Energy, Pitting and Thermal Quenching in ILC Cavities"
Over 20,000, nine-cell SRF cavities will be required
for construction of the International Linear Collider (ILC). Manufacturing
processes need to be improved to provide consistently high accelerating gradient
performance. Current goals require ILC cavities with accelerating gradients of
35MV/m. A large fraction (~1/3) of cavities undergo thermal quenching at an
unacceptably low gradient near 16 MV/m, often due to "pits" which appear in the
enlarged-grain, heat affected zone (HAZ) of e-beam welds along the cavity cell
equators. Pits tend to form at about the same distance (2/3 of the depth from
the weld) into the HAZ. This implies that the (T vs t) thermal cycle is
ILC SRF cavities are produced by deep-drawing of sheet, followed by welding and chemical processing. Deep-drawing of half-cells results in non-uniform strain, which is a maximum at the equator rim. We have used EBSD methods to examine crystal lattice distortion through the equatorial wall thickness of a JLab 1500 MHz cavity. The severe strain is not just a surface effect which can be removed by chemical treatment. Individual grains with severe strain are imbedded several hundred microns from the cavity interior surfaces. The severely distorted metal near the equator is unstable and transforms during heating in the HAZ, through various recovery and recrystallization processes. The highest energy pockets may lead to transformations of a different type, which may be the source of pit formation. Time-dependent thermal profiles can easily be calculated for pit locations with a commercial weld simulation package (such as ESI Groupís Sysweld), and, with microscopic study may provide an explanation of the isolated pit forming mechanism.
However, if we demonstrate that stored strain energy results in HAZ pitting, then the simple solution is to weld the niobium in a stabilized condition by applying a recrystallization anneal after deep drawing.
Thursday, February 26, 2009
p.m. - 4:30 p.m.
CEBAF Center, Room L102/L104
Talk Slides: (Slides)
For more information, please contact Dr. Alex Bogacz or Anne-Marie Valente.