Analysis of Primary Stripper Foils at the Spallation Neutron Source by an Electron Beam Foil Test Stand

Diamond films are used at the Spallation Neutron Source (SNS) as the primary charge exchange foils (i.e., stripper foils) of the accelerated 1 GeV (Gigaelectron volts) hydride ions. The most common type of film used is a nanocrystalline diamond film, typically 17 mm x 45 mm (millimeter) with an aerial density of 350 μg/cm² (microgram per square centimeter). The diamond film is deposited on a corrugated silicon substrate using plasma-assisted chemical vapor deposition. After the growth of the diamond film, 30 mm of the silicon substrate is etched away, leaving a freestanding diamond foil with a silicon handle that can be inserted into SNS for operation. Each stripper foil is typically used for a month of operation before being replaced by another stripper foil. Since SNS is a user facility, it is very difficult and expensive to experiment with different stripper foil properties that influence the lifetime. Therefore in 2009, a test stand was designed and developed that would research various stripper foil properties and their potential performance in SNS. This test stand consisted of a thirty thousand electron volt (keV) electron gun that would be used to study stripper foil degradation and how this impacts foil lifetime. An electron gun was chosen because of its ability to replicate the thermal load on the stripper foils as well as the sheer size of the electron gun that would allow it to become a tabletop test stand. The electron gun capabilities include: current up to 5 mA (milliampere), 0.300 mm² focused spot size, and rastering in the x- and y-directions. A 30 keV and 1.6 mA/mm² (milliampere per square millimeter) electron beam deposits the same power density on a diamond foil as a 1.4 MW (megawatt) SNS beam. Rastering of the electron beam exposes a similar area of the foil as SNS beams. Experiments were conducted using the foil test stand to study: foil flutter and lifetime; effects of corrugation patterns, aerial densities, foil crystallite size (micro vs. nano), and boron doping; temperature distributions and film emissivity; and conversion rate of nanocrystalline diamond into graphite.