13th iea/rfp workshop – stockholm october 9-11, 2008 2d characterization of thermal core topology...

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)


Outline

1. Physics issues.

2. Diagnostic set-up & data interpretation.

3. QSH control in experiments.

4. Results on QSH island and SHAx.


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


… 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?


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


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)


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


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).


RESULTS


Localized QSH and SHAx example

separatrix

SHAx

QSHi


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.


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


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


Evolution to a SHAx state 1/2

First example (ext. ref. + OPCD): from a QSH island to a SHAx


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


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


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).


Thanks


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


TS and emissivity poloidal and radial profiles


Normalized averaged density profiles

13th iea/rfp workshop – stockholm october 9-11, 2008 2d characterization of thermal core topology...

Documents

th iearfp workshop stockholm

qsh island

island qsh

shax slide

localized qsh

w ts qsh

qsh appearance

stephens slide