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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.

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Presentation on theme: "Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’ Control of Neoclassical Tearing Modes E. Westerhof FOM-Instituut."— Presentation transcript:

1 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.

2 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

3 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 ms identified as 3/2 tearing  ’ < 0 stable till 20 ms after mode onset triggered by sawtooth Koslowski et al., NF (2000)

4 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

5 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)

6 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)  =  current perturbation j 1 =  ’(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:  t w = 1.22  (  ’(w)  c 1 j ni1  c 2  1 j 0 )

7 Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’ Instability drive from bootstrap current perturbation bootstrap current j bs   1/2  p ’ profile is flattened across island by parallel transport j ni1 =  j bs in island, this drives mode growth  t w  j bs w / (w 2 + w c 2 ) with incomplete flattening w c  (   /  || ) 1/4 Sauter et al., PoP (1997)

8 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 Hegna, PoP (1998)

9 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 j CD  exp(  2 (r  r CD ) 2 ) contribution to  ’  ’ CD = I CD    (q 2 /r s 2 q’) F(  (r CD -r s )) Westerhof, NF (1990)

10 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 (2000)

11 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 j bs phased current drive term in Rutherford equation constant a J depends on local shear, phasing, and J CD (r) profile J 0 = I p (r s ) /  r s 2 CW current drive appears to be similarly effective –J 1 is created by transport within flux-surfaces Morris et al., 19th EPS and ICPP (1992) Yu et al., PoP (2000)

12 Electron Cyclotron Current Drive ASDEX-Upgrade 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 (2000)

13 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 (2000)

14 Control of neoclassical TMs, III Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’ finite w seed 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

15 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N SXR P NBI, P ICRH n=1, n=2 NTM Monster sawtooth triggered NTM JET, with central ICRH

16 Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’ example: IC resonance on low field side RF phasing -90 o 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)stabilisation JET, 42 MHz 2  cH H-minority

17 Rijnhuizen colloquium 18 January 2001 E. Westerhof, FOM-Instituut voor Plasmafysica‘ Rijnhuizen’ comparison of JET discharges with ICCD for sawtooth destabilisation and NBI power ramp to locate threshold  N 1.20 T, 1.20 MA further analysis ongoing Higher threshold  N for NTM with ICCD sawtooth destabilisation

18 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


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