and particles are strictly conserved within the FRANTIC geometry. When data is transferred to a generalized flux surface geometry, particle, momentum conservation, and plasma frame energy balance are conserved perfectly.

While the model provide qualitative description of the neutral transport, there are caveats

Simplified geometry - The geometry in FRANTIC is a set of nested cylinders of varying minor radius and shifted centers based on the major radius locations of centers of flux surfaces in the target plasma equilibrium geometry. Linear momentum transport is done in this shifted cyclinders geometry and then converted to torques by factoring in the cylinder major radius positions at the end.

It is a 1D model - Edge neutral sources enter the plasma from any direction with equal probability; there is no possibility for poloidal variation of the source. All results are of course also 1D, assuming that neutral density, temperature, and angular velocity are functions of flux surface only, which may not be very realistic. The results can be treated as flux surface averages, but FRANTIC itself cannot provide any 2D information.

Three neutral sources are considered, a 'warm' source due to wall recycling, a 'cold' source due to gas injection and a volumetric source due to neutral beam halo and recombination.

For each ion species j, up to a total of seven ion species, the neutral sources are defined according to following scheme:

The wall source is usually dominated by recycling. If RECYCB=1.0, the important energy parameters in terms of neutral penetration are set by the energy of the warm neutrals, i.e. E0IN(js), with js=1+3(j-1) for each ion species j.

General Settings

NSOMOD=1
to select the FRANTIC neutral transport model (at present this is the only model available).
NZONES_FRANTIC
This selects the number of radial zones used by FRANTIC (the default value is 20).
NLRECO
Set equal to TRUE to turn-on the recombination volume neutral source.
RECYCB
This determines how the specified recycling source is divided into warm and cold recycled neutrals. A fraction RECYCB of the source are considered warm neutrals while a fraction ( 1 - RECYCB ) are treated as cold.
FH0ESC
This indicates the fraction of escaping neutrals to return as "warm" recycling source neutrals when NRYCYOPT!=2. The remainder are returned cold. If NRYCYOPT=2 then escaping neutrals are considered lost and are not returned to the plasma.

Temperature of the edge neutral sources

The temperature of each neutral source js entering the plasma is selected with the namelist variable E0IN(js).

Temperature of the warm source

The temperature of the warm source is selected according to the following namelist variables, thus the value of E0IN(js), for js=1+3(j-1) does not matter.

MOD0ED=1
If a UFILE for the temperature of the recycling source is provided, then TRANSP uses this, otherwise it uses the namelist value TIEDGE.
MOD0ED=2
The temperature of the neutral source is set as a fraction TI0FRC of the central ion temperature.
MOD0ED=3
Use the electron temperature just outside the separatrix from the input data.
MOD0ED=4
Use the ion temperature just outside the separatrix from the input data.

Temperature of the volume source

For each ion species, the temmperature of the volume source is set at the local value of the ion temperature, thus the settings in E0IN(js), for js=2+3(j-1), have no effect on the calculations.

Temperature of the cold source

For each ion species j, the temperature of the cold source can be set in a UFILE PRETGF/EXTTGF, or set by the namelist variable E0IN(js), with js=3+3(j-1)

Angular velocity of the edge neutral sources

Similarly to the convention for the temperature, the variable OMEG0IN(js) controls the angular velocity of the edge neutral sources.

Angular velocity of the warm source

The angular velocity of the warm source is selected according to the following namelist variables, thus the value of OMEG0IN(js), for js=1+3(j-1) does not matter.

MOD0ED_OMEGA=1
If a UFILE PREORC/EXTORC for the angular velocity of the recycling source is provided, then TRANSP uses this, otherwise it uses the namelist value OMEG0IN(js), with js=1+3(j-1) for each ion thermal species j.
MOD0ED_OMEGA=2
In this case, for each thermal ion species j the angular velocity is set as a fraction OMEG0FRC of the edge ion angular velocity, i.e. the angular velocity just outside the plasma separatrix.

Angular velocity of the cold source

For each ion species j, the angular velocity of the cold source can be set in a UFILE PREOGF/EXTOGF, or set by the namelist variable OMEG0IN(js), with js=3+3(j-1)

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