An air‐cored betatron has been constructed whose coil is effectively a single turn made up from four circular turns connected in parallel. The design was facilitated by choosing independently the positions of the four parallel turns and the current distribution through them. The desired current distribution was obtained by making two of the turns in the form of toroidal coils whose inductance could be adjusted. The betatron has been used in combination with an azimuthal magnetic field for the production of an intense stream of runaway electrons, consisting of essentially all the electrons of a plasma. After a short interval (∼0.1 &mgr;sec) of normal acceleration, the electrons experience less‐than‐normal acceleration followed by deceleration and disruption of the stream. Intense bursts of microwave radiation occur during the current pulse. The observed stream behavior is reasonably well explained by the theory of the two stream instability.