Backlighter Setup

Spectral Profile

SPECT3D supports the use of several types of backlighter radiation sources:

• Continuum: a blackbody source defined by a single temperature.
• Line: a narrow-band source defined by a brightness temperature and frequency range.
• Line+Continuum: a combination of a blackbody source and a narrow-band line source.
• External Radiation Source: utilizes the same source parameters as those used to specify an external radiation source (which is used in computing photoionization/photoexcitation effects. To use this, the external radiation source must be on.
• File: specify a VISRAD-formatted file to use as a backlighter.
• X-ray scatt. source.

For several types of backlighters, time-dependent parameters can be specified. To do this, see Adding Backlighter Time Steps.

Spatial Profile

SPECT3D supports two backlighter spatial intensity profiles: Uniform and Distributed. The relevant options depend on the type of projection chosen in the Image Plane Setup:

• Orthographic projection: uniform backlighter only. In this case, the user input for Spot radius is ignored. See: uniform backlighter rays in the orthographic case.
• Point projection:
  • uniform backlighter: in this case, the backlighter is treated as a point source. The Spot radius affects only the magnitude of the spectral flux. Geometrically, the backlighter is still a point source, with a single line-of-sight ray drawn for each detector pixel. See: uniform backlighter rays in the point projection case.
  • distributed backlighter: the backlighter has a finite size, and an intensity profile according to the formula below. Each detector pixel draws a line-of-sight ray to each point in the backlighter distribution. See: distributed backlighter rays in the point projection case.
• Spherical crystal model: distributed backlighter only.

If a Distributed backlighter is specified, the intensity profile is modeled using a supergaussian function:

I(r) = I_o exp[ - ( r / r_o )ⁿ ],

where r is the radial coordinate, r_o is the spot size at the 1/e value of the intensity, and n is the supergaussian exponent. Flat, step-like profiles can be produced using supergaussian exponents of around 10 or more. Additionally, the number of grid points in the radial and azimuthal directions are specified. These determine the number of rays from the backlighter that will contribute to each line of sight. Finally, the angle of the backlighter foil is specified. An angle of 90 degrees means that the foil faces the target square on.

Source for X-ray Scattering:

To set up the radiation source for performing x-ray scattering simulations, select X-ray Scattering Source for the Backlight Spectral Type, and then click on the Set Parameters button. The parameters used in the original G. Gregori formalism are described here. Detailed description of the models can be found in: B. J. B. Crowley and G. Gregori, New J. Phys. 15 015014 (2013). When running a calculation that uses x-ray scattering, multi-threading should be switched off.

• Scattering source parameters:
  
  • The x-ray scattering source is characterized by:
    • Photon energy (eV): specifies the central wavelength in the source spectrum. For Gaussian and Lorentzian profile types, the central wavelength is the wavelength of the profile's peak intensity. For a table profile type, the central wavelength corresponds to 0 eV in the table.
    • Brightness temperature (eV): determines the specific intensity of the source according to Plank's law. Only used in the case of Gaussian or Lorentzian profile.
    • Profile: specifies the spectral profile of the source. This may be Gaussian, Lorentzian or a table of intensity and energy values. For the table option see Entering Table Data. In a table, 0 eV will actually correspond to the value of "Photon energy". Bins per pixel: this parameter should typically be given a value of 1. It was used in previous versions of the stand-alone XRTS code, and is mainly left here for backward compatibility. It will not significantly increase SPECT3D calculation speeds, and will cause reduction in spectral resolution. Warning: high values of this parameter can result in non-physical shifts is the XRTS spectrum.
    • Source position (cm): The source position may be entered in either Cartesian or Spherical coordinates,
    • Scattering energy (eV): These parameters control the photon energy range and grid where the scattering is computed. The range will be from "Photon energy " plus "Maximum loss" through "Photon energy " plus "Maximum gain", with steps (spectral resolution) of "Energy step".

• Aperture parameters:
  
  • An aperture may be used to limit the volume of plasma exposed to the x-ray source.
  • The orientation position with respect to the source is needed to establish the normal direction of the aperture.
  • The horizontal and vertical shifts allow the center of the aperture to be moved away from the line joining the source and the orientation point (cm). Shifts do not change the normal direction of the aperture.

• Plasma parameters:

• Use ion feature parameter: user-defined ion feature, set value for |f(k)+q(k)|²*S_ii(k). This value can also be changed in Hydro Overrides (This feature is a beta version and not available to all users)
• Override mean charge: total average ionization per ion (only for the first atomic species listed in the Atomic Data page of Spect3D). If selected, the electron density is also affected accordingly.
• Use Debye temperature: for ion feature calculation with Debye-Waller factor.
• Use band gap: use valence-to-conduction band gap in calculation.
• Use ion temperature: set ion temperature with respect to electron temperature. If not set, T_ion=T_cf, where T_cf is temperature of classical fluid giving same correlation energy as quantum fluid.
• Use XRTS flag from hydro file: this checkbox is enabled only for special exodus hydro files containing a variable called scatter_flag, which is used for omitting individual cells from the XRTS calculation. This variable must be an integer, where a value of 0 will omit XRTS for a given cell, and 1 will allow it. If this variable exists in the hydro file, its usage is controlled by this checkbox.
• Parameter space for scattering calculation: provides limits for temperature and density where the scattering will be computed. If either temperature or density is outside of those limits for a given volume element, the scattering from this volume element will not be computed.

 

• Modeling parameters:
  
  • The models presently available for the calculation of the total spectrum are:
    • RPA (Linux and Mac only)
    • Lindhard
    • Tsytovich
    • Static LFC
    • Dynamic LFC (Linux and Mac only)
    • BMA (Linux and Mac only)
    • BMA+SLFC model is not yet supported.
  • Override with Tystovich for alpha <: this allows for the total spectrum model to be forced to use the Tsytovich model for certain plasma conditions. The plasma conditions are defined by the parameter α, defined by:
    
    α = ( c_scat × λ ) √( ρ_e / T_eff_K ) / sin( θ / 2 )
    
    where λ is the X-Ray probe wavelength in m, ρ_e is the electron density in m^3, T_eff_K is the effective temperature in K, θ is the scattering angle, and c_scat is a scattering constant defined by:
    
    c_scat = √( qe^2 / ε0 × kB ) / 4π
    
    where qe is the elementary charge, ε0 is the vacuum permittivity, and kB is the Boltzmann constant. If this option is selected, the Tystovich model will be used if α is less than the value in the input field.
  • Static model : model for ion feature.
  • Bound-free model parameters, active if "Add bound-free" box is checked:
    • Bound-free model. Model for bound-free electron feature. Form-factor approximation (FFA); Impulse approximation (IA); Impulse approximation+Compton defect (IBA).
    • Effective electron mass: for FFA model only (in units of electron mass).
    • Continuum lowering model: Stewart-Pyatt (SP); Ecker-Kroll (EK); or user defined (USR). In the case of user defined, shift must be specified (eV).
    • Normalization for bound-free model: multiplies the bound-free model with normalization constant. FK is the normalization based on (1-f(k)²) for the elastic peak. This normalization preserves the total power and it should be used with FFA. NO uses no normalization (=multiplies by 1) and it should be used with IA or IBA. USR selects user defined normalization constant.
    • Use bound-free Doppler broadening: applies a Doppler broadening to bound-free electron feature. The Doppler correction is based on the value for ion temperature.
  • Use advanced mixing model: should be used in most calculations.
  • Use IRS model: use incipient Rydberg states model to correct for ionization and hopping.
  • Override hard sphere diameter: user-provided diameter of hard spheres in SOCPN model.
  • Override static local field factor: user defined value for the static local field factor.
  • Polarizability: set real and imaginary part of polarizability (important for calculations near an edge).
  • Generate input file for standalone XRTS code. If selected, XRTS inputs will be generated for each plasma zone.
  • Dawson: number of points for Dawson's integral.
  • Distribution: number of points for integral over electron distribution function integral.
  • PVI: number of points for principal value integration.
  • Relaxation: number of relaxation points.
  • FFT: number of points for Fast Fourier Transform.
  • Tolerance: smallness parameter for convergence test.

The geometry for an X ray source is shown below:

 

Plasma temperature and mass density for x-ray scattering calculation are determined by input hydro parameters, electron number density is calculated based on plasma conditions for each zone. The scattering angle is computed for each volume element based on the LOS and the line that connects the volume element center and the source, as shown in figure above.

Next