Historical research guide · 0469 · 1956
Experimental Study of Ionized Matter Projected across a Magnetic Field
Winston H. Bostick
In one minute
Winston Bostick built a small plasma gun at the Livermore radiation laboratory and fired ionised matter — titanium and deuterium ions, struck from deuterium-loaded titanium electrodes by a short, very high-current arc — across a steady magnetic field. The plasma did not smear out. It left the gun as an expanding ring shaped by its own magnetic field, and when it crossed the applied field it held together as a compact, self-organised object that Bostick named a plasmoid, a plasma-magnetic entity. Then he measured it. Speeds up to twenty million centimetres a second by time of flight; a transverse electric field of fifty volts per centimetre where the motion across the field predicts sixty; a dip of thirty gauss in the magnetic field inside the object, meaning it carries a trapped field of its own; a definite size and shape, a cylinder stretched along the field lines with its ions gathered in a shell. Fired at one another, two plasmoids bounced apart like billiard balls, curved together, spiralled to a stop, or smashed into fragments.
Connection
This is the paper that gave chapter 9 its noun. A plasmoid is not a metaphor and not a claim about anything unexplained: it is a laboratory object with a measured speed, size, magnetic moment and electric field, made on a bench in 1956 and photographed while it flew — which is exactly why the chapter can treat self-organising, self-confining plasma as a starting point rather than as something to argue for.
From the source
The apparatus and the speed. Bostick’s button-type pulsed source strikes a high-current arc — one thousand to ten thousand amperes, lasting between a tenth and a half of a microsecond — between the exposed ends of two metallic electrodes in vacuum, and projects titanium ions, deuterium ions and electrons forward at speeds up to two times ten to the seventh centimetres per second, or about two hundred kilometres a second, measured by time of flight in the vacuum chamber.Abstract and section ‘Production of a Plasmoid in Field-Free Space’, page 292
The plasma is emitted as a ring, not a blob. Probe traces and a Kerr-cell photograph taken with a half-microsecond exposure, showing the plasma in its own recombination light, both indicate that the matter leaves the source in a toroidal form shaped by the magnetic field of the arc current itself, with no external field applied at all. Bostick’s words for what that photograph settles: the plasma is emitted not as an amorphous blob, but in the form of a torus.Section ‘Production of a Plasmoid in Field-Free Space’, pages 292 to 293, Figures 2 and 3
The word. Bostick first proposed plasmon, following Wheeler’s geon; David Pines of Princeton pointed out that plasmon should be reserved for a quantum of plasma-oscillation energy and proposed plasmoid instead, which Bostick adopted. He defines it as a plasma-magnetic entity and states that the word will be employed as a generic term for all plasma-magnetic entities. Every later use of the term in plasma physics descends from this footnote.Footnote 1, page 292
Crossing the field, and the four measured quantities. Fired into an applied direct-current magnetic field, the plasma crosses it with no difficulty and arrives as a compact configuration elongated along the field lines with its ion density concentrated in a shell. Two probes half a centimetre apart measure the transverse electric field that motion across a magnetic field must produce: fifty volts per centimetre measured, against sixty predicted for a velocity of three million centimetres per second across two thousand gauss. Halving the field roughly halves the measured electric field, so the two track together as they should.Section ‘Effect of Firing a Plasmoid across a Magnetic Field’, pages 293 to 294, Figures 6 to 8
The plasmoid carries its own trapped magnetic field. A magnetic coupling loop 0.64 centimetres across, placed ten centimetres from the source in a field of two thousand gauss, registers the plasmoid passing as a diamagnetic signal of the right duration corresponding to a magnetic field decrease of thirty gauss inside it; the largest such perturbation measured at that distance corresponds to one hundred gauss. That depression is what confines the object sideways, and it is the reason Bostick calls the thing an entity rather than a cloud.Section ‘Effect of Firing a Plasmoid across a Magnetic Field’, page 294, Figure 6 third trace
Plasmoids interact, and Bostick says plainly that no theory predicted them. Fired at one another across the field, two plasmoids deflect like billiard balls in the centre-of-mass frame; at a poorer vacuum, near a thousandth of a millimetre of mercury, they spiral to a stop; with four guns firing at once they can be made to smash each other into fragments. He notes that despite considerable theoretical interest in plasmas and magnetic fields there had been no theoretical predictions concerning the existence of plasmoids or these effects, and offers his explanation afterwards, from the diamagnetic depression and the polarisation electric field. He closes by proposing that plasmoids form naturally, represent a higher-entropy state than a homogenised mixture of field and plasma, and may be what the jets of a contracting magnetised galaxy become.Sections ‘Interaction of Plasmoids with One Another’ and ‘Explanation of Interaction Effects of Plasmoids’, pages 297 to 299
The way in
https://doi.org/10.1103/PhysRev.104.292How to cite it
Winston H. Bostick (1956) Experimental Study of Ionized Matter Projected across a Magnetic Field doi:10.1103/PhysRev.104.292