Editing Stellarator (section)

=== Early devices ===
With the funding from the AEC, Spitzer began work by inviting [[James Van Allen]] to join the group and set up an experimental program. Allen suggested starting with a small "tabletop" device. This led to the Model A design, which began construction in 1952. It was made from 5&nbsp;cm [[pyrex]] tubes about 350&nbsp;cm in total length, and magnets capable of about 1,000&nbsp;gauss.{{sfn|Stix|1998|p=6}} The machine began operations in early 1953 and clearly demonstrated improved confinement over the simple torus.{{sfn|Ellis|1958|p=13}}

This led to the construction of the Model B, which had the problem that the magnets were not well mounted and tended to move around when they were powered to their maximum capacity of 50,000&nbsp;gauss. A second design also failed for the same reason, but this machine demonstrated several-hundred-kilovolt X-rays that suggested good confinement. The lessons from these two designs led to the B-1, which used ohmic heating (see below) to reach plasma temperatures around 100,000 degrees.{{sfn|Ellis|1958|p=13}} This machine demonstrated that impurities in the plasma caused large [[x-ray]] emissions that rapidly cooled the plasma. In 1956, B-1 was rebuilt with an ultra-high vacuum system to reduce the impurities but found that even at smaller quantities they were still a serious problem. Another effect noticed in the B-1 was that during the heating process, the particles would remain confined for only a few tenths of a millisecond, while once the field was turned off, any remaining particles were confined for as long as 10 milliseconds. This appeared to be due to "cooperative effects" within the plasma.{{sfn|Ellis|1958|p=14}}

Meanwhile, a second machine known as B-2 was being built. This was similar to the B-1 machine but used pulsed power to allow it to reach higher magnetic energy and included a second heating system known as magnetic pumping. This machine was also modified to add an ultra-high vacuum system. Unfortunately, B-2 demonstrated little heating from the magnetic pumping, which was not entirely unexpected because this mechanism required longer confinement times, and this was not being achieved. As it appeared that little could be learned from this system in its current form, in 1958 it was sent to the [[Atoms for Peace]] show in [[Geneva]].{{sfn|Ellis|1958|p=14}} However, when the heating system was modified, the coupling increased dramatically, demonstrating temperatures within the heating section as high as {{val|1000|u=eV}}.{{sfn|Stix|1998|p=6}}{{efn|The bulk temperature of the plasma was much lower, this was the temperature only within the heating section.}}

Two additional machines were built to study pulsed operation. B-64 was completed in 1955, essentially a larger version of the B-1 machine but powered by pulses of current that produced up to 15,000&nbsp;gauss. This machine included a [[divertor]], which removed impurities from the plasma, greatly reducing the x-ray cooling effect seen on earlier machines. B-64 included straight sections in the curved ends which gave it a squared-off appearance. This appearance led to its name, it was a "figure-8, squared", or 8 squared, or 64. This led to experiments in 1956 where the machine was re-assembled without the twist in the tubes, allowing the particles to travel without rotation.{{sfn|Stix|1998|p=7}}

B-65, completed in 1957, was built using the new "racetrack" layout. This was the result of the observation that adding helical coils to the curved portions of the device produced a field that introduced the rotation purely through the resulting magnetic fields. This had the added advantage that the magnetic field included ''shear'', which was known to improve stability.{{sfn|Stix|1998|p=7}} B-3, also completed in 1957, was a greatly enlarged B-2 machine with ultra-high vacuum and pulsed confinement up to 50,000&nbsp;gauss and projected confinement times as long as 0.01 second. The last of the B-series machines was the B-66, completed in 1958, which was essentially a combination of the racetrack layout from B-65 with the larger size and energy of the B-3.{{sfn|Ellis|1958|p=14}}

Unfortunately, all of these larger machines demonstrated a problem that came to be known as "[[pump out]]". This effect was causing plasma drift rates that were not only higher than classical theory suggested but also much higher than the Bohm rates. B-3's drift rate was a full three times that of the worst-case Bohm predictions, and failed to maintain confinement for more than a few tens of microseconds.{{sfn|Stix|1998|p=7}}