Beam Species Measurements on the MAST NBI system

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Beam Species Measurements on the MAST NBI system
Brendan Crowley Thanks to R King (CCFE) and A
Bharatti (IPR)
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Outline of discussion

• Neutral beams at MAST
• Ion Source Configurations
• Beam Species Model
• Beam Spectroscopy
• Results

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Neutral Beams at MAST

• Two on axis beams
• 75 keV 65 amps (D2)
• Up to 2.5 MW NBI per beam
• JET PINI type ion sources
• Getter pumped beam boxes
• The basis for several diagnostics
• CXRS
• MSE
• FIDA

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Neutral Beams at MAST

• Two on axis beams
• 75 keV 65 amps (D2)
• Up to 2.5 MW NBI per beam
• JET PINI type ion sources
• Getter pumped beam boxes
• The basis for several diagnostics
• CXRS
• MSE
• FIDA

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• Neutral beams are used for
• Heating
• Fuelling
• Diagnostics (e.g. MSE, CXRS,FIDA)
• Current drive
• Rotation drive
• How well they do this depends on
• Beam penetration depth (particle energy)
• Beam particle density
• Beam current
• Beam divergence (angular width)
• Mostly we need to know
• how much energy is deposited and
• where it gets deposited

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Ion Sources

• Filament or RF driven discharge
• Magnetic confinement achieved by using permanent
  magnets in multi-cusp configuration - strong
  field on the walls, field-free region in the
  centre of the source.
• Ions created in various electron-molecule and
  ion-molecule collisions.
• Typical ion source parameters
• Primary electron energy (Varc) 100eV
• Arc current lt1500 A
• Gas flow 10 -15 mbarl/s
• Gas pressure 10-3 mbar
• Parameters determined by the ion source
• (fixed magnetic configuration and gas flow)
• Arc efficiency Ibeam/Iarc
• Ion species fractions HH2H3

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MAST NBI Ion Source

• MAST injectors have ion sources with a supercusp
  magnetic filter field that is used to enhance the
  production of monatomic ions
• No primaries are available for further ionisation
  of the neutral source gas.
• The molecular ions are preferentially removed at
  low electron energies by dissociative attachment.

Magnetic configuration of the standard MAST
supercusp ion source.
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MAST NBI Ion Source

• Next Campaign Filter field will be removed
  leading to
• Primaries being available for further ionisation
  of the neutral source gas.
• The molecular ions are preferentially removed at
  low electron energies by dissociative attachment.
• Need to measure new species mix.

Magnetic configuration of the new MAST supercusp
ion source.
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DSS system
?
Beam emission shifted from Da by ????0(Vbeam/c)
cos?
Source parameters Species mix
(HH2H3) Beam parameters Perveance scans (
beam divergence ) Beam species distribution
Neutral beam fractions (H(E)H(E/2)H(E/3))Power
fractions (P(E)P(E/2)P(E/3)
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and continues in the neutraliser
.
There are three simple scenarios
H
H2
H
These three molecular ions have all been
accelerated through the same potential, and
therefore they all have the same energy. E.
H2
H2
H
H
H3
The collisions with the neutraliser gas can cause
the molecular ions to dissociate.
H2
H
H
H
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Channels for Beam Emission
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DSS analysis Corona model
H-alpha transition diagram

• iH,H2,H3(ext species)
• k0,1,-1(charge state)
• Nl(?) Excited neutral density at energy ?E,E/2
  and E/3.
• Nik Beam species density at energies
  E,2E/3,E/2 E/3.
• sik(?')Excitation cross section of a beam
  species.
• Fik(?') Fraction of beam existing in a charge
  state k.

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Species mix(source)
Beam fractions and power fractions
Species fractions(beam)
Specie fractions
C2 and C3 are intensity multiplication factors
and they depend on beam energy and target
thickness.
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Results

• Raw data for Da peak
• Doppler shifted full, half and third energy peaks
• Impurity peak at E/18

• Shifted and impurity peaks fitted by analysis
  code using constrained MPI fit.

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Results

• Species fractions as a function of arc current.

• Power fractions as a function of Extracted power.

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Species Composition for checkerboard source.
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Limitations of the Model

• Assumption that radiation is instantaneous.
• H(3s) has longer life times (158 ns)
• Major populating and depopulating processes are
  neglected.
• Cascading from upper states
• Collisional quenching of excited neutrals.
• Need to apply modified corona model.

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Summary and future work

• Progress so far
• Proof of principle experiment complete..
• Analysis code complete.
• Species fractions for filter source measured
• Power fractions to MAST confirmed.
• Future work.
• Confirm species fraction on filter and non
  filter source.
• Beam particle density
• Beam current
• Beam divergence

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Obtaining the relative fractions

• These are calculated with a simple expression
• This shows the number of third energy particles
  relative to the number of full energy particles
• F is flux of particles leaving the ion source, J
  is detected intensity
• C is a constant, which looks like

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this

• Thankfully, most of these terms can be ignored,
  because
• There will be no emission from ions (they have no
  electrons)
• We dont care how many ions there are, because
  they get removed anyway
• So we have a much simpler expression to deal with
• s is the excitation cross-section and f is the
  fraction

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Estimation of Beam fractions
Beam fractions at 20KeV
Formulation K H Berkner, R V Pyle
and J W Stearns, 1975, Nuclear fusion,
249-254,15. J
Kim and H H Haselton, 1979, J.Appl.Phys,3802-3807,
50. Cross sections C F Barnett,
Atomic data for nuclear fusion,
1990,Volume.1,Report ORNL-6086/V1,
Oak ridge national laboratory.