A spheromak is an arrangement of plasma formed into a toroidal shape similar to a smoke ring. The spheromak contains large internal electric currents and their associated magnetic fields arranged so the magnetohydrodynamic forces within the spheromak are nearly balanced, resulting in long-lived (microsecond) confinement times without external fields. Spheromaks belong to a type of plasma configuration referred to as the compact toroids. A spheromak can be made and sustained using magnetic flux injection, leading to a dynomak.

The physics of the spheromak and of collisions between spheromaks is similar to a variety of astrophysical events, like coronal loops and filaments, relativistic jets and plasmoids. They are particularly useful for studying magnetic reconnection events, when two or more spheromaks collide. Spheromaks are easy to generate using a "gun" that ejects spheromaks off the end of an electrode into a holding area, called the flux conserver. This has made them useful in the laboratory setting, and spheromak guns are relatively common in astrophysics labs. These devices are often, confusingly, referred to simply as "spheromaks" as well; the term has two meanings.

Spheromaks have been proposed as a magnetic fusion energy concept due to their long confinement times, which was on the same order as the best tokamaks when they were first studied. Although they had some successes during the 1970s and '80s, these small and lower-energy devices had limited performance and most spheromak research ended when fusion funding was dramatically curtailed in the late 1980s. However, in the late 1990s research demonstrated that hotter spheromaks have better confinement times, and this led to a second wave of spheromak machines. Spheromaks have also been used to inject plasma into a bigger magnetic confinement experiment like a tokamak.

The difference between a field-reversed configuration (FRC) and a spheromak is that a spheromak has an internal toroidal field while the FRC plasma does not. This field can run clockwise or counterclockwise to the spinning plasma direction.

The spheromak has undergone several distinct periods of investigation, with the greatest efforts during the 1980s, and a reemergence in the 2000s.

A key concept in the understanding of the spheromak is magnetic helicity, a value ${\displaystyle H}$ that describes the "twistedness" of the magnetic field in a plasma.

The earliest work on these concepts was developed by Hannes Alfvén in 1943, which won him the 1970 Nobel Prize in Physics. His development of the concept of Alfvén waves explained the long-duration dynamics of plasma as electric currents traveling within them produced magnetic fields which, in a fashion similar to a dynamo, gave rise to new currents. In 1950, Lundquist experimentally studied Alfvén waves in mercury and introduced the characterizing Lundquist number, which describes the plasma's conductivity. In 1958, Lodewijk Woltjer, working on astrophysical plasmas, noted that ${\displaystyle H}$ is conserved, which implies that a twisty field will attempt to maintain its twistiness even with external forces being applied to it.

Starting in 1959, Alfvén and a team including Lindberg, Mitlid and Jacobsen built a device to create balls of plasma for study. This device was identical to modern "coaxial injector" devices (see below) and the experimenters were surprised to find a number of interesting behaviors. Among these was the creation of stable rings of plasma. In spite of their many successes, in 1964 the researchers turned to other areas and the injector concept lay dormant for two decades.