# Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA
## 6 Conclusions  
The production of high-brightness ultra-short electron bunches has great potential. In this article we have given an overview of the aspects guiding the conceptual and technical design of a linear accelerator for external injection in LWFA.  
At the beginning of the paper we have identified and discussed the requirements for the electron-bunch allowing for an ideal match with the acceleration in the plasma chamber. Afterward we focused on the design of the linac and highlighted the parameters which play an essential role in the optimization of the electron bunch compression for the production of high-brightness fs long bunches. As a concrete example of linac design for external injection in LWFA, we have described the design of the SINBAD-ARES linac, which is currently under construction at DESY (Hamburg). The implementation of the conceptual design with some highlights concerning 3D integration solutions have been presented. Finally, in the last section of the article we discussed the technical challenges concerning the characterization and control of such ultra-short and low-charge electron bunches.  
The goal of our discussion is to show that the production of such high-brightness ultra-short electron probes is within reach of present accelerator technology, even though not yet experimentally shown. We believe the SINBAD-ARES linac to be the right tool for this type of research because of the following reasons:  
* Possibility of using different compression methods (velocity bunching, magnetic compression, hybrid scheme) that are limited by different factors.  
Figure 14: The ARES linac and the associated infrastructure in the former DORIS facility.  
* Wide range of experimental adjustment of important parameters for optimization of bunch compression (such as photo-cathode laser, cathode type) and jitter stability (e.g., momentum compaction factor in the dogleg and bunch compressor).
* Precise control of LLRF and temperature stabilization of the RF-gun and linac cavities.
* Integration of low-charge and short-bunch diagnostics along the beamline.  
The SINBAD-ARES linac at DESY aims therefore to produce ultra-short electron bunches in the upcoming years and possibly help the scientific community in understanding the limitations for the optimization of their brightness.  
[PERSON] conceived the SINBAD facility and the hosted ARES accelerator. [PERSON] and [PERSON] designed the concept of the ARES accelerator and contributed to its 3d integration and procurement. U.D. led the design and procurement of the SINBAD infrastructure, including the ARES linac. [PERSON] wrote the paper with significant contribution of [PERSON] and [PERSON].  
The authors would like to thank [PERSON] for the suggestions for improving the text of the article. Moreover the authors would like to thank the colleagues of the DESY Machine Physics (MPY) division and the technical groups for the essential support in the design and procurement of the SINBAD-ARES linac.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} RF & Radio-Frequency \\ DESY & Deutsches Elektronen-Synchrotron \\ SINBAD & Short INnovative Bunches and Accelerators at Desy \\ ARES & Accelerator Research Experiment at SINBAD \\ LWFA & Laser Wake-Field Acceleration \\ CSR & Coherent Synchrotron Radiation \\ C-S Parameters & Courant Snyder Parameters \\ DLA & Dielectric Laser Acceleration \\ TDA & THZ Driven Acceleration \\ RMS & Root Mean Square \\ FWHM & Full Width at Half Maximum \\ LCLS & Linac Coherent Light Source \\ ACHIP & Accelerator on a Chip International Program \\ WP & Working Point \\ PMQ & Permanent Magnet Quadrupole \\ BPM & Beam Position Monitor \\ BAC & Beam Arrival time Cavity \\ TDS & Transverse Deflection Structure \\ LLRF & Low Level RF \\ EMI & Electro-Magnetic Interference \\ FEL & Free Electron Laser \\ SC & Space Charge \\ TWS & Traveling Wave Structure \\ \end{tabular}