the current in the transformer has to be increased over time to produce the varying magnetic field. This places a limit on the product of confinement time and magnetic field, for any given source of power. In Z-pinch machines the current is generally provided from a large bank of capacitors and triggered by a spark gap, known as a Marx Bank or Marx generator. As the conductivity of plasma is fairly good, about that of copper, the energy stored in the power source is quickly depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature. History Early machines Pinch devices were among the earliest efforts in fusion power. Research began in the UK in the immediate post-war era, but a lack of interest led to little development until the 1950s. The announcement of the Huemul Project in early 1951 led to fusion efforts around the world, notably in the UK and in the US (see Perhapsatron, a z-pinch machine at LANL). Small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel. The problem became known as the "kink instability". Stabilized pinch By 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added external magnets that created a toroidal magnetic field inside the chamber. When the device was fired, this field added to the one created by the current in the plasma. The result was that the formerly straight magnetic field was twisted into a helix, which the particles followed as they traveled around the tube driven by the current. A particle near the outside of the tube that wanted to kink outward would travel along these lines until it returned to the inside of the tube, where its outward-directed motion would bring it back into the centre of the plasma. Researchers in the UK started construction of ZETA in 1954. ZETA was by far the largest fusion device of its era. At the time, almost all fusion research was classified, so progress on ZETA was generally unknown outside the labs working on it. However US researchers visited ZETA and realized that they were about to be outpaced. Teams on both sides of the Atlantic rushed to be the first to complete stabilized pinch machines. ZETA