# Streamer criterion : foreword

## General

As said before, in Flux, the dielectric breakdown study is based on Streamers inception criterion. This criterion does not give precise information about arc path and breakdown voltage. The aim is rather to identify the Streamer starting area and a “Streamer inception voltage”.

## Concerned dielectric medium

The “Streamer criterion” tool allows studying Streamers in gas medium. In fact, the gas has a lower breakdown voltage than liquids and solids.

## Access in Flux

The tool is available in Flux electrostatic application (2D plane, 2D axisymmetric and 3D), in postprocessing context.

It is available through the menu “Computation”.

## General principle

Each gas has a coefficient called **α _{eff}** defined by a Streamer
mathematical model depending on electric field and pressure. This model is determined by
fitting an experimental curve. Consequently, a model will be optimal for some
configurations (of electric field, geometry, pressure, temperature, etc.) but can not be
universal.

Air formula used in Flux:

With :

**Er**: reduced electric field, **E**: electric field in** kV/mm**, **P**:
pressure in bar

**K** is computed by integrating **α _{eff}** on a path
(Streamer path), and its value is compared to the threshold
value. If the value is higher than the threshold, then the Streamer has a high
probability to start.

As the law between the electric field and the electric potential is linear, a dichotomy
is applied on the electric field until **K** is equal to the threshold. It allows
finding the Streamer inception voltage on the considered path.

## Streamer path

Streamers paths are not predictable by modelization.

The paths used in the modeling are:

- Previously identified with experiments
- Or approximated by physical quantities evolution laws. In Flux, the electric field lines are considered.