Micro-tearing induced electron transport

A.R. Field1, R.J. Akers1, D.J. Applegate1, W. Guttenfelder2, M.Reshko3, C.M.Roach1, R. Scannell1, M.Valovic1 and the MAST team

1 Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OXON, OX14 3DB, UK2 Centre for Fusion, Space and Astrophysics, Dept. of Physics, University of Warwick, Coventry, CV4 7AL.

3 Department of Physics, University of York, Heslington, York, YO10 5DD, UK.

ACKNOWLEDGEMENTSThis work, carried out under the European Fusion Development Agreement, supported by the United Kingdom Engineering, Physical Sciences Research Council and the European Communities, has been carried out within the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission."

ITPA Confinement and Transport Meeting5th-7th October 2009

PPPL, USA

ST Transport paradigm

ExB

• High ExB shear, driven by tangential NBI heating, suppresses ion scale turbulence

• i reduced to ion neo-classical level but e remains highly anomalous

• Non-linear electron-scale (ETG) simulations exhibit radially elongated streamers

• Streamers result in eeExp ~ 5 m2/s, much larger than mixing length estimate

• Ion scale simulations also exhibit extended modes of tearing parity, -tearing modes

• Stochasticity from non-linearly saturated -tearing may enhance electron transport

GS2: Linear growth rates GS2: Non-linear electron scale

~

H-mode transport, low *

• Electron heat flux dominates total heat loss in low-* H-mode discharge

• Ion heat flux is close to the ion neo-classical level from NCLASS

ETG transport: Modelling and predictive calculations

• ETG (GYRO) simulations, incl. EB shear to quantify e dependence on q & • Comparison of exp/model e indicates large transport near mid-radius (very sensitive to gradients)

critTeTeTe

Te2e

e L

R

L

R

L

vF

s

GYRO simulations

Walter Guttenfelder

Linear micro-tearing simulations

Properties:

• Linear instability with (as ITG) and tearing parity, i.e. at tearing layer

• Driven by free-energy in Te gradient and unstable at high > 0.1

• Collisional drive (thermal force, electron de-trapping) not important in ST plasmas

• Magnetic drifts and electro-static effects play an important role

||

~J)1(~ Oρk i

Electro-static potential Parallel vector potential Parallel electric field Parallel current density

• Islands formed by at higher-order rational surfaces, e.g. 34/25

• Stochastic field at high mode amplitudes could enhance electron transport

• Exist in region where theory breaks down, current layer width and ee ων *~iρd ~

||

~J

Non-linear micro-tearing simulations

• Observed in gyro-kinetic simulations of MAST (GS2) and NSTX (GENE)

• Simulations challenging: extended || to B and fine scale structures to B

• Small time step to resolve fast e.m. fluctuations together with slow growth rate

• Non-linear simulations required 32000 CPU hours on COLUMBUS cluster

Electro-static field

Vector potential

Potential fluctuations

• Calculations made tractable by increasing ne and to reduce parallel extent

• Also, limited number (4) of poloidal modes ( )0.6 0.4, 0.2, 0,iy ρk

Improved collision operator in GS2

• Convergence problem due to dominance of largest kx component

• Gyro-averaged pitch-angle scattering operator – partial improvement

• Hyper-diffusion term (mimics shearing effect of eddy diffusion) successful

2

kDgC k

2

kDgC k

2222

,,

222 )()( yxkk

kkyxk kkφkkDgCyx

yx

Effect of gyro-averaged pitch angle scattering Effect of hyper-diffusion

• Higher resolution ( ) run performed to test convergence

• Greater resolution increases magnitude of field perturbation, i.e. not converged

• Phase change with sudden transition to stochastic field at threshold amplitude

0.7 ..., 0.1, 0,iy ρk

Transition to stochastic field at threshold amplitude

• J|| perturbation at plane of intersection of flux tube with LFS mid-plane

• Poincaré puncture plot of magnetic field shows transition to stochastic field

t = 532 t = 598

• Stochastic field would result in enhanced radial electron heat transport

• Need to quantify stochastisation and calculate the thermal diffusivity

• On-going studies of influence of sheared flow – D. Dickenson, York University

2010: Trial PCI high-k turbulence system

• Measure k ~ 10 cm-1 (ki ~ 6) near peak of ETG range

n/n ~ 4x10-4 ~10x below predicted turbulence level (GS2)

• Localization from change in field line angle along LoS

• Relatively simple and cheap c.f. -wave scattering – detector from NIFS

• Presently under consideration whether to proceed

CO2 laser housing

Present: Retro-reflectorProposed: Split 20% of power and relay to PCI imaging optics and sensitive HgCdTe detector array

LoS double-pass of plasma

Clive Michael

2009: NdYAG TS Upgrade

Stage 1 (M7a/b):

• Eight 30Hz, 1.5J lasers with ‘burst mode’ event triggering, NTMs, pellets, etc

Stage 2 (M7c from Sept. ‘09):

• 120 spatial points with ~ 1 cm resolution (300 point Ruby TS system retained)

R. Scannell et al., Rev. Sci. Inst. 2008

Collection lensSpectrometers

Possible experimental tests

Theory predicts strong dependences on:

• Beta - dimensionless scan (planned for MAST)

• Field curvature - aspect ratio scan (at fixed ande.g. DIII-D/NSTX/MAST)Measurable signatures might be:

• Parallel current fluctuations – fast polarimetry measurements?

Br perturbations due to J|| fluctuations may be detectable on radial polarimetry chord

• Stochastic regions around island chains – high-resolution TS measurements?

Statisical analysis of relation between LTe and q profile (high-resolution TS & MSE)

How to distinguish ETG from micro-tearing?

• ETG expected to have strong dependence on q/s

• Significant ETG transport requires development of streamers, kr << k

• ETG transport exhibits critical Te behaviour

• Theoretical statements on parametric dependencies required for micro-tearing