Defense Intelligence Reference Document Inertial Electrostatic Confinement Fusion

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are observed. For example, with the device of Figure 3.2b, the input power is about 2
kW with over 1.5 kW being carried out by the jet flow. Consequently, this configuration
provides a way to efficiently convert the energy stored by accelerated ions in the
symmetric microchannels into a directed beam or jet.
Comparison to a Conventional "Plasma" Thruster
The IEC thruster can be viewed as transforming a conventional ion thruster into a
spherical form. In the IEC thruster of Figure 3.2b, ions are produced in the gas
discharge region through the injection and oscillation of electrons about a guide grid
that is held to a slightly positive potential. A second grid extracts ions from the
discharge region and accelerates them towards the center of the device. This type of
ion formation in the IEC has been studied extensively by workers at Nambe Tech, a
"spin-off" company by LANL workers (Reference 3.8). In the case of a planar thruster,
ions are also formed in a discharge region. However, instead of a guide grid, a
magnetic field is used to contain electrons, providing a long path to maximize ionization
collisions. The ion source regions are basically the same, then, for both IEC and planar
ion thrusters, and the extraction/acceleration provided by the grids and the magnetic
fields play equivalent roles in the respective devices. However, the IEC jet thruster
concept must extract a net thrust from the spherical configuration. That is where the
two concepts (planar vs. IEC) diverge, and the star-jet operational mode becomes
essential. The micro-channel formation in the STAR mode acts to maintain and store
the accelerated ions until they are diverted directionally and escape out through the
plasma jet opening. The opening is biased, enabling the control of the potential gradient
between the grid and the opening, hence allowing for increased control of the intensity
of the plasma jet.
The maintenance of STAR mode is then fundamental to the success of IEC jet
operation. Both experimental data and particle-in-cell (PIC) code simulations show that
the energetic ions can be maintained in microchannels for hundreds of passes. The jet
opening allows escape into it in less than ten passes. Consequently, a majority of the
ions, >95 percent, escape at full energy. Thus, the star-jet provides an amazingly good
method to both store and direct the energetic ions.
Overall, the IEC configuration offers distinct advantages. First, the grid structures used
in the IEC thruster are much more open than those used in the conventional planar
thruster designs. When combined with microchannel ion focusing, this prevents ion-grid
collisions, greatly reducing grid erosion and significantly increasing the thruster lifetime.
Second, transforming the device into a sphere makes the overall unit more compact per
unit-ion-source volume, implying a weight reduction advantage. Third, unwanted
neutral propellant gas leakage would be reduced, due to the smaller net opening
required by the high-density jet, vs. a conventional planar thruster having multiple grid
openings. This advantage becomes especially important for thruster applications.
Assuming that the IEC thruster efficiency would be comparable to that of the planar Ion
thruster design, based on these operational improvements, the IEC thruster then offers
significant advantages. [Note that an alternate IEC thruster concept is described later in
the section on J. Khachan's work. It too looks very promising.]
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