Dust formation : speculated mechanism N i = density of particles with a size i R = nucleation rate (estimated from the chemical kinetics model) G = coagulation/agglomeration rate (two particles  larger particles) W = growth rate (surface growth - heterogeneous chemistry) T = particle losses due to transport : diffusion, thermophoresis, drag,... C, C 2, C 3 Ar + /H 3 + sputtering/chemical sputtering/erosion Gas phase Chemistry nucleation Surface growth Coagulation Agglomeration

Estimation of discharge main characteristics: flux and ion energy distribution or ion average energy on the cathode Extraction of C 1, C 2 et C 3 from the substrate surface Chemistry and molecular growth  Formation of C n=1,nl clusters, where n l is arbitrary chosen (n l =30 or 60) Nucleation of carbon dusts from clusters: Assumption of ‘Largest Molecular Edifice’ Growth, charging, transport and wall losses of dusts Feed back on the gas phase chemistry  heterogeneous process Size distribution of dusts Model of nucleation, growth and transport of dust in DC discharges ignited in Ar/H 2 (2)

Molecular growth modelling of carbon clusters and dusts n i,z = density of the cluster C i of charge z Nucleation Dust Transport N = nucleation C = coagulation A = condensation Molecular growth MobilityDiffusion Gas phase chemistry and molecular growth Production rate of the C i cluster  Determination of the average diamater d p clusters

Molecular growth of clusters –Rates computed according to formation enthalpies –Clusters have configurational isomers (chains, rings, multi-cycles) distinguished by cyclization entropy (20 kcal/mol/cycle) –Extrapolation for unknown values according to cluster periodicities Bernholc & Schweigert models (classical models) (**) : Carbon cluster growth reactions ** Growth = one single process (C n + C x  C n+x ), but take into account the stability of the C n clusters First version of the model took into account neutral clusters

 Low pressure discharge : p=1-10 Pa  Diffusion characteristic time =1-10 ms very short as compared to the growth chemistry  no possibility for growth of neutral  Need for species with higher residence time : Negative clusters And Trapping electric field configuration  Back to some basic discharge physics Molecular growth modelling of neutral carbon clusters and dusts

Electric field reversal and molecular growth of negative clusters Charging of dust particles only effective if electric field is confining ! Where is the confining electric field ?  Kolobov & Tsendin, Phys. Rev. A , Boeuf &PitchFord, J. Phys. D, (1994) –Self-consistent electric field reversal: confinement –Three electron populations: energetic, passing, trapped NG: Negative glow / FDS: Faraday Dark Space / PC: Positive Column and negative ions EeEe x sheath dcdc E0E0 x0x0 x1x1 ~ < 1 V NGFDS PC Trapped electrons (n e ) R Energetic electrons (  ) Passing electrons (j) 2 V 0 /d c

Negative carbon cluster growth reactions Attachment C n + e -  C n - – Rates computed according to electronic affinities Charge exchange C n - + C x  C n + C x - – Electronic affinities From Y. Achiba et al., J. Elect. Spect. Related Phen. 142, 231 (2005) Dust agglomeration (sticking) Detachment C n - + e -  C n + 2e -

carbon particles aerosol dynamic in a DC dicharge Particle charging is a key point : ==> Enhanced particle charging insures a significant trapping and long residence time ==> Enhanced particle charging prevents coagulation and growth Z U=zV Z Z' k coag ====> Z+Z' K coag (z,z’)

The only way to have growth ==> charge fluctuation and electron depletion Possible because particle charg ing is a discrete process  Dynamic fluctuation of small particles between positively and negatively charged states  Coagulation takes place between two particles that has opposite instantanous charges or no charge  involve small particles.  coag <<  fluctuation <<  trans Transport feels the average charge Coagulation feels the fluctuations Fluctuation

Molecular growth of negative clusters Negative clusters have significant densities  Growth rate is a function of the electric field profile in the discharge  An accurate knowledge of the field profile is required

Dust density Electric field reversal electron average energy in the NG  E  Cathode Anode EE npnp n p | max =10 13 cm -3 n p | max =5x10 11 cm -3 =0.1 eV =1 eV Field reversal

charge position (cm) 0.03 eV 0.1eV 1eV x x x x x x x10 -7 diam è tre (cm) position (cm) 0.03 eV 0.1 eV 1 eV Cathode Anode <e> <e> Dust average charge and diameter Cathode Anode It is indeed possible to explain particle formation through negative ion driven molecular growth  Discharge dynamic (field reversal) and sputtering kinetics are key-points Pbs : we need better description of the growth kinetics : Model  1 hour for dust formation (instead of few minutes) Take into account the size and charge distributions

CASIMIR Device (Chemical Ablation, Sputtering, Ionization, Multi-wall Interaction, and Redeposition ) 3 rd module : Redeposition chamber - Collection of the deposit : filter and substrate) 2 nd module : Microwave plasma source "surfaguide" Decoupling gas phase and surface process 1 st module : Sputering/erosion of carbon susbtarte (H 2 /Ar plasmas) - Multipolar microwave discharge - Gaz = H 2 /Ar, Pressure mbar - carbon Substrate (Controled temperature and voltage)

Measurement techniques Mass spectrometer / ion energy analyzer - Detection of neutral and radivcalar species in the plasma (m/z uma) - Detection of positive et négative ions - Measurement of IEDF (+/ eV) Optical Emission Spectroscopy (H/D et carbonated species) (temperature and density measurements and characterization of plasma species in CASIMIR) Analysis of the deposit microstructure by SEM and Raman

Results I. Mass spectrometry: Polarisation Sheath graphite disc substrate Photography of the negatively polarized disc substrate in Ar/H 2 Polarisation Sheath Plane Substrat Photography of the plane polarized substrate in Ar/H 2 plasma

Resultts b) Mass spectrometry and IEDF measurements : Ions in the discharge H +, H 2 +, H 3 + mass spectra (0,60 kW, 100 sccm) D +, D 2 +, D 3 + mass spectra (0,60 kW, 100 sccm) Ar 2+, Ar + mass spectra (0,60 kW, 10 sccm) D - mass spectrum (0,60 kW, 100 sccm) I. Spectromètre de masse / analyseur d’énergie :

Results c) IEDF D + and Ar + IEDF’s D+D+ Ar +

Results I.2) deuxième études : sur la tête uma a)Hydrocarbon production through erosion/sputtering in CASIMIR Mass spectra in H 2, Ar, et Ar/H 2 plasma I. Carbon detection : (1): E between 9,8 and 14,25 eV CH 3 + e - => CH e - (in the plasma) (2) : E > 14,25 eV CH 4 + e - => CH H + 2 e - (in the analyzer) Threshold mode detection of CH 3 radical CH 3 Detection of C, CH, CH 3,CH 4 et C 2

Results I. Mass spectrometry: b) Effetc of the polarisation on the erosion yield Voltage contrôle  microarcs 600 V – 2 A Mass spectrum in H 2 plasma With and without polarisation (Alim1) Comparaison of masse spectra obtained with the two contrôle modes in Ar/H 2 plasma Courant contrôle 300 mA – 1000 V