THE CUSP AS A PLASMA CONTAINER MAGNETIC BOTTLE

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THE CUSP AS A PLASMA CONTAINER (MAGNETIC BOTTLE)
Question of the day (1975) Are the holes rce,
rci or (rcerci)½ ?
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Basis of 24 CUSP Design

• Need ion gyroradius 1 cm and size of chamber gtgt
  ion gyroradius Make the ring cusp diameter 60
  cm (24).
• Need field-free plasma with single ion species
  Create at cusp null point by two-stage laser
  ionization of falling D2 pellet.
• Non-invasive cusp width measurement Use
  Thompson scattering of laser light.

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24 CUSP EXPERIMENT SCHEMATIC
Pellet is vaporized by Nd/Gl laser and ionized by
CO2 laser
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DEUTERIUM FILAMENT EXTRUDER AND CUTTER
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ACCURATE DROPPING REQUIRES D2 TO BE PLASTIC
70K too brittle
110K just right
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PELLET POSITION DETECTING AND LASER TRIGGERING
SYSTEM
Pellet is dropped 86 cm into 1mm diameter focal
spot of Nd-gl laser
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INTERFEROGRAM OF DEUTERIUM PLASMA
400 ns after CO2 laser spike. Radius 3 cm.
Total number of electrons 1019
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PLASMA DIAGNOSTICS (WITHOUT MAGNETIC FIELD)
LANGMUIR PROBE SIGNAL AT WALL
Maxwellian plasma of 2 ? 1019 ions Expanding at
7 ? 106 cm/s ( 50eV)
Only hydrogen lines
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PLASMA BOUNDARY DETECTED BY MAGNETIC PROBE
B 0 in this region
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ELECTRON DENSITY AND TEMPERATURE IN RING CUSP
MEASURED BY THOMSON SCATTERING OF RUBY LASER
LIGHT
Density from scattered signal amplitude.
Temperature from dispersion.
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ELECTRON DENSITY PROFILES IN RING CUSP AT
DIFFERENT TIMES
Units are 1015 electrons.cm-3 Each result is the
average of 2 4 shots
Te 15 eV
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A RING OF BRIGHT LIGHT IS SEEN WHERE THE ESCAPING
PLASMA HITS THE CHAMBER WALL AND WINDOW
Streak camera picture of light on window.
Duration 10msec.
Ring cusp light on interior chamber wall
Light on window 1cm wide
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CONCLUSIONS

• The gyroradius of 15eV electrons in
  1.7-2.1KGauss is 0.006 - 0.008 cm.
• The gyroradius of 50eV deuterons at these same
  fields is 0.7 - 0.9 cm.
• The loss width of a ring cusp, from laser
  scattering and wall light, is consistent with the
  ion gyroradius.
• Results were highly reproducible, indicating
  absence of instabilities
• SIGNIFICANCE OF THE RESULTS
• For MFE The size of the escape hole makes the
  cusp unsuitable as a long term plasma container.
• For IFE The experiment demonstrates the basic
  principle of Magnetic Intervention.

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EXTRUDER NOZZLE AND FILAMENT CUTTING WIRES
View from bottom
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