Extremely far from equilibrium: the multiscale dynamics of streamer discharges

Transcription

1 Extremely far from equilibrium: the multiscale dynamics of streamer discharges Ute Ebert 1,2 1 Centrum Wiskunde & Informatica Amsterdam 2 Eindhoven University of Technology

2 A gallery of streamer discharges

3 Plasma medicine ( Plasma bullets ) Air purification Disinfection Plasma assisted ignition combustion aviation

4 Corona reactors: Reduction of NO x and dust [Pemen, van Heesch et al. at TU Eindhoven]

5 Streamer diameters and velocities in STP air Thick streamers excellently convert pulsed electric power into chemical radicals for biogas cleaning, disinfection, sterilization [van Heesch, Pemen et al.] High field zone = self organized chemical reactors, electron accelerators

6 Figures from [Briels et al., J Phys D, 2006] More than 50% of energy converted into O*! [van Heesch et al., J Phys D 2008]

7 Positive and negative streamers in air 1 bar, 40 mm gap, t exp = 160 ns + - U < 40 kv: thin positive streamers, hardly any negative ones. U > 60 kv: positive and negative streamers more similar, positive ones ~20% faster. 20 kv 20 kv Positive streamers are always longer and faster. 46 kv 47 kv [Briels et al., J. Phys. D 2008] 83 kv 77 kv

8 Positive streamer in high purity 7.0 nitrogen: Avalanches created by single electrons [Nijdam et al. J Phys D 2010] Theory reproduces avalanche density. [Wormeester et al., Jap J Appl Phys 2011, Nijdam et al. J Phys D, 2011, Luque et al., Phys Rev E, 2011]

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10 ca. 45 lightning flashes/second worldwide important source of the green house gases NO x and O 3 (after 1-2 days)

11 [Briggs et al., JGR 2011; NASA]

12 Electron acceleration and Gamma-rays from a lightning leader Extend cross-sections to 100 MeV, include relativistic motion, Bremsstrahlung, Compton scattering Need model for stepped negative lightning leader. [Xu, Celestin & Pasko, GRL 2012]: Photon spectrum at

13 Positive streamers, negative counterstreamers, transition to leader

14 Hard X-rays (>200 kev) from positive sparks 1 m 1 MV over 1 m X-ray source: Negative streamers! Typical energies: ~200 kev [Kochkin, Nguyen, van Deursen, Ebert, J Phys D 2012]

15 Simulation methods Open source code and documentation under construction. I have removed material from the presentation that is not submitted yet. Find publications on

16 Fast processes in the ionization front: Electrons drift and diffuse in local E-field Elastic, inelastic and ionizing collisions with neutral molecules. Degree of ionization < 10-4 at sea level. E A + Models contain: 1. Ohm s law for electron motion A + - e - 2. Ionization reactions 3. Space charge effects

17 2D fluid model: Positive streamer in air (2008) Complete system: Zoom: n e, equi-φ n e n + n + -n e E + equi-φ Computation with comoving local grid refinement.

18 Positive streamer in air n + -n e E, equi-φ Local field enhancement: >30 kv/cm in STP air -> ionization, excitation, X-rays

19 2 interacting streamers in 3D (in 2008): Surfaces of equal electron density Quasispectral method for the Poisson equation [Luque et al., Phys Rev Lett 08, Research Highlight Nature 08, Review in J Comput Phys 2012]

20 Towards a multi-streamer theory Not yet submitted results on a better characterization of streamer trees (by dielectric breakdown models) are removed here. Expect our preprints soon. Include longer time scales: Chemistry after the primary excitations by electron impact, heating and transition to leader

21 Particle 3D hybrid model Particle Electron density 14 Charge density 14 Electric field Fluid Fluid to study electron run-away and fluctuation effects [Li et al., JAP 2007, JPD 2008,, J Comput Phys 2012].

22 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid [Li et al., Plasma Sources Sci Technol, 2012] 22

23 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 23

24 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 24

25 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 25

26 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 26

27 Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 27

28 3D: Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid 28

29 3D: Negative streamer in STP N 2 in overvolted gap (E back = -100 kv/cm) Classical fluid Extended fluid Super-particle Hybrid Local field approximation with gradient correction, flux coefficients from MC swarms [Li et al., J Comput Phys, 2010] 29

30 Extended density model Superparticle model Hybrid model 30

31 Extended density model Superparticle model Hybrid model Extended density model approximates propagation well, but branching occurs too late. [Li et al., PSST 2012] 31

32 3D particle simulation with electrode, adaptive mesh refinement, and adaptive (super-)particle management based on a fast and mesh-free k-d tree algorithm Method by Teunissen et al., submitted to J. Comput. Phys. 3D streamer simulation with needle will be submitted soon

33 1 electron in non-ionized ambient air at 70 kv/cm, (4 mm) 3 [Sun, Teunissen, Ebert, submitted to Geophys. Res. Lett.]

34 1 electron in ambient air with 10 3 O 2- /cm 3 at 70 kv/cm, (4 mm) 3 Natural background ionization creates homogeneous breakdown in overvolted gaps, not streamers. [A.B. Sun, J. Teunissen, U. Ebert, submitted to Geophys. Res. Lett.]

35 Streamer inception from needle in 200 mbar air S. Nijdam, published in [Ebert et al., Nonlinearity 2011]

36 Collision cross-sections for different gases Stochastic electron motion (Monte Carlo or Boltzmann) Transport and reaction coefficients for density model or new density model E A + Density or spatially hybrid model A Further model reduction to moving boundaries and to discharge trees as a whole e -