A spherical tokamak is a type of fusion power device based on the tokamak principle. It is notable for its very narrow profile, or aspect ratio. A traditional tokamak has a toroidal confinement area that gives it an overall shape similar to a donut, complete with a large hole in the middle. The spherical tokamak reduces the size of the hole as much as possible, resulting in a plasma shape that is almost spherical, often compared to a cored apple. The spherical tokamak is sometimes referred to as a spherical torus and often shortened to ST.
The spherical tokamak is an offshoot of the conventional tokamak design. Proponents claim that it has a number of substantial practical advantages over these devices. For this reason the ST has generated considerable interest since the late 1980s. However, development remains effectively one generation behind traditional tokamak efforts like JET. Major experiments in the ST field include the pioneering START and MAST at Culham in the UK, the US's National Spherical Torus Experiment - Upgrade (NSTX-U) and Russian Globus-M.
Research has investigated whether spherical tokamaks are a route to lower cost reactors. Further research is needed to better understand how such devices scale. Even in the event that STs do not lead to lower cost approaches to power generation, they are still lower cost in general; this makes them attractive devices for studying plasma physics, or as high-energy neutron sources.
Princeton Plasma Physics Laboratory cites three advantages. First, spherical tokamaks require less steel, concrete, and copper are needed for construction, which should make them cheaper to build than larger conventional tokamaks. Second, their geometry lets the magnetic field lines confine and stabilize the plasma more efficiently even with weaker magnets. Third, the spherical design allows plasma particles to spin faster, which helps break up small turbulent ripples that would otherwise leak heat out.
Background
Basic fusion physics The basic idea behind fusion is to force two suitable atoms close enough together that the strong force pulls them together to make a single larger atom. This process releases a considerable amount of binding energy, typically in the form of high-speed subatomic particles like neutrons or beta particles. However, these same fuel atoms also experience the electromagnetic force pushing them apart. In order for them to fuse, they must be pressed together with enough energy to overcome this Coulomb barrier.
The simplest way to do this is to heat the fuel to very high temperatures, and allow the Maxwell–Boltzmann distribution to produce a number of very high-energy atoms within a larger, cooler mix. For the fusion to occur, the higher speed atoms have to meet, and in the random distribution that will take time. The time will be reduced by increasing the temperature, which increases the number of high-speed particles in the mix, or by increasing the pressure, which keeps them closer together. The product of temperature, pressure and time produces the expected rate of fusion events, the so-called fusion triple product. To be useful as a net energy exporter, the triple product has to meet a certain minimum condition, the Lawson criterion.
In practical terms, the required temperatures are on the order of 100 million degrees. This leads to problems with the two other terms; confining the fuel at a high enough pressure and for a long enough time is well beyond the capabilities of any known material. However, at these temperatures the fuel is in the form of an electrically conductive plasma, which leads to a number of potential confinement solutions using magnetic or electrical fields. Most fusion devices use variations of these techniques.
Tokamaks are the most researched approach within the larger group of magnetic fusion energy (MFE) designs. They attempt to confine a plasma using powerful magnetic fields. Tokamaks confine their fuel at low pressure (around 1/millionth of atmospheric) but high temperatures (150 million Celsius), and attempt to keep those conditions stable for ever-increasing times on the order of seconds to minutes. Doing so, however, requires massive amount of power in the magnetic system, and any way to reduce this improves the overall energy efficiency of the system.
Energy balance Ideally, the energy needed to heat the fuel would be made up by the energy released from the reactions, keeping the cycle going. Anything over and above this amount could be used for power generation. This leads to the concept of the Lawson criterion, which delineates the conditions needed to produce net power.
When the fusion fuel is heated, it will naturally lose energy through a number of processes. These are generally related to radiating terms like blackbody radiation, and conduction terms, where the physical interaction with the surroundings carries energy out of the plasma. The resulting energy balance for any fusion power device, using a hot plasma, is shown below.
where: , is the net power out , is the efficiency with which the plant captures energy, say through a steam turbine, and any power used to run the reactor , is the power generated by fusion reactions, basically a function of the rate of reactions , is the power lost through conduction to the reactor body , is the power lost as light, leaving the plasma, typically through gamma radiation
To achieve net power, a device must be built which optimizes this equation. Fusion research has traditionally focused on increasing the first P term: the fusion rate. This has led to a variety of machines that operate at ever higher temperatures and attempt to maintain the resulting plasma in a stable state long enough to meet the desired triple product. However, it is also essential to maximize the η for practical reasons, and in the case of a MFE reactor, that generally means increasing the efficiency of the confinement system, notably the energy used in the magnets.
Beta number
A measure of success across the magnetic fusion energy world is the beta number. Every machine containing plasma magnetically, can be compared using this number.
This is the ratio of the plasma pressure to the magnetic field pressure. Improving beta means that you need to use, in relative terms, less energy to generate the magnetic fields for any given plasma pressure (or density). The price of magnets scales roughly with β, so reactors operating at higher betas are less expensive for any given level of confinement. Conventional tokamaks operate at relatively low betas, the record being just over 12%, but various calculations show that practical designs would need to operate as high as 20%.
Aspect ratio
One limiting factor in improving beta is the size of the magnets. Tokamaks use a series of ring-shaped magnets around the confinement area, and their physical dimensions mean that the hole in the middle of the torus can be reduced only so much before the magnet windings are touching. This limits the aspect ratio, , of the reactor to about 2.5; the diameter of the reactor as a whole could be about 2.5 times the cross-sectional diameter of the confinement area. Some experimental designs were slightly under this limit, while many reactors had much higher A.
History
Reducing aspect ratio
During the 1980s, researchers at Oak Ridge National Laboratory (ORNL), led by Ben Carreras and Tim Hender, were studying the operations of tokamaks as A was reduced. They noticed, based on magnetohydrodynamic considerations, that tokamaks were inherently more stable at low aspect ratios. In particular, the classic "kink instability" was strongly suppressed. Other groups expanded on this body of theory, and found that the same was true for the high-order ballooning instability as well. This suggested that a low-A machine would not only be less expensive to build, but have better performance as well.
In the traditional tokamak design, the confinement magnets are normally arranged outside a toroidal vacuum chamber holding the plasma. This chamber is known as the first wall, and defines the minimum distance between the magnets and plasma. In a production design, another layer, the blanket, sits between the first wall and magnets. The blanket serves two purposes, one is to protect the magnets from the high energy neutrons, which will damage them, and the other is to use those neutrons to breed tritium from lithium, producing more fuel for the reactor. However, this arrangement means there is considerable distance between the magnets and plasma, in most designs something on the order of a meter or more. This places significant limits on the achievable aspect ratio.
One attempt to improve the reactor geometry was attempted by a class of designs known as the "compact tokamak", typified by the Alcator C-Mod (operational since 1991), the Riggatron (conceptual, unbuilt) and IGNITOR (construction abandoned). The later two of these designs dispensed with the first wall and placed the magnets in direct contact with the plasma; in a production design the blanket would be outside the magnets. This greatly simplifies the physical design as well, as the toroidal vacuum vessel can be replaced with a cylinder. The decreased distance between the magnets and plasma leads to much higher betas, so conventional (non-superconducting) magnets could be used. The downside to this approach, one that was widely criticized in the field, is that it places the magnets directly in the high-energy neutron flux of the fusion reactions. In operation the magnets would be rapidly eroded, requiring the vacuum vessel to be opened and the entire magnet assembly replaced after a month or so of operation.
Around the same time, several advances in plasma physics were making their way through the fusion community. Of particular importance were the concepts of elongation and triangularity, referring to the cross-sectional shape of the plasma. Early tokamaks had all used circular cross-sections simply because that was the easiest to model and build, but over time it became clear that C or (more commonly) D-shaped plasma cross-sections led to higher performance. This produces plasmas with high "shear", which distributed and broke up turbulent eddies in the plasma. These changes led to the "advanced tokamak" designs, which include ITER.Charles Kessel, "What's an Advanced Tokamak", Spring APS, Philadelphia, 2003
Spherical tokamaks In 1984,Y-K Martin Peng, "Spherical Torus, Compact Fusion at Low Yield"., ORNL/FEDC-87/7 (December 1984) Martin Peng of ORNL proposed an alternate arrangement of the magnet coils that would greatly reduce the aspect ratio while avoiding the erosion issues of the compact tokamak. Instead of wiring each magnet coil separately, he proposed using a single large conductor in the center, and wiring the magnets as half-rings off of this conductor. What was once a series of individual rings passing through the hole in the center of the reactor was reduced to a single post, allowing for aspect ratios as low as 1.2. This means that STs can reach the same operational triple product numbers as conventional designs using one tenth the magnetic field.