13th iea/rfp workshop – stockholm october 9-11, 2008 2d characterization of thermal core topology...
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13th IEA/RFP Workshop – Stockholm October 9-11, 2008
2D characterization of thermal core topology changes in controlled RFX-mod QSH states
A. Alfier
on behalf of the RFX-mod team
(in collaboration with J. Reusch and H. Stephens)
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Outline
1. Physics issues.
2. Diagnostic set-up & data interpretation.
3. QSH control in experiments.
4. Results on QSH island and SHAx.
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
1. Starting from measurements …
During QSH states at Ip>1MA, the localized thermal structure associated to the dominant m=1, n=-7 mode may:
1) become large enough to occupy most of the plasma core (±20cm)
2) have 1keV Te plateau in the plasma core.
QSH with island (QSHi) SHAx
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
… some issues have come along together with SHAx
How can we compare Te and SXR measurements?
Is it a poloidally symmetric structure? (e.g. does the plateau uniformly fill the core?)
How does it evolve?
How can we compare Te and SXR measurements to magnetic reconstructions?
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Diagnostic set-up
Thomson scattering (TS)• 82.5°
• 40Hz time resol.• 1cm spatial resol. • [-0.96 , 0.84] r/a
• 84 Te points
Soft X Tomography (SXT)• 202.5°
• ~102 kHz time resol.• 78 chords
• brightness values• tomographic reconstr.
of emissivity
Soft X Multichord Camera (SXMC)• 262.5°
• ~ kHz time resol.• 4.5cm spatial resol.
• outer midplane• 10 line-integrated Te values
(double filter technique)
TS
SXMCSXT
Top view of RFX-mod vessel
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Interpretation of measurements during QSH state
We exploit the phase relation n to determine the poloidal position of the island O-point at the location of the three diagnostics (n=-7 for the m=1 dominant mode)
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
QSH control
Temporal phase (i.e.: QSH appearance)
The OPCD technique triggers the plasma transition from MH to QSH (i.e applying a 40Hz poloidal current which generates a toroidal field perturbation ).
Spatial phase (i.e O-point poloidal position)
An external reference of 1mT at the plasma edge for the n=-7 mode rotates at a frequency of 40Hz as a seed for the QSH state.
Standard Rot. Perturb. Rot. Perturb. + OPCD
OPCD cycle
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Plasma pulses
Plasma parameters:
Ip ~ 1.5MA I/N = 4-8 10-14 A/m F ~ -0.01 ~ 1.4;
OPCD + Rot. perturb. + TS laser trigger control to scan the island Te profile;
~ 30 shots;
O-point in the range 90°-270° @ TS time;
Measurements from TS – SXT – SXCM have been correlated, and compared with Poincarè plots obtained with ORBIT (see M. Gobbin presentation this afternoon).
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
RESULTS
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Localized QSH and SHAx example
separatrix
SHAx
QSHi
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Island width
1. Defined the magnetic width on Poincarè (sticky region) and thermal width on TS profiles.
2. wmag proportional to wTS
QSHiSHAx
QSHi
SHAx
Hint: some discrepancy due to the definition of
width.
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
2D Te reconstruction
SHAxQSHi
wTS has a poloidal dependence, slightly increasing (QSHl) or
decreasing (SHAx) approaching to the O-point poloidal position
Combining TS profiles, with respect to the island O-point
position, we can obtain a 2D Te reconstruction for localized QSHi
and SHAx
SHAxQSHi
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Comparison between emissivity and Te
Te and SXR brightness are related through the Bremsstrahlung
formula:
const
Interferometer inversion 2D map
Line integrated measurements from SXT (empty triangles) agree with
those simulated in the range 0-20%.
Hint: the simulation is based on nested circular surfaces, not helical surfaces!
TS measurementsTe approximation
TS measurementsTe approximation
Isoemissive ~ Isothermal
QSHi #24124 SHAx #24598
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Evolution to a SHAx state 1/2
First example (ext. ref. + OPCD): from a QSH island to a SHAx
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Evolution to a SHAx state 2/2
Second example (ext. ref.): from a centered wide emissive and thermal structure (not a typical QSH island) to a SHAx
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Recent analysis on MST Te profiles (thanks to J. Reusch & H. Stephens)
During PPCD experiment on MST, Te profiles similar to those found on RFX-mod have been measured. Adapting ORBIT to MST in a very approximate way*, the obtained Poincarè plot shows similar feature to those of RFX-mod
* the equilibrium is calculated with the &p model, reproducing the q-profile produced by MSTFIT.
This is a very preliminary analysis done on just 5 shots: it needs to be extended and improved, but it is very promising.
All modesn=6 mode
RFX-mod (OPCD + Rot. Pert.)
Ip=1.35MAne =3.5·1019m-3
I/N= 5.5·10-14Am
MST (PPCD)
Ip=400kAne =1019m-3
I/N= 5·10-
14Am
n=6
n=-7
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Conclusions
During the experimental campaign in 2008 TS, SXT and SXMC have been run simultaneously and measurements combined.
OPCD and external rotating reference have been successfully used to trigger QSH/SHAx states and control its position
2D map of electron temperature has been obtained for SHAx and QSH island
SHAx are poloidally asymmetric structure and occupy most of the plasma core
Isoemissive surfaces of SXT can be considered isothermal surfaces during both QSH and SHAx islands (discrepancy < 20%).
The transition from MH to SHAx may occur with features which are not unique (here we have shown just two examples)
Some MST Te profiles during PPCD show features similar to those of Te profiles during SHAx states on RFX-mod at similar I/N (very preliminary analysis).
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Thanks
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
In QSH thermal profile:
1. Peak Te linearly increased up to 1keV;
2. Te between 300-500eV;
3. ∇Te increased to 4-7 keV/m;
4. lowest values ever measured on RFX-mod of E,e (5-15 m2/s) on the island gradient.
-400 -200 0 200 4000
0.2
0.4
0.6
0.8
1
Radius (mm)
Te (
keV
)
Thomson scattering #21982 @ 75ms
Te (keV) Ip ~ 1.26MAne ~ 2.2·1019m-3
Best results:
Tepeak ~ 1 keV at 3.5·1019m-3
e,island ~ 10 m2/sE,e ~ 1.3 ms
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
TS and emissivity poloidal and radial profiles
13th IEA/RFP Workshop – Stockholm October 9-11, 2008
Normalized averaged density profiles
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