• Z. Liu
  IUCF, Bloomington, Indiana
• S.M. Cousineau, V.V. Danilov, J. Galambos, J.A. Holmes, M.A. Plum
  ORNL, Oak Ridge, Tennessee

The instability caused by the electron cloud effect (ECE) may set an upper limit to beam intensity in proton storage rings. This instability is potentially a major obstacle to the full intensity operation, at 1.5·10¹⁴ protons per pulse, of the Spallation Neutron Source (SNS). High intensity experiments have been done with different sets of parameters that affect the electron-proton (e-p) instability, of which bunch intensity and bunch shape are considered as two main factors. In the experiment, the phase and amplitude of the second harmonic RF cavity are used to modify the bunch shape. Simulation with the beam dynamics code ORBIT has been carried out to compare with experimental results and to understand the impact of bunch shape on electron cloud build-up and beam stability. We have also attempted to benchmark the e-p model to predict the frequency spectrum and the RF buncher voltage threshold values against experimental results. Details and discussion will be reported in this conference.

* M.T.F. Pivi and M.A. Furman, PRSTAB 6, 034201 (2003)
** V. Danilov et. al, 39th ICFA Advanced Beam Dynamics Workshop, 2006
*** B. Macek et. al, PAC 2003

• M.A. Plum, J. Galambos, S.-H. Kim, P. Ladd, Y. Polsky, R.W. Shaw
  ORNL, Oak Ridge, Tennessee
• C.F. Luck, C.C. Peters
  ORNL RAD, Oak Ridge, Tennessee
• R.J. Macek
  LANL, Los Alamos, New Mexico
• D. Raparia
  BNL, Upton, Long Island, New York

The diamond stripper foils in use at the Spallation Neutron Source worked successfully with no failures until May 3, 2009, when we started experiencing a rash of foil failures after increasing the beam power to ~840 kW. The main contributions to foil failure are thought to be 1) convoy electrons, stripped from the incoming H− beam, that strike the foil bracket and may also reflect back from the electron catcher, and 2) vacuum breakdown from the charge developed on the foil by secondary electron emission. In this paper we will detail these and other failure mechanisms, and describe the improvements we have made to mitigate them.