TY - CPAPER AU - Li, R.K. TI - Ultra-Low Charge, Ultra-High Brightness Frontiers of Photoinjectors: Challenges and Perspectives J2 - Proc. of FEL2015 AB - Photoinjectors deliver high brightness electron beams that are essential to the success of FELs. Higher beam brightness can boost the performances of existing facilities and enable new capabilities of future ones. Ultralow charge (a few pC and lower) is the choice to reach higher brightness thanks to the favorable scaling, as well as longitudinally coherent, single-spike FELs. This parameter space is particularly relevant for future high rep-rate machines. Despite the ultralow total charge, the charge density is still high and collective effects need to be studied in-depth. The extreme emittance, bunch length, spot size, and charge also pose challenges on the diagnosis of these beams. In this talk, we will discuss recent progress on the generation, manipulation, and diagnosis of ultralow charge beams, including e.g. shaping the space charge dominated beams to maximize the phase space density, new techniques to characterize the nm-rad-level emittance, and new advances in photocathode physics and rf structures. Many of the above progress are motivated by the recent development of ultrafast electron diffraction and microscopy using photoinjectors. Understanding and controlling the high brightness beams in accelerators with the high stability and precision typical in the electron microscopy community, constitutes an R&D frontier on electron beams and sources for the coming decades. PB - JACoW CY - Geneva, Switzerland ER -