2. pkm- NCSX CDR, 5/21-23/2002 1 Power and Particle Handling in NCSX Peter Mioduszewski 1 for the NCSX Boundary Group: for the NCSX Boundary Group: M. Fenstermacher 2, A. Grossman 3, A. Koniges 2, L. Owen 1, T. Rognlien 2, M. Umansky 2 1 Oak Ridge National Laboratory 2 Livermore National Laboratory 3 University of California at San Diego NCSX Conceptual Design Review Princeton, May 21-23, 2002

3. pkm- NCSX CDR, 5/21-23/2002 2 Objectives l The task of the plasma boundary group is to develop techniques for improving plasma performance by controlling the plasma boundary: l Heat removal: avoid excess temperatures, materials choice (C, W, Mo?), configuration of plasma-facing components (PFCs) l Impurity control: materials, wall conditioning, specific PFC design and location, control of plasma edge parameters (e.g. control Te, screening, etc.) l Recycling control: positioning of the recycling sources away from critical locations, baffled and/or pumped divertors, and wall conditioning.

4. pkm- NCSX CDR, 5/21-23/2002 3 Phased PFC Development will be Guided by Experiment and Modeling 1st generation of PFCs: initial poloidal limiters at  = 60º è Phase 1: Machine shake-down è Phase 2: Vacuum and flux surface mapping è Phase 3: Ohmic operation: 100-300 kW l 2nd generation of PFCs: wall armor for thermal-, fast particles, NBI è Phase 4: Auxiliary heating: 3 - 6 MW, first opportunity to measure finite beta edge configurations for a data base for divertor design l 3rd generation of PFCs: divertor operation in baffle mode è Phase 5: Confinement and beta limits l 4th generation of PFCs: divertor operation in pumping mode è Phase 6: Long-pulse operation: (~1.2 s pulse does not need real-time cooling)

5. pkm- NCSX CDR, 5/21-23/2002 4 1st Generation PFCs: Poloidal Limiters at  = 60º The initial set of limiters will protect the walls and allow for initial Ohmic operation. Poloidal cross-section at  = 60º poloidal limiters Plan view of vacuum vessel and plasma

6. pkm- NCSX CDR, 5/21-23/2002 5 2nd Generation PFCs: Graphite Wall Armor (Liner) n The 2nd generation of PFCs will consist on graphite panels attached to the vacuum vessel and bakeable to 350ºC. n This configuration will provide the first opportunity to diagnose the plasma boundary with finite beta plasmas. n Experiments and modeling in this configuration will provide the basis for the divertor design.

7. pkm- NCSX CDR, 5/21-23/2002 6 l Experience in W7-AS indicates that substantial plasma performance improvement can be expected through systematic control of plasma-wall interactions with a divertor:   E increases steeply with density,   p and  imp decrease with increasing density  Record value of ~ 3% achieved (at B = 1.25 T)  Full density control  Plasma heating at extremely high density with EBW l Although the NCSX configuration is somewhat different, the W7-AS results provide a compass for the direction of our boundary program. The key to good plasma performance in NCSX is most likely boundary control with divertor-like configurations.

8. pkm- NCSX CDR, 5/21-23/2002 7 Long Connection Lengths Are Needed Between LCMS and Divertor For Sufficient Temperature Separation. The divertor-LCMS temperature difference for NCSX can be calculated with the “2-point-model” The figure shows: Lc = 100 m -> sufficient divertor-separatrix temperature separation Lc = 5 m -> temperature separation insufficient, even at high ne

9. pkm- NCSX CDR, 5/21-23/2002 8 The NCSX Boundary Has Long Connection Lengths ! è Poincaré plots, generated with the MFBE* and Gourdon codes: 20 field-lines were started at the outer/inner midplane (0-1 cm) and followed for 20 toroidal revolutions (Lc~180 m) è Significant flux expansion è Long connection lengths: here up to 180 m è Kolmogorov lengths are ~30-50 m è Suitable for divertor operation A. Grossman *E. Strumberger 0º30º 60º 90º

10. pkm- NCSX CDR, 5/21-23/2002 9 3rd and 4th Generation PFCs : Divertor vacuum vessel divertor pump (e.g.Ti) 3rd Generation “baffle mode” 4th Generation “pumping mode” divertor plate and baffles

11. pkm- NCSX CDR, 5/21-23/2002 10 Neutrals Penetration Is Low in the Limiter and Liner Configurations Plasma efflux based on: n e =5x10 19 m -3, assuming  p = 20 ms, flux ampl.= 5x l modeling indicates low neutral density in the confinement zone n W7-AS has no problem with a smaller waist than NCSX (16 vs.24cm) Rec.loc.

12. pkm- NCSX CDR, 5/21-23/2002 11 In the Divertor Configuration, Neutrals Are Contained by the Divertor Plasma/Baffles

13. pkm- NCSX CDR, 5/21-23/2002 12 Summary l Improved performance in W7-AS has recently demonstrated that power and particle control are essential tools for improving plasma performance in stellarators. l For initial operation, our understanding of the NCSX boundary will be limited and we will start with a simple limiter configuration. l The boundary in NCSX is stochastic, with Kolmogorov lengths measuring several toroidal revolutions ! l This guarantees sufficiently long connection lengths which - in conjunction with the observed flux expansion - are suitable for divertor operation. l As our understanding of the boundary grows, we will improve impurity and neutrals control by developing divertor configurations.

14. pkm- NCSX CDR, 5/21-23/2002 13 Extras

15. 14  The new divertors enable access to a new regime with NBI at very high density (up to n e ~ 3.5 x 10 20 m -3 ) with promising confinement properties: -  E increases steeply with density, -  p and  imp decrease with increasing density courtesy of P. Grigull  Record value of ~ 3% achieved (at B = 1.25 T)  Full density control already without Ti- gettering.  Quasi-steady state operation also including partial detachment.  Radiation stays always peaked at the edge.  Plasma heating at extremely high density by HF (EBW 140 GHz) is successfully demonstrated. Divertor Operation has Dramatically Improved Plasma Performance in W7-AS

16. pkm- NCSX CDR, 5/21-23/2002 15 Toroidal Decay of Neutral Density Recycled at  = 0º

17. pkm- NCSX CDR, 5/21-23/2002 16 Initial Results of Poincaré Plots With Field-Line Diffusion- Field-Lines Launched Just Inside the Last Closed Magnetic Surface A. Koniges Field-line expansion at tips of bean section looks promising for divertor operation.

18. pkm- NCSX CDR, 5/21-23/2002 17 Initial Foot-Prints Used For Divertor Plate Design Indicate Nearly Full Toroidal Coverage of Divertor A. Koniges toroidal poloidal outermidplane

19. pkm- NCSX CDR, 5/21-23/2002 18 Kolmogorov Length for Field-Lines Launched At Outside Midplane L k ~ 40 m L c ~ 180 m Toroidal Distance (m) Distance between Field-Lines (m)