Transcript UW -IEC - Fusion Technology Institute

UW -IEC
Negative Ions in IEC Devices
David R. Boris
2009 US-Japan IEC Workshop
12th October, 2009
This work performed at
The University of Wisconsin Fusion
Technology Institute
IEC devices operating at 0.3-3 Pa
are a good environment
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for negative ion formation
• Negative deuterium ion formation occurs through a
variety of interactions between a deuterium plasma and
comparatively high densities of neutral deuterium.
– Low Energy (< 1 eV): Thermal electron attachment
D2  e  D2( m)  D   D
meta-stable lifetime τ = ~1 fs to ~1 ms
(Cathode Region)
– High Energy(1 keV to 100 keV): Charge transfer reactions
D3  D2  D   2 D   2 D
D2  D2  D   2 D   D
(Inter-grid Region)
D fast  D2  D   D   D
D   D2  D   2D 
BORIS et al., PHYSICAL REVIEW E, 80, (036408) 2009
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Examples of charge transfer and
thermal electron attachment within
an IEC potential well.
Thermal eattachment
In charge transfer events positive ions
undergo charge transfer within the
intergrid region and are accelerated out
of the device with a fraction of the
cathode energy
The thermal electron population within
the cathode, resultant from secondary
emission, produces negative ions that
attain the full cathode energy
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The presence of negative ions can
significantly impact particle flow
in an IEC
• Negative ions add a divergent particle flux to the
convergent ion flow of positive ions
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Two methods were used to
detect negative ions in the
HOMER IEC device
• A magnetic deflection energy analyzer was
used to measure the energy/nucleon of
negative deuterium ions.
• A Faraday trap diagnostic was used to
measure the particle flux of negative ions
leaving the IEC.
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The magnetic deflection energy
analyzer deflects negative ions into a
detector according to q/m ratio
• A variable magnetic field deflects D- and D2- into a detector according to:
• Where θd is the deflection angle, p is the ion momentum, and l is the spatial
extent of the magnetic field
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Scanning the magnetic field
isolates small portions of the
negative ion energy distribution
Detector
Trajectories generated used
SIMION charged particle
tracking software.
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Deuterium Anion Spectra
show a multi-peaked structure
D2+D+
D
D33+D
D-
D+D-
D2
D-
•Least squares fit to structure indicates a variety of processes:
•Charge transfer of ion species from source region explains
D3  D2  D   D2  D   D 
3 of the peaks.
Ex.
•Thermal electron attachment to neutral gas in the cathode
explains remaining peak. Ex. D2( J  ~ 20)  e  D2( m) τm= 1-1000 μs
D2  e  D2( m)  D   D
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τm= 10 fs
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The location of negative ion
formation within the potential well
determines the ion’s energy
Thermal eattachment
In charge transfer events positive ions
undergo charge transfer within the
intergrid region and are accelerated out
of the device with a fraction of the
cathode energy
The thermal electron population within
the cathode, resultant from secondary
emission, produces negative ions that
attain the full cathode energy
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The energies of the various peaks
scale linearly with cathode voltage.
D2m=1/2
• Anions from charge transfer attain kinetic energies of 1/2 to 2/3 the
cathode voltage
• Anions from thermal electron attachment attain the full cathode energy
Background gas pressure affects
which charge transfer reactions occur
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90 kV, 30 mA
D3+
D2+
D+
3.75 mTorr
3.0 mTorr
2.6 mTorr
2.0 mTorr
1.5 mTorr
1.0 mTorr
0.7 mTorr
0.35 mTorr
• The relative sizes of the three charge transfer peaks reflect
the changes in positive ion concentrations with varying
background gas pressure.
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Faraday trap diagnostic confirmed
the presence of negative ions
• Magnetic
filter
prevents
collection of source plasma,
and fast electrons.
• A negatively biased grid
prevents secondary e- emission
from the collector plate
• 8.5 µA/cm2 of negative ion
current was detected at 40 cm
from the IEC cathode
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Summary
• Using a magnetic deflection-energy analyzer, negative
deuterium ions resultant from both charge-transfer and
thermal electron attachment processes have been measured
in the HOMER IEC device.
• Among these negative ions were long lived D2- ions with
lifetimes of at least 0.5 µs.
• A Faraday trap diagnostic has confirmed the presence of
these negative ions and indicates that they make up a
significant portion of the particle flux within the HOMER
IEC device.
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The UW-IEC Lab
Questions?
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Deuterium Anion Spectra
show a multi-peaked structure
D2+D+
D
D33+D
D-
D+D-
D2
D-
•Least squares fit to structure indicates multiple species
•Charge transfer of ion species from source region




explains 3 of the peaks. Ex. D3  D2  D  D2  D  D
•Thermal electron attachment to neutral gas in the cathode
explains remaining peak. Ex. D2( J  ~ 20)  e  D2( m) τm= 1-1000 μs
D2  e  D2( m)  D   D
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τm= 10 fs
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The negative ion energy
distribution varies with
pressure and cathode voltage
VOLTAGE SCAN
PRESSURE SCAN
Fusion proton energy spectra from
FIDO diagnostic show significant
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0.25 mTorr
1.5 mTorr
counts/bin
counts/bin
0
2700
2900
3100
3300
3500
0
2500
0
2500
2900
3100
3300
3500
2500
100 kV
30 mA
2700 2900
3100 3300
D-D Proton Energy (keV)
3500
0
2500
2700
2900
3100
3300
3500
Gaussian Fits
Gaussian Fits
counts/bin
counts/bin
Gaussian Fits
2700
100 kV
30 mA
D-
2700 2900
3100 3300
D-D Proton Energy (keV)
3500
counts/bin
0
2500
200
150
counts/bin
50
2.5 mTorr
0
2500
100 kV
30 mA
D-
2700 2900
3100 3300
D-D Proton Energy (keV)
3500
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Pressure Scan on
Faraday Trap
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Voltage Scan on
Faraday Trap
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Two diagnostics have been
developed for negative ion
detection within IEC devices
Magnetic Deflection Energy Analyzer
Faraday Trap