Divertor and Edge Physics

Transcription

1 Divertor and Edge Physics Goals Recent results Near-term plans Five-year outlook Presented by B. LaBombard

2 Divertor and Edge Physics Research Goals! Advance physics understanding in key divertor and edge science areas: Edge plasma transport Neutral dynamics and fueling Impurity sources and transport Benchmark: predictive capability, scaleable to reactor! Identify and develop control techniques for present and future experiments: Density control Impurity control High divertor heat flux handling

3 Edge Plasma Transport Recent experiments have clarified key role of transport: Low Transport Ionization High Transport FluxTowards Divertor/Baffle Dominates! Impacts tokamak operation and divertor design Sets plasma/wall interaction level in main-chamber: Main-chamber neutrals (=> confinement?) Wall impurity sources and screening! Must be understood for predictive modeling Heat convection to wall may precipitate divertor detachment Particle transport is bursty => diffusive paradigm in SOL/Divertor simulations is inadequate! Ionization Flux Towards Limiter/Wall Dominates! May be key element in density limit physics Near Density Limit, cross-field convection dominates heat flux across SOL, impacts discharge power balance => empirical scaling of density limit may be set by physics of SOL/Edge transport!

4 Time (milliseconds) Edge Plasma Transport: Turbulence Probe data PDFs from Dα Diode data (10 20 m -3 ) Isat/<Isat> Near SOL Density Far SOL Distance into SOL (mm) Limiter Shadow Time (ms) µs 13µs Prob. Dist. Fn. Prob. Dist. Fn ρ=9 mm ρ=3 mm skewness=0.5 kurtosis= Light Signal Gaussians skewness=1.0 kurtosis= Light Signal! SOL: Two zone structure, near and far SOL! Bursty, non-gaussian statistics, rapid transport in far SOL => Consistent with main-chamber recycling phenomenon => Related to density limit physics(?) => What is the physics underlying the bursty transport behavior?

5 Edge Plasma Transport: Turbulence Imaging & Simulation Gas Puff Imaging (GPI) movie of plasma turbulence with PSI-3* camera,12 frames of 64x64 pixels at 4 µsec/frame S. Zweben, J. Terry GPI view R Light Intensity Cross-field propagation of intensity (density) blobs clearly seen. Size scales consistent with other measures. => consistent with rapid transport to main-chamber limiters Plasma Density 5.3 cm Local 3-D turbulence simulation (EM Braginskii fluid code) RBM dominates turbulence Similar transport as in experiment, BUT, a full spectrum of size scales => where are the blobs? R K. Hallatschek, B. Rogers *Princeton Scientific Instruments

6 Edge Plasma Transport: Fluctuation-Induced Fluxes s -1 Electrostatic probes have been used as a primary tool for diagnosing edge plasma turbulence (20+ years), including direct measurement of fluctuation-induced fluxes. Recent results: Probe-inferred fluctuation-induced fluxes are not indicative of fluxes in the unperturbed plasma! 10 1 Particle Flux Profiles Probe Particle Balance Distance into SOL (mm) Comparison with particle-balance derived fluxes:! Fluxes too high! Profile has wrong shape!! Similar puzzle reported on JET, DIII-D,... and evident in early literature! Why? => flux measurement is likely corrupted by the flux drawn to the probe itself => important implications for edge transport analysis! => e.g., Main-chamber recycling phenomena (flat flux profile) completely missed by probeinferred flux profile!

7 Edge Plasma Transport: Turbulence Studies Near Term Plans! Diagnostics Two Xybion cameras, PSI-5 camera => D α & He-II fluctuations Two-color fast-diode measurements => measure n,t e fluctuations Inner wall scanning probe, Diode comparisons of inside and outside SOL! Run plans Turbulence near density limit, in EDA H-modes, RF power scans Radial/poloidal dependence of fluctuation statistics! Theory Support/Numerical Simulations Klaus Hallatschek (IPP Garching) visiting PSFC for 6 months => Non-local turbulence simulations, density limit discharges Comparisons of turbulence imaging output with BOUT code simulations (Nevins, Xu, LLNL)

8 Edge Plasma Transport: Turbulence Studies Long Term Plans! Turbulence imaging upgrades Two-color camera and 2-D fast-diode array imaging =>infer n and T e fluctuating fields Time-delay correlation camera imaging =>track fluctuation propagation Imaging of deep-scanning gas-injection probe => turbulence inside separatrix! Close-coupling to Turbulence Simulations/Theory BOUT simulations Hallatschek/Rogers code ported to Beowulf cluster => Goal: simulate observed blobs, statistics, transport levels, and scalings! Determine validity range of probe-inferred fluctuations Consistency checks with special-geometry probes Direct comparisons of probe spectra (ω, k) with diode spectra Image probe-induced fluctuations with GPI-like system

9 Edge Plasma Transport: Density Limit Physics (ev) 'Deep' Probe-Scan Profile Density/n G Distance into Separatrix SOL (mm) Near Term Plans Electron Temperature Density limit discharges with GPI and deep probe insertion Numerical turbulence simulations (K. Hallatschek) Collaboration with JET to perform similar experiments ne/n G ! Near Density Limit, bursty transport zone begins to cross into closed flux surface regions => large convective heat losses depress T e near separatrix => suggests far SOL transport physics determines discharge density limit Long Term Plans Investigate edge turbulence/transport with improved turbulence diagnostics Numerical turbulence simulations Goal: reproduce evolution of edge turbulence/transport as density limit is approached

10 Neutral Dynamics/Fueling Plasma recycling and neutral density distributions in the divertor and edge plasma are determined by plasma transport, wall structures & neutral baffling, neutral-neutral collisions, affecting:! Fueling location of discharge! Density control capability via pumping! Core confinement(?) (via neutral-plasma interaction) Key Question: What are the relative roles of cross-field transport, limiter recycling, and divertor neutral baffling in setting the neutral density distributions in?

11 Neutral Dynamics/Fueling: Results! Main-chamber recycling comparison experiments: /DIII-D Same trends found: P mid affected by wall recycling, similar-shaped n & T e profiles in SOL, wall flux fraction similar Neutral Pressures vs X-pt Balance Neutral Pressure (mtorr) Midplane Lower Divertor Upper Divertor DN X-pt separation, mapped to midplane (mm) SN! X-point balance experiments P mid insensitive to operation with open vs closed divertor Near DN, upper divertor pressures are high => suggests plasma-plugging phenomenon => allows upper divertor to be used for cryo-pumping, lower divertor for heat flux handling! U. Toronto modeling of divertor neutral pressures OSM-Eirene (with n-n collisions) yields pressures factor of 2 to 10 too low! Indicates missing (or incorrect) physics

12 Neutral Dynamics/Fueling: Plans Upper Cyropump Near Term! SN/DN discharges, variable triangularity Scope dn/dt expected from an upper cryopump Secondary Scrape-off Layer! D 0 Wall inventory and pumping experiments Role of co-deposition (D. Whyte/UCSD)! Benchmark OSM-Eirene code Low density discharges, additional pressure/flow measurements Lower Cyropump! Long pulse experiments Look for D 0 wall-saturation Long Term! Cryopump operation and optimization

13 Impurities Sources and Transport Core and divertor plasma performance critically depends on impurity concentrations. Knowledge of impurity production and transport physics is key to optimizing core plasma performance and wall component lifetimes. Results! DIVIMP modeling of Ar & He divertor impurity compression can reproduce observations => Physics: plasma flows and firstflight impurity MFP! Impurities are better screened on high- vs low-field side! Boron nitride on RF antenna structures: -RF/edge plasma/wall interaction greatly reduced -Core Mo reduced -No impact on edge neutral pressures or density control Near Term Plans! Modeling of main-chamber impurity sources and screening! Inner wall impurity screening experiments => characterize transport outside secondary separatrix on high-field side! Boronization study => Does boronization lead to improved operation by directly controlling impurity sources?

14 High Heat Flux Handling! New inner divertor design Tiles shaped for high heat-flux handling, swept strike-point Thermal design /w inertial cooling accommodates energy of longpulse, full power discharges Near Term Plans! Test advanced divertor materials => Tungsten-brush tiles (SNL)! Evaluate divertor target performance in long-pulse discharges Long Term Plans! New outer divertor => full toroidal ring, no leading edges! Advanced divertor module tests: Tungsten-brush Flowing liquid-metal target

15 5-Year Outlook: New Tools for Science Areas! Turbulence diagnostics upgrades! Close-coupling to Turbulence Simulations/Theory! Upgraded edge Thomson profiles extended into far SOL! Improved plasma flow measurements High radial resolution doppler spectroscopy => assess interplay between plasma flows/turbulence/transport Impurity plume imaging => toroidal image of impurity dispersal => ExB shear! High spatial resolution inner & outer divertor probe arrays! Cryopump! Improved in-vessel neutral pressure measurements! IR imaging of inner and outer divertor target regions

16 5-Year Outlook: Physics Mission Support Program 2002 Unbalance DN operation MW LHCD, 2MA, 8T MW LHCD MW ICRF, Cryopump, 3-sec pulse length MW LHCD MW ICRF 2006 BP/AT optimization sec pulse length 2008 BP/AT comparison Divertor/Edge Support Density control scoping experiments SOL transport, heat fluxes, and impurity response to LHCD, Tungstenbrush tile tests Density control, heat flux handling, SOL profile control for LHCD coupling,bp dissipative divertor development Advanced divertor material tests Outer divertor upgrade, heat flux handling techniques, advanced materials tests BP dissipative divertor, He ash removal assessment for BP and AT scenarios, advanced divertor modules BPX prototype divertor tests

17 Divertor and Edge Physics: Summary! Fundamental contributions are being made in science areas: Edge plasma transport Neutral dynamics and fueling Impurity sources and transport! Scope is consistent with physics mission, providing vital support for overall physics program! Near term and long term plans are in place to maintain productive science output and to support Advanced Tokamak and Burning Plasma thrusts

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