Polywell

The polywell is a proposed design for a fusion reactor using an electric and magnetic field to heat ions to fusion conditions. The design is related to the fusor, the high beta fusion reactor, the magnetic mirror, and the biconic cusp. A set of electromagnets generates a magnetic field that traps electrons. This creates a negative voltage, which attracts positive ions. As the ions accelerate towards the negative center, their kinetic energy rises. Ions that collide at high enough energies can fuse.

Mechanism

Fusor heating

A Farnsworth-Hirsch fusor consists of two wire cages, one inside the other, often referred to as grids, that are placed inside a vacuum chamber. The outer cage has a positive voltage relative to the inner cage. A fuel, typically, deuterium gas, is injected into this chamber. It is heated past its ionization temperature, making positive ions. The ions are positive and move towards the negative inner cage. Those that miss the wires of the inner cage fly through the center of the device at high speeds and can fly out the other side of the inner cage. As the ions move outward, a Coulomb force impels them back towards the center. Over time, a core of ionized gas can form inside the inner cage. Ions pass back and forth through the core until they strike either the grid or another nucleus. Most nucleus strikes do not result in fusion. Grid strikes can raise the temperature of the grid as well as eroding it. These strikes conduct mass and energy away from the plasma, as well as spall off metal ions into the gas, which cools it. In fusors, the potential well is made with a wire cage. Because most of the ions and electrons fall onto the cage, fusors suffer from high conduction losses. Hence, no fusor has come close to energy break-even.

Diamagnetic plasma trapping The Polywell is attempting to hold a diamagnetic plasma - a material which rejects the outside magnetic fields created by the electromagnets.

Most plasma in most fusion reactors (such as Magnetic mirrors, tokamaks and Stellarators) are considered magnetized. A Magnetized plasma occurs when the external field is so strong that it completely penetrates and controls the plasma, such that the material behavior is dominated by the external field. Some fusion plasmas are self-magnetized (such as field-reversed configurations, or Dynomaks) all of which can create their own weak magnetic fields through the formation of loops of plasma currents and other structures. Both the Polywell and the high beta fusion reactor pre-suppose that the plasma self-generated field is so strong that it will reject the outside field. Bussard later called this type of confinement the Wiffle-Ball. This analogy was used to describe electron trapping inside the field. Marbles can be trapped inside a Wiffle ball, a hollow, perforated sphere; if marbles are put inside, they can roll and sometimes escape through the holes in the sphere. The magnetic topology of a high-beta polywell acts similarly with electrons. In June 2014 EMC2 published a preprint providing (1) x-ray and (2) flux loop measurements that the diamagnetic effect will impact the external field.

According to Bussard, typical cusp leakage rate is such that an electron makes 5 to 8 passes before escaping through a cusp in a standard mirror confinement biconic cusp; 10 to 60 passes in a polywell under mirror confinement (low beta) that he called cusp confinement; and several thousand passes in Wiffle-Ball confinement (high beta). In February 2013, Lockheed Martin Skunk Works announced a new compact fusion machine, the high beta fusion reactor, that may be related to the biconic cusp and the polywell, and working at β = 1.

Other trapping mechanisms

Magnetic mirror Magnetic mirror dominates in low beta designs. Both ions and electrons are reflected from high to low density fields. This is known as the magnetic mirror effect. The polywell's rings are arranged so the densest fields are on the outside, trapping electrons in the center. This can trap particles at low beta values.

Cusp confinement

In high beta conditions, the machine may operate with cusp confinement. This is an improvement over the simpler magnetic mirror. The MaGrid has six point cusps, each located in the middle of a ring; and two highly modified line cusps, linking the eight corner cusps located at cube vertices. The key is that these two line cusps are much narrower than the single line cusp in magnetic mirror machines, so the net losses are less. The two line cusps losses are similar to or lower than the six face-centered point cusps. In 1955, Harold Grad theorized that a high-beta plasma pressure combined with a cusped magnetic field would improve plasma confinement. A diamagnetic plasma rejects the external fields and plugs the cusps. This system would be a much better trap. Cusped confinement was explored theoretically and experimentally. However, most cusped experiments failed and disappeared from national programs by 1980.

Beta in magnetic traps

Magnetic fields exert a pressure on the plasma. Beta is the ratio of plasma pressure to the magnetic field strength. It can be defined separately for electrons and ions. The polywell concerns itself only for the electron beta, whereas the ion beta is of greater interest within Tokamak and other neutral-plasma machines. The two vary by a very large ratio, because of the enormous difference in mass between an electron and any ion. Typically, in other devices the electron beta is neglected, as the ion beta determines more important plasma parameters. This is a significant point of confusion for scientists more familiar with more 'conventional' fusion plasma physics. Note that for the electron beta, only the electron number density and temperature are used, as both of these, but especially the latter, can vary significantly from the ion parameters at the same location.

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