February 2013: Cornell ERL electron injector surpasses long standing record
This is a photo of a GaAs photocathode used for generating electrons. The beam from a high-power fiber laser is directed at one of the four silver spots on the cathode. Thru the photoelectric effect, electrons are produced at the cathode, and then they are accelerated to produce a beam.
The goal of the Energy Recovery Linac (ERL) project at Cornell is to create a new type of continuous-duty x-ray source, and doing so requires making high average power, ultra-low emittance electron beams and accelerating and recovering their energy in a superconducting linear accelerator. One of the challenges is building an electron injector to provide the necessary high quality beam. The National Science Foundation has funded a prototyping project to demonstrate that such an injector can be built.
Many technologies need to come together for such a high-power accelerator to work, including lasers, high voltage, superconductivity, cathodes (see figure), materials, vacuum science, and electronics, to name a few. Also, experts from many fields of engineering and science are needed to design, build and successfully test this machine. For example, an electron gun creates a beam of electrons by shining a high-power laser onto a photocathode target and the ejected photoelectrons are accelerated to nearly the speed of light using superconducting radio-frequency technology. The electrons are created using the photoelectric effect: electrons are emitted from many materials as they absorb energetic light from a laser.
A full-scale ERL x-ray light source requires an average current of 100 mA at 5 GeV beam energy. While many accelerators operate at 5 GeV, none that use photocathode sources come close to providing 100 mA. The previous world record for high average current from a photocathode source was 32 mA (by Boeing Corp. in 1993). Recently, the Cornell prototype electron injector produced a beam with 65 mA, over twice the previous record, and well on the way to the desired 100 mA level. Because no one has ever created an ERL light source operating with high-energy particle beams, the Cornell results bolster confidence in the ERL technology approach. The recent results make it clear that the original design goals of the ERL are within reach.
This major accomplishment in the field of accelerator physics is a result of a 5-year effort on the part of CLASSE (Cornell Laboratory for Accelerator Sciences and Education), which was led by Dr. Dunham with active participation from accelerator physics faculty Bazarov, Hoffstaetter, and Liepe, many research scientists and other faculty, PhD students, and support personnel in Wilson and Newman labs. Results were recently reported in the journal Applied Physics Letters 102, 034105 (2013) (http://apl.aip.org/resource/1/applab/v102/i3/p034105_s1