Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Date: Nov 27, 8:10 pm
Poster: Mark D. Hugo

On Nov 27, 8:10 pm, Mark D. Hugo wrote:

Engineering Issues of Gridded Inertial Electrostatic Confinement (IEC) Devices
John M. De Mora, Luis Chacon, George H. Miley

Fusion Studies Laboratory
100 NEL, 103 South Goodwin Avenue
Urbana, Illinois, U.S.A., 61801-2984

The inertial electrostatic confinement (IEC) concept employs a grounded spherical chamber, filled with a gaseous fusionable fuel, and an internal cathode biased at a high negative voltage. Several alternatives may be considered for the inner cathode: an inner spherical semi-transparent grid biased at a high negative potential is the simplest approach, although overheating (due to collisions of energetic ions) and neutron irradiation may hamper the integrity of the grid. Other approaches have replaced the inner grid by a virtual cathode, created by confining an electron cloud within the IEC chamber. This is done either using quasi-spherical-cusp magnetic fields (POLYWELL=99), or with penning traps. Although the grid losses are avoided in both, there is no experimental confirmation of their ion focusing properties, and great concern exists about the quality of the electron confinement and the dispersion of the ion density distribution in velocity space.

On the contrary, gridded IEC systems have shown, under certain experimental conditions, the successful formation of ion microchannels, which entail very good ion focusing properties. In addition, the scaling of these systems for commercial exploitation is very attractive due to the simplicity of the power core (which should result in a low cost), the compatibility with direct conversion, and low projected maintenance costs. Nevertheless, major concern exists about the integrity of the grid due to plasma heating and neutron irradiation.

The integrity of the grid can be preserved, for a fixed fusion power, by increasing its surface (i.e., its radius) to enhance radiation cooling, and by providing active cooling within the grid wires. In this study, a hollow tantalum grid, actively cooled by a pressurized water circuit is considered. Radiation cooling is disregarded, hence providing for a security margin in the design. The heat transfer analysis will show that, for a grid maximum temperature of 1000 C, and assuming reactor gains of ~ 5, grid radii ~ 1.5 m are possible for fusion powers of 800 MWth. In such conditions, typical heat fluxes across the grid wires surface are ~ 0.32 KW/cm2. However, the issue remains if a grid of this radius and transparency can retain adequate focusing for IEC operation. To study this, a correlation of focal spot vs. grid radius and transparency is being developed and benchmarked against present smaller diameter IEC devices.


1. G. H. Miley et al., Dense Z-pinches, eds. M. Haines and A. Knight, AIP Conf. 299, AIP Press, NY, 675-689 (1994).

2. R. W. Bussard, Fusion Technol., 19, 273-293 (1991).

3. D. C. Barnes et al., Proc. International Sherwood Fusion Theory Conference, Dallas, TX, 2C28 (1994).

4. J. M. De Mora et al., Proc. IEEE-NPSS 16th Symposium of Fusion Energy, eds. G. H. Miley and C. M. Elliott, IEEE, Piscataway, NJ, 1486-1489 (1996).

5. R. W. Bussard et al., Fusion Technol., 26, 1326-1336 (1994). DEAR ALL: Dr. Miley calls his devices "Inertial Electrostatic Confinement" devices. In his two papers (contact him directly please! Or at least the U. of IL Fusion studies department.) he properly credits Farnsworth. The last time I communicated with him on the progress, about 3 months ago, he had been turned down for at least one DOE grant, and I believe I heard a third party tell me he now has the ignominious distinction of having been turned down for two more grants. Boggles the mind I'd say. Particularily because through Diemler Benz Dr. Miley is offering the devices at $60K a crack as "switchable neutron sources". Similar "accellerator based" neutron sources sell for closer to $250K, and demand 50KW input, versus the 5KW or so that Dr. Miley is using.