17th Toki Conference & 16th Stellarator/Heliotron Workshop, 2007.

Searching for a Flux Expansion
Divertor in TJ-II.
F. Castejón1,2, A. López-Fraguas1, A. Tarancón2,3 and J. L. Velasco2,31) Lab. Nacional de Fusión –Asociación Euratom/Ciemat, 28040-Madrid, Spain
2) Instituto de Biocomputación y Física de Sistemas Complejos, 50009-Zaragoza, Spain
3) Universidad de Zaragoza, 50009-Zaragoza, Spain#0Outline• Introduction and Motivation.
• The chosen configuration in TJ-II.
• ISDEP code and the plasmaconditions.
• Results.
• Conclusions and future work.

#1Divertor Concepts
Power exhaust and density control are mandatory for astellarator-reactor (R. König et al. PPCF,2002). Concepts:• X- point (Tokamaks).
• Helical Divertor.
• Island Divertor.
• Flux Expansion Divertor• The goals:– Concentrate plasma-wall interaction in favourablezones (with plates and pumping).
– Diminish the incoming fluxes on the plates (ad hoc
magnetic configuration; increasing the plate size).
– Hinder the neutrals to enter the plasma: Long path or
steep gradient pressure profile.
#2

Island Divertor
W7-AS -> W7-X

Y. Feng et al. NF 2006
#3

Helical Divertor
LHD(N. Oyabu et al. nf 1994)#4

X-point
H- mode obtained in JET with theX-point on the limiter plate=
Flux Expansion?

P. Rebut. PPCF 2006

#5

Flux Expansion Divertor: NCSX
00

R. Maingi. 33rd EPS Conference, 2006.
900

300

600

• Most flux expansion at 00 - best for divertor plates• Green field lines launched at inner midplane• Red field lines launched at outer midplane#6Requirements• Helical Divertor and Island Divertor requirerobust magnetic topology, almost unchanged
during plasma operation.
• Those concepts are not suitable for:–Configurations based on bootstrap current (NCSX,QPS).
–Equilibrium topology strongly dependent on beta.
–Flexible devices that can vary the rotational transport
profiles (TJ-II).
• Flux Expansion Divertor could be the solution.
#7

Flux Expansion Divertor in TJ-II?

••
•
•
•

Large flux expansion configurations do exist.
No ergodic zones appear in TJ-II. Look for a strategy for creating them.
The inner part of the configuration should be unaffected.
Lithium evaporation onto these target plates?
Pump behind plates?
#8

The Magnetic Configuration• Poincaré plots of the magnetic surfaces (rotated 4φ to be compared).
• Maximum flux expansion around φ=π/4 and θ=π/2. Although wide φ−range
envisaged.

#9ISDEP code: Examples of 3D orbits• Following particle
trajectories rather thanfield lines (105-106
particles). Grid
computing inside EGEE.
(F. Castejón et al. PPCF 2007)• Guiding centre
approximation.

• i-i & i-e Coulomb
collisions.

• Considering electrostatic
potential.

• No assumptions on orbit
size or diffusive transport.
#10Measured Profiles (used in the simulatiuon)• Two regimes: low density ECRH plasma andhigh density NBI plasma.

• Simulations valid for a wide range ofparameters.

• ne and Te, from Thomson Scattering.
• Ti from CX-NPA.
• Potential from HIBP.

#11

The Map of Losses• Maximum plasmawall interaction onthe groove.
• PWI close to theplasma bulk.
• Up-down
asymmetric flux.

#12

Obtaining the Map of Fluxes• We locate Nφ x Nθ = 4 x 32 = 128 plates in each period.
• A single plate (ρ,i,j):! > !0;2"
2"i<# <(i + 1);N#N#2"2"
j <$ <( j + 1)
N$N$• θ rotated -4φ with respect to the horizontal plane (groovemagnetic axis line: θ=0).
• The toroidal range of θ=−π/2 :#13Flux Map: ECRH
0<φ<π/8π/8<φ<π/4π/4<φ<3π/83π/8<φ<π/2• The plates areindependent.
• The shadow
effect is notconsidered inthis calculation.
• Clear poloidal
and toroidalstructure.
• Small changesfor ρ>1.

θ/πθ/πθ/πθ/π#14

Flux Map: NBI
0<φ<π/8π/8<φ<π/4π/4<φ<3π/83π/8<φ<π/2θ/πθ/πθ/πθ/π#15Flux on the chamber: ECRH0<φ<π/8π/8<φ<π/4π/4<φ<3π/83π/8<φ<π/2θ/πθ/πθ/πθ/π• Fraction of
trajectories on theplates: 50%.

• Fraction ofcollisions on thegroove: from 60%
to 35%#16

Flux on the chamber: NBI
0<φ<π/8π/8<φ<π/4π/4<φ<3π/83π/8<φ<π/2θ/πθ/πθ/πθ/π• Fraction of
trajectories on theplates: 63%.

• Fraction ofcollisions on thegroove: from 50%
to 35%#17

Conclusions…
• A promising configuration for Flux ExpansionDivertor has been found in TJ-II.
• ISDEP code has been used to calculate the maps ofthe fluxes in ECRH and NBI plasmas.
• The flux map shows a strong poloidal structure: It ispossible to locate divertor plates to suppress a large
fraction of the total flux onto the wall.
• The flux on the groove can be strongly reduced,
which is critical to diminish the plasma-wallinteraction in TJ-II.
#18

… and Future Work• Refine the design of the plates.
• Explore the possibility of creating anergodic layer outside the LCFS.
• Estimate the recycling & transport ofneutrals.
• Consider the Li coating?
• Thinking of experiments.
#19

#20

DIVERTOR: aplicación a TJ-II¿existen configuraciones“naturales” adecuadas
para un divertor?
Bombeo(Ti getter)#21

Equivalente Limitador rho=0.8, 0.9#22

DIVERTOR: aplicación a TJ-II

Divertor sin “islas”,
Utiliza la expansion de flujo#23