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| Fig. 1: A simple IEC reactor built by the author. (Source: F.Dayton) |
Nuclear fusion is the process in which two atomic nuclei fuse together forming a larger nucleus and thereby releasing energy. The heat of the plasma is captured to create steam to turn a turbine. The goal of all fusion reactors is to have the electricity from the steam generated exceed the total input electricity to the system.
There are dozens of proposed nuclear reactor designs. Broadly, these designs can be broken into two classes that differ in how they contain plasma: magnetic confinement and inertial confinement.
Magnetic confinement uses strong magnetic forces to squeeze a plasma of hot hydrogen to fusion conditions. The magnetic fields also prevent the plasma from melting the container. Magnetic confinement designs include toroidal machines, such as the tokamak, stellarator designs, and sphereomak designs. [1]
Inertial confinement takes a different approach to achieving fusion conditions: hydrogen pellets are compressed by means of a strong laser beam. [2] The French Laser Megajoule and the American National Ignition Facility are examples of reactors using this approach. [2]
A third type of fusion reactor design, inertial electrostatic confinement, shares some characteristics of magnetic and inertial confinement designs but is unique in others.
IEC uses a strong voltage differential to accelerate ions towards the center of a sphere with the goal of causing some to collide and fuse. [3] The idea of confining a plasma in such a way for nuclear fusion was first introduced in a scientific paper by Elmore et al. [3] They proposed the following hypothetical reactor design: The chamber is first pumped to a near vacuum. Inside the chamber are two concentric, spherical metal "grids". The outer grid is grounded, while the inner grid is given a very high voltage. This creates a very high voltage differential between the two grids. Next, hydrogen gas is introduced into the chamber. The voltage differential between the grids causes the electrons on the hydrogen atoms to be stripped off and attracted to the outer, more positive grid. The remaining positive hydrogen nuclei are accelerated towards the inner grid, pass through it, and have a chance of colliding, thus fusing, in the center. If positive nuclei do not collide, they pass through the center, decelerate, then fall back towards the center. These oscillations can repeat many times, increasing the chances of a collision. [3] The potential difference between the two grids is maintained, keeping the separation between the positive ions and electrons.
The ionization of hydrogen and concentration of ions at the center of the reactor create visible plasma. The goal of an IEC reactor is the concentrate this plasma to an ion density that supports enough fusion to generate heat that exceeds the energy needed to maintain the reaction.
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| Fig. 2: Farnsworth's reactor design. [4] |
The first reactor design based on this principle is Farnsworth's fusor. [4] The apparatus resembles the one described by Elmore et al. in that it has two concentric electric grids to contain the plasma. But it also features four ion guns added to assist with fuel injection. Fig. 2 Shows the fusor design. [4]
Fusors generally follow this blueprint: they have multiple concentric grids and with the possible (but not necessary) addition of ion guns. Today, fusors are the most widely built IEC reactors owing to their relatively easy construction and the fact that visible plasma and neutrons from fusion can be observed with just a 15 keV voltage differential. Fig. 1 shows a fusor project built by the author. Farnsworth's reactor laboratory experiments showed fusion neutron production at rates of ~1010 n/s. [5]
Another IEC design is the Polywell, proposed by Robert W. Bussard in 1989. [5] The Polywell seeks to achieve a denser plasma than the fusor. The Polywell augments the fusor design with the addition of strong electromagnets placed outside the grids. The magnetic fields further compress the electron cloud, maintaining a higher electron density under stable conditions, therefore allowing for potentially more fusion. [5]
The main problem with IEC reactors is maintaining a dense enough ion concentration at the center of the reactor. This requires strong electric and magnetic fields. Net-positive fusion conditions require a denser, hotter ion concentration that is not possible to contain and keep stable with current IEC designs. The authors of the 1959 IEC paper understood this, saying that static solutions for which ions are contained do not exist for sufficiently high ion densities. [3] They state clearly in the conclusion that the system is unstable for ion densities sufficiently high to obtain appreciable thermonuclear yield. [3]
The gain of ITER is Q ≥ 10. A theoretical IEC reactor, which is maintained with 23 GW, is only ouputs 590 MW. Or Q = 0.026, a far cry from net fusion. [6]
Most scientists believe net positive fusion can never be achieved with an IEC reactor design given these drawbacks. [6,7]
© Finn Dayton. 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.
[1] M. Tekant, "Magnetic Fusion Machines," Physics 241, Stanford University, Physics 241, Winter 2013.
[2] R. Islam, "Inertial Confinement Fusion: A Promising Alternative?" Physics 241, Stanford University, Winter 2015.
[3] W. C. Elmore, J. L.Tuck, K. M. Watson, "On the Inertial-Electrostatic Confinement of a Plasma," Phys. Fluids 2, 239 (1959).
[4] P.T. Farnsworth, "Method and Apparatus for Producing Nuclear-Fusion Reactions," U.S. Patent 3,386,883, 4 Jun 68.
[5] R. W. Bussard, "Some Physics Considerations of Magnetic Inertial-Electrostatic Confinement: A New Concept for Spherical Converging-Flow Fusion," Fusion Technol. 19, 273 (1991).
[6] W. M. Nevins, "Can Inertial Electrostatic Confinement Work Beyond the Ion-Ion Collisional Time Scale?" Phys. Plasmas 2, 3804 (1995).
[7] T. H. Rider, "A General Critique of Inertial-Electrostatic Confinement Fusion Systems," Phys. Plasmas 2, 1853 (1995).