Use of coaxial plasma guns to start up field-reversed-mirror reactors Page: 4 of 16
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neutral-bean injection. In this report we consider the design (based on
preliminary scaling laws) of a coaxial plasma gun for start-up in the field-
re versed-mirror pilot reactor of Ref. 2.
THE MAGNETIZED COAXIAL PLASMA GUN
The basic arrangement for formation of field-reversed plasma rings with
a coaxial plasma gun is shown schematically in Fig. la. The plasma gun is a
cylindrically-symmetrie, annular-breeched device in which a ring-shaped plasma
is formed and accelerated by discharge current flowing radially across the
anode-cathode gap. If the gun's external solenoid windings and the solenoid
windings inside the center electrode are properly designed, the central, axial
magnetic flux can be constrained largely within the center electrode. The
field lines emerge radially at the ends of the center electrode and return
outside the body of the gun, forming a magnetic-cusped field at the muzzle end
of the gun.
While the axial magnetic field in the anode-cathode gap may be quite
small, a toroidal field will be imbedded in the plasma during acceleration
owing to the large, axial current flowing in the center electrode. As the
plasma leaves the gun it encounters the radial-field component of the magnetic
cusp, as shown in Fig. lb. Inertia and J x B acceleration, together with hi$i
conductivity, cause the plasma to "push" the radial field, thus capturing
magnetic flux as it passes through the cusp. Behind the ring the extended
field lines break and reconnect so that the ring emerges downstream from the
cusp—with its "captured flux"—as an annular plasma confined by an
axially field-reversed, closed magnetic field. The plasma ring may sub-
sequently drift into a magnetic-mirror trough where it could be trapped and a
fusion plasma bum initiated, as shown in Fig. lc. Moreover, upon leaving the
gun, the plasma could also be made to enter a region of increasing axial field
for the purpose of heating the fusion fuel by magnetic compression prior to
confinement in an axial-mirror well.
GUN START-UP IN THE BETA II EXPERIMENT
The construction of a magnetized coaxial plasma gin for starting up a
field-re versed plasma in Beta II was proposed in August 1978 (7). The goals
of the experimental program on Beta II were stated to be: (a) formation of the
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Smith, A.C. Jr.; Carlson, G.A.; Eddleman, J.L.; Hartman, C.W. & Neef, W.S. Jr. Use of coaxial plasma guns to start up field-reversed-mirror reactors, report, March 19, 1980; United States. (digital.library.unt.edu/ark:/67531/metadc1092646/m1/4/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.