Demonstration of ultra-low emittance beams in a kHz laser wakefield accelerator and application to electron diffraction

We present a compact, cost-effective method for measuring the emittance of kHz-repetition-rate laser-wakefield accelerated electron beams using a permanent solenoid. The measured normalized emittance, ${\epsilon }_n=124\mathrm{n}\mathrm{m}\cdot \mathrm{r}\mathrm{a}\mathrm{d}$ ($\simeq 0.04\pi \mathrm{m}\mathrm{m}\cdot \mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$) at $2.7$MeV, is comparable to that of ultra-low emittance radiofrequency guns used for electron diffraction. Leveraging this low emittance, we successfully applied the electron beam to electron diffraction. We demonstrate diffraction images obtained from a single crystal Silicon nano-membrane sample, clearly resolving diffraction peaks across multiple orders.