rfx-mod workshop – padova, 20-22 january 2009 1 experimental qsh confinement and transport fulvio...

RFX-mod Workshop – Padova, 20-22 January 2009 1

Experimental QSH confinement and transport

Fulvio Auriemma on behalf of RFX-mod team

Consorzio RFX, Euratom-ENEA Association, Padova, Italy


Outline

magnetic configuration in QSH – SHAx states

improved confinement and energy transport

particle confinement: density and impurity behaviour

conclusion


QSH in the high current regime Ip > 1 MA

10 E

QSHi SHAx

Magnetic topology of

dominant mode computed by

FLiT[1]

Helical plasma configuration

[1] R. Lorenzini et al, APS Conference (2008)





conclusion


diagnostics for thermal properties of QSH state

TS:

84 points for local Te along an equatorial line of sight

SXT:

78 chords for SXR tomographic reconstruction

SXMC:

10 chords for time resolved Te profile evolution


thermal structures in RFX-mod

[2] F. Bonomo et al, submitted to Nucl.Fus[3] R.Lorenzini et al. accepted by PoP

•steep Te gradient in

radial profile 1/LTe20 m-1

•Thermal structures

related to magnetic

topology both for QSHi

and SHAx[2]

•SXMC (triangles) accords

to TS at different toroidal

position: Te constant on

helical flux surfaces SXT provides good experimental

approximation of the poloidal

map of central hot region

L. Marrelli

presentation

[3]

QSH


energy confinement time

SHAxQSHi

Poloidal scan TS measurements[2]: wider hot region hence higher energy content in SHAxs

Energy confinement time E increases by 50-80% when the plasma transits to the SHAx:

this is both due an increase of the thermal energy content and a reduction of the Ohmic input power[4]

[2] F. Bonomo et al, submitted to Nucl.Fus[4] P. Piovesan et al, EPS Conference (2008)

2D Te Thomson scattering map:


Energy diffusion calculation

[5] L.Carraro, submitted to Nucl. Fusion

E computed by 1D single fluid approach and solving the power balance equation (symmetric approximation) in SHAx regimes E strongly decreases in the gradient region: thermal transport barrier In SHAxs regimes the magnetic turbulence could become so small that other transport mechanisms could act [5]





conclusion


Density behaviour in QSH

[2] F. Bonomo et al, submitted to Nucl.Fus.

neutral density profile

QSH confining

region

Inverted density profileAverage line density

Average density profiles still flat during QSH/SHAx [2]:

lack of particle source (about 0.1%) inside the confining structures,

as predicted by NENE code


Mapping electron density ne on helical flux

The ne profile shape, which is modified injecting solid Hydrogen pellets[1]

Homologous interferometer chords show asymmetries that are well matched assuming ne constant on helical flux surfaces.

[1] R. Lorenzini et al, APS Conference (2008)[7] A. Alfier et al. proposal #91 TF3 (2009)

More information also from ne TS measurement with new calibration procedure [7]


pellet injection: additional information

The pellet injection experiments show:

1. Asymmetries due to topology

2. Ablated pellet particles are “confined” (flat density time evolution)


Pellet particle deposition and FLiT (by D. Terranova)

We follow magnetic field lines for a chosen number of turns starting at the pellet positions in time and see where these lines hit the poloidal cross section of the interferometer.

Pellet toroidal angle Interferometer toroidal angle

Each colour indicates the points corresponding to different time instants.

[6] D.Terranova et al, RFP-WS (2008)


Pellet particle deposition and FLiT

The time evolution of the chords’ signals is compatible with the reconstructionof the path of the internal magnetic field lines by means of the FLiT code.

H from the pellet entering the plasma

Pellet toroidal angle Interferometer toroidal angle



Pellet and global particle confinement

tpre t2 tp

Multiple Helicitypre = 4.2 ms

Pellet ablation

Growing QSH (ne diffusion)2 = 6.9 ms

Single Helical Axis (ne sustained)p = 8.6 ms

This could be an alternative way for having QSH or SHAx at high density. We still need more experimental evidence.

dtdNp

N



Impurities injection experiments: LBO

Ni XVII 249 Å and Ni XVIII 292 Å observed: impurity reaches the hot

helical structure but

1D collisional-radiative impurity transport -> no evidence of

improved impurity confinement, according with ORBIT simulation[5]

[5] L.Carraro, submitted to Nucl. Fusion

M. Gobbin

presentation


conclusion

Helical magnetic topology observed also on kinetic quantities (Te and ne)

80% higher energy confinement time in SHAXs than in QSHi

Energy diffusion coefficient e damped by one order of magnitude in SHAxs

Particle confinement in SHAx structures seen in pellet injection experiments:

improved global particle confinement time by a factor 2

No improved impurities confinement in SHAx structure due to their high

collisionality

TO DO:

Perform transport simulation in helical geometry both for energy and for particles (new tools to be developed)

New efforts to inject particle source in QSH confining structures

New LBO experiments with other impurities


Asymmetries (2/2): enhanced pellet ablation

H emission from the ablation of the pellet increases abruptly as the pellet hits the edge of the island where large Te gradients are present.


Asymmetries (1/2): line integrated density

Homologous chords do show a different time evolution [5].



Pellet and global particle confinement (2)

p = 7.9 ms (SHAx)p = 2.4 ms (diffusion)

p = 8.0 ms (SHAx)p = 12.5 ms (SHAx)

Clear evidence of improved global particle confinement time (by a factor 2 to 3) when the internal magnetic field

configuration changes from MH to QSH and to SHAx.

Plasma electron density profiles (in flux coordinates) for three pellets with increasing penetration length.


Mapping soft X-ray measurements on helical flux

The soft X-rays tomography is the diagnostic with the highest space resolution in RFX-mod

The emissivity has been assumed to be a simple 3-parameter function of


• Single pulse ruby laser (7J @ 694nm, 30ns at FWHM) focused on a 3mm pin-hole in vacuum.

• Sapphire lens & prism deflect beam by 30° and image the pin-hole in vacuum vessel.

• A camera lens (f=83mm / F#1.2) collects light at ~150° from 16 positions over =1mm fibers:

12 scattering volumes for Te, ∼10mm resolution;

4 measuring points for detecting background plasma light.

• The entrance port hosts the input system & the collection window stable alignment.

• Fibers are arranged in a 4x4 pattern and fed into a 4 spectral channels spectrometer.

• An Intensified CCD (ICCD) acquires the Thomson and the Background signal

Edge TS /LBO system

rfx-mod workshop – padova, 20-22 january 2009 1 experimental qsh confinement and transport fulvio...

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