Observation of radial equilibrium and transition to a helix of a gas-embeddedZpinch

A laser-initiated, gas-embedded $Z$ pinch is operated with a preheat current which adjusts the plasma parameters by Joule heating and expansion so that when the main current is injected, the plasma core is heated under conditions of pressure balance. This results in a pinch which has a constant radius during the time of current rise. The pinch is stable during this time and thereafter undergoes a rapid transformation into a radially expanding helical structure. The growth time and wavelength of the instability is in good agreement with the theoretical predictions of the $m=1\mathrm{}$ magnetohydrodynamic instability.