To enhance the performance of the next-generation X-ray free electron laser (XFEL), a photocathode RF gun capable of providing the required high-quality electron beam with a small emittance has been a significant research objective. In comparison to the conventional L-band or S-band RF gun, the C-band RF gun features a higher acceleration gradient above 150 MV/m and the ability to generate a small-emittance beam. Low-emittance electron beams are critical for enhancing XFEL coherence and brightness, driving demand for advanced RF gun designs. For a bunch charge of 100 pC, a normalized emittance of less than 0.2 mm·mrad has been expected at the gun exit.

This paper presents the design of an emittance measurement device, which can accurately measure such a small emittance at the C-band RF gun exit to ensure beam quality for XFEL applications.

To achieve the desired accuracy, the primary parameters —slit width, slit thickness, and beamlet-drift length—have been systematically optimized through numerical simulations using Astra and Python based on the single-slit-scan method. Dynamic errors, including motor displacement and imaging resolution, were quantified to ensure measurement reliability.

The evaluations indicate that the measurement error of 95% emittance is less than 5%, achieved by employing a slit width of 5 μm, a slit thickness of 1 mm, and a beamlet-drift length of 0.11 m under dynamic conditions.

This optimized emittance measurement device supports precise beam quality characterization for XFELs, offering potential for further advancements in electron beam diagnostics.