Analysis of primary stripper foils at SNS by an electron beam foil test stand

Diamond foils are used at the Spallation Neutron Source (SNS) as the primary strippers of hydride ions. A nanocrystalline diamond film, typically 17 × 45 mm with an aerial density of 350 μg/cm², is deposited on a corrugated silicon substrate using plasma-assisted chemical vapor deposition. After growth, 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. An electron beam test facility was constructed to study stripper foil degradation and impact on foil lifetime. The electron beam capabilities include: current up to 5 mA, 0.300 mm² focused spot size, and rastering in the x- and y-directions. A 30 keV and 1.6 mA/mm2 electron beam deposits the same power density on a diamond foil as a 1.4 MW 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.