Corrections for systematic errors in slit-profiler transverse phase space measurements

TL;DR

This work tackles systematic errors in slit-screen transverse phase-space measurements used to determine beam emittance in photo injectors. It develops analytic corrections for slit size, slit thickness, imaging resolution, PSF, profiler step size, residual space-charge, and noise cuts, validated by PITZ measurements and ASTRA simulations. The corrections bring measured emittance values into close agreement with true emittance (about 2% accuracy) for representative 250 pC and 1 nC beams, and reveal how these corrections alter the inferred optimum gun-solenoid settings. The resulting framework enhances the reliability of emittance measurements, improving beamline tuning, simulations benchmarking, and overall injector performance.

Abstract

In photo injectors, the transverse emittance is one of the key measures of beam quality as it defines the possible performance of the whole facility. As such it is important to measure the emittance in photo injectors and ensure the accuracy of these measurements. While there are many different methods of measuring the emittance, this paper focuses on quantifying the systematic errors present in transverse phase space measurements taken with slit-profiler methods, i.e. scanning a narrow slit over the beam and continually measuring the passed beamlets' divergence with a downstream profiler. The measurement errors include effects of the slit size, beamlet imaging, and residual space charge. While these effects are generally small, they can have significant impact on the measured emittance when the 2D phase space is strongly coupled. The systematic effects studied and corrections are demonstrated with simulations and measurements from the Photo Injector Test facility at DESY in Zeuthen (PITZ) using a slit-screen emittance scanner.

Figures (13)

• Figure 1: Slit-screen phase space measurement scheme. The slit allows only a small beamlet to pass through which is measured downstream. Shown here is a vertical slit which is used to measure the horizontal phase space Raffael_thesis.
• Figure 2: Section of PITZ beamline used for emittance measurements. The gun is surrounded by a main and bucking solenoid for focusing. Two dipoles are used for energy measurements before and after the booster. The diagonal lines represent screen stations and the screen directly after the second dipole is used for imaging the beamlets. The slits for emittance measurements are inserted at 'EMSY' (Emittance Measurement SYstem).
• Figure 3: (a) The beamlet phase space can be modeled with a rectangle, however, a trapezoid more accurately models the slope of the phase space ellipse across the slit. (b) The resulting beamlet phase spaces at the profiler for a rectangular and trapezoidal beamlet. (c) The projection of the rectangular beamlet is symmetric, but the trapezoidal beamlet has an asymmetric projection that depends on the slopes of the edges. (d) The resulting reconstructed phase space using the trapezoidal beamlet for a uniform distribution shows a clear coupling due to the asymmetry caused by the slit opening.
• Figure 4: Simulated emittance using a 50µ m and 100µ m slit. After correction with Eqs. \ref{['eqn:slit_profile_effect']}-\ref{['eqn:slit_coupling_effect']} the measurements match the true emittance (50µ m corrected results not shown). 50µ m slits are commonly used at PITZ for emittance measurements.
• Figure 5: Top: Phase space acceptance of a slit in terms of the slit opening and thickness. The thickness can cut particles when the divergences are large. Bottom: example of simulated and analytic transmission of a 1n C beam with a 10µ m wide, 1m m thick slit. The simulated results show a moving average to reduce noise.