This page outlines the general directions in which the code is now being developed. Plasma simulations are a research field of their own. A conference, published as an ASP Conference volume 247, Spectroscopic Challenges of Photoionized Plasmas, Ferland & Savin, editors, summarized the current state of plasma codes and what needs to be done next.
The goal from the beginning has been to do a complete simulation of what happens in nature. The capabilities of the code have always been limited by available computer power. As machines grow ever faster it becomes possible to increase the fidelity of the simulation. The code is developed with a structure that is designed to facilitate these capabilities.
The atomic - molecular data base
This is listed as a major item due to the large amount of ongoing human effort involved. The conditions in a non-equilibrium plasma are determined by the underlying microphysics. This in turn rests on the foundation of basic atomic and molecular cross sections and rates. The data handling needs of just keeping Cloudy current with the atomic database are extensive. Roughly 100 papers appear each year with new cross sections or rates (a web search on the Astrophysics part of ADS will find about a third of these – many more are in the Physics and Chemistry literature). Cloudy now predicts the intensities of well over 106 emission lines. New oscillator and collision strengths appear for roughly 104 transitions per year, and it is necessary to produce temperature fits to the collision strengths. Extensive sets of photoionization cross-sections with autoionization resonances and experimental / theoretical recombination rates are now appearing. The atomic data must be continuously updated just to stand still. Finally, large changes in the atomic rates may affect the stability of the code's solvers or change important predictions. Any change to the code has a finite possibility of introducing error, so all changes must be crosschecked. All of this is labor intensive but does not, in itself, result in publications.
All clouds are actually dynamical flows of some sort. In collaboration with Robin Williams, Will Henney, and Jane Arthur, I am incorporating hydrodynamics and time-dependent rates and advection, with the initial goal of simulating the flows seen in star-forming regions. This project is described in The Hydrodynamics of Photoionized Flows, 2002, G.J. Ferland, W.J. Henney, R.J.R. Williams, S.J. Arthur, Rev Mex AA (Serie de Conferencieas), 12, 43-49, available here. The goal is to have an essentially zero-free parameter description of the evolution of these systems.
Shocks, time-dependent ionization
The code was able to do time-dependent calculations early in its history (Ferland & Truran 1981, ApJ 244, 1022-1032). This was not developed along with the rest of the infrastructure since there was no compelling scientific case for doing so. Once the dynamics work is complete and fully stable it will be straightforward to extend the treatment to shocks and time-dependent ionization. The current development is being done in a manner to facilitate this extension.
Exact radiative transfer
Nearly all plasma codes now use escape probabilities to do line and continuum transport due to limits imposed by available processor power. Computers are now becoming powerful enough to do the radiative transport with exact methods. Most sources are optically thick in the Lyman continuum and some lines, while the line radiative transfer effects are important in most sources. From the beginning, the intention was to eventually incorporate exact radiative transfer methods in Cloudy, and the code was designed to receive this. Work is now underway to prepare the infrastructure for this extension.
Higher order dimensionality
The code is currently 1D. Higher order dimensionality mainly affects the transfer of the diffuse fields, which in turn affects predictions at the 10 - 15% level. The exact radiative transfer approach will use ALI with short characteristics, and the extension to 2D or 3D geometries will be straightforward.