We also work on the various applications of IEC fusion discharges such as spacecraft electric propulsion and neutron sources.
Inertial electrostatic confinement (IEC) is a method of extracting power from the fusion of light nuclei, and relies on spherically symmetric electric fields to inertially confine and accelerate ions to thermonuclear energies. In conventional devices a spherical, gridded metal electrode (the cathode) produces the required electric field. The large negative voltage applied to the cathode produces a potential well which causes an influx of ions to the centre of the device. If the potential through which the ions have been accelerated is large enough, a portion of the collisions between these energetic ions will result in fusion, producing energy in the process. Uncollided ions will pass through the cathode and out the other side, and will be returned to the centre by the influence of the radial electric field.
The greatest disadvantage of the design is ion bombardment of the cathode, which is a source of significant energy loss and results in the cathode's erosion by heating and sputtering. Future development of IEC as a power source is dependent on the discovery of a method to produce the radial convergent electric field without exposing electrodes to the destructive thermonuclear plasma.
The use of physical cathodes in IEC is fraught with problems, as much efficiency is lost through ion bombardment of the electrode, and the metal is quickly eroded. Hence the major focus of the IEC community currently is the production of a "virtual" cathode; a region of negative space charge which will attract and confine positive ions. Last year the fusion group successfully produced a 4 kV, short-lived virtual cathode by pulsing a conventional IEC device in reversed polarity. Also observed were large amplitude oscillations, believed to be indicative of a collapsing and expanding electron cloud.
An alternative fusion method, termed the periodically oscillating plasma sphere (POPS), involves driving the collapse and expansion of a sphere of plasma in order to periodically heat and compress the plasma to fusion densities. The oscillations observed are believed to be a form of the POPS regime. Current research is focusing on extending the lifetime of the virtual cathode, and investigating the potential application of the observed electron oscillations to confining fusion plasmas.
The Polywell is a fusion reactor concept that combines elements of IEC and magnetic confinement fusion. The Polywell concept aims to replace the physical cathode with one that is formed by trapping energetic electrons in a magnetic cusp arrangement. The potential well would then accelerate monoenergetic positive ions to the centre, where the ions would either collide with other high energy ions to produce fusion or scatter through the well, at which point they will fall back in to the well, resulting in ion confinement.
The magnetic field configuration is created by pairs of opposing current loops each creating a cusp. In a cube configuration, these point cusps are arranged so that they sit around the faces of a cube, one pair on each axis. The magnetic field is zero at the center due to symmetry, creating a null point. Magnetic flux that enters the Polywell through the coil faces is balanced by the fluxes leaving through the spaces between the coils. As a result there is a magnetic mirror effect, along the three orthogonal axes, on a particle located at the center. During operation, electrons are confined by reflection from the magnetic field configuration.
Moreover, the number of collisions of electrons and ions with the magnetic field coils is greatly reduced due to deflection by local fields, which loop around the coils. The magnetic field geometries in the Polywell are inherently MHD stable because they are everywhere convex toward the centre.