CAE/GAE modes –link to electron transport in MAST?

A.R. Field1, R.J. Akers1, L. Appel1, D. Dunai3, H. Smith2, M. Turnyanskiy, M.Valovic1, E. Verwichte2, S. Zoletnik3, O. Zolotukhin1 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 RMKI/KFKI, Hungarian Association of Science, Budapest, Hungary

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

Introduction

Fast magneto-acoustic modes:

• CAE or GAE modes in frequency range 0.2 < /ci < 1, (ci/2 ~ 2-4 MHz)

• GAEs have k|| >> k (shear), whereas CAEs have k >> k|| (compressional)

• May resonate with electron transit frequencies resulting in stochastic losses?

Core electron transport:

• Central Te profile (r/a < 0.3) is often observed to be flat in NBI heated discharges

• High electron heat diffusivity hard to explain by micro-instabilities driven by Te

• Fast-ion pressure profile peaked with typically Pfast/Ppl 0.3

• High levels of high frequency MHD activity driven by fast-ions with vb ~ 2 vA

Fast-ion diffusion:

• Anomalous fast-ion loss or slowing-down modified heating profile modified e

• TAE activity 50 kHz < fMHD < 300 kHz causes fast-ion redistribution and losses

• Dfast 4 m2/s can be required to match neutron rate and Wpl in TRANSP

ASTRA Transport Simulations (GLF23)

Results:

• Match to Te for r/a > 0.4

• Flat Te in core not reproduced

• Initial rise in core Ti too slow

• Acceptable match to Ti for r/a > 0.4

• ExB shear stabilisation included

• GLF23 is not ideal for MAST

O. Zolotukhin

O. Zolotukhin

Flat core Te in presence of CAEs

• OMAHA high-frequency coil array (Br,z, coils at 9 locations, 5 MHz, n 20)

• Modes at f ~ 0.8 MHz identified as CAEs from polarisation measurements

• CAEs typically observed 0.8-2 MHz range with n = 4-10

CAEsCAEs

Polarisation of HF modes

• CAEs or GAEs are elliptically polarised, plane of polarisation

• Shear GAEs have , compressional CAEs have

• Polarisation measurements on MAST indicate high-frequency modes are CAEs

• In contrast, lower frequency TAE modes identified as shear modes

tan eee

Plane of polarisation Mode polarisation (en.B0 ~ 0)

00 BB nn ee 00 BB nn ee

L C Appel, T. Fulop, et al., PPCF, 50 (2008) 115011

B~

Calculations of CAE eigenfunctions

• CAE mode structures calculated using cold, Hall-MHD equation for ST plasma

• Lower order modes - standing-waves, Higher-order modes - travelling waves

• Eigenmodes are radially extended peaking at r/a ~ 0.5

• Decreasing the aspect ratio localises modes more to the outboard side

H. Smith, E. Verwichte, PPCF, 51 (2009) 075001

B|| eigenfunctions (n=5) Mode frequencies

Evidence for fast-ion redistribution

• Matching neutron rate and Wpl requires assumption of Dfast ~ 2 m2/s

• Fast-ion induced MHD results in redistribution of NBI power deposition

• Energy dependent Dfast evoked in some cases, e.g. low-ne L-mode ‘ITB’ discharges

• Preferential loss of energetic fast-ions would reduce Qe (hence e) and neutron rate (also sensitive to NBI species ratio, power calibration, etc)

• Planned FIDA and neutron camera diagnostics will help with interpretation

2010: Neutron Camera and FIDA system

Fast-ion distribution:

• Important to diagnose fast-ion spatial and energy distributions

• Determines heating and momentum sources and NBI driven current density

Neutron Camera:

• Neutron emission dominated by beam-thermal component

• Spatial profile sensitive to fast-ion distribution

• Four channel, proof-of-principle neutron camera for M8

FIDA Diagnostic:

• Measures spectrum of Doppler-shifted D emission from fast-ion CX neutrals

• Multi-channel system planned for M8 (EFDA Priority Support)

Neutron Camera

FIDA Spectrum

2010: 2D BES turbulence imaging system

APD array

• 2D BES system for density turbulence measurements

• f 1 MHz, kr, 1.6 cm-1, ki 1, ne/ne few 0.1%

• Custom 8x4 channel APD camera

• Developed in collaboration with HAS at RMKI Budapest

Pre-amplifier

APD camera

Interferencefilter

In-vesseloptics

Mirror drive

A.R. Field, et al., RSI, 80 (2009) 073503

Summary & Outlook

NSTX PRLs:

• Gates et al. (2001):

Fast-ion driven CAEs fast-ion velocity diffusion enhanced ion heating

• Stutman et al. (2009):

Fast-ion driven GAEs stochastic e- diffusion increased electron losses

MAST results:

• Fast magneto acoustic modes identified as CAEs from polarisation

• Flat core Te profiles often observed in presence of fast-ion driven MHD

• Anomalous fast-ion redistribution/loss evoked for consistent interpretation

Plans for 2010:

• Diagnose fast-ion distribution and refine interpretation of influence on transport

• Encourage further studies to characterise fast-ion driven MHD

• Dedicated experiments, e.g. power or energy scan, 2D BES measurements, etc