Spheromaks

What spheromaks are: Spheromaks are plasmas with very large internal currents and internal magnetic fields that are aligned so as to be in a nearly force-free equilibrium, i.e., the currents are very nearly parallel to the magnetic fields. The spheromak equilibrium is a `natural' state since magnetic turbulence tends to drive magnetically dominated plasmas towards the spheromak state.

 

 

Planar spheromak gun on 1.4 m diameter vacuum tank
Spheromak technology: Laboratory spheromaks involve very large currents, typically 100's of kiloamperes and high voltages, typically kilovolts. These currents and voltages are obtained using high energy capacitor banks which are switched in microseconds. The formation geometry is arranged such that magnetic flux cuts across the electrodes connected to the capacitor bank. This configurations generates helicity (twistedness) in the flux tube going from one electrode to the other. With enough helicity a spheromak is formed.
Why spheromaks are interesting: Spheromaks are inherently three-dimensional and involve the concept of magnetic helicity which is a measure of the twistedness of a magnetic flux tube. Spheromaks have been proposed as the basis of magnetic fusion confinement schemes and as a means for refueling tokamaks. The physics of spheromaks is closely related to the physics of astrophysical jets. What we are doing: We are interested in learning more about the topology of spheromaks as they form. To do this we are using an ultra-fast camera (10 ns shutter speed) to photograph the various stages of a spheromak as it forms. We have observed very distinctive and reproducible twisted flux tubes. We are measuring the internal magnetic fields in these flux tubes.

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Spheromak formation movies 

Links to related activities elsewhere

Making spheromaks: Making spheromaks is analogous to blowing bubbles: the component of the magnetic stress tensor parallel to the magnetic field acts like the surface tension in the soap film while the perpendicular component acts like the air pressure inflating the bubble. When the destabilizing stress due to the perpendicular component overwhelms the stabilizing stress due to the parallel component, a detached spheromak breaks off.