Paper VT-TuP1
Vacuum Pressure Simulation for the Hard X-Ray Insertion Device Beamline 17A at NSLS

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

Built in the 1980’s, the insertion device beamline-17 at the X-ray storage ring of the Brookhaven Lab’s National Synchrotron Light Source (NSLS) has been using superconducting-wiggler generated hard X-rays to facilitate cutting edge research. By sharing the wiggler’s horizontal beam fan, three inline and one adjacent beamlines (17B1-B3 and 17C) have been designed to perform material stress-strain mapping, mineral phase transition under high-pressure, laser heating, and diffraction crystallography. To meet present-day high demand of hard X-rays for nano-structure probing via surface and interface scattering experiments and for large-volume high-press studies, a new beamline dubbed 17A, which also shares the wiggler’s beam fan, has been constructed at immediately downstream to the common monochromator for all the branch beamlines at 17. For the purpose of improving beam quality, user safety and system vacuum, the degraded monochromator was replaced during the 17A construction by a custom-made monolithic unit to accommodate (1) a Si-crystal for the white beam bending (7.6-deg) into the 17A line, (2) a water-cooled white-beam filter followed by a collimated aperture for beam steering, (3) a Hevi-Met alloy of tungsten for bremsstrahlung shielding, (4) an ASME-certified burst disk for high pressure release, and (5) a sputter ion pump for outgas removal and high vacuum upkeep. Flanged to the beam exit port at the SS monochromator chamber is a round SS spool piece and a copper-brazed Be-window, installed to separate the beamline-17A vacuum from its upstream beamline-17 vacuum. At 1.4-meter downstream of the Si-crystal in monochromator is a 6-way cross, set to install a phosphor screen and a CCTV for the beam image viewing and profile recording. Along the beam path of 2.6-meter from the 6-port optical enclosure, a 200 L/s sputter ion pump is hooked and sealed beneath the round beam pipe to remove desorbed gases from photon-stimulated scattering amid two Be-windows. For beam size confinement, residual gas analysis, synchrotron radiation blockage and shock wave monitoring, a tungsten slit, a tee-port, a tantalum-plated safety shutter and a Be-window are respectively installed at 0.7-, 1.5-, 1.8- and 2.5-meter off the ion pump. Prediction of pressure profile along the 17A was performed using the Monte-Carlo based Molflow code for gas conductance estimate and the finite-difference based Vaccalc code for pressure distribution calculations. Details of beamline vacuum versus pre-cleaned and pre-baked assemblies encompassing the segmented beampipe will be presented. (Work performed under auspices of the US DOE, under contract DE-AC02-98CH10886)