|Fig. 1: The structure of the Fusor invented by Farnsworth-Hirsch.  (Source: Wikimedia Commons)|
Fusion power is a form of power generation in which energy is generated by using fusion reactions to produce heat for electricity generation. In contrast to fission, fusion creates energy by combining lighter elements into heavier elements. The advantages for fusion are (1) abundant fuels, (2) no radioactive waste produced and (3) controlled side reaction. The potential of fusion to create clean energy has attracted many scientists over several decades. One idea for a safe and efficient reactor for fusion is called the "Fusor". But unfortunately, no Fusor designs have yet achieved a positive energy gain, due to the difficulty of controlling the fusion reaction. In other words, the energy needed to initiate the reaction is more than the energy produced by the reaction. The Fusor employs inertial electdrostatic confinement - the use of an electric field to confine the plasma.
The Fusor device typically consists of two wire cages called grids. The inner cage holds a negative voltage compared to the outer cage. When the fusion fuel is introduced (ioned gas), the voltage will ignite the fuel. The huge electric field accelerates the ions into fusion conditions.
For example, Fig. 1 shows the structure of the reactor invented by Farnsworth.  The Hirsch-Meeks fusor, a design which utilizes many of the same principles as the Farnsworth- Hirsch fusor, is widely applied in multiple fields now. A generally circular cathode has concentrically positioned a generally circular anode having multiple openings for the free flow of electrons.  The anode maintains as a tangible structure to provide the enough space for electrons. And the electron emission is ignited in the inter-electrode space between the anode and the cathode. Then the electron penetrated into the tangible space and established a negative charged space. The advantage of the design is that the shielded anode structure can reduce the high energy electron loss caused by electron interception greatly, even approaching zero.
|Fig. 2: Homemade fusion reactor. (Source: Wikimedia Commons)|
Recently, scientists have tried to find more efficient design of the Fusor that would reduce the energy loss. For example, Fig. 2 shows the homemade fusor running under the inertial electrostatic confinement. The star mode is a regime in which the effective transparency of the accelerating grid is greatly enhanced over the value one would traditionally expect from considering the fraction of area taken up by the grid wires. Reducing the heating of the grid wires, the star mode should in principle allow smaller devices. 
The development of inertial electrostatic confinement devices may be a critical step for harnessing fusion energy. According to Miley et al., the spherical inertial electrostatic confinement (IEC) devices are capable of providing 107 neutrons per second steady-state.  In August 2014, a high-yield neutron generator device invented by Phoenix Nuclear Labs used an electric field to heat ions to fusion conditions. The device was claimed to sustain 1011 neutrons per second over a 24-hour period. These efforts reflect continued strong interest in non-destructive evaluation and neutron activation analysis applications of IEC devices.
© Hongpeng Gao. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. T. Farnsworth, "Method and Apparatus for Producing Nuclear-Fusion Reaction," US Patent US3386883, 4 Jun 68.
 R. L. Hirsch and G. A. Meeks, "Electrostatic Containment in Fusion Reactors," US Patent US3664920A, 23 May 72.
 B. Klopfer, "The Fusor," Physics 241, Stanford University, Winter 2012.
 G. H. Miley et al., "Discharge Characteristics of the Spherical Inertial Electrostatic Confinement (IEC) Device," IEEE 640696, IEEE Trans. Plasma Sci. 25, 733 (1997).