At the onset of the atomic age, governments on both sides of the iron curtain sought to harness the power of nuclear fusion. Researchers at the Princeton Plasma Physics Laboratory in New Jersey stood at the forefront of the American effort when, in 1953, they began using Stellarators — one of the earliest controlled fusion systems.
Early fusion research in the western world nearly immediately split into two halves after the end of WWII, with one subset of researchers observing super-compressed fusion materials at very short timescales, the others — including Dr. Lyman Spitzer, chair of the Department of Astronomy at Princeton University — observing these materials at a lower compression for longer times. Spitzer’s invention served this purpose wonderfully. The Stellarator that Spitzer invented in 1950 is designed to hold superheated, electrically-charged plasma — a most vital and basic component of nuclear fusion research — within a designated field using electromagnetic currents.
As a 1996 report by UCSD for the Fusion Energy Science Advisory Committee explains, these devices are essential for expanding our understanding of plasma physics:
The stellarator drives the development of three-dimensional (3-D) plasma physics, which is needed throughout the toroidal fusion program and for the study of naturally occurring plasmas. For example, resistive wall modes and field error effects are 3-D equilibrium problems. 3-D effects provide fundamental limits on the performance of nominally axisymmetric devices like tokamaks and RFP’s. Electron orbits in the magnetosphere are studied using the magnetic coordinate and drift Hamiltonian techniques developed for stellarators.
Normally when you heat a gas into plasma, it will naturally dissipate given the chance and, since there aren’t really any physical materials that can endure the enormous amounts of heat energy plasma emits, the particles must be confined using electromagnetic repulsion — specifically, the Lorentz force. When a wire helix is wrapped around a tube-shaped support structure and electrified, it generates a magnetic field that radiates out along the diameter of the wire. Since the field is stronger on the inside of the coil than the outside, it effectively compresses the plasma and prevents it from actually touching the support wall. And to keep the plasma from shooting out of the ends of the coil like soda out of a straw, the two ends are simply connected to form a doughnut of wire with a superheated particle filling.
The problem with this design was that the highest-energy — and therefore most valuable — particles would eventually spin themselves clear out of the confinement field. Spitzer’s Stellarator minimized this occurrence by twisting and stretching the ring shape into a figure 8 with a 180 degree twist in the middle. That way, any high energy particles on the outside edge of the ring (where the field is weakest) would be flipped back to the center of the beam by the time it entered the next curve.
Virtually all plasma physics research throughout the 1950s and 1960s occurred on Stellarators. The Model C, above, was the largest of these devices. “The machine was in the form of a racetrack t,200 cm in length with 5-7.5 cm minor radius for the plasma,” Thomas H Stix describes in Highlights in Early Stellarator Research at Princeton. “The toroidal magnetic field was typically operated at 35,000 gauss. One of the two straight legs of the racetrack contained a divertor, the other one a section to supply 4 megawatts of 25 MHz ion cyclotron resonance heating (ICRH). l:2 and l:3 helical windings installed on the U-bends provided a rotational transform up to about 180′.”
It entered service in 1962 and immediately blew the doors off of the earlier figure-8 design. It incorporated a pair of major innovations — the divertor, which sucked unwanted waste particles out of the stream without disrupting the confinement field, and ICRH that uses radio waves to force the ions to spin around the centre axis of the field the same way the wire helix of the earlier models wound around the central core of their support matrix — mitigated earlier models’ issues with plasma loss. This design also directly influenced the design development of the newer Tokamak experiments. In fact, in 1969 the PPPL converted the Model C itself to a tokamak design, renaming it the Symmetric Tokamak.
These machines were capable of heating ions to 1.6 keV, electrons to 3.5 keV, and packing as many as 3 x 1020 particles into a cubic meter of interior space, which is roughly what the Tokamak technology that supplanted Stellarators in the 1970s were capable of. However, the tokamaks didn’t suffer from anomalous plasma loss — they run the electrical current through the plasma itself rather than through the external wire helix — and returned better results than the earlier technology. This led to Stellarators to fall out of favour with the research community for nearly three decades until shortcomings in the Tokamak technology spurred a rekindling in interest for the Stellarator design in the 1990s. Today the Wendelstein 7-X in Germany, the Helically Symmetric Experiment in Wisconsin and Japan’s Large Helical Device carry on the work and the spirit of the original Stellarators. [IOP – Wiki – JSPF – UCSD – PPPL]