Roger Jaspers 1

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How to take measurements in a plasma at 150
million C

• Roger Jaspers

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A typical tokamak plasma

• Parameters Fusion reactor
• TeTi 150 MK
• ne1020 m-3
• Bt 5 T
• Ip15 MA
• high neutron flux
• ? gt 1016 n/m2s

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Questions

• What do we want to measure ?
• Why do we want to measure this ?
• How are we going to do this?
• What is possible?

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What do we want to measure ?

• Brainstorm ..
• Temperature - neutrons
• Density - helium
• Impurities - pressure
• Current - electric field
• Magnetic field - Rotation
• Radiation - Plasma position
• ..and many more things..
• and its profiles
• and its time evolution ..

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What can we measure ?

• Fields
• Radiation
• Particles

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What can we measure ?

• Passive Methods
• Magnetic coils
• Spectroscopy
• Mm wave emission
• Neutral Particle analysis
• Thermography
• Active Methods
• Interferometry
• Polarimetry
• Reflectometry
• Thomson Scattering
• Charge Exchange Spectroscopy
• Laser induced fluoresence
• Heavy ion beam probe
• Pellet injection
• ..many more..

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Why do we want to measure this ?

• Machine control
• Plasma position
• Bt, Ip, ne
• ? feedback control
• Physics understanding
• profiles, time evolution, fluctuations

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Examples
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Interferometry

• Refractive index of plasma
• Nkc/w (1- wp2/ w2)1/2 ? 1- wp2/ 2w2
• (for perp. propagation, O-mode)
• IR wavelength O (100 mm)
• Phase shift
• Line integrated signal

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Interferometry

• Mach-Zehnder interferometer

Shortcomings - fringe jumps - amplitude
variations due to absorption/refraction
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Geometry

• Flux surfaces
• 1D,2D,3D, ..

scattering
Local
Line-averaged
Multi-chord
tomography
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Abel inversion

• Theory abel-inversion

• shortcoming
• Hollow profiles

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Density measurement

• TEXTOR setup Typical Measurement

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Polarimeter

• Principle polarimeter
• Linear polarized wave
• Different refractive index of the two circular
  components
• X en O-mode
• ? Faraday rotation
• For propagation
• parallel to magnetic field

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Polarimeter
ITER

• Setup - polarimeter

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Temperature Measurements

• Thomson scattering Te (z)
• high localization, high accuracy, no calibration
• low (no) repetition rate
• Electron Cyclotron Emission (ECE) ?Te (r)
• high repetition rate, good localization and
  accuracy
• difficult calibration, only optically thick
  plasmas
• Charge Exchange Spectroscopy ? Ti

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Thomson Scattering

• Electrons scatter photons from laser puls
• ? observed photons Doppler shifted
• Spectrum represents
• velocity distribution
• ? width ?Te
• number of scattered
• photons ne

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Thomson Scattering

• Intensity of scattered light ne
• For maxwellian EVDF, width of scattered spectrum
  Te
• Relativistic effects are important for Te gt 0.5
  keV (mass / headlight)
• Scattering yield is very lowPs/P0 2 ? 10-15
  for ne 5 ? 1019 m-3, DL 5 mm,W 5 ? 10-3 sr
  and 20 transmission of optical system
• Excellent spatial resolution (mm range)
• Bad time resolution

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Thomson Scattering

• Typical Setup
• 25 J Ruby Laser
• (694.3 nm)
• Presently under
• Development
• Multi-burst system

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Thomson Scattering

• Typical Setup

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Thomson Scattering

• Example Textor

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Thomson Scattering
Two Te profiles in a single shot, just before
and after a sawtooth crash
Te and ne profiles for a plasma with m2 islands
with peaked ne inside the island
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Thomson Scattering
LIDAR
LIght Detection And Ranging features 180
scattering Time-of-flight information of
scattering from short pulse (300 ps ? 9 cm)
gives position information
LIDAR system at JET
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Electron Cyclotron Emission

• cyclotron motion
• wce eB/gme ? emission in 100 GHz range
• Radial resolved ? BB0R0/R
• If density is sufficiently high large fraction
  of photons reabsorbed ? Equilibrium between
  radiation field and emitting/absorbing medium ?
  Blackbody Radiator
• Planck
• Raleigh-Jeans

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Electron Cyclotron Emission

• Optical depth
• Resonances N1
• Cutoffs N0
• Use 2nd harmonic

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Electron Cyclotron Emission

• Localisation
• harmonic overlap
• relativistic effect wce1/g ? R, few cm r
• Doppler broadening (antenna)
• ECE-I
• 2D array
• Example TEXTOR

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Electron Cyclotron Emission
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Reflectometry

• Inject mm waves into plasma
• reflection at wpe
• optical wavelength
• determines position
• of reflection
• A) interferometry
• B) Pulse radar
• Also for fluctuation
• studies!

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Reflectometry

• Fluctuations

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Active Beam Spectroscopy

• H0/D0 beam injected to heat plasma
• Energy 50-100 keV/amu
• Also Diagnostic capabilities
• Charge Exchange Recombination Spectroscopy
• Beam emission Spectroscopy
• Motional Stark Effect Diagnostic

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Charge Exchange Spectroscopy

• resonant process
• exchanged electron
• in excited state
• Maximum cross-section in
• range 30-60 keV/amu

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Charge Exchange Spectroscopy

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Charge Exchange Spectroscopy

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Charge Exchange Spectroscopy

• High intensity
• Local Measurement
• Ti(r)
• V(r)
• nz(r) ? also He!

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Charge Exchange Spectroscopy

• But also complications
• passive line
• other spectral lines
• spectral deformation
• - due to scx f (Erel)
• - due to radial averaging
• contribution from excited beam neutrals
• beam attenuation
• Zeeman/Paschen Back splitting

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Charge Exchange Spectroscopy

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Beam emission Motional Stark effect
Balmer a emission (n3?n2, 6562 A)

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Motional Stark Effect

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The magnetic field structure
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The magnetic field structure

• The magnetic field helicity is described by the
  safety factor q
• The magnetic field structure is determined by the
  current density profile
• Low rational q-values like 1, 4/3, 3/2, 2, 5/2,
  3, 7/2 are related to - MHD instabilities -
  the transport properties - barriers

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Balmer-? line

• Hydrogen or deuterium
• Transition n3 ? n2
• Balmer-? line

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Motional Stark Effect (MSE)

• ADVANTAGES
• - local emission
• - full profile

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Motional Stark Effect (MSE)

• ADVANTAGES
• - local emission
• - full profile

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Motional Stark Effect (MSE)

• ADVANTAGES
• - local emission
• - full profile

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Motional Stark Effect (MSE)

• ADVANTAGES
• - local emission
• - full profile

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Different methods

• polarisation of one line TFTR,
  DIII-D,JET,ASDEX

• Stark splitting not sensitive to Bp

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Different methods

• polarisation of one line TFTR,
  DIII-D,JET,ASDEX

• Stark splitting not sensitive to Bp

• Intensity ratio I?/I? TEXTOR (ITER??)

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

• The direction of the electric field

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

• The direction of the electric field

• The beam density

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

• The direction of the electric field

• The beam density

• The composition of the injected beam

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Diagnostic information

• The location of the observation volume

• The absolute value of the magnetic field

• The direction of the electric field

• The beam density

• The composition of the injected beam

• The radial electric field

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