Is plasma important? Influence molecule formation?

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1 Is plasma important? Influence molecule formation? Plasma Structure (space & time) Influence? Daan Schram Eindhoven University of Technology

2 Plasma: transient state! Diffuse and dark clouds: medium seen as (chemically active) gas: Ignores extra dimension of plasma state?: the ability to change! Electrons and ions may be dominant in chemistry, despite lower density And: Fast cluster/ dust formation; surfaces, modified surface processes. Plasma dissociates and surface associates Plasma in B 0 easily influenced by E fields. Similar: E fields give currents, power (and also column rotation)

3 Illustration : earthly plasmas: thermal plasma source & remote plasma Cascade arc source Thermal Ar + (ionizing) plasma source: local sun! T e thermal: 1 ev; n e /n Ar ~ 0.1 Expanding recombining plasma: (no loss of ionization in argon): T e 0.2 ev, n e /n Ar ~ 0.01 With C n H m seeded in: Mainly charge transfer + dissociative recombination Idea: high efficiency conversion of molecules: Deposition. More ions: more chemistry (New) molecules formed at (stainless) steel walls 3

4 with injection C 2 H 2 Example: a-c:h deposition with Ar + remote source + C 2 H 2 injected CH* emission: blue Detailed data available on n e, T e, radicals, molecules, temperature, flow patterns, shock, velocities C 2 H 2 CH* (and C 2 *, CN*) Cemission 2 H 2 sign of formation of radicals by dissociative recombination of parent molecules: C 2 H 2 and HCN, reformed at surfaces. H 2 formation at surfaces & carbon loss in gas phase : first surface (clusters) then molecules? Thus plasma: More ions: more chemistry

5 Ionization loss in H 2 Hydrogen plasma expansion: loss of ionization: MAR molecular assisted recombination H 2 (r,v) Additional electron & ion recombination (MAR) Atomic lines (from molecular processes): H + + H 2 H 2+ + H & diss rec H α e + H 2 H - + H & mutual rec H n=5 H 2 (r,v) At higher downstream pressure By increasing power density MAR can be reduced, thus n e enhanced! H 2 (r,v) H 2 molecules may help other chemistry What produces H 2? H 2 (r,v) H + H 2+ H + H α red light e H - + H n+ H βγδ blue light

6 H 2 produced at surface Measured: VUV LIF: 4 shifted UV lines scanned together and fitted to a 2T distribution: absolute local densities! In plasma: H 2 () molecules formed at H covered surface: H 2 () excited: in hot process: Thus recombined molecules have high internal energy H 2 are the precursors for MAR! O Gabriel, D C Schram R Engeln, Physical Review E78 (2008) Plasma: arrival rate radicals >> LH desorption rate: saturated surface!

7 H 2, D 2, HD generated at surface Plasma generated H/D atom recombination at surfaces: H 2,HD, D 2. In same plasma distributions species dependent O Gabriel, D C Schram R Engeln, J. Chem. Phys. 132 (2010) H 2 : may diminish ion flow, thus chemistry

8 Even in Tokamaks: Fusion plasma JET: Also here MAR: see H α at surface. Also clusters and dust close to surface Phenomena general! JET in H α

9 plasmas in magnetic field B 0 : hydrogen plasma in magnetic field: electrons confined; current confined: power density remains higher & reduced losses: Higher n e, T e ; conductivity remains Coulomb: higher P/Vol higher n e! small current, low B field High current, high B field W.E.N. Harskamp et al Plasma Sources Sci. Technol. 21 (2012) A.E. Shumack et al Physical Review E 78, With magnetic field: higher current density & power density: higher electron density total number electrons and thus higher chemical capacity! Thus essential : current/ power/ dissipation: determines fate!

10 Magnetized plasma: conditions Questions are: are electrons magnetized? (Coulomb collisions dominate): Ω e τ e > 1: H ei = T eev 3/2 B 0 /(n e lnλ) > 1 If electrons magnetized: then also ions confined! Then strong reduction diffusion losses perpendicular to magnetic field : n e higher at same power density : and thus more chemical potential Other question: are electron ion collisions dominant?: n e /n H2 > 10-3 T eev 5/2 : τ ei = T e 3/2 /(n e lnλ) < τ e0 ~ /(n H2 T e ) Then optimum power input from Joule dissipation n e and T e can be made higher in transients?!!

11 Filaments general phenomena? If plasma free to move: plasma tends to make channels with higher local current density and to achieve higher n e and thus locally Coulomb resistivity! locally higher T e and then higher n e : small radius plasma channels with high conductivity, confined by high neutral density. Filamentary discharge in liquid from Paul Ceccato (full extent of the image is 3cm)

12 To the Clouds: structured! The new far-infrared Herschel view shows in spectacular detail the scene playing out around the Horsehead Nebula. This star-forming region appears obscured by dark dust lanes in visible light images, but blazes in full glory in the far-infrared Herschel view.

13 questions: numbers dark clouds Electrons magnetized? (dominance Coulomb collisions): Ω 22 3/2 e τ e > 1: H ei = T eev B 0 /(n e lnλ) >> 1 Yes, for B 0 order µgauss: T; n e = m -3, T e > ev (10K) Then also ions confined & strong reduction diffusion perpendicular to magnetic field : n e higher at same power density : and thus more chemical potential

14 Questions 2 numbers dark clouds Dominance of electron-ion (Coulomb) collisions?: n e /n H2 > 10-3 T e 5/2 : T e in ev τ ei = T e 3/2 /(n e lnλ) < τ e0 ~ /(n H2 T e ) : Yes, Coulomb collisions dominate, if T e = T gas!? Is this a coincidence? I do not think so! Conductivity: σ C = T eev 3/2 / lnλ ~ [A/Vm] for T e = 10-3 ev, T e will increase with current until τ ei ~τ e0 ; then n e and T e increase further. In transients? if local dissipation heats electrons Note: relation between estimated T and n e /n 0? Higher T gas goes together with higher n e /n 0? can see to get a rough estimate?

15 Plasma structure: structure power density! IF: locally higher neutral density : then more CR (and secondary) ionization: Higher T e (?!) and n e (& ion densities), more radicals formed/ more clusters/ and: In magnetic field, magnetic confinement: plasma expands along magnetic field. Currents? because of p e /n e e potential? dissipation & heating! Only V/m already enough? Thus locally: higher electron densities, higher radical densities, more clusters, surface: Electron temperature T e higher: clusters more charged, more chemistry, More molecules formed, more hydrogen atoms, if size > critical then also higher neutral densities: thus enhances effect of local higher density. B 0

16 Plasma? 1. Essential in chemistry 2. Modified surface processes: excited molecules (!?) 3. Ability to carry current in weak electric fields: thus adding power, higher T e 4. Filaments formation? By higher T e, higher conductivity, more power, more n e, more chemistry? 5. Higher T e important also for cluster charge. 6. Tendency to contract? Smaller radius to achieve more power density and n e. 7. Energy balance in current driven plasmas Joule dominated. Also here? 8. Chemistry at periphery? Molecular signatures give larger radius? The matter of current and dissipation is probably center of discussion. It would be nice to have a map of n e and T e!!

17 Consequences Currents: depends on conductivity and E field: E field: > T e in ev/ scale length (10 12 m): 1/10 12 ~ V/m Conductivity: if Coulomb: σ C = T e 3/2 /lnλ J = σ C E = 10-9 A/m 2, P joule = W/m 3. >> power needed to ensure ionization by CR and sec ion: P ion = k CR n 0 ξ P (E ion + 5/2kT e ) ~ ξ P n 0 = ξ P n 0 W Thus Joule dissipation can be more important! Leading to higher T e and thus charge clusters! If Radial size >> λ 0i, then neutrals will be confined by charged particles, which in turn are confined by magnetic field: this will give more neutral density, thus more CR and sec ionization, better confinement and thus will make neutral density (and electron density) even higher. Thus active plasma filaments in a recombining (lower T e ) background of much larger dimensions.