Over time the nature of the neutrons was explored and came to be understood as isolated events caused by instabilities inside the plasma. Earlier "gross" instabilities had been successfully dealt with in ZETA, but correcting these had simply turned up another set to be fixed. The new ones were being caused by turbulence within the plasma. Some progress on suppressing these had been made by E. P. Butt and others, but they were not well understood. Robinson was put on the task of better understanding the nature of the turbulence, running a series of experiments to characterize it. These experiments led to a better understanding of the theoretical nature of the problem, which in turn led to major work by John Bryan Taylor on a general theory of high-current electric discharges in magnetic fields. This work was a major advance in plasma physics, and through it introduced the concept of reversed field pinch, a field of study to this day. As the nature of these problems became clear, the ZETA team turned from attempting fusion to developing dramatically improved diagnostic tools for characterizing the plasma. Instead of measuring the spectroscopy of the ions, it is possible to directly measure the velocity of electrons through Thomson scattering. However, this requires a bright and highly monochromatic light source to be effective. The introduction of the laser in the 1960s provided just such a source, and beginning in 1964 the Harwell team became experts in this system. From the mid-1950s the Soviets had been quietly developing the tokamak device. In configuration, the tokamak is largely identical to the z-pinch devices like ZETA, consisting of a ring of magnets surrounding a toroidal vacuum tube, with a large transformer used to induce current into the plasma. The magnetic field of the two sources mixed to produce a single helical field that winds around the plasma. Where the two systems differed was primarily in the ratio of the power of the fields; ZETA's field was generated almost entirely by the transformer current, while the tokamak used more powerful ring magnets to balance the two more closely. This seemingly minor change has enormous effects on the dynamics of the plasma; ZETA's helix wound slowly around the plasma, the tokamak's was fairly "twisty". This is measured by the "safety factor". By the mid-1960s, experimental machines demonstrated that the tokamak concept was a dramatic improvement over older designs. However, the Soviets waited, perhaps wanting to avoid another ZETA debacle, until they were absolutely sure their machines were producing the numbers the measurements suggested they were.