Streamer properties and associated x-rays in perturbed air

Streamers are ionization waves in electric discharges. One of the key ingredients of streamerpropagation is an ambient gas that serves as a source of free electrons. Here, we explore thedependence of streamer dynamics on different spatial distributions of ambient air molecules. We varythe spatial profile of air parallel and perpendicular to the ambient electric field.

We consider localsinusoidal perturbations of 5%–100%, as induced from discharge shock waves. We use acylindrically symmetric particle-in-cell code to simulate the evolution of bidirectional streamers andcompare the electron density, electric field, streamer velocity and electron energy of streamers inuniform air and in perturbed air.

In all considered cases, the motion is driven along in decreasing airdensity and damped along increasing air density. Perturbations of at most 5%–10% change thevelocity differences by up to approximately 40%. Perturbations perpendicular to the electric fieldadditionally squeeze or branch streamers. Air variations can thus partly explain the difference ofvelocities and morphologies of streamer discharges.

In cases with large perturbations, electrons gainenergies of up to 30 keV compared to 100 eV in uniformly distributed air. For such perturbationsparallel to the ambient electric field, we see the spontaneous initiation of a negative streamer; forperpendicular perturbations, x-rays with energies of up to 20 keV are emitted within 0.17 ns.