Thomas Schwarz-Selinger. Max-Planck-Institut for Plasmaphysics, Garching Material Science Division Reactive Plasma Processes

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1 Max-Planck-Institut für Plasmaphysik Thomas Schwarz-Selinger Max-Planck-Institut for Plasmaphysics, Garching Material Science Division Reactive Plasma Processes personal research interests / latest work

2 my research background Plasma-Wall-Interaction at low Particle Energies: Deposition, Erosion and Hydrogen Retention of Amorphous, Hydrogenated Carbon Thin Films (a-c:h) model studies with lab experiments for Divertor conditions (mev- 100 ev)

3 my focus: growth, stability and erosion of a-c:h films power reactive gas methane bulk plasma neutrals radicals ions sheath electrons chemistry physics abstraction addition cross linking annealing growth zone synergisms implantation sputtering displacement bulk

4 growth and erosion of hydrogenated, amorphous carbon Motivation: Tritium Redeposition Joint European Torus JET Divertor graphite tile hydrocarbon (a-c:h) flakes (later a-c:t)

5 Max-Planck-Institut für Plasmaphysik (IPP), Garching, Germany IPP Greifswald: founded 1994 W7 X IPP Berlin: founded 1992 Plasma generator PSI-1 IPP Garching: founded 1960 W7 AS ASDEX Upgrade aim of research: Investigate basic conditions for a fusion power plant that generates energy similar like the sun from fusion of atomic nuclei Garching: Tokamak: ASDEX Upgrade Stellarator: Wendelstein W7-AS Plasma Heating Theory Plasma Wall Interaction

6 film growth and erosion studies: two approaches 1. quantified beam experiments 2. plasma experiments substrate electrode ECR cc-rf plasma pulsed ICP H Ar + CH 3 el l i p so met r y F T I R diagnostics: ellipsometry FTIR QMS, PM, RFA cavities... substrate + "easy" interpretation + isolation of microscopic mechanism "artificial plasma" + real life interpretation ambiguous complex particle zoo quantification of fluxes? modelling of fluxes

7 sticking coefficients of hydrocarbons at low energy PhD work Klaus Tichmann Setup: ion gun mass selected ions energy down to 10 ev in-situ ellipsometry Deposition pattern measured by 2-d ellipsometry First experiment: 150 ev CH 3 + ions on a-c:h surface measurement of sample current number of impinging carbon atoms 2D ellipsometry of grown surface number of deposited carbon atoms sticking probability of selected ion in this case: about 1 (still subject to large uncertainty)

8 sticking coefficients of hydrocarbons at low energy MD simulations with HCParCas Code (K. Tichmann, U. von Toussaint) sticking of C x H y ions on a-c:h surfaces sticking # of C atoms per impact s(c 2 H 6 ) s(c 2 H 2 ) s(ch 3 ) Bombardment of a-c:h films with C x H y ions Determination of sticking coefficients Comparision with well definded experiments Other data from MD: reflection coeff. refl. species energy of species ion energy (ev)

9 film growth and erosion studies: two approaches 1. quantified beam experiments 2. plasma experiments substrate electrode ECR cc-rf plasma pulsed ICP H Ar + CH 3 el l i p so met r y F T I R diagnostics: ellipsometry FTIR QMS, PM, RFA cavities... substrate + "easy" interpretation + isolation of microscopic mechanism "artificial plasma" + real life interpretation ambiguous complex particle zoo quantification of fluxes? modelling of fluxes

10 plasma chemistry of CxHy plasmas Balzers QMG 422 gassupply H I D E N E Q P p l a s ma mo n i t o r EDMS energy dispersive mass spectrometer + retarding field analyzer pumps plasma rf-bias load lock substrate MBMS molecular beam mass spectrometer shutter quartz ICP coil ellipsometry

11 plasma chemistry of CxHy plasmas (Dipl. work M.Pietzka + PhD work M.Sode) aim: time resolved measurements of ions, neutrals and radicals to CH 3 verify input parameters of plasma boundary codes signal QMS (counts/sec) CH 4, 2 Pa E mean = amu/e open flag plasma on plasma off σ cps plasma on τ = 0.04 sec equivalent β = 2.2 x 10-3 CH 3 (ITMS) τ = 3 sec equivalent β = 3 x scan voltage U e time (ms)

12 challenge for film removal: gaps everywhere plasma exposed surface remote surfaces animated view inside JET EU Prototype of Inner Vertical Target with CfC & W Armour

13 erosion of carbon in tile gap structures by oxygen plasmas model system for redeposited films: amorphous, hydrogenated carbon (a-c:h) different oxygen plasmas at p 1 Pa ECR afterglow (thermal energies) ECR plasma (ion energy 15 ev) CCP mode (ion energy 300 ev) analysis by ellipsometry before exposure 2D scan during exposure locally ex-situ 2D scan of all surfaces

14 erosion in ECR oxygen afterglow (thermal particle energies) side wall erosion drops faster for smaller gaps! eroded film thickness side walls (Å) ECR afterglow / 4 mm (r/l) / 2 mm (r/l) / 1 mm (r/l) / 0.5 mm (r/l) ellipsometry penetration depth (mm) p = 0.5 Pa, P MW = 75 Watt t = 5740 min

15 film characterization 3. Thermal desorption spectroscopy movable tubular oven T max = 1300 K + a-c:h removal + a-c:h characterization sample position sample position for MBMS

16 Thermal desorption of amourphous hydrogenated carbon thin films T. Dürbeck, T. Schwarz-Selinger, U. von Toussaint, W. Ja signal QMS (cps) species monitoring with QMS linear T ramp from RT to 1300 K different hold temperatures different hold times different ramps (3 to 40 K/min) T=875K H K/min oven temperature (K) signal QMS (cps) signal QMS (cps) H 2 ref hold 10 min restart cycle Nb H 2 CH oven temperature (K) oven temperature (K) sample temperature (K) cycle Nb

17 Variation of T ramp: Redhead analysis of TDS spectra T. Dürbeck, T. Schwarz-Selinger, U. von Toussaint, Binding energy distribution (BED) for H 2 release from a-c:h 13 Redhead analysis: Peak temperature vs. heating ramp ev BED 5pt av. ln(c*t 2 /heating rate) N(t) = -dσ/dt = ν σ 0 exp(-e act /kt) with T = T 0 + βt rate constant (pre-exponential factor) ν = 5.1 * s -1 population density (1/eV) ev / T peak (10-3 K -1 ) binding energy (ev). Determination of pre-exponential factor and binding energy distribtion (BED) [one BED explains all results for the different ramps from 3 K/min up to 40 K/min] Diffusion does not seem to play a dominant role in thermal decomposition of a-c:h.