Fast Ion Confinement In MST

1
Fast Ion Confinement In MST
Ben Hudson
G. Fiksel1, A.D. Beklemishev2, D. Craig1, V.I.
Davydenko2, D.J. Den Hartog1, A.A. Ivanov2, R.M.
Magee1, R. O'Connell1, S.C. Prager1, J.S. Sarff1,
Yu.A. Tsidulko2 1Department of Physics,
University of Wisconsin - Madison, Madison, WI,
USA and Center for Magnetic Self-Organization in
Laboratory and Astrophysical Plasma 2Budker
Institute of Nuclear Physics, Novosibirsk, Russia
IEA / RFP Workshop 2005 Madison, WI
2
Outline of Talk

• Motivation/Experimental setup
• Determination of fast ion confinement time
• Effect of ion guiding center drifts
• Fast ion confinement during reconnection events
• and NBI counter-injection
• Summary

3
What is the degree of confinement of fast ions
from NBI?

• The magnetic field of the RFP is typically
  stochastic.
• Dm has been measured to be 10-4 m
• A field line diffusing from near axis to wall
  1600 m
• For a 20keV deuterium ion, V 1.4x106 m/s
• An ion streaming along a stochastic field line
  would reach the
• wall in 1ms
• Energetic NBI ions have drifts that significantly
  affect their motion.
• What is the confinement of these ions in the
  stochastic magnetic field?

4
Experimental Setup
NBI
Neutral deuterium injected (20 keV, 1.3 ms, 25A)
PMT
Scintillator
MST Top View
5
Experimental Setup
NBI
Neutral deuterium injected (20 keV, 1.3 ms,
25A) Fast ions born within plasma
PMT
Scintillator
MST Top View
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Experimental Setup
NBI
Neutral deuterium injected (20 keV, 1.3 ms,
25A) Fast ions born within plasma D-D fusion
with bulk plasma ions produces neutrons Neutron
emission gives an indication of fast
ion population
PMT
Scintillator
MST Top View
neutrons
neutrons
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Neutron Flux Observed During NBI
NBI on 1.3ms

• Approximately linear rise in neutron flux during
  injection followed
• by slow signal decay.

8
Modeling of Neutron Signal

• Fast ion Coulomb collisional energy loss rate

• Te measured with Thompson scattering.
• Fast ion distribution, nfast(r), calculated based
  on ionization of neutral beam along injection
  chord.
• Plasma density, ni(r), measured with FIR
  interferometer.
• The loss time, tloss, reflects some imposed loss
  mechanism,
• (CX, stochastic diffusion, etc.)

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Estimate of Fast Ion Confinement Times
t 8 t 20ms t 1ms
NBI on
Fast ion confinement time gt 20 ms
Confinement time far exceeds the estimated
stochastic loss time.
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Fast Ions Are Less Susceptible To Stochastic
Diffusion
We believe the magnetic field in MST is
stochastic. If the guiding centers of charged
particles follow field lines, the large parallel
velocity of NBI ions would make them escape
1ms. We just saw that the fast ions are not
affected by the stochastic magnetic field of MST.
Why not?
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Ion Guiding Centers Do Not Follow Magnetic Field
Lines
Safety Factor For Magnetic Field Lines
Equilibrium field reconstructed from experimental
data.
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Ion Guiding Centers Do Not Follow Magnetic Field
Lines
Magnetic Islands
Equilibrium field reconstructed from experimental
data. Magnetic perturbations modeled with 3D MHD
code (DEBS)
Field lines
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Ion Guiding Centers Do Not Follow Magnetic Field
Lines
Safety Factor For Ion Guiding Center
Equilibrium field reconstructed from experimental
data. Magnetic perturbations modeled with 3D MHD
code (DEBS) QIGC shifted due to drifts.
Ion start radius
Field lines
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Ion Guiding Centers Do Not Follow Magnetic Field
Lines
Ion Guiding Center Islands
Equilibrium field reconstructed from experimental
data. Magnetic perturbations modeled with 3D MHD
code (DEBS) QIGC shifted due to drifts. Ion
islands analogous to magnetic islands.
Ion start radius
n5
Ions
Field lines
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Ion guiding center
n 5
n 6
Field lines
Ion guiding center punctures
n 5
16
Ion guiding center
n 5
n 6
Field lines
Jump across n 5 separatrix
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Ion guiding center
n 6
Field lines
n 6
18
Ion guiding center
n 6
Field lines
Ion becomes stochastic
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Ion guiding center
n 6
Field lines
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Rapid Losses Observed During Sawtooth Crash
One of the main features of a sawtooth crash is
increased magnetic fluctuations (factor of 2-5
is typical).
Duration of ST 0.1 - 0.5 ms Loss time during
crash lt 1 ms
NBI on
50 decrease in fast ion population
t 0.4ms
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Fast Ions Can Diffuse Stochastically During A
Sawtooth Crash
Increase mode amplitudes by factor of 3. n 5
ion island now overlapped. Rough estimate of
confinement time
Safety factor (sawtooth)
Ion start radius
0.3 ms

• Very brief confinement followed by rapid
  diffusion to the wall.
• Diffusion occurs on the timescale observed in the
  experiment.

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Counter-Injection Increases First-Orbit Losses
With counter-injection, ions are bent outward (in
radius) in the dominantly poloidal magnetic field
at the edge of MST.
Co-injection
Counter-injection
Confined
Confined
Fast ions
Fast ions
Beam Populations
Beam neutrals
Beam neutrals
Injection chord (m)
Injection chord (m)

• We know that the edge ions in counter-injection
  will be lost.
• 50 drop in signal amplitude expected due to
  prompt losses.

23
Counter-Injection Shows Increased Prompt Losses
Neutron Signal
Co-Injection Counter-Injection

• The raw signal shows 50 decrease in the neutron
  emission
• for counter-injection.

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Counter-Injection Shows Degraded Confinement
Neutron Signal
Normalized Neutron Signal
t 8 t 20ms t 4ms
Co-Injection Counter-Injection

• The raw signal shows 50 decrease in the neutron
  emission
• for counter-injection.
• The normalized signal indicates a fast loss
  mechanism for counter-injection, approaching the
  timescale of estimated stochastic losses.

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Counter-Injection Tends To Increase Ion
Stochasticity
Changing direction of Bq lowers qIGC instead of
raising it.
Safety factor (counter-injection)
Ion start radius
IGC start
n11
n8
The ion is immediately stochastic. This allows
transport towards the plasma edge where the
increased neutral density would lead to greater
charge-exchange losses.
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Summary

• Analysis of the fast ion population infers gt 20
  ms confinement time, which cannot be due to
  stochastic losses.
• Fast ions from NBI are detected through neutron
  emission after undergoing D-D fusion in the
  plasma.
• Particle orbits of fast ions are not stochastic
  because guiding center drifts take them out of
  resonance with the background magnetic
  perturbations.
• With counter-injection, prompt losses and poor
  confinement are observed experimentally.
  Modeling within the ion guiding center paradigm
  explains the observations.

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Future Work

• Collimate neutron detector to get a fast ion
  profile.
• Measure fast ion current near wall to directly
  verify the loss rate and possibly observe the
  stochastic transition of the well confined
  population.
• Modeling of NBI effect on plasma (heating,
  momentum, current drive)
• Experiments to measure NBI effect on plasma
  (longer pulse and/or higher power neutral beam
  needed)