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Fast Ion D-Alpha (FIDA) measurements at ASDEX Upgrade

B. Geiger, M. Garcia Munoz, W. W. Heidbrink, G. Tardini, V. Igochine, R. Fischer, R. Mc Dermott and the ASDEX Upgrade team

Advanced course of European Ph.D Network, Garching, October 01, 2010

Max-Planck-Institut für Plasmaphysik

Outline:• Motivation• Principles of the FIDA technique• Diagnostic setup at AUG• Results• Summary and Outlook

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Motivation - Fast-ions in fusion plasmas

Fast-ion redistribution/loss mechanismsFast-ion redistribution/loss mechanisms• Prompt fast-ion losses of NBI, ICRH and Prompt fast-ion losses of NBI, ICRH and

fusion originfusion origin• Magnetic field configuration e.g. rippleMagnetic field configuration e.g. ripple• Anomalous transport; ELMs, Anomalous transport; ELMs,

Microturbulence and MHD; vMicroturbulence and MHD; vfast fast > v> v Alfven Alfven

Fast-ion sourcesFast-ion sources • 3.5 MeV 3.5 MeV αα-particles produced in -particles produced in

thermonuclear reactionsthermonuclear reactions• NBI & ICRF heatingNBI & ICRF heating

Fast-ion confinement essential forFast-ion confinement essential for• Heating and current drive efficiencyHeating and current drive efficiency• Safety operation; First wall damageSafety operation; First wall damage

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The 6D distribution function of fast ions (reduced to 3D)

Fast Ion D-Alpha (FIDA) technique enables to observe a part of the distribution function

explored by W. W. Heidbrink, DIIID, 2004

Pitch v||/vtotal

(Projection the velocity vector on the magnetic field)

Simulated by TRANSP

Simulated by TRANSP

pitch of NBI

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Overview

• Motivation

• Principles of the FIDA technique

• Diagnostic setup at AUG

• Results

• Summary and Outlook

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Observation of Balmer alpha light: λ0=656.1, n=3-2

• Fast ions are neutralized by charge exchange reactions along NBI (localization of the measurement)

• Dα emission (n=3-2) with λ0=656.1 nm + shift

Energy 20keV 60keV 100keV

ΔλDoppler, α=0° 3.03 nm 5.26 nm 6.80 nmc

mEpitch /20

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FIDA radiance IFIDA contains information on the density nfast ions

+ Decay from higher n-states

+ Excitation from lower n-states by electron/ion impact

n

beambeamnCXbeam

beamnionsfastneutralsfastn vvnnn )3(,3

dlEnI neutralsfastnFIDA 233

dl : Integration along a given line of sight

E3→2: Transition probability from n=3 to n=2: Einstein coefficient

nbeam: Density of injected neutrals with full, half, and third energy (species mix) and Halo neutrals: Cloud of thermal neutrals around NBI, produced by charge reactions between injected neutrals and thermal D-ions.

σCX: Cross section for charge exchange

vbeam: relative velocity between fast ions and beam neutrals

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Interpretation of FIDA measurements

FIDA measurements are difficult to unfold Forward model to check

theoretical distribution function

FIDASIM code (Heidbrink, DIIID) :• Monte Carlo code using a 3D grid• 3D density profiles of beam neutrals nbeam (injected and halo neutrals)• Artificial FIDA spectra representing a theoretical fast ion distribution function

(e.g. from TRANSP)

Inputs:• Fast ion distribution function

(TRANSP)• Atomic rates and cross sections• Kinetic profiles (Te, ne, Ti, ni, vtor…)• Equilibrium• Geometry

Calculation:• Attenuation of injected NBI neutrals

and generation of Halo neutrals• Probability for charge exchange

reactions of fast ion• Collisional radiative model along

path of a fast neutral• FIDA spectra (Doppler, Stark Effect)

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FIDA emission must be separated from other spectral contributions

• Halo emission: Halo neutrals are thermally distributed. Their emission can be approximated with a Gaussian curve.

• Bremsstrahlung: Radiation from the whole plasma. Flat shape in spectra but limits FIDA technique to low densities and Zeff

Passive:• Edge D-alpha: very intense passive

radiation of D atoms at the edge

• Impurity line radiation

• Beam emission: injected neutrals get excited and emit D-alpha radiation. Shape determined by species mix, Doppler shift and Stark splitting

Active D-Alpha components:• FIDA

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Overview

• Motivation

• Principles of the FIDA technique

• Diagnostic setup at AUG

• Results

• Summary and Outlook

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Existing CXRS diagnostic (CER) used for FIDA measurements!

• Focused on 60kV source NBI 3

• 25 tangential lines of sight

• 400 µm fibers

• Movable grating, 2400l/mm

• CCD camera (PI) operated in frame transfer mode

• ~9nm spectral range

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BEAM emission

core

edge

PASSIVE D-Alpha spectra at AUGACTIVE D-Alpha spectra at AUG

Measurement!

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• Clean spectra thanks to small low Z-impurity concentrations (e.g. C)

Background can be estimated as a flat line (Bremsstrahlung)

Continuous FIDA measurements possible! (Beam modulation not necessary)

• 10 ms exposure time

• instrument function of ~0.2 nm(200µm entrance slit)

• 661.0nm central wavelength

Example of typically observed active and passive spectra

Tungsten coating very good for FIDA:

CII

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Radial FIDA profiles

• Radial FIDA profiles can be calculated by integrating over a given wavelength range for every line of sight

• Relative small and offset-like uncertainty when estimating background with a flat line

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Weighting function

Which part of the phase space is observed?

Weighting function (rho=0.18)• Doppler effect• Stark effect (9 components)• Charge exchange cross section

into n=3 state

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Fast ions injected by the AUG NBI sources can be observed

Product of distribution function with weighting function (λ=659.5-660.5 nm)

60kV 60kV+93kV60kV+93kV

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Overview

• Motivation

• Principles of the FIDA technique

• Diagnostic setup at AUG

• Results

• Summary and Outlook

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FIDA technique resolves off- and on-axis NBI heating

On axis (NBI8)

Off axis (NBI6)

Integration from λ=659.5 to 660.5 nm

#25698: Fast ions by NBI 6 and NBI 8 in addition to NBI 3

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Comparisons to the FIDASIM code: #25528

• Low density• On-and off-axis heating by NBI 8 and NBI 6• Continuous and modulated heating with NBI 3

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Simulated (FIDASIM) and measured FIDA spectra

Multiplied by 1.3!

FIDA, Simulation

FIDA, Simulation

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Simulated and measured FIDA profiles

Good agreement is found between TRANSP predicted classical FIDA profiles (classical fast ion distribution) and the measured profiles in MHD quiescent plasmas.

Analysis with beam modulation:

Technical problem of NBI with acceleration voltage and divergence when modulating

Clear off axis contribution visible but deviances in the plasma center

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Beam emission shows that NBI 3 does not inject with full power and energy

• Intensity of Beam emission lower than 50% when modulating (reduced power)

• Shift of Beam emission indicates that NBI 3 operates with reduced voltage (less than 60kV)

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Sawtooth like crash at ~0.565s

• Crash at 0.565s shown by neutrons rate and Te

• NBI 3 fuelling continuously

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Sawtooth like crash observed by Soft X-Ray

Inversion radius at rho ~0.4

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Sawtooth like crash caused by the collapse of a double tearing mode

Z. Chang et al, ’Off-axis sawteeth and double-tearing reconnection in reversed magnetic shear plasmas in TFTR’, 1996 Phys. Rev. Lett. 77, 3553

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Fast ions are moving outwards!

Estimated density profiles of fast ions with energies between 25keV and 60keV, pitch < -0.4

(Density of injected and halo neutrals has been accounted for)

• Temporal evolution of FIDA measurements shows redistribution of fast ions during a sawtooth-like crash

• Inversion radius at about rho=0.4 comparable to observations from Soft-X-Ray

Neutron rate

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Summary and outlook

SUMMARY

• FIDA measurements are possible at ASDEX Upgrade with tangential view of CER diagnostic

• Good agreement between simulated and measured FIDA spectra/radial profiles

• Temporal evolution of fast ion densities can be studied (10ms time resolution)

OUTLOOK

• Analysis of MHD effects and different NBI injection geometries on the fast ion distribution

• Construction and installation of an independent FIDA (BES) diagnostic