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Inertial electrostatic confinement

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Inertial electrostatic confinement (often abbreviated as IEC) is a concept for retaining a plasma using an electrostatic field. The field accelerates charged particles (either ions or electrons) radially inward, usually in a spherical but sometimes in a cylindrical geometry. Ions can be confined with IEC in order to achieve controlled nuclear fusion.

Approaches to IEC

US3530497 — Hirsch–Meeks fusor. The two concentric electrodes can be seen in the center of the reaction chamber.

The best-known IEC device is the Farnsworth-Hirsch Fusor.[1] This system consists largely of two concentric spherical electrical grids inside a vacuum chamber into which a small amount of fusion fuel is introduced. Voltage across the grids causes the fuel to ionize around them, and positively charged ions are accelerated towards the center of the chamber. Those ions may collide and fuse with ions coming from the other direction, may scatter without fusing, or may pass directly through. In the latter two cases, the ions will tend to be stopped by the electric field and re-accelerated toward the center. Fusors can also use ion guns rather than electric grids.

The fusor's popularity is largely due to the fact that simple versions can be built for as little as $500 to $4000 (in 2003 US dollars), making it accessible to hobbyists, science fair contestants and small universities. Even these simple devices can reproducibly and convincingly produce fusion reactions, but no fusor has ever come close to producing a significant amount of fusion power. They can be dangerous if proper care is not taken because they require high voltages and can produce harmful radiation (neutrons, gamma rays and x-rays). The basic IEC device has been developed as a commercial neutron generator for industrial applications: first with the trade name FusionStar and now NSD-Fusion.

Two newer approaches both try to solve a problem found in the fusor, which is that some ions collide with the grids. This heats the grids, sprays high-mass ions into the reaction chamber, pollutes the plasma, and cools the fuel. The Polywell uses a magnetic field to trap a quantity of electrons, fuel ions are then accelerated directly into the middle where they are trapped by the electron cloud that forms a "virtual electrode". Another modern approach uses a Penning trap to trap electrons in a system otherwise similar to the Polywell.[2][3]

In 2001, a Japanese team was able to directly measure the double well of an IEC machine using laser-induced fluorescence.[4]

Critique

According to Todd Rider in A general critique of inertial-electrostatic confinement fusion systems, net energy production is not viable in IEC fusion for fuels other than D-T, D-D, and D-He3, and breakeven operation with any fuel except D-T is unlikely. The primary problem that he discusses is the thermalization of ions, allowing them to escape over the top of the electrostatic well more rapidly than they fuse. He considers his paper optimistic because he assumes that core degradation can be countered.

Nevins makes an argument similar to Rider's in [W.M. Nevins, Phys. Plasmas <2> (10), 3804 (October, 1995)], where he shows that the fusion gain (ratio of fusion power produced to the power required to maintain the non-equilibrium ion distribution function) is limited to 0.1 assuming that the device is fueled with a mixture of deuterium and tritium. A fusion gain of about 10 is required for net energy production. Other fusion researchers such as Rostoker and Monckton have disagreed with these analyses, claiming their assumptions do not always apply, and proposing nonthermal schemes that they calculate can produce net power,[5] and theorists at LANL have proposed[6] a new electrostatic plasma equilibrium that should mitigate this problem. This concept, called Periodically Oscillating Plasma Sphere (POPS), has been confirmed experimentally[7]. POPS oscillation maintains equilibrium distribution of the ions at all times, which would eliminate any power loss due to Coulomb collisions, resulting in a net energy gain for fusion-power generation[8].

References

  1. ^ R. Hirsch, "Inertial-Electrostatic Confinement of Ionized Fusion Gases," Journal of Applied Physics 38, 4522 (1967).
  2. ^ R.W. Bussard, "Some Physics Considerations of Magnetic Inertial-Electrostatic Confinement: A New Concept for Spherical Converging-flow Fusion," Fusion Technology 19, 273 (1991).
  3. ^ D.C. Barnes, R.A. Nebel, and L. Turner, "Production and Application of Dense Penning Trap Plasmas," Physics of Fluids B 5, 3651 (1993).
  4. ^ [1]Current Status of IEC (Inertial Electrostatic Confinement) Fusion Neutron/Proton Source Study
  5. ^ Science 17 July 1998: Vol. 281. no. 5375, p. 307
  6. ^ R.A. Nebel and D.C. Barnes, “The periodically oscillating plasma sphere,” Fusion Technology 38, 28 (1998).
  7. ^ J. Park et al., “First experimental confirmation of periodically oscillating plasma sphere (POPS) oscillation,” submitted to Physical Review Letters.
  8. ^ Inertial-Electrostatic-Confinement Fusion Periodically Oscillating Plasma Sphere (POPS) research at LANL
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