THE CUSP AS A PLASMA CONTAINER (MAGNETIC BOTTLE) Question of the day (1975): Are the holes r ce, r ci or (r ce r ci ) ½ ?

Basis of 24” CUSP Design Need ion gyroradius ~ 1 cm and size of chamber >> 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 D 2 pellet. Non-invasive cusp width measurement: Use Thompson scattering of laser light.

24” CUSP EXPERIMENT — SCHEMATIC Pellet is vaporized by Nd/Gl laser and ionized by CO2 laser

DEUTERIUM FILAMENT EXTRUDER AND CUTTER

ACCURATE DROPPING REQUIRES D 2 TO BE PLASTIC 7 0 K — too brittle11 0 K — just right

Pellet is dropped 86 cm into 1mm diameter focal spot of Nd-gl laser PELLET POSITION DETECTING AND LASER TRIGGERING SYSTEM

INTERFEROGRAM OF DEUTERIUM PLASMA 400 ns after CO 2 laser spike. Radius ~ 3 cm. Total number of electrons ~ 10 19

PLASMA DIAGNOSTICS (WITHOUT MAGNETIC FIELD) LANGMUIR PROBE SIGNAL AT WALL Maxwellian plasma of 2  ions Expanding at 7  10 6 cm/s (~ 50eV) Only hydrogen lines

PLASMA BOUNDARY DETECTED BY MAGNETIC PROBE B = 0 in this region

ELECTRON DENSITY AND TEMPERATURE IN RING CUSP MEASURED BY THOMSON SCATTERING OF RUBY LASER LIGHT Density: from scattered signal amplitude. Temperature: from dispersion.

ELECTRON DENSITY PROFILES IN RING CUSP AT DIFFERENT TIMES Units are electrons.cm -3 Each result is the average of 2 – 4 shots T e = 15 eV

A RING OF BRIGHT LIGHT IS SEEN WHERE THE ESCAPING PLASMA HITS THE CHAMBER WALL AND WINDOW Light on window ~ 1cm wide Streak camera picture of light on window. Duration ~ 10  sec. Ring cusp light on interior chamber wall

CONCLUSIONS The gyroradius of 15eV electrons in KGauss is ~ cm. The gyroradius of 50eV deuterons at these same fields is ~ 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.

EXTRUDER NOZZLE AND FILAMENT CUTTING WIRES View from bottom

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