pkm- ncsx cdr, 5/21-23/2002 1 power and particle handling in ncsx peter mioduszewski 1 for the ncsx...

1pkm- NCSX CDR, 5/21-23/2002pkm- NCSX CDR, 5/21-23/2002

Power and Particle Handling in NCSXPower and Particle Handling in NCSX

Peter MioduszewskiPeter Mioduszewski11

for the NCSX Boundary Group:for the NCSX Boundary Group:

M. FenstermacherM. Fenstermacher22, A. Grossman, A. Grossman33, A. Koniges, A. Koniges22, L. Owen, L. Owen11, T. , T. RognlienRognlien22, M. Umansky, M. Umansky22

1 1 Oak Ridge National LaboratoryOak Ridge National Laboratory2 2 Livermore National LaboratoryLivermore National Laboratory

33University of California at San Diego University of California at San Diego

NCSX Conceptual Design ReviewNCSX Conceptual Design Review

Princeton, May 21-23, 2002Princeton, May 21-23, 2002


ObjectivesObjectives

The task of the plasma boundary group is to develop techniques for improving plasma performance by controlling the plasma boundary:

Heat removal: avoid excess temperatures, materials choice (C, W, Mo?), configuration of plasma-facing components (PFCs)

Impurity control: materials, wall conditioning, specific PFC design and location, control of plasma edge parameters (e.g. control Te, screening, etc.)

Recycling control: positioning of the recycling sources away from critical locations, baffled and/or pumped divertors, and wall conditioning.


Phased PFC Development will be Guided by Phased PFC Development will be Guided by Experiment and Modeling Experiment and Modeling

1st generation of PFCs: initial poloidal limiters at = 60º Phase 1: Machine shake-down Phase 2: Vacuum and flux surface mapping Phase 3: Ohmic operation: 100-300 kW

2nd generation of PFCs: wall armor for thermal-, fast particles, NBI

Phase 4: Auxiliary heating: 3 - 6 MW, first opportunity to measure finite beta edge configurations for a data base for divertor design

3rd generation of PFCs: divertor operation in baffle mode Phase 5: Confinement and beta limits

4th generation of PFCs: divertor operation in pumping mode

Phase 6: Long-pulse operation: (~1.2 s pulse does not need real-time cooling)


1st Generation PFCs: Poloidal Limiters at 1st Generation PFCs: Poloidal Limiters at = = 60º60º

The initial set of limiters will protect the walls and allow for initial Ohmic operation.

Poloidal cross-section at = 60º

Plasma

Vacuum Vessel

poloidal limiters

Plan view of vacuum vessel and plasma


2nd Generation PFCs: Graphite Wall Armor 2nd Generation PFCs: Graphite Wall Armor (Liner) (Liner)

The 2nd generation of PFCs will consist on graphite panels attached to the vacuum vessel and bakeable to 350ºC.

This configuration will provide the first opportunity to diagnose the plasma boundary with finite beta plasmas.

Experiments and modeling in this configuration will provide the basis for the divertor design.

= 0o

2nd generation: wall armor

wall armor

plasma


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 <ß> ~ 3% achieved (at B = 1.25 T) Full density control Plasma heating at extremely high density with EBW

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.


Long Connection Lengths Are Needed Between LCMS and Long Connection Lengths Are Needed Between LCMS and Divertor For Sufficient Temperature SeparationDivertor 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

.

0 1 2 3 4 50

40

80

120

160

200

Tsep100

Tdiv100

Tsep5

Tdiv5

nsep (1019m-3)

3 MW

0 1 2 3 4 50

40

80

120

160

200Tsep100

Tdiv100 Tsep5

Tdiv5

nsep (1019m-3)

6 MW


The NCSX Boundary Has Long Connection The NCSX Boundary Has Long Connection Lengths ! Lengths !

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)

Significant flux expansion Long connection lengths: here up to 180 m Kolmogorov lengths are ~30-50 m Suitable for divertor operation

A. Grossman

*E. Strumberger

50 cm

0º 30º 60º 90º


3rd and 4th Generation PFCs : Divertor 3rd and 4th Generation PFCs : Divertor

vacuum vessel

divertor pump (e.g.Ti)

3rd Generation“baffle mode”

4th Generation“pumping mode”

divertor plateand baffles

10

pkm- NCSX CDR, 5/21-23/2002pkm- NCSX CDR, 5/21-23/2002

Neutrals Penetration Is Low in the Neutrals Penetration Is Low in the Limiter and Liner ConfigurationsLimiter and Liner Configurations

Plasma efflux based on: ne=5x1019m-3, assuming p= 20 ms, flux ampl.= 5x modeling indicates low neutral density in the confinement zone

W7-AS has no problem with a smaller waist than NCSX (16 vs.24cm)

Rec.loc. Rec.loc.

11


In the Divertor Configuration, Neutrals Are In the Divertor Configuration, Neutrals Are Contained by the Divertor Plasma/Baffles Contained by the Divertor Plasma/Baffles

10 7

10 8

10 9

10 10

10 11

0 5 10 15 20 25Radius in outer midplane (cm)

Core SOLDivertor

φ=0poloidalcrosssectionsimulation

12


SummarySummary

Improved performance in W7-AS has recently demonstrated that power and particle control are essential tools for improving plasma performance in stellarators.

For initial operation, our understanding of the NCSX boundary will be limited and we will start with a simple limiter configuration.

The boundary in NCSX is stochastic, with Kolmogorov lengths measuring several toroidal revolutions !

This guarantees sufficiently long connection lengths which - in conjunction with the observed flux expansion - are suitable for divertor operation.

As our understanding of the boundary grows, we will improve impurity and neutrals control by developing divertor configurations.

13


ExtrasExtras

14


The new divertors enable access to a new regime with NBI at very high

density (up to ne~ 3.5 x 1020 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 <ß> ~ 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 Divertor Operation has Dramatically Improved Plasma Performance in W7-ASImproved Plasma Performance in W7-AS

15


Toroidal Decay of Neutral Density Recycled at Toroidal Decay of Neutral Density Recycled at = 0º= 0º

0

5 10 10

1 10 11

1.5 10 11

0 10 20 30 40 50 60

n(H0

) (cm

-3)

Toroidal angle φ( .)deg

Just insideseparatrix

Just inside the vacuum vessel wall

Midplane simulations( ) Recycle puff at φ=0

16


Initial Results of Poincaré Plots With Field-Line Diffusion- Initial Results of Poincaré Plots With Field-Line Diffusion- Field-Lines Launched Just Inside the Last Closed Field-Lines Launched Just Inside the Last Closed

Magnetic Surface Magnetic Surface A. Koniges

Field-line expansionat tips of bean section looks promising for divertor operation.

17


Initial Foot-Prints Used For Divertor Plate Initial Foot-Prints Used For Divertor Plate Design Indicate Nearly Full Toroidal Coverage of Design Indicate Nearly Full Toroidal Coverage of

DivertorDivertor

A. KonigesA. Koniges

toroidaltoroidal

poloidalpoloidal

outeroutermidplanemidplane

18


Kolmogorov Length for Field-Lines Kolmogorov Length for Field-Lines Launched At Outside MidplaneLaunched At Outside Midplane

Lk ~ 40 m

Lc ~ 180 m

0.001

0.01

0.1

1

0 50 100 150 200 250

Kolmogorov Length: FL 1 and 2 mm outside

Dist:1-2 (m)

y = 0.010152 * e^(0.025256x) R= 0.79601

Dist:1-2 (m)

Tor.Distance (m)Toroidal Distance (m)

Dis

tan

ce b

etw

een

Fie

ld-L

ines

(m

)

pkm- ncsx cdr, 5/21-23/2002 1 power and particle handling in ncsx peter mioduszewski 1 for the ncsx...

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