An Irresistible Force Meets Its Match
With trillions of hard x-ray photons squeezed into a needle-thin beam, the Linac Coherent Light Source (LCLS) will pack quite a punch. "Its power density is tremendous," says SLAC's Mike Saleski. "It can bore a hole through several inches of copper and iron."
A very complex system is needed to keep the beam from straying. With this in mind, Saleski, John Arthur and other SLAC researchers have devised an advanced containment system for the electron and x-ray beams that will be generated by the LCLS.
Like the linac's current containment system, the LCLS system will use a combination of devices including collimators and stoppers. Both collimators and stoppers are used for beam containment. Stoppers move into the beam in order to block it completely, while collimators are fixed apertures which limit the size of the beam.
Together, these devices constrain the beam within a designated path and prevent radiation from leaking into unprotected areas. But the stoppers and collimators themselves could be damaged by too intense or too prolonged a beam. To prevent this, sensors monitor the beam and automatically shut it off if a problem is detected.
For the LCLS's extremely intense, fast x-ray pulses, new stoppers have to be developed. "Common stopper materials such as lead or copper can't take the heat," Arthur says.
SLAC's answer to this is a beam stopper with two parts: a 10 millimeter layer of a light material to scatter the photons, and a thicker layer of a dense substance such as tungsten to absorb them. Since the beam is very narrow, the stopper will only need to be a couple of inches in diameter. For the scattering material in the first layer, SLAC researchers are considering using boron carbide, which is light, hard, and heat-resistant.
When construction is complete, the linac will feed both the LCLS and the B Factory. The new electron beam containment system has been designed for two injectorsthe original linac injector and the new LCLS injectorto run simultaneously. Any fault in the system will immediately shut down the injector in the beamline experiencing the problem. The various shutoff paths and mechanisms have been carefully designed to minimize the interference between the PEP-II and LCLS programs.
Installation of the new LCLS containment system will begin in August.
Image: This x-ray collimator is one of the many components of the SPEAR3 beam containment system. SLAC researchers are currently designing an even more advanced system for the LCLS. (Click on image for larger version.)