High ion temperatures in W7AS
J. Baldzuhn, Plasma Phys. Contr. Fus., 40 967 (1998)
R. Jaenicke, Plasma Phys. Contr. Fus., 37A 163 (1995)
PECRH = 400 kW, PNBI = 900 kW
transport is neoclassical (lmfp) except at the edge
In this type of discharge:
Maximum Ti (= 1.6 keV) achieved in W7AS
so far
I / 31
Resumee on neoclassical transport in stellarators
At low collisionality ( * (v /R) 1, „long mean free path“ regime) particle and energy transport are well described by the neoclassical predictions
large drift from flux surfaces between collisions can be reduced by
radial electric fields (multiple roots possible)
quasisymmetry
At higher collisionality transport is anomalously enhanced as compared to neoclassical predictions (in particular towards the boundary)
turbulence driven transport
sheared flow driven by the (neoclassical) electric field can reduce anomalous transport
internal transport barriers
I / 32
Global confinement scaling (stellarators and tokamaks)
U. Stroth et al, Nucl. Fusion 36 106 (1996)
LHD: Stell. News 62 (1999)
International Stellarator Scaling
E = W/(P dW/dt)
ISS95 ~ a 2.21 R 0.65 P 0.59 n 0.51 B 0.83 0.4
~ a (nV/P) 0.5 (B2 ) 0.5 (approx.)
common scaling for Heliotrons/Torsatrons
confinement in W7AS 2x higher than in Heliotron/Torsatrons (except LHD)
? low shear favourable
? result of W7AS optimization
similar (Lmode) scaling of stellarators and tokamaks
I / 33
Edge thermal transport barrier in LHD
N. Ohyabu et al, Phys. Rev. Lett. 84 103 (2000)
PNBI = 3.5 MW, n = 2.2 1019 m3
pedestal
Improved global confinement in LHD (with respect to ISS95) is related to an edge transport barrier forming a temperature pedestal
(related = 1/1 at the edge ??)
I / 34
Rational surfaces and anomalous transport in W7AS
R. Brakel, submitted to Nucl. Fusion (2001)
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temperature profiles |
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PECRH= 340 kW |
PECRH= 450 kW |
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n= 2.3x1019m-3 |
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n= 4x1019m-3 |
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confinement in low shear stellarators strongly depends on the rotational transform
maxima close to low order rational numbers low density of other rationals numbers
Hypothesis: transport is enhanced at rational surfaces heat conductivity model
I / 35
Stellarator discharges
high performance (high B, P, n) |
steady state |
short pulse length |
low performance (low B, P, n) |
W7AS @ B = 2.44 T |
ATF @ B = 0.53 T |
density control lost
1 hour !!
high performance at steady state: use superconducting coils and divertor (LHD, W7X)
I / 36
Towards steady state: |
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K. Kawahata, Plasma Phys. Contr. Fus., 42B 51 (2000) |
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0.9 MW ICRH, 68 s |
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1500 |
0.5 MW NBI, 80 s |
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Wp17170 |
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PICRH |
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PNBI |
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ne ~ 1.5 x 1019 m3 |
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Prad17170 |
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1.8 keV |
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Ti(Doppler) |
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recently: |
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envisaged: |
1 hour at 3 MW with improved divertor and heating systems |
I / 37 |
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Divertor concepts (particle and power exhaust)
Local island divertor (CHS):
field line diversion in a n/m = 1/1 local island produced by a perturbation field
Helical divertor (LHD):
inherent field line diversion in torsatron
Island divertor (W7AS, W7X): field line diversion in the n/m islands inherent to the configuration W7AS: n = 5 , W7X: n = 10
I / 38
Local island divertor experiments in CHS
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S. Masuzaki, J. Plasma Fus. Res. 1 310 (1998) |
without LID |
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without LID |
with LID |
with LID |
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without LID |
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with LID
with local island divertor (LID):
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the density is reduced |
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the radiation power is reduced (oxygen) |
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the temperature increases |
I / 39
Island divertor in W7AS
P. Grigull, Plasma Phys. Contr. Fus., submitted (2001)
5x2 divertor modules
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Targets |
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Baffles |
separatrix |
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5/9-island |
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Titanium |
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evaporators |
Bottom divertor Probe arrays Baffles
Target
I / 40