Development of Ion-Bombardment Surface Treatments for Suppression of Secondary Electron Emission in Accelerator Vacuum Chambers and Other Structures

Introduction Particle accelerators have been essential tools for elementary particle physics research for many years. They are now becoming increasingly important for applied science, typically done at user facilities such as light sources and neutron sources. The performance of future high intensity positron and proton accelerators is likely to be limited by the development of electron plasma, typically referred to as an electron cloud (EC), in the accelerator vacuum chambers [1-5]. At the Cornell Electron-Positron Storage Ring (CESR), the CESR Test Accelerator (CESRTA) program has been underway for the last 3 years. The principal goals of the program are to develop and understand methods to mitigate EC production and to characterize the impact of EC on ultra-low-emittance positron beams [6]. One of the promising techniques for suppressing EC formation in regions with magnetic fields is the use of longitudinally grooved chamber surfaces, which help suppress the escape of secondary electrons from the walls into the central volume of the vacuum chamber. The use of macroscopic grooves increases the vacuum chamber impedance and can adversely impact high intensity beams, particularly if the beam motion has a significant component perpendicular to the direction of the grooves. A possible way to obtain the same “geometric” suppression of the electron cloud with less harmful effect on the beam is to use ion bombardment to produce vacuum chamber surfaces with micron or nanometer scale features. A research effort to prepare such surfaces on standard accelerator vacuum chamber materials is the topic of this proposal. Electron clouds have been observed in a number of accelerators; EC can cause heating of cold vacuum chamber bores in designs with superconducting magnets, degradation in beam quality, and instabilities. Electron and positron rings in which EC has been observed include the Advanced Photon Source [7], the Beijing Electron-Positron Collider [8], CESR [9], the KEK B Factory [10], the PEP-II B Factory [11] and the ANKA light source [12]. Proton and ion rings in which EC has been observed include the CERN Proton Synchrotron [13] and Super Proton Synchrotron [14], the Fermilab Main Injector [15], the Relativistic Heavy Ion Collider [16], and the Spallation Neutron Source accumulator ring [17]. This has led to a significant investment of effort into the development of mitigation techniques [18,19]. Another accelerator application where secondary electron yield (SEY) mitigation is critical is for the performance of RF windows and RF couplers: in this case, electrons are accelerated by the electromagnetic field, impact the surface and produce secondary electrons; if SEY > 1, the density of the electron cloud increases exponentially (“multipacting”). Secondary emission in couplers and windows can be a problem for both normal conducting cavity systems [20] and superconducting cavity systems [21]. Thus we anticipate that the successful development of a new surface mitigation technology will have broader application than just to accelerator vacuum chambers.