rijnhuizen colloquium 18 january 2001 e. westerhof, fom-instituut voor plasmafysica‘ rijnhuizen’...

Trilateral Eureg io Cluster

FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M

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Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’

Control of Neoclassical Tearing Modes

E. Westerhof

FOM-Instituut voor Plasmafysica “Rijnhuizen”

acknowledgement:

colleagues from EFDA-JET Task force M: O. Sauter et al.

colleagues from Trilateral Euregio Cluster: R. Koslowski et al.



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CONTENTS• Introduction

– experimental beta limits– Rutherford equation for magnetic island evolution

• Control of Neoclassical Tearing Modes– ways to influence island growth– experimental realisations: COMPASS-D, ASDEX-U– ways to influence ‘seeding’ of NTM– sawtooth and NTM control on JET

• Outlook– open issues and opportunities



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-limit in TEXTOR RI-mode

• sudden confinement reduction

• mode grows to saturation in 10-20 ms

• identified as 3/2 tearing• ’ < 0 stable till 20 ms after

mode onset• triggered by sawtooth

Koslowski et al., NF 40 821 (2000)



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NTM limiting performance in JET

• cascade of NTM as NBI power is ramped and N increased

• 3/2: confinement deterioration

• 2/1: can cause disruptions

• experiments at low field as JET is “under powered”

Buttery et al., 26th EPS 1999



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Projection to ITER

• almost linear scaling of N at 3/2 mode onset with poloidal ion Larmor radius *

• results from different devices corrected for collisionality variation

• but, underlying physics not fully understood

Buttery et al., PPCF 42 B61 (2000)



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Rutherford equation

• in nonlinear regime inertia is negligible

• flux surface average of Ohm’s law (1 pert. of helical flux)

<t1>= <(j1 jni1) 1j0 >• current perturbation j1 = ’(w) 1 / w (constant 1 approx.)

• stability parameter ’(w) = jump in 1 derivative across magnetic island width w

w = 4 (1 / 0’’)1/2

• Rutherford equation:

tw = 1.22 (’(w) c1jni1 c21j0)



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Instability drive from bootstrap current perturbation• bootstrap current

jbs 1/2 p’

• profile is flattened across island by parallel transport

• jni1 = jbs in island, this drives mode growth

tw jbs w / (w2 + wc2)

• with incomplete flattening

wc (/||)1/4

Sauter et al., PoP 4 1654 (1997)



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Generelized Rutherford equation• including neoclassical effects

• sign of ion-polarisation term depends on mode rotation and is highly disputed

• phase diagram with ’ < 0

• a critical seed island is

required for mode growth

3ip

2

22c

2p2/1

1w

),(ga

ww

wa'22.1

dtdw

Hegna, PoP 5 1767 (1998)



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Control of neoclassical TMs, I

• current profile control in order to increase ’

• all previous work on classical TM applies

for example, by driven current jCD exp(2(rrCD)2)

contribution to ’

’CD =

ICD (q2/rs2q’) F((rCD-rs))

Westerhof, NF 30 1144 (1990)



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Lower Hybrid Current Drive COMPASS-D • 2/1 neoclassical tearing mode

during high power ECRH (1MW)• stabilised by 90kW off-axis LHCD• increase of after stabilisation • change to ’ estimated to be

sufficient for NTM stabilisation

Warrick et al., PRL 85 574 (2000)



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Control of neoclassical TMs, II

• co-current drive inside island to substitute missing jbs

• phased current drive term in Rutherford equation

constant aJ depends on local shear, phasing, and JCD(r) profile

J0 = Ip(rs) / rs2

• CW current drive appears to be similarly effective– J1 is created by transport within flux-surfaces

0

CDJ J

Jw1

a'22.1dtdw

Morris et al., 19th EPS and ICPP (1992)

Yu et al., PoP 7 312 (2000)

Electron Cyclotron Current Drive ASDEX-Upgrade



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• 3/2 neoclassical tearing mode during high power NBI (10 MW)• stabilised by 1 MW off-axis co-ECCD (continuous)

Gantenbein et al., PRL 85 1242 (2000)



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Electron Cyclotron Current Drive ASDEX-Upgrade, modelling• MHD modelling of ECCD

stabilisation• very sensitive to co-CD location• with counter-ECCD no effect

• also heating acts to stabilise• heating and co-CD about equally

effective • ctr-CD and heating balance

Gantenbein et al., PRL 85 1242 (2000)

Control of neoclassical TMs, III



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• finite wseed needed for growth: control seeding of mode– seeding is a rather neglected topic in NTM literature

• seeding of 3/2 NTM mostly from sawtooth precursor– toroidal coupling of 2/2 component to the 3/2 mode

• thus: sawtooth control = NTM seed island control– sawtooth stabilisation risk of “monster sawtooth” BAD– destabilisation small period, small amplitude small seed– recent experiments on JET by Task force M



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• 2.5 T, at such fields NTM never triggered with NBI-only

• NTM triggered by large sawtooth with 800 ms period

• low threshold:

N = 1.1

N

SXR

PNBI, PICRH

n=1, n=2 NTM

Monster sawtooth triggered NTMJET, with central ICRH



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• example: IC resonance on low field side

• RF phasing -90o for current drive

• B-field ramped down from 1.65 to 1.40 T

• clear effect on period as resonance crosses sawtooth inversion

ICCD sawtooth (de)stabilisationJET, 42 MHz 2cH H-minority



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• comparison of JET discharges 51994 with ICCD for sawtooth destabilisation and 51995

• NBI power ramp to

locate threshold N

• 1.20 T, 1.20 MA

• further analysis ongoing

Higher threshold N for NTMwith ICCD sawtooth destabilisation



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Outlook• open issues:

– natural mode rotation and ion-polarisation current

– causes and processes of seed island generation

– experimental scaling of NTM -limit, extrapolation to reactor

– …

• opportunities:– TEXTOR in same collisionality regime as ITER-FEAT

– 1MW, 140 GHz ECCD system opens ways to affect NTM

– DED is an experiment on seed island generation

– ECRH on JET-EP well suited for NTM and sawteeth studies

– we have expertise, experimental tools, and codes to contribute to fundamental questions raised by NTM physics

rijnhuizen colloquium 18 january 2001 e. westerhof, fom-instituut voor plasmafysica‘ rijnhuizen’...

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