For more than a decade, Cornell scientists have been developing a groundbreaking new synchrotron X-ray technology called the Energy Recovery Linac (ERL). A new X-ray laser on the West Coast is taking advantage of their developments.
Now in early planning stages, Stanford University’s Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory (formerly Stanford Linear Accelerator Center) is upgrading its Department of Energy free electron laser X-ray. Much of the fundamental technology that will allow them to make the upgrade – the brightness of the X-ray beams and the acceleration system with superconducting surfaces – will be using work done by Cornell accelerator physicists.
“Our work on the ERL has been very important for the proposed accelerator at SLAC,” said Georg Hoffstaetter, professor of physics and principal investigator of the Cornell ERL project.
Stanford plans an X-ray laser in which a highly focused beam of light arrives in strobelike pulses to capture high-resolution images of dynamic processes, including atomic-resolution images of biological molecules like proteins, cell membranes and viruses.
Because the X-rays need to pulse a million times per second, a new way of powering the accelerator and making the X-ray beams had to be designed. In working on the ERL technology, which is closely related to the science of free electron lasers, Cornell researchers devised a way to power a linear accelerator continuously, as opposed to pulsing it on and off.
Led by Hoffstaetter, Cornell scientists have pioneered the production of very narrow electron beams with high currents and the use of superconducting radio frequency (SRF) cavities that are fitted along the accelerator beam pipe. Both technologies are essential for the new X-ray laser: The electron beam must be exceedingly narrow to produce a coherent X-ray beam, and the electrical power to accelerate the beam is kept low by using superconducting surfaces to produce the fields that accelerate the electrons.
“The only way for a linear accelerator to be on all the time is for current in the accelerating structures to flow without loss, and that is what superconductivity achieves,” Hoffstaetter said.
The Stanford X-ray laser will require a continuous beam of electrons to focus on a very narrow spot. Cornell ERL researchers have developed a source of electrons that has unprecedented brightness, meaning it puts the largest possible electron current onto the smallest possible spot.
The ERL that has been designed at Cornell will have an even larger electron current than the upgrade to the LCLS. Accelerating this current would require far more electrical power, if not for the principle of energy recovery that Cornell researchers are using. The energy from the spent beam, after its use for X-ray experiments, is reused to accelerate a new beam – hence, “recovery.” The process requires the accelerator to be on all the time, a problem solved by the low energy loss in the superconducting surfaces of SRF cavities.
With construction expected to start this year at SLAC, the X-ray laser upgrade is projected to begin operating in 2019. Cornell and several other U.S. laboratories are collaborating with SLAC to build the new X-ray laser.