Fusion Group
Harnessing nuclear fusion as a viable energy source
Our research in Inertial Electrostatic Confinement fusion range from the mainstream gridded systems and hollow cathodes, through to more exotic concepts such as POPS and the Polywell.
Our fusion energy research focuses on Inertial Electrostatic Confinement (IEC). This approach enables fusion to be carried out on a bench top while still enabling the physics of larger fusion devices to be studied. The approaches used to study the physics of IEC use optical diagnostics such as emission and laser spectroscopy amongst other methods.
Plasma theory and modelling supplement the experimental work in order to gain a clearer picture of the physics involved. This research area has also given rise to applications of the IEC discharge to a new type spacecraft electric propulsion, which has also become one of the research areas of the fusion group.
Nuclear fusion is a reaction in which two light nuclei collide to form a single heavier nucleus. In this process a significant proportion of the mass of the reacting nuclei is converted directly in to energy. As a result of this, nuclear fusion is one of the most energetic reactions known to humanity.
As all nuclei are positively charged, there is a very strong repulsive force between two nuclei in close proximity. For a fusion reaction to occur, the two nuclei must be brought close enough together for the attractive nuclear forces to take over. This requires a very large amount of energy. When a gas is heated to thermonuclear temperatures, over 10,000 degrees, many of the particles in the gas will have the energy necessary to produce fusion. At these temperatures the nuclei and electrons of the gas have separated, forming a new state of matter called a plasma. A substantial proportion of the collisions between the energetic nuclei will result in a fusion reaction. The energy released in these reactions will heat the plasma further and hence sustain the fusion reactions occurring within it. This process occurs naturally in stars and is the fundamental source of most of the energy on the earth.
The major technological barrier to achieving commercial fusion on earth as a power source is the difficulties associated with confining a plasma at thermonuclear temperatures. Any contact by the plasma with the containing walls will result in large energy loss, which may result in a cessation of nuclear reactions and damage the walls of the containing vessel. All major approaches to developing fusion as a power source focus on addressing the problems of providing energy to the nuclei so they may fuse, and confining the hot plasma which results.
The most highly developed fusion reactor design relies on magnetic fields and a toroidal (doughnut) geometry to contain the plasma. First conceived in Russia in the 1950's, a prototype reactor of this design is currently under construction in the south of France, and is expected to produce 500 MW of power.
The reactor vessel in which the plasma is contained is shaped like a doughnut. Wrapped around the vessel are many coils of wire, through which large currents are passed. This produces a magnetic field inside the vessel which forms a large loop around the length of the doughnut. The charged particles which compose the plasma spiral around this magnetic field and are kept from making contact with the vessel walls. A current is passed around the vessel in the plasma. This current produces a second magnetic field and heats the plasma, much like a heater element is heated by the current passing through it. This second magnetic field corrects for the drift of particles towards the walls resulting from the curvature of the first magnetic field. The combination of magnetic fields has proven highly successful in confining plasmas with temperatures over 1 million degrees.
Difficulties arise, however, as the plasma itself creates magnetic and electric fields itself which can counteract the confining fields. These instabilities need to be closely monitored and controlled if the plasma is to remain contained. It is this for this reason that the progression to a prototype reactor has taken over 60 years since the idea's conception. Over the many decades of the reactor design's development, the size of the vessel has steadily increased, as this was found to be necessary to combat the instabilities in the plasma. As a result, operating Tokamaks will be the size of very large buildings, and there is little hope for miniaturisation of the design.
An alternative design for a fusion reactor which holds much promise as a compact, portable fusion power source is termed Inertial Electrostatic Confinement. This design was conceived in the early 1960's but has received a fraction of the research effort that the Tokamak design has enjoyed. It is this plasma confinement method which is the subject of the research performed by the Fusion Group at the University of Sydney.
Through the School of Physics we an active member of the Australian ITER Forum supporting large scale fusion experiments overseas. The fusion group also has affiliations with the National fusion facility at the ANU in Canberra.
We hosted the 13th US-Japan Workshop on Inertial Electrostatic Confinement on December 7 – 8, 2011 in Sydney. We have links and collaborations with other groups in plasma physics worldwide. A selection of these is listed below:
University of Wisconsin – Fusion Technology Institute
Kyoto Institute of Advanced Energy
Aix-Marseille Université - Physique des Interactions Ioniques et Moléculaires (PIIM)
Eindhoven University of Technology - Science and Technology of Nuclear Fusion
Dresden Technical University – Institute of Aerospace Engineering
Our projects in Inertial Electrostatic Confinement (IEC) fusion range from the mainstream gridded systems and hollow cathodes, through to more exotic concepts such as POPS and the Polywell.
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.
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