Optical Accelerator: Scaling Laws and Figures of Merit

Abstract

Abstract. Indications that solid-state lasers will reach wall-plug to light efficiencies of 30% or more make a laser-driven vacuum-accelerator increasingly appealing. Since at the wavelength of relevant lasers, dielectrics may sustain significantly higher electric field and transmit power with reduced loss comparing to metals, the basic assumption is that laser accelerator structures will be dielectrics. For structures that have typical dimensions of a few microns, present manufacturing constraints entail planar structures that in turn, require re-evaluation of many of the scaling laws that were developed for azimuthally symmetric structures. Moreover, structures that operate at a wavelength of a few centimeters are machined today with an accuracy of microns. In future it will not be possible to maintain 4-5 orders of magnitude difference between operating wavelength and achievable tolerance. An additional difference is, that contrary to present accelerators where the number of electrons in a micro-bunch is of the order of a 10 10 , in an optical structure this number drops to a few thousands. Consequently, the relative impact of individual electrons may be significantly larger. Acceleration structures with higher degree of symmetry, similar to optical fibers, have also some inherent advantages however thermal gradients as well as heat dissipation may become critical impediments. The impact of all these factors on the performance of a laser accelerator structure needs to be determined. Efficiency, wakes and emittance scaling laws that have been developed recently will be presented. It will be shown that there are some inherent advantages in combining the accelerator structure and the laser cavity in one system.

Design of a multi-channel photonic crystal dielectric laser accelerator

“…Due to the broadband nature of the MIMOSA, we anticipate the long-range wakefield effects to be insignificant. The short-range wakefield effects and beam loading properties are similar to those in dual grating DLAs [1,32,33], and a brief discussion is presented in Appendix D. The space charge effect in MIMOSA is similar to previous single channel DLAs [2,14], which is briefly discussed in Appendix E. Moreover, for applications like medical therapy, the required average number of electrons per micro-bunch per channel is only slightly above 1 (Appendix C). Therefore, both the space charge and wakefield effects are negligible in the MIMOSA operating for medical applications.…”

“…From [1,32], we find that k W = cZ C 4λ 2 β , where Z C is the characteristic impedance of the MIMOSA, and k H = c λ 2 Z H , where Z H is the broadband wakefield impedance. The characteristic impedance is Z C = ξ 2 βλ D Z 0 , where Z 0 = µ 0 / 0 is the impedance of vacuum, ξ is the ratio of the unloaded gradient and the incident electric field, i.e.…”