Max-Planck-Institut für Plasmaphysik EURATOM Assoziation Interaction of nitrogen plasmas with tungsten Klaus Schmid, A. Manhard, Ch. Linsmeier, A. Wiltner, T. Schwarz-Selinger, W. Jacob, Stephan Mändl

K. Schmid 2009 Introduction Summary Outline Experiment & Results N accumulation & sputtering of W Nitride phase formation

K. Schmid 2009 Introduction ASDEX Upgrade (now full W) experiments show performance increase for N 2 seeding in the divertor compared to Ne or Ar [1]: Why N and W ? [1] A. Kallenbach, et. Al. Nuclear Fusion 49 (2009) Question: Influence of nitrogen ions on tungsten PFCs ? Sputtering of tungsten by nitrogen Nitrogen accumulation in tungsten Thermodynamics and thermal stability of the tungsten-nitrogen system Perform experiments on: + Improved energy confinement + Smaller ELMs N 2 gas puffing is used for edge plasma cooling in high-Z PFC fusion experiments

K. Schmid 2009 Experiment & Results N-accumulation and Sputtering of W by N grid Sample holder U DC, bias ECR plasma ion source with freely expanding plasma beam Homogenous irradiation of samples Ion energy by DC biasing up to 500V Water cooled sample holder Ion flux & energy distribution measured by retarding field analyzer: 3 to 4x10 18 N y + (m -2 s) Molecular ion distribution measured by plasma monitor Dominantly N 2 + ions PLAQ Samples W Cu Si PVD tungsten layer ~ 500 nm PVD copper interlayer ~100 nm stress relief Silicon substrate well-defined surface for RBS analysis Implantation setup

K. Schmid 2009 Experiment & Results N-accumulation and Sputtering of W by N N 2 plasma flux and composition Retarding field analyzer measurements Flux :3 - 4 x N 2 + / cm 2 s in main peak Most ions have energy corresponding to bias Plasma monitor measurements Plasma dominated by N 2 + ions N 3 + ions occur at highest pressures ~1Pa N + ions maintain constant level with pressure Implantations were performed at 0.25 Pa 90% N 2 +

K. Schmid 2009 Experiment & Results N-accumulation and Sputtering of W by N Measuring N accumulation in W by nuclear reaction NRA with 3.8 MeV 3He Reaction used: 14N ( 3He, p ) 16O Sensitivity: N/cm cts/ C Peak integral of p1 + p2 protons is evaluated to obtain the N areal density Measuring W sputtering via thickness change of W layer Rutherford backscattering (2.3 MeV 4He) Yields tungsten layer thickness Sensitivity: W atoms / cm 2

K. Schmid 2009 Experiment & Results N-accumulation and Sputtering of W by N Implantation in plasma ion source PlaQ Low energy ions Sample temperature room temperature Quick saturation of retained N amount TRIDYN calculations predict similar levels Diffusion of N in W low (similar to C in W) N accumulation controlled by the implantation range For our low implantation energies one expects accumulation in the range of:

K. Schmid 2009 Experiment & Results N-accumulation and Sputtering of W by N Sputter yield of W by N from an N 2 plasma Sputter yield is much lower that expected from static TRIM Accumulation of N in W surface reduces partial sputter yield Good agreement between dynamic TRIM and experiment (dyn. Surface evolution) For Ne no accumulation in W surface Ne Sputter yield matches static TRIM N accumulation in W surface shields W from erosion by N Could partly be the reason for the good AUG performance with N-puffing

K. Schmid 2009 Experiment & Results Nitride phase formation Bombardment of W by 3keV N ion beam XPS analysis to identify nitride formation Literature data on W4f shift due to nitride formation varies strongly We find a shift of 0.45eV for nitride peaks. (lies within the literature data range) The intensity of nitride phase decreases with temperature

K. Schmid 2009 Experiment & Results Nitride phase formation W-N Phase diagram calculated by ThermoCalc TM Based on very little available data N 2 gas phase suppressed At ambient pressures WN instable above ~600K XPS measurements confirm thermodynamic modeling WN decomposes at high temperatures and N is lost as degassing N 2.

K. Schmid 2009 Plasma immersion ion implantation at IOM Leipzig 10 kV pulses Sample heated by pulses Implantation fluence comparable to plasma implantation Retained amount of N decreases with temperature above 650K Experiment & Results Nitride phase formation Decay above ~650K Nicely fits predictions by thermodynamic modeling As expected: Total accumulated amount higher than in our plasma implantation due to higher energies

K. Schmid 2009 Summary N accumulation quickly saturates once implantation range is filled with nitride phase Total amount of N is determined by N energy via the implantation range N accumulation in the surface leads to a reduction in the partial W sputter yield Advantage over noble gas seeding species XPS measurements of nitride fraction & measurements of total N amount indicate that the nitride decomposes at elevated temperatures This is line with thermodynamic calculations by ThermoCalc TM