Critical edge gradients and flows with reversed magnetic field in Alcator C-Mod

Transcription

1 Critical edge gradients and flows with reversed magnetic field in B. LaBombard, N. Smick, M. Greenwald, J.W. Hughes B. Lipschultz, K. Marr, J.L. Terry, Team Contributed talk JO Presented at the 48th Annual Meeting of the APS Division of Plasma Physics October 30 November 3, 2006 Philadelphia, PA

2 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal

3 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal 'Critical gradient' behavior of L-mode pressure profiles ( Achievable value of affected by topology (LSN/USN) α MHD ) near sep.

4 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal 'Critical gradient' behavior of L-mode pressure profiles ( Achievable value of affected by topology (LSN/USN) ) near sep. Strong (near-sonic) plasma flows just outside the LCFS Flow direction connected to magnetic topology (LSN/USN)

5 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal 'Critical gradient' behavior of L-mode pressure profiles ( Achievable value of affected by topology (LSN/USN) ) near sep. Strong (near-sonic) plasma flows just outside the LCFS Flow direction connected to magnetic topology (LSN/USN) Potential link between 'critical gradient' behavior and SOL flows

6 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal 'Critical gradient' behavior of L-mode pressure profiles ( Achievable value of affected by topology (LSN/USN) ) near sep. Strong (near-sonic) plasma flows just outside the LCFS Flow direction connected to magnetic topology (LSN/USN) Potential link between 'critical gradient' behavior and SOL flows esults were obtained with 'normal' direction of B T and Ip ( down) But LSN/USN discharges operate with different divertor geometry......measurement locations not identical in LSN/USN

7 Background: ecent experiments have revealed important aspects of transport physics at the SOL interface......which may be fundamental to understanding the edge pedestal 'Critical gradient' behavior of L-mode pressure profiles ( Achievable value of affected by topology (LSN/USN) ) near sep. Strong (near-sonic) plasma flows just outside the LCFS Flow direction connected to magnetic topology (LSN/USN) Potential link between 'critical gradient' behavior and SOL flows esults were obtained with 'normal' direction of B T and Ip ( down) But LSN/USN discharges operate with different divertor geometry......measurement locations not identical in LSN/USN This talk: New results with 'reversed' direction of B T and Ip

8 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality

9 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space ~ nt e B 2 ~ p e I p 2 2 L p e mm outside sep. Inaccessible I P (MA) inverse collisionality parameter ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < 0.53 Nuclear Fusion 45 (2005) 1658.

10 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space ~ nt e B 2 ~ p e I p 2 2 L p e inverse collisionality parameter 2 mm outside sep. Inaccessible I P (MA) ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < makes contact with Electromagnetic Fluid Drift Turbulence simulations 'Phase Space' of EMFDT Inaccessible Transport Increasing α d <== increasing ne Nuclear Fusion 45 (2005) ogers, Drake, and Zeiler, Phys. ev. Lett. 81 (1998) 4396.

11 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space Extended BT, Ip, range ('normal') and lower vs upper-null topologies investigated in 2005/6 ~ nt e B 2 ~ p e I p 2 2 L p e mm outside sep. Inaccessible I P (MA) inverse collisionality parameter ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < 0.53 Nuclear Fusion 45 (2005) 1658.

12 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space 'eversed field' lower and upper-null topologies investigated in 2006 ~ nt e B 2 ~ p e I p 2 2 L p e mm outside sep. Inaccessible I P (MA) inverse collisionality parameter ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < 0.53 Nuclear Fusion 45 (2005) 1658.

13 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space Measurements from Scanning Langmuir-Mach probes on High and Low-field SOLs... ~ nt e B 2 ~ p e I p 2 2 L p e mm outside sep. Inaccessible I P (MA) inverse collisionality parameter ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < 0.53 Nuclear Fusion 45 (2005) 1658.

14 Background: Pressure gradients near sep. are 'clamped' at a value of, dependent on collisionality ~ nt e B 2 ~ p e I p 2 Data from 2000 campaign: Discharges with different BT, Ip, ne map to same dimensionless space 2 L p e inverse collisionality parameter 2 mm outside sep. Inaccessible Nuclear Fusion 45 (2005) I P (MA) ( ) 1/ 4 1 λ ei L ~ α n d <== increasing ne Low-power Ohmic L-mode discharges Density: 0.14 < n/n G < 0.53 Measurements from Scanning Langmuir-Mach probes on High and Low-field SOLs in 'Near SOL', i.e., 1-2 mm from sep. 1.0 Density (10 20 m -3 ) 0.1 Near SOL Far SOL Limiter SOL Distance into the SOL (mm)

15 'Normal Field' esults (2005 & 2006) - Pressure gradients near sep. consistently scale as Ip 2... but value depends on lower / upper X-point topology nt e MA 0.8 MA ev m -3 mm -1 nt e MA 1.1 MA 0.8 MA 1 MA 0 ( ) 1 λ ei L ~ n α d 1/ 4

16 'Normal Field' esults (2005 & 2006) - Pressure gradients near sep. consistently scale as Ip 2... but value depends on lower / upper X-point topology nt e ev m -3 mm -1 nt e MA MA 2 1 MA MA MA 2 MA 1 0 ( ) 1 λ ei L ~ n α d 1/ 4 ~ Edge plasma states again align in EMFDT phase-space, but in two bands nt e I p ρ = 1 mm Lower null achieves higher values of compared to upper null at high collisionality

17 'Normal Field' esults (2005 & 2006) - Pressure gradients near sep. consistently scale as Ip 2... but value depends on lower / upper X-point topology nt e ev m -3 mm -1 nt e MA MA 1.1 MA 0.8 MA 1.1 MA 0.8 MA 0 ( ) 1 λ ei L ~ n α d 1/ 4 ~ Edge plasma states again align in EMFDT phase-space, but in two bands nt e I p ρ = 1 mm Lower null achieves higher values of compared to upper null at high collisionality Is the x-point effect real? Look at LSN/USN with reversed fields...

18 'eversed Field' Data (2006) - Upper x-point discharges yield highest eversed field, 0.8 MA 0.8 ρ = 1 mm 0.6

19 'eversed Field' Data (2006) - Upper x-point discharges yield highest eversed field, 0.8 MA Forward field, 0.8 MA 0.8 ρ = 1 mm 0.8 ρ = 1 mm towards x-point results in higher at high collisionality

20 'eversed Field' Data (2006) - Upper x-point discharges yield highest eversed field, 0.8 MA Forward field, 0.8 MA 0.8 ρ = 1 mm 0.8 ρ = 1 mm towards x-point results in higher at high collisionality... but 'phase space' mapping is different in reversed vs. forward B

21 'eversed Field' Data (2006) - Upper x-point discharges yield highest eversed field, 0.8 MA Forward field, 0.8 MA 0.8 ρ = 1 mm 0.8 ρ = 1 mm towards x-point results in higher at high collisionality... but 'phase space' mapping is different in reversed vs. forward B Differences may be related to SOL flows, which is next topic...

22 Near-sonic parallel plasma flow seen on high-field side SOL in single-null discharges Parallel Flow vs. Inverse Collisionality mm outside separatrix Parallel Flow Mach Number Nuclear Fusion 44 (2004) 1047.

23 Near-sonic parallel plasma flow seen on high-field side SOL in single-null discharges Parallel Flow vs. Inverse Collisionality mm outside separatrix (2006) Parallel Flow Mach Number Direction of flow depends on x-point location NOT Similar magnitude with forward and reversed magnetic fields Nuclear Fusion 44 (2004) 1047.

24 Near-sonic parallel plasma flow seen on high-field side SOL in single-null discharges Parallel Flow vs. Inverse Collisionality mm outside separatrix Parallel Flow Mach Number Direction of flow depends on x-point location NOT Similar magnitude with forward and reversed magnetic fields => High-field side flows are primarily 'transport driven' Nuclear Fusion 44 (2004) 1047.

25 Parallel plasma flows on low-field side SOL depend on B-field direction, modulated by x-point location Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix Parallel Flow Mach Number - -

26 Parallel plasma flows on low-field side SOL depend on B-field direction, modulated by x-point location Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix Parallel Flow Mach Number - -

27 Parallel plasma flows on low-field side SOL depend on B-field direction, modulated by x-point location Parallel Flow Mach Number Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix - - Direction of parallel flow depends on B-field direction => Mix of co-current Toroidal otation + Pfirsch-Schluter components

28 Parallel plasma flows on low-field side SOL depend on B-field direction, modulated by x-point location Parallel Flow Mach Number Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix - - Direction of parallel flow depends on B-field direction => Mix of co-current Toroidal otation + Pfirsch-Schluter components Magnitude of flow is sensitive to x-point location

29 Parallel plasma flows on low-field side SOL depend on B-field direction, modulated by x-point location Parallel Flow Mach Number Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix I p - - I p Direction of parallel flow depends on B-field direction => Mix of co-current Toroidal otation + Pfirsch-Schluter components Magnitude of flow is sensitive to x-point location Parallel flows are strongest when 'transport-driven' SOL flow is in co-current direction => toward x-point

30 Data suggest 'transport-driven' SOL flows and x-point topology may affect local achieved in L-mode

31 Data suggest 'transport-driven' SOL flows and x-point topology may affect local achieved in L-mode Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix Parallel Flow Mach Number - -

32 Data suggest 'transport-driven' SOL flows and x-point topology may affect local achieved in L-mode Parallel Flow Mach Number Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix vs. Inverse Collisionality 1 mm outside separatrix ρ = 1 mm

33 Data suggest 'transport-driven' SOL flows and x-point topology may affect local achieved in L-mode Parallel Flow Mach Number Parallel Flow vs. Inverse Collisionality 1 mm outside separatrix - - I p I p vs. Inverse Collisionality 1 mm outside separatrix ρ = 1 mm Trend of higher when 'transport-driven' flow is in co-current direction => toward x-point

34 Summary Extended Ip, B T range and 'reversed-field' operation have allowed important tests of edge 'critical gradient' and flow observations in

35 Summary Extended Ip, B T range and 'reversed-field' operation have allowed important tests of edge 'critical gradient' and flow observations in Near SOL of L-mode plasma organizes toward a ~critical gradient ( ); attainable depends on collisionality Topologies with toward x-point lead to higher

36 Summary Extended Ip, B T range and 'reversed-field' operation have allowed important tests of edge 'critical gradient' and flow observations in Near SOL of L-mode plasma organizes toward a ~critical gradient ( ); attainable depends on collisionality Topologies with toward x-point lead to higher Near-sonic flows in high-field SOL depend on x-point location NOT Supports 'transport-driven flow hypothesis Co-current SOL flows persist in low-field SOL with toward x-point => when transport-driven flow can 'spin-up' plasma in co-current direction

37 Summary Extended Ip, B T range and 'reversed-field' operation have allowed important tests of edge 'critical gradient' and flow observations in Near SOL of L-mode plasma organizes toward a ~critical gradient ( ); attainable depends on collisionality Topologies with toward x-point lead to higher Near-sonic flows in high-field SOL depend on x-point location NOT Supports 'transport-driven flow hypothesis Co-current SOL flows persist in low-field SOL with toward x-point => when transport-driven flow can 'spin-up' plasma in co-current direction Observations suggest a connection between SOL flows and attainable => Higher obtained in L-mode with 'favorable', co-current SOL flows