Storage Ring Absorbers
Photon absorbers are placed on the downstream end of Storage Ring vacuum chambers to protect chamber components (chamber walls, flanges, RF spacers and fingers) from high-intensity x-rays. Additional absorbers are provided to collimate beams going into the front-ends (wedge absorbers), to compensate for larger aperture of the RF cavities (RF flange and taper absorbers), and to protect un-cooled components such as gate valves and ceramic chambers (transition absorbers).
Absorbers at the end of straight vacuum chambers (Sections 1,3 and 5), are named as end absorbers EA1, EA3 and EA5. The absorbers at the end of curved chambers (Sections 2 and 4) are called crotch absorbers C2 and C4 owing to their locations between the legs of circulating particle beam and exit photon beam. Adjacent to crotch absorbers C2 and C4 are the wedge absorbers W2 and W4. The locations of various absorbers in a standard sector, and how they intercept x-ray fans of the bending magnet, is illustrated in this drawing.
Four beam scrapers (one horizontal scraper, two vertical scraper assemblies and one window scraper) are installed (click here for locations) in the Storage Ring to scrape the particle beam. Their blades are designed to survive sustained exposure to bending magnet x-ray fans.
Crotch absorbers in the APS storage ring intercept 2/3 of the radiated power from bending magnets at a maximum normal power density of 145 watts/mm. As shown in Fig. 1, a crotch absorber consists of two distinct features: a central nose region and two side wings. Water channels and internal fins in the Glidcop body are made by the EDM (electric discharge machining) process. A copper plug, brazed in the central round opening, directs water flow to the side wings and into the internal fins of the nose.
RF Taper Absorber
The rf cavities of the APS storage ring have 140-mm circular aperture. A set of four cavities is used in each of the four rf straight sections. Downstream of the last cavity, the aperture is gradually reduced to the elliptical aperture of the vacuum chamber. A surface providing this transition would intercept x-rays from the upstream dipole at a normal power density of 26 watts/mm.
Vertical Beam Scraper
A vertical beam scraper, consisting of upper and lower independently driven blades, is used in the storage ring for beam diagnostics. Figure 5 shows a part of the upper blade subassembly. A rectangular block of tungsten (a high Z material) is used for scraping the electron beam. Because of its poor thermal conductivity, the tungsten block cannot absorb 53 watts/mm of the incident x-ray beam power.
To shield the tungsten block from intense x-rays, a Glidcop absorber is attached on its upstream side. The front face of the absorber intercepts the beam at 25° to reduce the incident power density. External surface fins are used (as in the crotch absorber) to split the beam footprint. Internal fins in the water channels enhance heat transfer and reduce deformations due to water pressure.
A transition absorber built within a double-sided 6"(152 mm) flange allows it to be inserted in tight spaces. Water tubes are brazed to the flange, to an absorber plate with an elliptical aperture, and to an external manifold. The absorber plate, made from Glidcop or OFHC copper depending on the incident beam power, is also brazed to the flange for structural strength.
The elliptical aperture is approximately 4 mm smaller all around than the nominal vacuum chamber aperture. The latter is an ellipse with major and minor radii of 42 mm and 21 mm, respectively. This absorber can, therefore, protect downstream components that fall within its shadow, which extends up to 0.5 meters for the maximum specified beam deviation of 8 mrad. A nearly parabolic profile of the elliptical aperture is selected to spread the incident beam and to reduce rf impedance of the aperture. In addition, a bevel groove is machined in the plate where an rf spring is inserted to provide rf continuity with the mating flange.
PhotosCrotch Absorber - Water Channels
Crotch Absorber - FE Analysis
Vertical Scraper - Blade Components
RF Taper Absorber - Glidcop Body
Beam Dump - Water Channels
Crotch Absorber - EDM Electrodes
Crotch Absorber - Assembly