# Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA
## 5 Challenges for Beam Stability, Control and Diagnostics
### Characterization of Low Charge Beams  
Bunches comprising few or even a single electron can be diagnosed e.g., in beam test facilities. Anyhow transporting low charge beams over long distances and being able to fully characterize them, is not trivial and typically linear accelerators, such as FELs, work with a minimum e-bunch charge of about 10 pC. We shall try to give here an overview of part of the ongoing developments in diagnostics to allow full control and characterization of few pC or even sub-pC bunches with fs or sub-fs duration. * **Beam Position Monitors** (BPMs) are critical for measuring the trajectory of the beam in a non-destructive way in order to monitor e.g., drifts of the working point during the measurement time due e.g., to temperature drift in some parts of the beamline. BPMs placed in a dispersive region also allow online energy measurements for energy feedback systems. Cavity BPMs typically provide the best resolution and are being employed in modern FELs, such as the SwissFEL [81], to allow beam transport down to 10 pC charge or even lower charge. In order to be able to monitor the stability of the trajectory of the low charge ARES beam, high resolution cavity BPMs (<5 \(\upmu\)m at 0.5 pC charge averaging over 50 bunches) are currently under development also at SINBAD [82]. * **Beam Arrival-time Cavities** (BACs) are essential for many reasons: improving feedback systems for minimization of arrival time jitter, online measurement of the \(R_{56}\) in a chicane etc. Recently a novel design for those devices has been proposed, working down to 1 pC bunch charge [83]. This design is currently still under testing. * Low charge **monitors** have been developed e.g., in electron diffraction sources and are being further improved to reach even better resolution values [84; 85; 86]. Figure 13: Displacement and divergence of the beam at the injection point as a function of the laser displacement at the photocathode for WP2 (**upper row**) and WP4 (**lower row**) of the ARES linac. For both WP2 and WP4 Flat-Top transverse laser profile and Gaussian longitudinal laser profile were used. The RMS laser spot-size were 0.18 mm (WP2) and 0.17 mm (WP4), while the laser RMS duration were 3 ps (WP2) and 290 fs (WP4). * Ultra-low charge (few pC to sub-pC) beams can deliver down to **nanometer emittance** values. Methods for measuring single-shot emittance values for ultra-low charge bunches at the RF gun exit have been developed at UCLA [87]. Low emittances have also been measured, again at the RF gun exit, by using the well-known solenoid scan technique [88]. Nanometer slice emittances of a 30 fC beam, a few ps long, have been measured at SwissFEL Injector Test Facility [89] at about 250 MeV by using conventional emittance reconstruction in a multi-screen lattice combined with high resolution transverse profile monitors. * **Bunch length measurement** of fs long electron bunches is challenging and there is much research and development currently going on into this topic. Many different technologies have been proposed to reach the necessary resolution: an X-band RF deflector [90] succeeded in achieving a sub-fs resolution at SLAC. Recently at UCLA the first sub-10 fs high brightness beam has been characterized using the same method [91]. New designs for RF deflectors involving novel technologies, such as plasma Transverse Deflection Cavities [92] and THz streaking devices [93; 94], have also been proposed and are currently in the development phase. * **Longitudinal phase space characterization** is even more challenging. RF deflectors intrinsically perturb the measurement by inducing an energy spread modulation. For high streaking voltages this contribution is dominant [95]. * A new project for the realization of the PolariX TDS (Polarizable X-band Transverse Deflection Structure) is currently taking place thanks to a collaboration between DESY, CERN and PSI [96]. This cavity has the potential of facilitating a **3D reconstruction of the beam charge distribution** with sub-fs resolution [97].