Atomic Processes Setup

In the Atomic Processes setup widget, the user specifies whether the plasma is assumed to be in LTE.

When LTE is specified, atomic level populations are determined using the Saha equation and Boltzmann statistics.

For non-LTE plasmas, atomic level populations are determined by solving a coupled set of atomic rate equations. These rate equations include the following atomic processes:

• collisional ionization and (3-body) recombination
• radiative recombination
• autoionization, dielectronic recombination, and electron capture
• collisional excitation and deexcitation
• spontaneous emission.

There are a number of parameters that can be adjusted for non-LTE plasma calculations. These are shown by clicking on the Advanced button.

Rate Multipliers Tab

To investigate the role of individual processes in establishing atomic level populations/ionization distributions, an option is provided for applying a multiplier to the rate coefficients for each of the processes listed above. To show these multipliers, press the Advanced button.

For example, to assess whether photoionization has a significant effect in a calculation, the multiplier for its rate coefficient could be set to zero. This would effectively turn off photoionization (as well as its inverse process, stimulated recombination) by setting the photoionization rates for all transitions to zero.

Rate multipliers affect the calculation of atomic level populations, and, because of this, they can indirectly affect the calculation of emissivities, opacities and resulting spectra. Note, however, that the multipliers are not used directly in the calculation of emissivities and opacities. The emissivities and opacities are only affected through the change in the atomic level populations.

When both the photoexcitation and photoionization multipliers are set to zero, a calculation may require significantly less CPU time because the calculation of these radiation field-induced rates is bypassed.

Transitions Tab

Other Advanced parameters that affect atomic rate calculations are listed on the Transitions tab:

• Set Stark broadening modifiers: The calculated Stark widths for selected transitions can be modified. For details see Modifying the Stark Widths.
• Include dense plasma shift : If checked, spectral lines will be shifted in dense plasmas (Stark shift). This feature is in beta stage and is not available to all users.
• Set osc. strength/wavelength modifiers: See Modifying Transition Energies and Strengths for details on how to specify photon energies and oscillator strengths for selected transitions.
• Continuum Lowering:
• Pressure ionization model. Available options are: None, Hummer & Mihalas occupation probability (default), Stewart & Pyatt, Ecker & Kröll. This feature is in beta stage and is not available to all users.
• Energy scaling factor. Scaling factor can be applied to ΔI computed by pressure ionization model. Default is 1.0. This feature is in beta stage and is not available to all users.
• Multiply Excited Levels with Implicit Transitions: When checked, detailed calculations of implicit doubly-excited states will be performed. This is effective for ATBASE versions greater than 4.2. If the detailed calculations are included, the calculation will take considerably longer.

Populations Solution Method Tab

Solution of the atomic rates equations utilizes iterative methods. The Population Solution Method tab allows users to attempt alternative approaches if convergence problems occur.

• An accelerated convergence algorithm can be applied when calculating atomic level populations. This is sometimes helpful if problems occur with the atomic level populations solver.