Validity of using the photo-cathode gun for a compact X-ray Free-Electron Laser

Abstract

The X-ray free-electron laser (XFEL) based on self amplified spontaneous emission (SASE) is an unprecedented coherent hard X-ray source [1,2]. However, due to its huge size and construction cost, it would not be easy to have this magnificent scientific facility in your neighborhood. Therefore, there have been studies on why it must be so big (why its electron beam energy must be so high and why its undulator is so long) and how its size can be reduced (how its electron energy can be lowered) without degrading radiation quality [3, 4]. Size reduction of a XFEL machine is basically reduced to how a lower energy electron beam can generate a XFEL without degrading the radiation quality. The reason a high energy electron beam is needed is that a very small electron beam emittance is needed for lasing as the relation = n/γ indicates. Here, is the electron beam emittance, n is the gun emittance, and γ is the relativistic factor. Hence, if n is low enough, a lower electron energy can make the same . A compact XFEL without its quality being degraded obviously means a XFEL with high FEL efficiency. A FEL’s efficiency can be roughly estimated by using the FEL parameter ρ. We can make ρ high enough, even at a low electron beam energy, by using a low emittance gun as explained above. Therefore, the issue is reduced to developing an electron gun with improved gun emittance. The emittance performance of the photo-cathode gun was widely known to be around 1−1.2 μrad, and the design of Linac Coherent Light Source (LCLS) was based on this number [5]. However, in the recent LCLS com-