Tutorial: changing the gas and the chemistry

Table of Contents

• Cross sections and transport data
• Converting to a format that the code can read
• Updating the config file

Using a different gas requires the following steps:

1. If no suitable transport data file is already avaible, electron-neutral cross sections for the gas have to be obtained and electron transport data has to be computed, see Generating electron transport data
2. Depending on the application, additional chemical reactions have to be obtained.
3. If applicable, photoionization parameters for the gas have to be obtained. Unfortunately, there is often a lack of information on these parameters.
4. Updating the config file

Below, we will provide a basic outline of these steps for a somewhat case of simulations in pure argon, without any additional chemistry or photoionization. Note: all the referenced files can be found in the folder tutorials/changing_gas

Cross sections and transport data

We will skip a detailed explanation here, since this step is covered at Boltzmann solvers. Here it is assumed that the Linux version of Bolsig+ is used, which is called bolsigminus. However, the output can be obtained with the Windows version, or the online version available at lxcat. Coupled with the program is a set of Siglo cross sections that we will use, which are also available from lxcat. We will use the Bolsig input file bolsigminus_argon_input.txt available at tutorials/changing_gas.

Converting to a format that the code can read

After executing Bolsig, a file called argon_swarm.dat is produced. This file can be converted using tools/bolsig_convert.py:

tools/bolsig_convert.py <path>/argon_swarm.dat <path>/argon_transport_data.txt

which produces a file argon_transport_data.txt. Sometimes an error like the following can occur ‘ValueError: could not convert string ’-0.2757-206' to float64`. These wrongly formatted numbers should then be corrected in the Bolsig output.

We will now add a minimal chemistry, which will only consist of an ionization reaction:

reaction_list
-----------------------
# Only a single ionization reaction
e + Ar -> e + e + Ar+,field_table,C3 Ar Ionization 15.80 eV
-----------------------

The resulting file is available as argon_chemistry.txt.

Updating the config file

Note that we will not include photoionization here, so that the following parameters should be changed:

# Whether photoionization is enabled:
photoi%enabled = F

# Gas component names:
gas%components = Ar

# Gas component fractions:
gas%fractions = 1.0

# Input file with transport (and reaction) data:
input_data%file = <path>/argon_chemistry.txt

An example of config file for axisymmetric simulations is available at tutorial_argon.cfg.