Laser and Particle Beams

Research Article

Effects of dielectric discontinuity on the dispersion characteristics of the tape helix slow-wave structure with two metal shields

Yu Zhanga1 c1, Jinliang Liua1, Shiwen Wanga1, Xuliang Fana1, Hongbo Zhanga1 and Jiahuai Fenga1

a1 College of Opto-electronic Science and Engineering, National University of Defense Technology, Changsha, China

Abstract

In the tape helix slow-wave system, discontinuous dielectrics have great effects on the dispersion characteristics. In this paper, the tape helix slow-wave system, including an inner and an outer metal shield, tape helix, nylon support and de-ionized water as filling dielectric, was analyzed. Effects of dielectric discontinuity caused by the support dielectric and filling dielectric on the dispersion characteristics were studied in detail. The dispersion relations, phase velocities, slow-wave coefficients and electric lengths of the spatial harmonics in the system were calculated. Results showed that, if the permittivity of support dielectric was smaller than that of the filling dielectric, frequencies of the spatial harmonics in the system rose, phase velocities and slow-wave coefficients increased, the slow-wave effect of the system was weakened so that the previous electric length was shortened. The reverse condition corresponded to the reverse results, and the electromagnetic simulation also proved it. By use of the helical pulse forming line of accelerator based on the studied tape helix slow-wave system, the electric lengths of the system were tested as 188.5 ns and 200 ns in experiment when the thicknesses of nylon support were 6 mm and 3 mm, respectively. The theoretical calculation results 198 ns and 211 ns basically corresponded to experimental results, which only had relative errors as 5 and 5.5%, respectively.

(Received July 25 2011)

(Accepted September 02 2011)

Keywords

  • Accelerator;
  • Dielectric discontinuity;
  • Dispersion relation;
  • Electric length;
  • Pulse forming line;
  • Spatial harmonics;
  • Tape helix slow-wave structure

Correspondence:

c1 Address correspondence and reprint requests to: Yu Zhang, College of Opto-electronic Science and Engineering, National University of Defense Technology, Changsha, 410073, China. E-mail: zyu841227@yahoo.com.cn