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ultipacting is a phenomenon of resonant electron multiplication e.g. in the particle accelerators of the high energy physics, in which a large number of electrons build up an electron avalanche, leading to remarkable power losses and heating of the walls, so that it becomes impossible to increase the cavity fields by rising the incident power. In superconducting structures a large rise of temperature can lead to a thermal breakdown. Furthermore, a heavy bombing of multipacting electrons may break the ceramic windows in the power feed lines leading to disastrous consequences.

The research group has in collaboration with DESY (Deutsches Elektronen-Synchrotron), Germany, developed computational methods for the numerical simulation of electron multipacting in particle accelerators. The developed methods have been applied to analyze multipacting in accelerator cavities and input couplers, and find to an optimal biasing DC voltage to suppress multipacting.

Research Topics

  • Electromagnetic field computing in axisymmetric structures.
  • Analysis of multipacting in cavities and input couplers with ceramic windows.
  • Suppressing multipacting by a biasing DC voltage.

The two figures below show the electric field and a multipacting electron trajectory in the superconducting TESLA 1.3 GHz accelerator cavity. The field solution in found by the axisymmtric boundary integral equation method.


  • P. Ylä-Oijala and S. Järvenpää: Finite element method for the electromagnetic field computation in cylindrically symmetric RF structures, Electromagnetics, Vol. 22, pp. 261-273, 2002.

  • P. Ylä-Oijala and Marko Ukkola: Suppressing Electron Multipacting in Ceramic Windows by DC Bias, Nuclear Instruments and Methods in Physics Research, Section A , Vol. 474, pp. 197-208, 2001.

  • P. Ylä-Oijala and D. Proch: MultiPac - Multipacting Simulation Package with 2D FEM Field Solver, The 10th Workshop on RF Superconductivity, Tsukuba, Japan, 2001.

  • Y. Kang, et. al. (P. Ylä-Oijala): Electromagnetic Simulations and Properties of the Fundamental Power Couplers for the SNS Superconducting Cavities, Proceedings of the 2001 Particle Accelerator Conference, PAC 2001, Chicago Illinois, USA, pp. 1122-1124, 2001.

  • G. Giovati, et. al. (P. Ylä-Oijala): Superconducting Prototype Cavities for the Spallation Netron Source (SNS) Project, Proceedings of the 2001 Particle Accelerator Conference, PAC 2001, Chicago Illinois, USA, pp. 484-486, 2001.

  • P. Ylä-Oijala, M. Ukkola, S. Järvenpää and J. Lukkarinen: MultiPac 2.0 - Multipacting simulation toolbox with 2D FEM field solver and MATLAB graphical user interface, Rolf Nevanlinna Institute, Reseacrh Reports C35, pp. 1-47, 2000.

  • P. Ylä-Oijala and Marko Ukkola: Suppressing electron multipacting in TTF III cold window by DC bias, HIP-2000-27/TECH, Helsinki Institute of Physics, pp. 1-16, 2000.

  • P. Ylä-Oijala: Electron Multipacting in TESLA Cavities and Input Couplers, Particle Accelerators, Vol. 63, pp. 105-137, 1999.

  • E. Somersalo, P. Ylä-Oijala, D. Proch and J. Sarvas: Computational Methods for Analyzing Electron Multipacting in RF Structures, Particle Accelerators, Vol. 59, pp. 107-141, 1998.

  • P. Ylä-Oijala: Suppressing electron multipacting in coaxial lines by DC voltage, DESY Print 1997, TESLA Reports 97-21 , pp. 1-14, Hamburg Germany 1997.

  • P. Ylä-Oijala: Analysis of electron multipacting in coaxial lines with traveling and mixed waves, DESY Print 1997, TESLA Reports 97-20 , pp. 1-21, Hamburg Germany 1997.

  • E. Somersalo, P. Ylä-Oijala and D. Proch: Analysis of Electron Multipacting in Coaxial Lines, Proc. of Particle Accelerator Conference in Dallas, USA, pp. 1500-1502, 1995.

  • E. Somersalo, P. Ylä-Oijala and D. Proch: Electron multipacting in RF structures, DESY Print 1994, TESLA Reports 94-14, pp. 1-19, Hamburg Germany 1994.

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