Max-Planck-Institut
für Plasmaphysik
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
Outline:
• Motivation
• Principles of the FIDA technique
• Diagnostic setup at AUG
• Results
• Summary and Outlook
Advanced course of European Ph.D Network, Garching, October 01, 2010
1
Motivation - Fast-ions in fusion plasmas
Fast-ion sources
• 3.5 MeV α-particles produced in
thermonuclear reactions
• NBI & ICRF heating
Fast-ion confinement essential for
• Heating and current drive efficiency
• Safety operation; First wall damage
Fast-ion redistribution/loss mechanisms
• Prompt fast-ion losses of NBI, ICRH and
fusion origin
• Magnetic field configuration e.g. ripple
• Anomalous transport; ELMs,
Microturbulence and MHD; vfast > v Alfven
2
The 6D distribution function of fast ions (reduced to 3D)
Simulated by
TRANSP
pitch of
NBI
Simulated by
TRANSP
Pitch v||/vtotal
(Projection the velocity vector on the magnetic field)

Fast Ion D-Alpha (FIDA) technique enables to
observe a part of the distribution function
explored by W. W. Heidbrink, DIIID, 2004
3
Overview
• Motivation
• Principles of the FIDA technique
• Diagnostic setup at AUG
• Results
• Summary and Outlook
4
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
pitch 2 E / m
  0 
c
Energy
20keV
60keV
100keV
ΔλDoppler, α=0°
3.03 nm
5.26 nm
6.80 nm
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FIDA radiance IFIDA contains information on the density nfast ions
I FIDA   nn 3 fast neutrals  E32 dl
dl :
Integration along a given line of sight
E3→2: Transition probability from n=3 to n=2: Einstein coefficient
nn3 fast neutrals  n fast ions    nn,beam   CX ( n3) vbeam  vbeam
n beam
+ Decay from higher n-states
+ Excitation from lower n-states by electron/ion impact
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:
Calculation:
•
•
•
•
•
•
Fast ion distribution function
(TRANSP)
Atomic rates and cross sections
Kinetic profiles (Te, ne, Ti, ni, vtor…)
Equilibrium
Geometry
•
•
•
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
Active D-Alpha components:
•
•
•
FIDA
Beam emission: injected neutrals get
excited and emit D-alpha radiation.
Shape determined by species mix,
Doppler shift and Stark splitting
Halo emission: Halo neutrals are
thermally distributed. Their emission
can be approximated with a Gaussian
curve.
Passive:
•
•
Edge D-alpha: very intense passive
radiation of D atoms at the edge
Impurity line radiation
•
Bremsstrahlung: Radiation from the
whole plasma. Flat shape in spectra
but limits FIDA technique to low
densities and Zeff
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Overview
• Motivation
• Principles of the FIDA technique
• Diagnostic setup at AUG
• Results
• Summary and Outlook
9
Existing CXRS diagnostic (CER) used for FIDA measurements!
• Focused on 60kV source NBI 3 • Movable grating, 2400l/mm
• CCD camera (PI) operated in
• 25 tangential lines of sight
frame transfer mode
• 400 µm fibers
• ~9nm spectral range
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PASSIVE
ACTIVE D-Alpha
D-Alphaspectra
spectraat
atAUG
AUG
Measurement!
BEAM emission
edge
core
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Example of typically observed active and passive spectra
•
10 ms exposure time
•
instrument function of
~0.2 nm
(200µm entrance slit)
•
661.0nm central
wavelength
CII
Tungsten coating very good for FIDA:
•
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)
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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
60kV
60kV+93kV
60kV+93kV
Product of distribution function with
weighting function (λ=659.5-660.5 nm)
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Overview
• Motivation
• Principles of the FIDA technique
• Diagnostic setup at AUG
• Results
• Summary and Outlook
16
FIDA technique resolves off- and on-axis NBI heating
#25698: Fast ions by NBI 6
and NBI 8 in addition to NBI 3
On axis (NBI8)
Off axis (NBI6)
Integration from
λ=659.5 to 660.5 nm
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Comparisons to the FIDASIM code: #25528
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•
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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
FIDA, Simulation
FIDA, Simulation
Multiplied by 1.3!
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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)
21
Sawtooth like crash at ~0.565s
•
•
Crash at 0.565s shown by neutrons rate and Te
NBI 3 fuelling continuously
22
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 doubletearing reconnection in reversed magnetic shear
plasmas in TFTR’, 1996 Phys. Rev. Lett. 77, 3553
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Fast ions are moving outwards!
Neutron rate
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
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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
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