Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic confinement fusion (MCF) designs. Most IEC devices directly accelerate their fuel to fusion conditions, thereby avoiding energy losses seen during the longer heating stages of MCF devices. In theory, this makes them more suitable for using alternative aneutronic fusion fuels, which offer a number of major practical benefits and makes IEC devices one of the more widely studied approaches to fusion.
IEC devices were the very first fusion products to reach the commercial market in 2000, as neutron generators. A company called NSD-Gradel developed a compact IEC device that fused ions and created neutrons and sold the product for several hundred thousand dollars.
As the negatively charged electrons and positively charged ions in the plasma move in different directions in an electric field, the field has to be arranged in some fashion so that the two particles remain close together. Most IEC designs achieve this by pulling the electrons or ions across a potential well, beyond which the potential drops and the particles continue to move due to their inertia. Fusion occurs in this lower-potential area when ions moving in different directions collide. Because the motion provided by the field creates the energy levels needed for fusion, not random collisions with the rest of the fuel, the bulk of the plasma does not have to be hot and the systems as a whole work at much lower temperatures and energy levels than MCF devices.
One of the simpler IEC devices is the fusor, which consists of two concentric metal wire spherical grids. When the grids are charged to a high voltage, the fuel gas ionizes. The field between the two then accelerates the fuel inward, and when it passes the inner grid the field drops and the ions continue inward toward the center. If they impact with another ion they may undergo fusion. If they do not, they travel out of the reaction area into the charged area again, where they are re-accelerated inward. Overall the physical process is similar to the colliding beam fusion, although beam devices are linear instead of spherical. Other IEC designs, like the polywell, differ largely in the arrangement of the fields used to create the potential well.
A number of detailed theoretical studies have pointed out that the IEC approach is subject to a number of energy loss mechanisms that are not present if the fuel is evenly heated, or "Maxwellian". These loss mechanisms appear to be greater than the rate of fusion in such devices, meaning they can never reach fusion breakeven and thus be used for power production. These mechanisms are more powerful when the atomic mass of the fuel increases, which suggests IEC also does not have any advantage with aneutronic fuels. Whether these critiques apply to specific IEC devices remains highly contentious.
Mechanism
For every volt that an ion is accelerated across, its kinetic energy gain corresponds to an increase of temperature of 11,604 kelvins (K). For example, a typical magnetic confinement fusion plasma is 15 keV, which corresponds to 170 megakelvin (MK). An ion with a charge of one can reach this temperature by being accelerated across a 15,000 V drop. This sort of voltage is easily achieved in common electrical devices; a typical cathode-ray tube operates in this range.
In fusors, the voltage drop is made with a wire cage. However high conduction losses occur in fusors because most ions fall into the cage before fusion can occur. This prevents current fusors from ever producing net power.
History
1930s
Mark Oliphant adapts Cockcroft and Walton's particle accelerator at the Cavendish Laboratory to create tritium and helium-3 by nuclear fusion.
1950s
Three researchers at LANL including Jim Tuck first explored the idea, theoretically, in a 1959 paper. The idea had been proposed by a colleague.W. H. Wells, Bendix Aviation Corporation (private communication, 1954) The concept was to capture electrons inside a positive cage. The electrons would accelerate the ions to fusion conditions.
Other concepts were being developed which would later merge into the IEC field. These include the publication of the Lawson criterion by John D. Lawson in 1957 in England."Some Criteria for a Power Producing Thermonuclear Reactor" J D Lawson, Atomic Energy Research Establishment, Harwell, Berks, 2 November 1956 This puts on minimum criteria on power plant designs which do fusion using hot Maxwellian plasma clouds. Also, work exploring how electrons behave inside the biconic cusp, done by Harold Grad group at the Courant Institute in 1957.Grad, H. Theory of Cusped Geometries, I. General Survey, NYO-7969, Inst. Math. Sci., N.Y.U., December 1, 1957Berkowitz, J., Theory of Cusped Geometries, II. Particle Losses, NYO-2530, Inst. Math. Sci., N.Y.U., January 6, 1959. A biconic cusp is a device with two alike magnetic poles facing one another (i.e. north-north). Electrons and ions can be trapped between these.
1960s
In his work with vacuum tubes, Philo Farnsworth observed that electric charge would accumulate in regions of the tube. Today, this effect is known as the multipactor effect.Cartlidge, Edwin. The Secret World of Amateur Fusion. Physics World, March 2007: IOP Publishing Ltd, pp. 10-11. . Farnsworth reasoned that if ions were concentrated high enough they could collide, and fuse. In 1962, he filed a patent on a design using a positive inner cage to concentrate plasma, in order to achieve nuclear fusion.US Patent 3,258,402 June 28, 1966 During this time, Robert L. Hirsch joined the Farnsworth Television labs and began work on what became the fusor. Hirsch patented the design in 1966US Patent 3,386,883 June 4, 1968 and published the design in 1967. The Hirsch machine was a 17.8 cm diameter machine with 150 kV voltage drop across it and used ion beams to help inject material.
Simultaneously, a key plasma physics text was published by Lyman Spitzer at Princeton in 1963.Lyman J Spitzer, "The Physics of Fully Ionized Gases" 1963 Spitzer took the ideal gas laws and adapted them to an ionized plasma, developing many of the fundamental equations used to model a plasma. Meanwhile, magnetic mirror theory and direct energy conversion were developed by Richard F. Post's group at LLNL."Mirror Systems: Fuel Cycles, loss reduction and energy recovery" by Richard F. Post, BNES Nuclear fusion reactor conferences at Culham laboratory, September 1969. A magnetic mirror or magnetic bottle is similar to a biconic cusp except that the poles are reversed.
1980s
In 1980 Robert W. Bussard developed a cross between a fusor and magnetic mirror, the polywell. The idea was to confine a non-neutral plasma using magnetic fields. This would, in turn, attract ions. This idea had been published previously, notably by Oleg Lavrentiev in Russia."Confinement d'un Plasma par un Systemem Polyedrique a' Courant Alternatif", Z. Naturforschung Vol. 21 n, pp. 1085–1089 (1966) Bussard patentedR.W.Bussard in U.S.Patent 4,826,646, "Method and apparatus for controlling charged particles", issued May 2, 1989 the design and received funding from Defense Threat Reduction Agency, DARPA and the US Navy to develop the idea.Dr. Robert Bussard (lecturer) (2006-11-09). "Should Google Go Nuclear? Clean, cheap, nuclear power (no, really)" (Flash video). Google Tech Talks. Google. Retrieved 2006-12-03.
1990s
Bussard and Nicholas Krall published theory and experimental results in the early nineties."Inertial electrostatic fusion (IEF): A clean energy future" (Microsoft Word document). Energy/Matter Conversion Corporation. Retrieved 2006-12-03. In response, Todd Rider at MIT, under Lawrence Lidsky developed general models of the device. Rider argued that the device was fundamentally limited. That same year, 1995, William Nevins at LLNL published a criticism of the polywell. Nevins argued that the particles would build up angular momentum, causing the dense core to degrade.
In the mid-nineties, Bussard publications prompted the development of fusors at the University of Wisconsin–Madison and at the University of Illinois at Urbana–Champaign. Madison's machine was first built in 1995. George H. Miley's team at Illinois built a 25 cm fusor which has produced 107 neutrons using deuterium gas and discovered the "star mode" of fusor operation in 1994.Miley Abstract Accomplishments, www.avrc.com/Miley_abstract_accomplishments.doc The following year, the first "US-Japan Workshop on IEC Fusion" was conducted. This is now the premier conference for IEC researchers. At this time in Europe, an IEC device was developed as a commercial neutron source by Daimler-Chrysler Aerospace under the name FusionStar. In the late 1990s, amateur fusion hobbyist Richard Hull began building fusors in his home."Living with a nuclear reactor" The Wall Street Journal, interview with Sam Schechner, https://www.youtube.com/watch?v=LJL3RQ4I-iE In March 1999, he achieved a neutron rate of 105 neutrons per second."The Neutron Club", Richard Hull, Accessed 6-9-2011, https://prometheusfusionperfection.com/category/fusor/ Hull and Paul Schatzkin started fusor.net in 1998. Through this open forum, since 1998, a community of amateur fusioneers have built homemade fusion reactors using fusors.
2000s
Despite demonstration in 2000 of 7200 hours of operation without degradation at high input power as a sealed reaction chamber with automated control the FusionStar project was canceled and the company NSD Ltd was founded. The spherical FusionStar technology was then further developed as a linear geometry system with improved efficiency and higher neutron output by NSD Ltd. which became NSD-Fusion GmbH in 2005.
In early 2000, Alex Klein developed a cross between a polywell and ion beams."The Multipole Ion-beam eXperiment", Presentation, Alex Klien, 7–8 December 2011, 13th US-Japan IEC workshop, Sydney 2011
In response to Riders' criticisms, researchers at LANL reasoned that a plasma oscillating could be at local thermodynamic equilibrium; this prompted the POPS and Penning trap machines. At this time, MIT researchers became interested in fusors for space propulsionPh.D. Thesis "Improving Particle Confinement in Inertial Electrostatic Fusion for Spacecraft Power and Propulsion", Carl Dietrich, Massachusetts Institute of Technology, February 2007 and powering space vehicles.Ph.D. Thesis "Improved lifetimes and synchronization behavior in Mutlt-grid IEC fusion devices", Tom McGuire, Massachusetts Institute of Technology, February 2007 Specifically, researchers developed fusors with multiple inner cages. In 2005, Greg Piefer founded Phoenix Nuclear Labs to develop the fusor into a neutron source for the mass production of medical isotopes."Phoenix Nuclear Labs meets neutron production milestone", PNL press release May 1, 2013, Ross Radel, Evan Sengbusch