Alcator C-Mod Particle Transport in the Scrape-off Layer and Relationship to Discharge Density Limit in Alcator C-Mod B. LaBombard, R.L. Boivin, M. Greenwald, J. Hughes, B. Lipschultz, D. Mossessian, C.S. Pitcher, J.L. Terry, S.J. Zweben, Alcator C-Mod Group Invited talk BI.6 Presented at the 42nd Annual Meeting of the APS Division of Plasma Physics combined with the th International Congress on Plasma Physics October 23-27, 2 Qu bec City, Canada
Motivation: Cross-field particle transport (Γ ) in SOL and its dependence on discharge conditions impacts tokamak operation & design Γ determines level of plasma/wall interaction in main-chamber neutral pressures (=> confinement) impurity sources => impacts divertor design Low Transport High Transport SOL Core Plasma Core Plasma ions neutrals Divertor C-Mod Q: What will be operating regime in a reactor? Heat convection across separatrix and its scaling may play role in tokamak density limit Goals: Characterize and understand Γ : - empirical scalings - underlying turbulence - relationship to tokamak density limit
Outline of Talk Main-Chamber Recycling Diagnostics: Effective Cross- Field Particle Diffusitives (Deff) & Scalings Cross-Field Heat Convection Horizontal Scanning Probe Midplane Dα Fluctuations Behavior Near Discharge Density Limit 6 5 2 4 Vertical Scanning Probe 8 3 6 Divertor Probes
Scrape-off Layer Density Profiles Exhibit a "Two-Exponential" Decay 2 SOL Density Profiles n e /n G =.43 [Ohmic L-Mode] Limiter Shadow (m-3) 9 8 Near SOL n e /n G =.6 5 5 2 25 Separatrix Far SOL Distance into SOL (mm) Near SOL: steep decay, λ n ~ 2 to 8 mm Far SOL: shallow decay, λ n ~ 8 to > mm At low ne, density at limiter edge is less than ~/ of separatrix density Density at limiter edge increases sharply with increasing ne => Always some level of main-chamber (limiter) recycling
Main Chamber Ionization and Fluxes in the Far SOL Are Large Compared to Flows Towards Div./Baffle 23 Main Chamber Ionization (Dα) Ion Flux (D + /s) 22 Flux on Divertor (Div. Probes) Flow Towards Divertor/Baffle (Mach Probe)..2.5 2 ne/n G Recycling in Main Chamber SOL is primarily balance by fluxes onto main-chamber walls Poloidal flows to divertor/baffle are weak Main-Chamber Recycling Regime (MCR) persists over wide parameter range M.V. Umansky, et al. Phys. Plasmas 5, 3373 (998).
A New View of Particle Transport Processes in SOL Old Paradigm: SOL density decays "exponentially" because... plasma drains along field lines towards divertor/baffle SOL Profiles Density Limiter Shadow Ionization FluxTowards Divertor/Baffle Dominates Separatrix SOL drained by parallel flow New Paradigm: SOL density decays "exponentially" because... cross-field transport velocity increases across SOL, maintaining cross-field flux towards wall SOL Profiles Density Limiter Shadow Flux Towards Limiter/Wall Dominates Velocity Ionization Separatrix
Outline of Talk Main-Chamber Recycling Diagnostics: Effective Cross- Field Particle Diffusitives (Deff) & Scalings Cross-Field Heat Convection Fluctuations Edge Thomson Scattering Tangential- Viewing Ly α Array Horizontal Scanning Probe Behavior Near Discharge Density Limit
-5 5 5 2-5 5 5 2-5 5 5 2 (m (m -3 s - ) (m -2 s - ) -3 )2 (m 2 s - ) In MCR Regime, Cross-Field Diffusion Coefficient Profiles (Deff) can be Inferred Directly from Profile Measurements 9 23 22 2 2.. n Sion Γ Deff. -5 5 5 2 Separatrix Distance into SOL (mm) Limiter Shadow - Density n e /n G.43.27.9 - Ionization Source (from Lyα profiles) - Cross-Field Flux Profile - Effective Diffusion Coefficient: Deff = -Γ / n Persistent trend of Deff increasing by ~ or more with distance from separatrix => variation in Deff reflects variation in n Deff increases with discharge density => Γ gets larger, n gets smaller Method benchmarked against UEDGE modeling Deff (χ eff ) increasing seen before: ASDEX, JT-6, JET,...
Magnitude of Deff in Near SOL is Correlated with Collisionality in Near SOL. Regression Analysis of D eff, 2 mm into SOL Te -3.8 n. Ip -.3 BT.8 64 Ohmic L-Mode Datapoints:.8 < ne < 2.5x 2 m -3.6 < Ip <. MA 4 < B T < 6 tesla.4 < ne/n G <.47 (m2 s-)... (λei/l) -.7 Multiple Correlation Coefficient =.8 4 Parameter Regression: => Suggests (B T /Ip), q, or L dependence (m2 s-). Multiple Correlation Coefficient =.75.... (m2 s-) Parameter Regression: => Statistics point to (λei/l) as most relevant parameter => Deff correlates with local collisionality: Deff ~ (λei /L) -.7 Trend: ne/n G => λei/l => Deff near sep.
5 5 2 5 5 2 Cross-Field Heat Convection to Limiter/Wall Competes with Parallel Conduction Losses to Divertor at Moderate ne/n G Finite Te on open field lines => power conducted to divertor: Qdiv T 7/2 /L δρ' ρ Cross-field particle fluxes (Γ ) => power convected: Qconv ~ 5 Te Γ A sep 5 Electron Temperature (ev). Power Through Flux Surface ρ (MW)... ne/n G =.9 Qdiv ne/n G =.9 ne/n G =.43 ne/n G =.43 Qconv Qconv Qdiv 5 5 2 ρ (mm) Limiter Distance into SOL Shadow Qconv > Qdiv only in far sol Qconv > Qdiv over entire SOL At low density, heat losses in Near SOL are dominated by parallel conduction to Divertor At moderate density, cross-field heat convection to Limiter/Wall exceeds conduction losses to Divertor/Baffle over entire SOL
A New View of Heat Transport Processes in SOL Old Paradigm: (in absence of a "radiating mantle") Parallel e - conduction to divertor dominates heat losses in Near SOL region Te at separatrix (Tsep) is a weak function of λ T e and SOL power (P sol): Psol Tsep (Psol / λ T e )2/7 Modified Paradigm: Parallel e - conduction and cross-field heat convection contribute to heat losses in Near SOL region (charge exchange also can contribute) Psol At low collisionality, parallel conduction regulates Tsep: Tsep (Psol / λ T e )2/7 At high collisionality, heat convection becomes large, Tsep is reduced and is no longer "regulated" by this law!
Outline of Talk Main-Chamber Recycling Diagnostics: Effective Cross- Field Particle Diffusitives (Deff) & Scalings Cross-Field Heat Convection Limiter-Shadow Particle Flux Probe Horizontal Scanning Probe Fluctuations Turbulence Imaging Behavior Near Discharge Density Limit
..5 Time (milliseconds)..5..5 Time (milliseconds)..5..5 Time (milliseconds)..5..5 Time (milliseconds)..5 2 2 2 2 2 Fluctuations Exhibit Different Character in Near and Far SOL Regions ( 2 m -3 ). Near SOL SOL Density Profile Far SOL.2 5 5 2 Distance into SOL (mm) Isat/<Isat> "Snapshots" τ ac ~.2µs 2.2µs 5.5µs 3µs µs..5..5 Time (ms) Limiter Shadow 25 Auto-Correlation Functions Isat/<Isat> (µs) Near SOL (steep n profile): -> moderate amplitude, "random" fluctuations Far SOL (flatter n profile): -> large amplitude, intermittent Isat "bursts" => Consistent with Deff with distance into SOL
2-D Turbulence Imaging: Intermittent, ~ cm Scale "Blobs" of Emission Extend into Far SOL Near SOL Separatrix Density Far SOL Limiter Shadow Turbulence Imaging: 2-D Images of Da emission, looking along field lines at a "plume" from D2 gas nozzle ~ mm spatial resolution 2 µs exposure times 7 ms between exposures 2 cm ~ cm scale blobs intermittently occupy Far SOL zone, and extend to Limiter Shadow Intermittent "Blobs" (2-D imaging) and "Bursts" (probes) are consistent with large density and temperature (?) perturbations rapidly transporting particles and energy to Limiter/Walls More on 2-D edge turbulence Imaging: S.J. Zweben - oral JO.5 Tuesday PM J.L. Terry - poster UP.6 Thursday AM
..2.4 seconds.6.8...2.4 seconds.6.8. (MA) Collisionality at the Separatrix and Heat Convection to Limiter/Wall Increases as Discharge Density Limit is Approached (MW) Diverted Discharge with Ramping ne/n G. 5 Plasma Current.5 2..5 Input Power ne/n G Line Averaged Density Radiated Power λei/l....2.4.6.8. (seconds) Horizontal Scanning Probe records profiles at three times 4 3 2 ( 2 m -3 ).8.4 Heat Convection to Limiter/Wall based on Limiter Particle Flux Probe λei/l, mm into SOL As density limit is approached: λei/l near separatrix drops dramatically Radiation and Convection to Limiter/Wall are comparable and mostly account for input power Near density limit: Radiation + Convection to Walls ~ Input Power
-2 2 4 6 8 2 2 4 6 8 2-2 2 4 6 8 2 Near Density Limit: Large Amplitude, Long- Correlation Time Fluctuations Envelop Entire SOL and Cross Separatrix n / n G at separatrix does not increase!.4. SOL Profiles Density / n G ne/n G.82.4.34 (ev) 6 4 2-2 Electron Temperature (µs) -2 Auto-Correlation Times (Vf data).3. -2 2 4 6 8 2 Separatrix Distance into SOL (mm) RMS Isat/<Isat> Near density limit: SOL n & Te profiles become flat, Tsep low ~ 25 ev! Fluctuations characteristic of "Far SOL" now occur everywhere, even across the separatrix => Consistent with large Convection Losses
Summary C-Mod SOL Core Plasma Divertor SOL density profiles exhibit a "twoexponential" decay: Near and Far SOL Yet, Main-chamber plasma exhausts primarily onto Limiter/Wall surfaces! - Why? - => New Particle Transport Paradigm: Density "decays exponentially" because... cross-field transport (Deff ) increases rapidly with distance into SOL... not because parallel flows "drain" SOL plasma Particle (Deff) and heat convection near separatrix increases with collisionality: Deff ~ (λei /L) -.7 => Heat Transport Paradigm Modified: At moderate collisionality (ne/n G ~.5), crossfield heat convection exceeds conduction losses Tsep no longer regulated by "conduction law": Tsep (Psol / λ T e )2/7
Summary (page 2) Fluctuation behavior supports picture of particle & energy transport increasing with distance into SOL Near SOL: (steep density profile) low amplitude "random" fluctuations Far SOL: (flat density profile) large amplitude intermittent "bursts" in Isat and ~ cm "blobs" in Dα, extending into Limiter Shadow => New Insight on Density Limit Physics: As density limit is approached, λei/l near separatrix drops and transport across the SOL increases dramatically Heat Convection to Limiter/Wall becomes large fraction of input power "Bursty" fluctuations (large transport) occur over entire SOL and begins to attack plasma on closed flux surfaces ~ at limit: Radiation + Convection to Wall ~ Input Power Rapid increase of Heat Convection as edge plasma cools may play role in thermal instability leading to disruption