Formation of a High-Current Electron Beam in Modified Betatron Fields

VOLUME PHYSICAL REVIEW LETTERS 53, NUMBER 3 Formation of a High-Current 16 JUL+ 1984 Electron Beam in Modified Betatron Fields H. Ishizuka, G. Lindley, B. Mandelbaum, A. Fisher, and N. Rostoker Department of Physics, University of California, Irvine, California 9271 7 (Received 2 February 1984) An electron beam with a current exceeding 200 A has been generated and sustained by continuous injection of electrons into a rising toroidal and vertical magnetic field. There is a large mismatch between the electron energy and the vertical field. When the vertical field reaches a certain strength, accelerated beam electrons hit the outer wall of the torus, destroy- ing the beam. By control of this vertical field the lifetime of the beam is extended yielding an electron energy of 1 MeV. PACS numbers: 52.75.Di, 29. 20.Fj The production of a multikiloampere electron beam in a betatron has recently attracted interest. ' In order to focus an intense beam against the space-charge electric field, a modified betatron' ' employs a toroidal magnetic field in addition to the betatron (or vertical) field. In this paper we report the results of the formation of a high-current beam in a modified betatron and discuss the beam's characteristic features. The schematic of the exper- imental apparatus is shown in Fig. 1 and the major parameters are given in Table I. The vertical field is produced by a pair of air-core coils, 62. 5 cm in radius and 64 cm apart, and a center solenoid, 42 cm in diameter and 120 cm long, provides part of the toroidal electric field. Usually the center solenoid is connected in series with the vertical field coils in which case the beta- tron flux condition, (B)/B=2, is met near the minor axis of the torus. The one-turn accelerating voltage is typically 150—250 V. The toroidal mag- netic field, produced by a set of coils wound closely to the glass torus, is much stronger than the vertical field. Injection of electrons is accomplished by ap- plying negative voltage to a heated cathode, which V TORO IDAL F ELD COI L I V : C. S. AUX VERT ICAL FIELD COIL C. S, : CENTER SOLENOID G AUX: AUX L ARY I v G: GLASS TORUS FIG, 1. Schematic cross section of apparatus. I LOOPS is normally located 1 cm from the outer wall of the torus. A typical sequence of operation is shown in Fig. 2. The toroidal magnetic field [Fig. 2(a)] is fired prior to the vertical field [Fig. 2(b)] and the injec- tion voltage. When the vertical fiekl attains a cer- tain value the beam forms. This formation is re- vealed by a drastic reduction in the injection current [Fig. 2(c)] and the beginning of a beam current measured by a Rogowski loop wound around the torus [Fig. 2(d) shows two traces of the beam current]. The beam current increases up to a peak value, then decreases gradually, and finally van- ishes abruptly. When the beam vanishes a burst of x rays is detected as shown in Fig. 2(e). The beam is created only if the toroidal magnetic field is larger than a few kilogauss and the emission from the in- jector is greater than approximately 3 A. It should be noted that the beam life suffers little change if the toroidal magnetic field is reversed, indicating that the vertical component of the field produced by the toroidal field coils is sufficiently small. The life and parameters of the beam are nearly independent of the strength of the toroidal field above approxi- mately 3 kG. The beam characteristics are similar for both types of injectors (tangential and nondirec- tional) and with the accelerating gap in the screen liner either open or shorted. In conventional circular accelerators the vertical field B, the orbit radius R, and the momentum p of a particle with charge e are related by p = eBR. This is not the case with a high-current beam. In Fig. 3, open circles show the vertical field at the start of the beam as a function of the injection voltage. When the injector cathode is located 1 cm from the outer wall of the torus, the beam is formed at 40 G (50 6) for an injection voltage of 10 kV (40 kV). On the other hand, p = eBR gives B = 8. 5 6) for 10-keV (40-keV) electrons which take a gyroradius of 0. 4 m. Closed circles in Fig. 3 indi- cate the vertical fields at the end of the beam. The