XP-815: Investigation of SOL widths in NSTX H-mode plasmas

1. XP-815: Investigation of SOL widths in NSTX H-mode plasmas J-W. Ahn1, R.Maingi2, J.A.Boedo1,V.Soukhanovskii3 Acknowledgement: B. LeBlanc and R. Kaita for TS data 1University of California – San Diego 2Oak Ridge National Laboratory 3Lawrence Livermore National Laboratory J-W.Ahn, NSTX Results Review, 08/06/2008

2. Introduction • Primary Objectives of XP-815: • 1. To find dependence of near SOL widths on plasma parameters •  comparison with existing scaling laws •  develop empirical scaling laws for NSTX H-mode plasmas • 2. To compare upstream SOL widths and target heat flux width to investigate parallel (and/or perpendicular) SOL transport • 3. To find relation of SOL widths with edge turbulence characteristics • Diagnostics: • Reciprocating mid-plane fast probe – 0.5ms temporal, 1mm spatial resolution for Te and ne measurement • Divertor target IR camera (33ms temporal, 6mm spatial resolution) – target heat flux profile measurement • Thomson Scattering – 60Hz temporal, 1-2cm spatial resolution in the SOL • Edge turbulence measurement: Probe Isat tip, GPI, reflectometry, USXR, mid-plane Dβ camera, etc J-W.Ahn, NSTX Results Review, 08/06/2008

3. Edge density profile is ‘fixed’ during the H-mode density ramping period 1. Te,sep estimated from 2 point model  TS profiles as a function of R-Rsep 2. ne,sep appears to be fixed irrespective of nebar 3. This enables us to ensemble TS profiles at different time slices for identical H-mode shots to construct a profile with higher spatial resolution J-W.Ahn, NSTX Results Review, 08/06/2008

4. Density scan: λTe, λne, and λq t=648ms t=382ms 1. nebar continuously rises during the H-mode, by a factor of ~2, with ne,sep fixed 2. λq stays at 4-5mm over the whole density ramping period 3. λTe and λne also stay constant J-W.Ahn, NSTX Results Review, 08/06/2008

5. Ip scan1: λjsat and λq 1. Ip ↑  λjsat ↓ and λq ↓ 2. R-Rsep for fast probe Jsat data is from equilibrium reconstruction  The apparent radial ‘shift’ of Jsat profiles with respect to each other isthought to be due to the uncertainty in LCFS position determined by equilibrium reconstruction J-W.Ahn, NSTX Results Review, 08/06/2008

6. Ip scan 2: λTe and λne Ip ↑ λTe decreases and λne stays constant J-W.Ahn, NSTX Results Review, 08/06/2008

7. Power scan: λTe and λne 1. PNBI↑  λTe decreases and λne stays constant 2. λq decreases from 0.7cm to 0.5cm J-W.Ahn, NSTX Results Review, 08/06/2008

8. Comparison with H-mode SOL width scaling laws from conventional tokamaks from AUG1 from AUG1 from JET2 1. λTe , λne , and λq dependence on Ip and power: consistent with scaling laws 2. λq dependence on density: consistent with scaling law 3. λTe and λne dependence on density: different trend from scaling laws 1K. McCormick, JNM 266-269 (1999) 99 2W.Fundamenski, NF 44 (2004) 20 J-W.Ahn, NSTX Results Review, 08/06/2008

9. Relation between λq and λTe in Near SOL (conduction-limited regime) Dominant || electron conduction with ||Te (2) Assume offset exponential profiles (1) Assume simple exponential profiles Conventional relation Additional factor introduced J-W.Ahn, NSTX Results Review, 08/06/2008

10. λTe/λq=2 λTe/λq=1 ELM-free H-mode Type-V ELM H-mode λTe/λq ratio for ELM-free and Type-V ELM H-modes λTe/λq = is expected if e-conduction is dominant, considering flat Te and ne profiles in the far SOL (ie, λTe/λq=2-2.5) 2. Experimental results: – ELM-free H-mode: λTe/λq ~ 2 – Type-V ELM H-mode: λTe/λq≤ 1 ELMs playing a role in reducing λTe/λq ratio???, eg multiple stripes at the divertor target, therefore broadening the measured (time averaged) λq??? Detailed physical mechanisms to be investigated using the fast IR camera J-W.Ahn, NSTX Results Review, 08/06/2008

11. Summary and conclusions Confirmed dependence of λq, λTe, λjsat, and λne on Ip, power, and nebar in H-mode plasmas  negative dependence on Ip and power  no dependence on nebar  λq change appears to be driven primarily by λTe change 2. λne remains unchanged by any parameter scan 3. Comparison with H-mode scaling laws from conventional tokamaks  λTe , λne , and λq dependences on Ip and power consistent with scaling laws λq dependence on density consistent with the scaling law λTe and λne dependence on density shows different trend from scaling laws 4. NSTX near SOL plasma appears e-conduction dominated without ELMs 5. ELMs reduce λTe/ λq ratio? Physical mechanisms to be investigated (eg, due to target λq broadening, etc) J-W.Ahn, NSTX Results Review, 08/06/2008

12. Future Work 1. More data point and confidence  SOL width scaling laws  Comparison with theoretical models eg, SOLPS results and analytic models (J.Canik, R.Maingi)  Extrapolation to future machines 2. Detailed investigation of SOL transport  roles of heat conduction and convection associated with ELMs  perpendicular particle transport (eg, Deff from edge Dβ profile) 3. Investigate relation of SOL widths with edge turbulence characteristics (collaboration with J.Boedo) J-W.Ahn, NSTX Results Review, 08/06/2008

13. Backup: Ensembled TS profiles, reliable??? Ip=1MA, PNBI=1MW Type-V ELM H-mode 45 TS profiles Te,sep=35eV (from 2pt model) Te,sep=70eV 1.χ2stays low up to Te,sep=45eV 2. For Te,sep > 45eV, ensembled profile becomes more scattered  Te,sep=35eV is in the ball park?? 3. However, some ensembled profiles show monotonic χ2 increase from lower Te,sep J-W.Ahn, NSTX Results Review, 08/06/2008

14. Backup: Ip scan, λTe and λne 1. Te and ne profiles from the fast probe 2. Large R-Rsep values for the profiles & the respective profile ‘shift’  due to uncertainty in LCFS position determined by LRDFIT???  Te,sep is expected to be ~30eV from power balance consideration J-W.Ahn, NSTX Results Review, 08/06/2008