rijnhuizen colloquium 18 january 2001 e. westerhof, fom-instituut voor plasmafysica‘ rijnhuizen’...
TRANSCRIPT
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
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.
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
-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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
• 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)
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
• 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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
• 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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
• 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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
• 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
Trilateral Eureg io Cluster
FO M -Instituut vo o r PlasmafysicaA sso ciatio n EU RATO M -FO M
TEC
Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’
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