High-throughput, high-brightness, ultrashort 90 keV electrons at 40 kHz

Radiofrequency-compressed keV electron sources for ultrafast electron diffraction (UED) face competing demands: short pulses require low charge, yet weak scatterers demand high flux; high repetition rates enable signal averaging, yet most systems operate $\lesssim$1 kHz with low detection efficiency. Here, we demonstrate a 90 keV DC-RF source operating at 40 kHz with direct electron detection that address these challenges simultaneously. THz streaking retrieves compressed pulse durations of 97 $\pm$ 3 fs (FWHM) at 370 aC and 114 $\pm$ 47 fs (FWHM) at 2.8 fC. Long-term $t_0$ timing drifts, characterized independently both by convolution analysis of compression data and direct THz streaking measurements, lie between 65 - 95 fs (FWHM), among the lowest reported for RF-based systems. At low charge (17 aC), we report an intrinsic pulse duration of 56 fs (FWHM) from comparison of simulations to measured compression data, among the shortest for keV UED at $>$16 aC. Moreover, 2.8 fC bunches, combined with 40 kHz repetition rate and direct detection, produce a detectable normalized throughput that is one (three-to-four) orders of magnitude higher than existing keV (MeV) sources. This enables practical UED studies of weakly scattering samples and processes previously impractical due to low cross-sections and long acquisition times.