Experimental Study of Runaway Electrons in a Plasma Betatron

Abstract

Runaway electrons generated by induction acceleration in a plasma betatron with axially symmetric guide field Bz(r,z,t)=B0(r0/r)nf(z) sin 2πt/T; (n = 0.43, T = 16μsec, r0 = 4.8 cm) together with auxiliary azimuthal guide field Bθ(r) = const/r; (2 kG ≤ Bθ(r0) ≤ 5 kG) have been studied for a wide range of induction electric fields Eθ; (2.5 V/cm ≤ Eθ(r0) ≤ 26 V/cm). Experiments have been carried out in argon and krypton at ambient pressure of (0.5–2) × 10−3 Torr. Runaway currents of ≈ 1 A were observed as immersed in a background conduction current of 25–100 A. After a period of normal-betatron-acceleration-beam disruption occurs, and the energetic runaways strike the vacuum-chamber walls. The beam-disruption time tx has been measured as a function of Eθ and Bθ for several half-cycles of the betatron guide field. A space-charge theory is developed in order to explain the small runaway currents. Reasonably good agreement with the experimental results is obtained over a wide range of parameters. The reason for the beam disruption and subsequent x-ray emission is unexplained.