MODIFICATIONS TO IMPROVE ENTRANCE SLIT THERMAL-STABILITY FOR GRASSHOPPER MONOCHROMATORS

As new monochromators are designed for high-flux storage rings, comput er modeling and thermal engineering can be done to process increased h eat loads and achieve mechanical stability. Several older monochromato rs, such as the Mark 2 and Mark 5 Grasshopper monochromators, which we re designed in 1974, have thermal instabilities in their entrance slit mechanisms. The Grasshoppers operating with narrow slits experience c losure of the entrance slit from thermal expansion. In extreme cases, the thermal expansion of the precision components has caused permanent mechanical damage, leaving the slit uncalibrated and/or inoperable. F or the Mark 2 and Mark 5 Grasshopper monochromators at the Synchrotron Radiation Center, the original 440 stainless steel entrance slit jaws were retrofitted with an Invar (low expansion Fe, Ni alloy) slit jaw. To transfer the heat from the critical components, two flexible heat straps of Cu were attached. These changes allow safe operation with a 10 mum entrance slit width where the previous limit was 30 mum. After an initial 2 min equilibration, the slit remains stable to 10%, with 1 00 mA of beam current. Additional improvements in slit thermal stabili ty are planned for a third Grasshopper.