Superconducting Helical Undulator with Superimposed Focusing Gradient for High Efficiency Tapered X-Ray FELs

SASE x-ray FELs are limited to <100 GW because of limited electron beam brightness. This limits the amount of photons that can be used to image samples at these wavelengths. Shorter, more intense x-ray pulses will allow more information to be gathered before the sample is destroyed by the x-ray pulse. Advanced FELs are needed to increase the extraction efficiency of radiation from the electron beam beyond 0.1%. Technical Approach A tapered helical undulator promises a large improvement in radiation extraction efficiency as compared to the planar undulators currently used. Further, superimposing a focusing quadrupole field on the undulator will allow longer quadrupoles, which increases the focusing strength and, therefore, decreases the beam size, resulting in a beam brightness increase. The undulator proposed for this Phase II project accomplishes all of these goals to produce an FEL with nearly 10% extraction efficiency. Phase I Results During Phase I, we advanced the undulator design from several parameters on paper to a 3D magnetic model using commercially available superconducting wire. We have also shown that the tolerances on this undulator are, in principle, tractable. Further, we have developed a design for a phase shifter that will fit within the small breaks between undulators. Phase II Plan The Phase II project is to build a section of this undulator, including extensive prototyping of the constituent parts. We will also perform a magnetic measurement for this difficult-to-measure system and show that the fabrication of the device meets the tolerance requirements developed during Phase I. Commercial Applications and Other Benefits The principal users of these magnets are large national laboratories upgrading or building hard x- ray free-electron lasers or synchrotron lightsources. Other potential users are EUV and beyond lithography sources for semiconductor chip manufacture, synchrotron sources world-wide and university laboratories that want to make use of high-energy photons from accelerator sources.