Defense Intelligence Reference Document Inertial Electrostatic Confinement Fusion

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Jet Extraction
To obtain thrust from an IEC device, a valley or trough must be created in the
electrostatic potential, and a hole must be physically cut into the ground sphere. This
allows high-speed ions to escape in the form of a plasma jet as described in the
preceding section. Ions are generated near the ground potential with the aid of electron
emitters and additional grids. A central spherical electrical grid accelerates ions to the
core region. A cylindrical "channel" grid with the same electrostatic potential as the
central spherical grid creates a passage through which ions can escape to the outside.
Thus this trough in the electrostatic potential profile across the centerline (thrust axis)
of the IEC thruster is a unique feature compared to the linear profile for a planar
device.
The ion beams finally exit through an opening in the ground sphere of the IEC device.
The channel grid must be well insulated from the ground potential to prevent short-
circuiting or arc-over; thus a separate insulated feed-through cable maintains the
negative potential on the inner spherical and channel grids. Makeup propellant gas is
fed into the ionization region through needle-valve-controlled tubing located around the
chamber wall. Ionization of the propellant uses the electron emitter-guide grid design
described earlier. The inner grid serves to both extract and accelerate ions, forming the
microchannels. To control neutralization of the plasma jet, additional electron emitters
are attached close to the jet discharge hole. A combination of electron emission rates
and jet grid bias can be used to control the beam space charge, hence the growth of
focusing of the beam during propagation. In present experiments, however, the
electron rate is fixed at a suitable value while the jet grid bias is varied.
Control of the jet diameter and focus is obtained in two ways: first, the channel grid will
be separately hinged with a small servo motor such that its axis can be moved over a
volume defined by a 10 percent cone angle; second, the grid bias can be varied over a
range up to the chamber potential to provide focus control over the jet flow. A large
negative grid bias will cause a narrow focus while small potential values will allow the
plasma to expand giving a broad cross section beam.
Experimental Jet Design and Performance
Figure 3.3 shows the thruster experiment components. It is also of interest to consider
the typical dimensions involved to illustrate the compactness of the unit. The
experimental studies use an existing spherical IEC chamber of ~30-cm with a 1-cm
diameter port on one side of it for beam extraction. (This is somewhat larger than
envisioned for application to micro-satellites). An 8-cm diameter spherical electrical
tungsten or tantalum wire grid, having a geometric transparency of ~90 percent, will
mount inside the chamber. A ~1-cm diameter hole will be cut into the side of the wire
grid, and this hole will be aligned with the hole in the chamber wall and connected to it
by a 2-cm diameter cylindrical guide grid. The insulator covering the grounded wall
must be of sufficient size to prevent arc-over from the ground to the cylindrical grid.
The inner electrical grids are connected to a 500-kV de power supply through the
insulated feed-through cable. A positively-charged outer grid with a variable voltage of
~ 10-100 Vis mounted on a swivel connector at the outside of the beam extraction
port of the chamber, in combination with four electron emitters, generates ions. The
choice for the voltages on the outer grids is flexible, so long as a sufficient ion
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