Evidence for Electromagnetic Fluid Drift Turbulence Controlling the Edge Plasma State in Alcator C-Mod

Transcription

1 Evidence for Electromagnetic Fluid Drift Turbulence Controlling the Edge Plasma State in B. LaBombard, J.W. Hughes, D. Mossessian, M. Greenwald, J.L. Terry, Team Contributed talk CO.00 Presented at the 45th Annual Meeting of the APS Division of Plasma Physics October 7-3, 003 Albuquerque, N.M.

2 Motivation: Quantitative, Physics-Based Description of Edge/SOL Transport is Still Lacking......yet, impact on reactor operation (edge pedestal, main-chamber recycling, density limit physics,...) is apparent Progress is being made on two fronts: - 3-D plasma-fluid turbulence simulation codes DALFTI, NLET, BOUT, PARTURB,... - Detailed diagnosis of profiles/turbulence in experiments Probes, BES and Gas-Puff Turbulence Imaging,... Questions: To what degree do the turbulence-simulations capture experimentally observed plasma response? Do they contain the essential physics? Focus of this talk: Dimensionless 'phase-space' occupied by the plasma edge and its relationship to turbulence theory/simulation

3 3-D Turbulence Theory/Simulations Identify Dimensionless Parameters that Determine Turbulence & Transport State Physics: ElectroMagnetic ~ parallel inductance, finite B, parallel resistivity, Fluid Drift non-linear drift-wave, curvature, 3-D Turbulence "ballooning-like" asymmetries, x-point effects Primary Dimensionless Parameters: Scott []: RDZ []: Collisionality a d ( ) C ~ qr l ei ~ ( l ei ) / q(rl ) / 4 ^ linear drift-resistive ballooning normalization Beta Gradient b ~ nt e B Ê qr Á Ë a MHD ~ n(t i + T e ) B Result: Turbulence character & transport level determined primarily by location in (b, C) or (a MHD, a d ) 'phase-space' Transport = f ( b,c,...) or = g(a MHD,a d,...) q R Example from DALFTI [3]: 0 c e I 0 - Electron Heat Diffusivity a M => strong dependence on a MHD for a MHD > 0. [] Scott, PPCF 39 (997) 635 [] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 8 (998) 4396 [3] Scott, to be published in Phys. Plasmas

4 In Actual Experiments, Gradients are Not Controlled But Constrained to Satisfy Particle & Power Balance Heat fluxes set by input power, particle fluxes set by fueling Electron Heat Diffusivity 0 lines of ~constant heat flux "critical gradient" c e 0 increase x 00 If: Test: - EMFD dimensionless parameters are setting transport levels And: - Transport is a strong function of local gradient 0 0 I Then: - Edge gradients ~(b or a MHD ) may appear as a function of the "other" key dimensionless parameters ~(C or a d ) => edge plasma state constrained to a characteristic curve in (b, C)- or (a MHD, a d )-space 0 - Look at "edge plasma state" in discharges with different l ei and a M q

5 Time (milliseconds) Tests for EMFD transport physics in the SOL should focus on region near separatrix (the Near SOL) Two-zone SOL profiles: (0 0 m -3 ) Isat/<Isat> Near SOL Density Far SOL Limiter Shadow Distance into SOL, r Time (ms) 5 Da-Light Probability Distributions Near SOL Far SOL r=9 mm r=3 mm skewness=0.5 kurtosis=0.5 Gaussians skewness=.0 kurtosis=. 0 3 Da Light: Signal - Mean Far SOL: flattened time-averaged gradients, intermittent 'bursty/blobby' transport Near SOL: steep time-averaged gradients, less 'bursty'

6 Experiment: Collect Edge Profile Data at Different n e and q in Otherwise Identical Discharges Goal: Examine "edge plasma state" with respect to EMFD parameters In particular, vary and l ei q Low-power Ohmic discharges L- and H-mode 7 6 BT, Ip, q Parameter Space q y95 = 6.5 Density:.4 < n/n G <.53 Plasma current: 0.5 < Ip <.0 MA Toroidal field: 4 < BT < 6 tesla Lower single-null Forward & reversed Ip, BT BT (tesla) q y95 = 5 q y95 = Ip (MA) L Elm-Free EDA L:Rev BT, Ip

7 Experiment: Examine "Edge Plasma State", mm into SOL => Location where electron pressure-gradient scale-lengths exhibit minima in all discharges 0 NL=0.66 x0 0 m - NL=0.83 NL=.0 Ohmic L-mode Profiles Edge Thomson Scanning- Probe r Elm-Free EDA r - "Transport barrier" near separatrix is present in both L- and H-mode Ohmic H-mode Profiles Notes: - Cross-checks between Edge Thomson & Scanning Probe look good Gradient scale-lengths similar; similar trends with discharge

8 Result: Gradient Scale-Lengths Near Separatrix Correlate with Collisionality, Scaled According to EMFD Parameters! 0 Data taken at r =.0 mm q y Discharge Conditions: L-mode, normal BT, Ip 0.5 < Ip <.0 MA 4 < BT < 6 tesla L n 30.4 < n/n G <.53 0 q y Ê Á Ë l ei R Ê l ei Á Ë qr / / I P (MA) and L n map to a simple function of C or a d over the full parameter range! Ê l Á ei q Ë R / ~ C / Ê R Á Ë / a d ~ Ê l Á ei që R / Ê Á Ë R / 4 Note: No fitted-parameters! a d computed from directly from data.

9 Result: Pressure-Gradients Near Sep. Scale with a MHD (or b) => Edge plasma state clusters around curve in (a MHD,a d ) space! b ~ R a MHD a MHD ~ nt e B ~ nt e I p q R a d ~ / / 4 Ê l Á ei Ê R Á që R Ë - Electron pressure gradients near the separatrix increase as I p, yielding similar values of a MHD or b for the same value of a d Data taken at r =.0 mm Inaccessible q y I P (MA) increasing plasma density - A sharp boundary in (a MHD, a d )-space for a d < 0.3 is implied by the data, defining a region of inaccessible states - similar behavior suggested by some turbulence simulations... Edge Plasma Phase-Space Suggested by 3-D Turbulence Simulation (Ref. []) Inaccessible Transport Increasing [] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 8 (998) 4396

10 MID_Alpha_Diamag () Result: Direction of Magnetic Field Influences Mapping of Edge Plasma State in EMFD-Space => Indicates additional parameter(s) "control" transport-gradient relationships 0 Data taken at r =.0 mm Similarities in Reversed B T, Ip Discharges: Core density, power input, particle & power fluxes across SOL 3 Differences in Reversed B T, Ip Discharges: Normal BT, Ip. Reversed BT, Ip Larger gradient scale-lengths in edge (~50%) Smaller values of a MHD (x ~/) a MHD 0.8 => same transport fluxes but gradients are weaker a d Note: No Ohmic H-mode observed with reversed field => Suggests link between "additional control parameter(s)" and L-H threshold

11 Summary: Strong Evidence for Electromagnetic Fluid Drift Turbulence Controlling Edge Plasma State - Plasma profiles near separatrix examined in ohmic discharges with range of densities, currents, fields Edge plasma state clusters around curve in (b, C)- or (a MHD, a d ) -space => consistent with transport being a strong function of EMFD parameters, as seen in 3-D turbulence simulations - Reversed-field data map to different characteristic curve in EMFD-space => evidence for additional parameter(s) controlling flux-gradient relationships => possible link to L-H threshold conditions Conclusion: - endorsement for EMFD description of edge turbulence and transport but, need to: - resolve additional control parameter(s) (theory and experiment) - resolve quantitative differences in flux-gradient relationships (theory vs. exp.) - connect with transport phenomena in Far SOL ('blob' propagation zone)