Editing Tokamak (section)

=== Magnetic confinement ===
When heated to fusion temperatures, the [[electron]]s in atoms dissociate,{{clarify|date=January 2025}} resulting in a fluid of nuclei and electrons known as [[plasma (physics)|plasma]]. Unlike electrically neutral atoms, a plasma is electrically conductive, and can, therefore, be manipulated by electrical or magnetic fields.{{sfn|Bromberg|1982|p=15}}

Sakharov's concern about the electrodes led him to consider using magnetic confinement instead of electrostatic. In the case of a magnetic field, the particles will circle around the [[lines of force]].{{sfn|Bromberg|1982|p=15}} As the particles are moving at high speed, their resulting paths look like a helix. If one arranges a magnetic field so lines of force are parallel and close together, the particles orbiting adjacent lines may collide, and fuse.{{sfn|Shafranov|2001|p=838}}

Such a field can be created in a [[solenoid]], a cylinder with magnets wrapped around the outside. The combined fields of the magnets create a set of parallel magnetic lines running down the length of the cylinder. This arrangement prevents the particles from moving sideways to the wall of the cylinder, but it does not prevent them from running out the end. The obvious solution to this problem is to bend the cylinder around into a donut shape, or torus, so that the lines form a series of continual rings. In this arrangement, the particles circle endlessly.{{sfn|Shafranov|2001|p=838}}

Sakharov discussed the concept with [[Igor Tamm]], and by the end of October 1950 the two had written a proposal and sent it to [[Igor Kurchatov]], the director of the atomic bomb project within the USSR, and his deputy, [[Igor Golovin]].{{sfn|Shafranov|2001|p=838}} However, this initial proposal ignored a fundamental problem; when arranged along a straight solenoid, the external magnets are evenly spaced, but when bent around into a torus, they are closer together on the inside of the ring than the outside. This leads to uneven forces that cause the particles to drift away from their magnetic lines.{{sfn|Shafranov|2001|p=839}}{{sfn|Bromberg|1982|p=16}}

During visits to the [[Laboratory of Measuring Instruments of the USSR Academy of Sciences]] (LIPAN), the Soviet [[Nuclear physics|nuclear research]] centre, Sakharov suggested two possible solutions to this problem. One was to suspend a current-carrying ring in the centre of the torus. The current in the ring would produce a magnetic field that would mix with the one from the magnets on the outside. The resulting field would be twisted into a helix, so that any given particle would find itself repeatedly on the outside, then inside, of the torus. The drifts caused by the uneven fields are in opposite directions on the inside and outside, so over the course of multiple [[orbit]]s around the long axis of the [[torus]], the opposite drifts would cancel out. Alternately, he suggested using an external magnet to induce a current in the [[Plasma (physics)|plasma]] itself, instead of a separate metal ring, which would have the same effect.{{sfn|Shafranov|2001|p=839}}

In January 1951, Kurchatov arranged a meeting at LIPAN to consider Sakharov's concepts. They found widespread interest and support, and in February a report on the topic was forwarded to [[Lavrentiy Beria]], who oversaw the atomic efforts in the USSR. For a time, nothing was heard back.{{sfn|Shafranov|2001|p=839}}