Polywell – introduction

For my own  project: for the moment, I am not absolutely sure that the polywell will be the configuration that I will adopt to carry out my experiments. But the concept is particularly attractive and remains one of my favorite goals.

sources: 

  • The “Polywell” Approach to Fusion
  • The World’s Simplest Fusion Reactor Revisited  – Tom Ligon – 4/30/2007 and 4/28/2008
  • Thomas ligon polywell interview – January 21, 2009 Transcript by Matt Moynihan, April 29, 2010

The polywell concept is associated with an original and controversial idea related to the quest for an efficient nuclear fusion reactor. It was invented in 1984 by Dr. Robert Bussard, one of the fathers in the United States of America’s research on fusion energy. For the next eleven years, he and his company created for the occasion (EMCC or EMC2) developed and tested his idea, but under the embargo of publication related to the financing of the work by the US defense. Bussard’s work was made public in 2006 after the termination of the US Navy funded program.

Dr. Bussard spent the last year of his life struggling to find new funding for his research program. He died in October 2007. Since then, his work has been pursued by various organizations and private companies. The demonstration of the validity of the concept has only really begun in recent years, with in particular the involvement of universities such as Sydney, which has initiated a rigorous and documented experimentation of operating principles.

Behind the word

Polywell stands for  polyhedral magnets and electrostatic potential well

 

The basis of this concept relies on magnets arranged in a polyhedron geometry for generating an electrostatic potential well.

1 / An hybrid concept

Polywell is sometimes classified in the family of electrostatic inertial confinement fusion devices. It is, however, a hybrid machine that combines the ideas of magnetic confinement fusion in the form of cusps with the ideas of IEC machines (Inertial Electrostatic Confinement). That’s why you will find it also classified as a Magnetic Electostatic Plasma Confinement (MEPC). The basic function of the machine is to create a virtual cathode by magnetically confining the electrons in the form of a quasi-spherical cusp magnetic field. The polyhedron arrangement of the magnets makes it possible to create these magnetic cusp fields, and the virtual cathode created in the center of the device by the confinement of the electrons activates the electrostatic potential well which must generate the concentration of the ions up to the fusion conditions.

 / General principle of Polywell

The electrons are injected into the Polywell and are confined by the magnetic field generated by the coils inside the rings in the form of cusps by the magnetic mirror effect. Confined electrons produce a negative space charge region that creates a potential sink of sufficient depth to accelerate the ions to energies that allow them to fuse. Continuous electron injection ensures that the system remains electrically non-neutral and compensates for the loss of electrons escaping from the cusp field. The magnetic field necessary to confine energetic electrons is much less important than the corresponding field necessary for the confinement of energetic ions because of their very different masses. Hence the goal of a Polywell device that is to transfer the problem of energy ion confinement to that of energy losses of electrons due to transport across field lines. To produce a net power, the power required to maintain the virtual cathode must be less than the fusion power produced by ion convergence.

 

This approach theoretically offers a way to overcome some of the difficulties that still face today’s research on fusion. Theoretical and experimental results suggest that this “polywell” approach to fusion may generate net power. However, this approach, as will be seen in the following documents, is sorely lacking in experimental data. But it is also what makes it particularly attractive, giving way to the design of experimental devices and offering multiple interests for the physics enthusiast.

In summary

Ion heating (& confinement) is provided by electrostatic fields from excess electrons (from the ideas of Elmore, Tuck, Watson, Farnsworth, Hirsch and others):

  • the (the) electron beam (and / or grid) accelerates the electrons in the center
  • The excess of electrons forms a potential well
  • Potential accelerates / confines ions
  • The energetic ions generate a fusion near the center

The confinement is ensured by magnetic cusps and can stably confine energy electrons at high plasma pressure.

 

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