Riggatron
Encyclopedia
A Riggatron is a magnetic confinement fusion
reactor design created by Robert W. Bussard
in the late 1970s. It is tokamak
on the basis of its magnetic geometry, but some unconventional engineering choices were made, in particular the use of copper magnets positioned inside the blanket, which was hoped to lead to much lower construction costs. Originally referred to as the Demountable Tokamak Fusion Core (DTFC), the name was later changed to refer to the Riggs Bank
, which funded development along with Bob Guccione
, publisher of the adult magazine Penthouse
.
. The lithium serves two purposes, one is to absorb the neutron
s from the fusion reactions and produce tritium
which is then used to fuel the reactor, and as a secondary role, as shielding to prevent those neutrons from reaching the magnets. Without the lithium blanket the neutrons degrade the magnets quite quickly.
This arrangement has two disadvantages. One is that a magnetic field must be produced not only in the plasma, where it is needed, but also in the blanket, where it is not, significantly raising the construction costs. The other is that the core, where the magnetic coils penetrate the machine along its axis, must be large enough to contain the shielding, which limits the achievable aspect ratio
. A higher aspect ratio generally results in better performance.
Another advantage of the parameters chosen was that ignition appeared to be possible with ohmic heating
alone, as opposed to more expensive systems like ion injection normally required. The first proposal, made in the late 1970s, projected that the device would be able to produce about four times the power in fusion reactions as it used in powering the heaters and magnets. This represents a fusion energy gain factor
(or simply "fusion gain") of three. Whether or not this was possible (given the history of fusion research since that time, it seems unlikely) the project was never completed as Guccioni was unable to secure the $150 million needed to build the full-sized device (much of which would have been for a large homopolar generator
).
With the demise of the original project, Bussard moved on to new inertial electrostatic confinement
fusion designs with extremely high claimed performance. This culminated in his final development of IEC before his death in October 2007 - the Polywell
device.
Magnetic confinement fusion
Magnetic confinement fusion is an approach to generating fusion power that uses magnetic fields to confine the hot fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, the other being inertial confinement fusion. The magnetic approach is...
reactor design created by Robert W. Bussard
Robert W. Bussard
Robert W. Bussard was an American physicist who worked primarily in nuclear fusion energy research. He was the recipient of the Schreiber-Spence Achievement Award for STAIF-2004. He was also a fellow of the International Academy of Astronautics and held a Ph.D...
in the late 1970s. It is tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
on the basis of its magnetic geometry, but some unconventional engineering choices were made, in particular the use of copper magnets positioned inside the blanket, which was hoped to lead to much lower construction costs. Originally referred to as the Demountable Tokamak Fusion Core (DTFC), the name was later changed to refer to the Riggs Bank
Riggs Bank
Riggs Bank was a Washington, D.C.-based commercial bank with branches located in the surrounding metropolitan area and offices around the world. For most of its history, it was the largest bank in the nation's capital. Riggs had been controlled by the Albritton family since the 1980s, but they lost...
, which funded development along with Bob Guccione
Bob Guccione
Bob Guccione was the founder and publisher of the adult magazine Penthouse. He resigned from his publisher position in November 2003.-Early life:...
, publisher of the adult magazine Penthouse
Penthouse (magazine)
Penthouse, a men's magazine founded by Bob Guccione, combines urban lifestyle articles and softcore pornographic pictorials that, in the 1990s, evolved into hardcore. Penthouse is owned by FriendFinder Network. formerly known as General Media, Inc. whose parent company was Penthouse International...
.
Conventional tokamak design
In a conventional tokamak design the confinement magnets are arranged outside a "blanket" of liquid lithiumLithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
. The lithium serves two purposes, one is to absorb the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s from the fusion reactions and produce tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
which is then used to fuel the reactor, and as a secondary role, as shielding to prevent those neutrons from reaching the magnets. Without the lithium blanket the neutrons degrade the magnets quite quickly.
This arrangement has two disadvantages. One is that a magnetic field must be produced not only in the plasma, where it is needed, but also in the blanket, where it is not, significantly raising the construction costs. The other is that the core, where the magnetic coils penetrate the machine along its axis, must be large enough to contain the shielding, which limits the achievable aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
. A higher aspect ratio generally results in better performance.
Riggatron improvement
The Riggatron re-arranged the layout of the conventional design, reducing the role of the lithium to producing tritium only. The magnets were to be directly exposed on the inside of the reactor core, bearing the full neutron flux. This precluded the use of superconducting magnets, and even copper magnets would have to be disposed in as little as 30 days of operation. The Riggatron was laid out to make this core replacement as easy and fast as possible. After removal and replacement, the magnets would then be melted down and reprocessed. Although this process would be costly, the smaller magnetized volume, the larger aspect ratio, and the reduction in complexity by avoiding superconducting magnets was a tradeoff that would, it was hoped, pay off.Another advantage of the parameters chosen was that ignition appeared to be possible with ohmic heating
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
alone, as opposed to more expensive systems like ion injection normally required. The first proposal, made in the late 1970s, projected that the device would be able to produce about four times the power in fusion reactions as it used in powering the heaters and magnets. This represents a fusion energy gain factor
Fusion energy gain factor
The fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1 is referred to as breakeven.In a fusion power reactor a plasma must be...
(or simply "fusion gain") of three. Whether or not this was possible (given the history of fusion research since that time, it seems unlikely) the project was never completed as Guccioni was unable to secure the $150 million needed to build the full-sized device (much of which would have been for a large homopolar generator
Homopolar generator
A homopolar generator is a DC electrical generator comprising an electrically conductive disc rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim, the electrical polarity depending on the direction of...
).
Fusion research establishment consideration
Studies carried out at the time suggest the Riggatron was not considered as much of a "sure thing" by other members of the fusion research establishment. Existing experimental tokamaks generally do not include a lithium blanket, and are thus fairly similar to the Riggatron in layout, yet none of these reactors is close to generating a fusion gain of one, let alone the three that was being claimed for the Riggatron. In retrospect it appears the Riggatron concept likely would not have worked, due to the various plasma instabilities that were only being discovered coincident with its design process. Interest in the Riggatron has essentially disappeared.With the demise of the original project, Bussard moved on to new inertial electrostatic confinement
Inertial electrostatic confinement
Inertial electrostatic confinement is a concept for retaining a plasma using an electrostatic field. The field accelerates charged particles radially inward, usually in a spherical but sometimes in a cylindrical geometry. Ions can be confined with IEC in order to achieve controlled nuclear fusion...
fusion designs with extremely high claimed performance. This culminated in his final development of IEC before his death in October 2007 - the Polywell
Polywell
A polywell device is a type of fusion reactor that was originated by Robert Bussard under a U.S. Navy research contract. It traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then accelerate positively charged ions for the purpose of achieving...
device.