  We investigate negative and positive streamers in C4F7N-CO2 mixtures through
simulations. These mixtures are considered to be more environmentally friendly
than the insulating gas SF6 that is widely used in high voltage technology.
Simulations are performed using a 3D particle-in-cell model. Negative streamers
can propagate when the background field is close to the critical field. We
relate this to their short conductive channels, due to rapid electron
attachment, which limits their field enhancement. Positive streamers also
require a background field close to the critical field, and in addition a
source of free electrons ahead of them. In our simulations these electrons are
provided through an artificial stochastic background ionization process as no
efficient photoionization process is known for these gases. In 3D, we can only
simulate the early inception stage of positive discharges, due to the extremely
high electric fields and electron densities that occur. Qualitative 2D
Cartesian simulations show that the growth of these discharges is highly
irregular, resulting from incoming negative streamers that connect to existing
channels. The inclusion of a stochastic background ionization process also has
an interesting effect on negative discharges: new streamers can be generated
behind previous ones, thereby forming a chain of negative streamers.
