The pinhole must be placed in a vacuum to prevent air breakdown by the focused beam. These filters are used to counter the effects caused by self-focusing. A compact torus with comparable toroidal and poloidal magnetic fields. The plasma is roughly spherical and is usually surrounded by a close-fitting conducting shell or cage. Both the poloidal and toroidal magnetic fields are generated by plasma currents.
External force is supplied by poloidal field coils outside the plasma separatrix.
The resulting configuration is approximately a force-free magnetic field. The spheromak machine geometry can be simpler than a tokamak, but the close-fitting wall is a source of impurities and the current cannot be inductively driven. After early experiments failed to achieve a reasonable beta, interest has ebbed. Contrast with stimulated emission. Spectroscopic measurements of plasmas using the pressure-based and motional Stark effects are useful for diagnostic purposes.