Spheromak

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.

The earliest work on these concepts was developed by Hannes Alfvén in 1943,[5] which won him the 1970 Nobel Prize in Physics.

[6] Starting in 1959, Alfvén and a team including Lindberg, Mitlid and Jacobsen built a device to create balls of plasma for study.

[10] Another approach to fusion was the theta pinch concept, which was similar to the z-pinch used in ZETA in theory, but used a different arrangement of currents and fields.

To increase fusion rates, research has focused on the "triple product" a combination of the plasma temperature, density and confinement time.

[12] Fusion devices generally fell into two classes, pulsed machines like the z-pinch that attempted to reach high densities and temperatures but only for microseconds, while steady state concepts such as the stellarator and magnetic mirror attempted to reach the Lawson criterion through longer confinement times.

Taylor's work suggested that self-stable plasmas would be a simple way to approach the problem along the confinement time axis.

In 1979 Rosenbluth and Bussac published a paper describing generalizations of Taylor's work, including a spherical minimum energy state having zero toroidal field on the bounding surface.

He had moved to the University of Miami and started gathering funding for a device combining two of his earlier conical theta-pinch systems, which became Trisops.

In Japan, Nihon University built the PS-1, which used a combination of theta and zeta pinches to produce spheromaks.

Harold Furth was excited by the prospect of a less-expensive solution to the confinement issue, and started the S1 at the Princeton Plasma Physics Laboratory, which used inductive heating.

This was this era's largest and most powerful device, generating spheromaks with surface currents of 1 MA, temperatures of 100 eV, and peak electron betas over 20%.

[15] CTX experimented with methods to re-introduce energy into the fully formed spheromak in order to counter losses at the surface.

In spite of these early successes, by the late 1980s the tokamak had surpassed the confinement times of the spheromaks by orders of magnitude.

CTX gained additional funding from the Defence Department and continued experiments until 1990; the last runs improved temperatures to 400 eV,[17] and confinement times on the order of 3 ms.[18] Through the early 1990s spheromak work was widely used by the astrophysics community to explain various events and the spheromak was studied as an add-on to existing MFE devices.

[22] Hammer, Hartman et al. showed that spheromaks could be accelerated to extremely high velocities using a railgun, which led to several proposed uses.

Among these was the use of such plasmas as "bullets" to fire at incoming warheads with the hope that the associated electric currents would disrupt their electronics.

[23][24] Other proposed uses included firing spheromaks at metal targets to generate intense X-ray flashes as a backlighting source for other experiments.

[21] In the late 1990s spheromak concepts were applied towards the study of fundamental plasma physics, notably magnetic reconnection.

In 1994 T. Kenneth Fowler was summarizing the results from CTX's experimental runs in the 1980s when he noticed that confinement time was proportional to plasma temperature.

[27] Also of note is the steady inductive helicity injected torus experiment (HIT-SI) at the University of Washington headed by Professor Thomas Jarboe.

[28] The success of sustaining spheromaks with evidence of pressure confinement[29] on this experiment motivated the creation of a new spheromak-based fusion reactor concept called the Dynomak that is projected to be cost competitive with conventional power sources.

This[clarification needed] is similar to the field configuration of a tokamak, except that the field-producing coils are simpler and do not penetrate the plasma torus.

[citation needed] Spheromaks are subject to external forces, notably the thermal gradient between the hot plasma and its cooler surroundings.

[citation needed] Spheromaks form naturally under a variety of conditions, enabling them to be generated in a number of ways.

As the current scales up it approaches the conditions of a traditional tokamak, but in a much smaller size and simpler form.