M. Hron ETFP workshop Kraków 11/09/2006 Presented by M Hron J Stockel, J Brotankova, I Duran, J Adamek, M Stepan, O Bilykova M Spolaore, E Martines, P

M. Hron ETFP workshop Kraków 11/09/2006

Presented by M Hron

J Stockel, J Brotankova, I Duran, J Adamek, M Stepan, O BilykovaM Spolaore, E Martines, P Devynck, P Peleman, G Van Oost,

L van de Peppel

Institute of Plasma Physics, Academy of Sciences of the Czech RepublicEURATOM Association IPP.CR, Prague, Czech Republic

In collaboration with EURATOM Associations: ENEA Padova (Padua, Italy),

CEA (Cadarache, France), Etat Belge (Ghent University, Belgium),

Edge plasma studies on the CASTOR tokamak


CASTOR tokamak

1958built in Kurchatov Institute, Moscow

1977put in operation in IPP Prague

1985reconstructed (new vessel)


CASTOR tokamak

MAIN PARAMETERSMAIN PARAMETERS

major radius 0.4 m

minor radius 85 mm

plasma volume 0.1 m3

plasma current 10 kA

toroidal magnetic field 1.3 Tesla

pulse length 30 ms

plasma density 1-2*1019 m-3

plasma temperature 150 eV

edge plasma density 2*1018 m-3

edge plasma temperature 15 eV

Manpower 20 My

MAIN PHYSICS TOPICSMAIN PHYSICS TOPICS

Edge plasma physicsfluctuation measurements, biasing

Wave plasma interactionfast particle generation, wave propagation

Diagnostics developmentSXR spectroscopyadvanced probes


Diagnostics


Edge plasma diagnosticsElectric

probes

• Classical Langmuir probesIV characteristics, local Te, ne, Ufl at the plasma edge, routine measurements

• Radial & Poloidal & 2D arrays of Langmuir probes for spatially / temporaly resolved measurements of plasma fluctuations

• Oriented probesRotating Mach probe, Gundestrup probe for flow measurements during biasing experiments

• Advanced probes Tunnel probe - a quite novel concept for fast Te

measurements


60 mm

Poloidal array of 124 probesPoloidal resolution = 2.9 deg (3 mm)64 fast channels available - signals of one half of the ring can be monitored simultaneously.

Rake probe• Distance between the tips 2.5 mm• Total length 35 mm• Movable on the shot to shot basis• Ufloat or Isat mode of operation

Probe arrays


Poloidal distributionRadial distribution at the top of the torus

Measured by the rake probe in a single shot

Measured by the poloidal ring in four shots

Floating potential profiles


Ring represents the poloidal limiter

Plasma is not centered, but downshifted

Separatrix is not defined by the limiter

Tips at the top – localized in the SOLConnection length >> 2R to amaterial surface (shield)depends on the local helicity of magnetic field lines - q(a)

Tips at the bottom - Closed MagneticField Lines

Respective position of separatrix and probes


Turbulence in the SOL


Poloidally periodicpatterns (bipolar) propagating poloidally are evident.

Po

loid

al d

irec

tio

n

LFS

TOP

HFS

Bottom

Time 0.5 ms

Potential “valley” Potential “hill”

Ufl(, t) – raw data


Po

loid

al d

irec

tio

n

Time lag [ms]

Poloidal periodicityas confirmed by cross-correlationanalysis

The reference probeis located at the top of the torus

Poloidal periodicity


Dominant poloidal mode number is found to be m = 6-7 (standard discharge conditions on CASTOR)

Poloidal mode analysis


The safety factor q(a) was increased in time by ramping down the plasma current. q

(a)

Time [ms]

Dominant mode number m clearly follows the evolutionof the edge safety factor q(a)

m

8

7

6

5

4

8

7

6

5

4

Poloidal mode analysis


Conclusion - Turbulence in SOL

Flute-like structure elongated along the magnetic field lines

Radial dimension ~ 1 cmPoloidal dimension ~ 1 cmLifetime ~ 1-40 sPoloidal wavelength ~ 5-15 cm

Only a single (bipolar) turbulent structure exists in the SOL.

Snakes q-times around the torusm=q, n=1 mode

Starts (and ends) on the Ion (and Electron) side of the poloidal limiter

Propagates poloidally due to the local ExB drift

experimental data folded on the toroidal surface (toroidal angle = time)


Biasing


Motivation

Generate electric fields in the edge plasma

manipulate with ion flows via ExB drift

reduce plasma fluctuations

improve particle&heat confinement

Massive electrode is inserted

in the edge plasma and biased

with respect to the vessel

Biasing experiments

density

H_alpha

U_bias

I_bias

biasingphase

1050 15 20 25t [ms]


Biased flux tube - originates at the electrode and extends upstream and downstream

Peaks - Intersection of the biased flux tube with the poloidal ring

Electrode is localized within the SOL and biased with respect to the vessel

Poloidal distribution of floating potential

SOL biasing


Poloidal position of the biased flux tube versus the local safety factor (#14076 &14077)

PROJECTION OF THE ELECTRODE

4

3

2

1

5

Position of peaks 2 and 3is independent on q-value, but determined by the poloidal extentof the biasing electrode.

Peaks 1,4,5 vary with the edge safety factor as expected

SOL biasing


• Terminates on the electron and ion side of the poloidal limiter at the bottom part of the torus. • Intersects q-times a poloidal cross section

• Originates at the electrode• Extends upstream and downstream along the magnetic field lines

Unfolded torusPoloidal cross section

SOL biasing


EpolxBtor drift

in radial direction

IsatBias/Isat

OHErad

Convective cells

BIAS

ohmicElectrode

A significant modificationof density profile is

observed during the SOLbiasing


Er(r) during Vfl peaks

10 s

• Sudden rise of oscillating behaviour during the biasing phase

• The effect involves a wide radial region

Edge plasma biasing


Creation and collapse of sheared region

Ufl

Erad

• Huge radial electric fields characterize the edge region in between Ufl oscillations

• High sheared region (transport barrier) propagates towards the wall at about 200 m/s becoming progressively thinner before being destroyed


• More clear evidence of a periodic radial propagation of high density structures is provided by the fluctuating part of Isat signal

Ufl

Isat

Ejection of particles


• Highly time resolved (1 MHz) measurement of H allowed a detailed investigation of it behavior during bias: a quite clear periodic oscillation is observed also on H

• The H periodic increase exhibit a good correlation with the radial propagations of density streams

H response


• Mach numbers show an equivalent behaviour with the 10 kHz • poloidal and toroidal flows swap during the relaxations.

MII

M

~100 s

0.1

0.2

0.3

0.4

0.5

11.6 11.8 12.0time [ms]

0

Modification of flows


Summary - Biasing

Biasing experiments resulted in effective inducing of an improved plasma confinement, characterized by steeper gradients of density and radial electric field.

SOL biasing creation of a bised flux tube in the SOLradial drift of particles (Epol x Btor)modification of the density profile

Edge plasma biasingperiodic creation and collapse of a transport

barrier (high shear region) at 10 kHz

critical gradients achieved both on floating potential and plasma density

radial propagation of high density structuresresponse of the neutral particle influx from the

wall

Documents

M. Hron ETFP workshop Kraków 11/09/2006 Presented by M Hron J Stockel, J Brotankova, I Duran, J Adamek, M Stepan, O Bilykova M Spolaore, E Martines, P