Vacuum flux surfaces measurements (W7AS)
R. Jaenicke, Nucl. Fusion 33, 687 (1993)
unperturbed nested fluxsurfaces |
m=2 and m=10 islands occur at = 1/2 |
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Vacuum flux surface measurements (LHD)
1/1 island
Magnetic surfaces at B = 0.0875 T measured by the fluorescing mesh method
A. Komori, Stell. News 58, 1 (1998)
1/1 island
Calculated magnetic surfaces including the earths magnetic field (BE ~ 5∙10 5 T BE / B ~ 5∙10 4 )
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Resumee on vacuum field properties
stellarators can be accurately build
to provide the desired vacuum field properties
the technical feasibility of the modular stellarator concept is proven
the first superconducting stellarator (LHD) is operational
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Plasma equilibrium
pressure equilibrium: |
p = j B, j = 0 |
diamagnetic current ( B): |
jdia = B p / B2 = B1 dp/dr e |
PfirschSchlüter current (|| B): |
jPS ~ 2 j cos e |
Effects of the PScurrent:
jPS produces a vertical dipole field BPS
BPS shifts the plasma outward (Shafranov shift )
limits the maximum pressure (equilibrium limit)
may „dig“ a magnetic well (improves stability limit)
jPS modifies the rotational transform
Optimization of W7AS/W7X
= reduction/minimization of PScurrent
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Methods to verify the plasma equilibrium
Magnetic flux measurements measure the flux of the dipole field arising from PScurrents
Softx radiation measurements measure the contours of constant emissivity (= flux surfaces if T = T(r), n = n(r))
compare results with equilibrium code predictions
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Magnetic flux measurements
/ (B a2) loop 0
loop ~ < > (B0 a2)/
< >/%
H. Renner, Contr. Fusion & Plasma Heating, 19th EPS Conf. (1992)
Experiment (W7AS):
P = 0.1 1.6 MW
B0 = 1.25 2.6 T a = 0.11 0.18 m= 0.3 0.63
Theory:
MHD equilibria from NEMEC and KW code
The agreement of experiment and theory confirms the concept of stellarator optimization by reduction of PScurrents
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Equilibrium at high beta
J. V. Hofmann, Plasma Phys. Contr. Fus. 38A 193 (1996)
Experiment (W7AS):
NBI discharge
PNBI = 2.5 MW, ne = 2 1020 m3 , < > = 1.8 %
contours of constant emissivity (tomographic reconstruction of softX data)
pressure profile from Thomson scattering (2 x electron pressure, Te = Ti)
= 2 |
0 |
n k T |
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B2 |
|||
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Theory:
MHD equilibrium from NEMEC code
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Measurement of Shafranov shift
J. V. Hofmann, Plasma Phys. Contr. Fus. 38A 193 (1996)
Experiment:
outward displacement of the plasma center measured by Softx emission
Theory:
Shafranov shift for a classical l =2 stellarator
confirms the reduction of PScurrents and Shafranov shift by a factor of 2 as predicted for W7AS
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High operation in CHS
S. Okamura, Plasma Phys. Contr. Nucl. Fus. Res., vol 1, 351 (1995)
stability analysis:
the magnetic well region expands with < >
Mercier stable for < >1.4%
MHD activity: decreases with < >
no hard stability limit observed
= 2.1%
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High operation in LHD
S. Sakakibara, Stell. News, 76, 8 (2001)
pellet |
Maximum transient (~ 3.0 %) in stellarators so far |
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stability analysis: |
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pressure gradient |
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deep in the unstable region |
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MHD activity: |
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n/m = 1/2 @ r/a = 0.5, |
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saturates with < > |
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no hard stability limit |
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observed |
n/m = 1/2
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