pedestal

Pedestal Module

The pedestal model can be activated by selecting in the namelist the following boundary conditions:

MODEEDG = 5 ! Use boundary conditions for the electron temperature profile set by the pedestal module

MODIEDG = 5 ! Use boundary conditions for the ion temperature profile set by the pedestal module

MODNEDG = 5 ! Use boundary conditions for the electron density profile set by the pedestal module

NOTE: it is possible to use pedestal boundary conditions only for one profile, for example electron temperature, but not for the ion temperature. However, since TRANSP enforces the boundary conditions for Te and Ti to be the same (as of April 2018), the boundary condition for Te will be in this case taken at XIBOUND.

The pedestal module can be activated also when the density and temperature profiles are read from UFILE. In this case, the calculated pedestal width and height are an output, but they are not used as boundary conditions.

Users can impose the values of the pedestal width and height from the namelist or use a predictive model.

To impose the pedestal width from the namelist:

NMODEL_PED_WIDTH = 0

TEPEDW = <value> ! Electron pedestal width in cm (+) or x (-)

TIPEDW = <value> ! Ion pedestal width in cm (+) or x (-)

XNEPEDW = <value> !Electron density pedestal width in cm (+) or x (-)

Input a negative value to express width in sqrt of norm toroidal flux. The default value is 0, meaning that the top of the pedestal is assigned at the plasm boundary.

To impose the pedestal height from the namelist:

NMODEL_PED_HEIGHT = 0 ! Pedestal height model choice:

TIPED = 100.0d0 ! Ion pedestal temperature in eV

TEPED = 100.0d0 ! Electron pedestal temperature in eV

XNEPED = 1.0d12 ! Electron pedestal density in cm**-3

To use the predictive model for the pedestal:

NMODEL_PED_HEIGHT = 1 ! predictive model for the pedestal height

NMODEL_PED_WIDTH = 1 ! predictive model for the pedestal width

When the pedestal height is predicted, the calculated density and temperature can be rescaled using the following namelist variables:

SCALE_TEPED = 1.0d0 ! Scale factor for Te pedestal height

SCALE_TIPED = 1.0d0 ! Scale factor for Ti pedestal height

SCALE_NEPED = 1.0d0 ! Scale factor for ne pedestal height

As of April 2018 new options have been added to the original NTCC module, summarized in the table and described below.

It should be noted that all pedestal models calculate the pedestal pressure, thus the pedestal temperature is derived for a given pedestal density. The resulting pedestal temperature depends on how the pedestal density is calculated.

The original NTCC module calculated the pedestal density as nped = 0.71 nlin, where nlin is the line averaged density.

This option - which is still the default for all models and is the only option available for the NTCC module - can be selected by setting LPED(2) = 1

Other options include:

  • LPED(2) = 2

to calculate the pedestal density from a fit of the density profile, according to the parametrization below.

The fit returns the width of the pedestal in normalized poloidal flux coordinates and the value of the density at the pedestal location.

where

r1 is the ratio of the density at the pedestal to the central value, ne,ped = r1 ne,0

r2 is the ratio of the density at the separatrix to the central value, ne,sep = r2 ne,0

The mid-pedestal location

is taken at a pedestal half-width to the right of the pedestal.

The pedestal top is defined as a pedestal half-width to the left of the pedestal location.

  • LPED(2) = 3

to calculate the pedestal width as a fraction (>1) of the density at the separatrix, ne,ped=CPED(4)xne,sep.

This setting is helpful when predicting the density profile, since the density is not an input to TRANSP.

Options for predictive pedestal height, selected with LPED(3)

Original NTCC model options:

  • LPED(3) = 1 Pedestal model using the pedestal width based on magnetic and flow shear stabilization (see section 3.1 in pedestal_models.ps for detailed information).

    • LPED(3) = 2 Pedestal model using the pedestal widhth based on flow shear stabilization (see section 3.2 in pedestal_models.ps for detailed information)

  • LPED(3) = 3 Pedestal temperature model using the pedestal width based on normalized pressure (see section 3.3 in pedestal_models.ps for detailed information)

  • LPED(3) = 11 Pedestal temperature model based on thermal conduction model I (see section 3.4 in pedestal_models.ps for detailed information)

  • LPED(3) = 12 Pedestal temperature model based on thermal conduction model II (see section 3.5 in pedestal_models.ps for detailed information)

The description of the NTCC PEDESTAL module can be found here and the standalone code can be downloaded from the NTCC directory.

The models included in the NTCC module are described in the paper by T. Onjun et al, Phys. Plasmas 9 (2011) 5018

Models based on EPED1 [P. Snyder et al, Nucl. Fusion 51 (2011) 103016]:

  • LPED(3) = 100

This model uses a multi-dimension interpolation over the space of parameters that are input to the EPED1 calculations. The lookup table consists of over 6000 EPED1 calculations that cover the operational parameter space of ITER and that are described here.

NOTE: this lookup table has been derived for ITER parameters, do not use otherwise.

  • LPED(3) = 111

Neural network based on EPED1 calculations [O. Meneghini et al, Nucl. Fusion 57 (2017) 086034].

This neural network has been trained from a total database of about 15000 calculations for ITER (including the ITER lookup table), xxx calculations for DIII-D, xxx for JET and xxx for KSTAR.

The output of the model is the pedestal pressure and width for a given pedestal density.

Models based on MHD stability calculations [P. Maget et al, NF 53 (2013) 093011]:

  • LPED(3) = 21

This model needs the pedestal width in cm as an input, using CPED(3).

Alternatively, the pedestal width can be taken from a fit over the pedestal density, when the density profile is prescribed with UFILE. In this case set CPED(3)=0.0 and LPED(2)=2

NOTE: this parametrization has been derived for ITER, do not use otherwise.

  • LPED(3) = 22

This model needs the pedestal width in cm as an input, using CPED(3).

Alternatively, the pedestal width can be taken from a fit over the pedestal density, when the density profile is prescribed with UFILE. In this case set CPED(3)=0.0 and LPED(2)=2

NOTE: this parametrization has been derived for ITER, do not use otherwise.

  • LPED(3) = 213

This model combines the option LPED(3)=3 for the calculation of the pedestal width and LPED(3)=21 for the calculation of the height. In this case, even when LPED(2)=2 is selected, the pedestal width is calculated from the NTCC model. There is no need for specifying the value of CPED(3) with this option.

NOTE: this model uses a parametrization that has been derived for ITER, do not use otherwise.

  • LPED(3) = 223

This model combines the option LPED(3)=3 for the calculation of the pedestal width and LPED(3)=21 for the calculation of the height. In this case, even when LPED(2)=2 is selected, the pedestal width is calculated from the NTCC model. There is no need for specifying the value of CPED(3) with this option.

NOTE: this model uses a parametrization that has been derived for ITER, do not use otherwise.

An option has been implemented to reconstruct a pedestal shape from a tanh function (see equation above for the density profile). This option is activated by setting LPED(9)=1. For backward compatibility this option is not available for the NTCC module, which uses a linear profile between the pedestal and the plasma boundary.

The figure on the left shows (top) a density profile that uses the parametrization above; the figure inset indicates the position of the pedestal and of the pedestal top.

The bottom plot shows the calculated temperature profile; the black profile uses a hyperbolic tangent parametrization at the pedestal location, while the red profile does not (default setting in the NTCC pedestal module).

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