Developing a 50 MeV LPA-based Injector at ATHENA for a Compact Storage Ring

The laser-driven generation of relativistic electron beams in plasma and their acceleration to high energies with GV/m-gradients has been successfully demonstrated. Now, it is time to focus on the application of laser-plasma accelerated (LPA) beams. The "Accelerator Technology HElmholtz iNfrAstructure" (ATHENA) of the Helmholtz Association fosters innovative particle accelerators and high-power laser technology. As part of the ATHENAe pillar several different applications driven by LPAs are to be developed, such as a compact FEL, medical imaging and the first realization of LPA-beam injection into a storage ring. The latter endeavour is conducted in close collaboration between Deutsches Elektronen-Synchrotron (DESY), Karlsruhe Institute of Technology (KIT) and Helmholtz Institute Jena (HIJ). In the cSTART project at KIT, a compact storage ring optimized for short bunches and suitable to accept LPA-based electron bunches is in preparation. In this conference contribution we will introduce the 50 MeV LPA-based injector and give an overview about the project goals. The key parameters of the plasma injector will be presented. Finally, the current status of the project will be summarized

Status Report of the 50 MeV LPA-Based Injector at ATHENA for a Compact Storage Ring

Laser-based plasma accelerators (LPA) have successfully demonstrated their capability to generate high-energy electron beams with intrinsically short bunch lengths and high peak currents at a setup with a small footprint. These properties make them attractive drivers for a broad range of different applications including injectors for rf-driven, ring-based light sources. In close collaboration the Deutsches Elektronen-Synchrotron (DESY), the Karlsruhe Institute of Technology (KIT) and the Helmholtz Institute Jena aim to develop a 50 MeV plasma injector and demonstrate the injection into a compact storage ring. This storage ring will be built within the project cSTART at KIT. As part of the ATHENA (Accelerator Technology HElmholtz iNfrAstructure) project, DESY will design, setup and operate a 50 MeV plasma injector prototype for this endeavor. This contribution gives a status update of the 50 MeV LPA-based injector and presents a first layout of the prototype design at DESY in Hamburg

Experimental Results on an OPCPA Seed System for a Laser-Plasma Acceleration Drive Laser

Laser-plasma acceleration[1], among others[2,3],promises to be a powerful technology for driving future compact light sources. LUX, which is operated by University of Hamburg,is such a laser-plasma accelerator. It is driven by the 200 TW Ti:Sapphire double chirped pulse amplification (CPA)laser system ANGUS, designed for long-term stability.The laser, as well as the electron beam line,are integrated in a control system, enabling continuous operation for many hours. This has been demonstrated with day-long measurement runs where electron beams with energies above 600 MeV and spontaneous undulator radiation well below 9 nm were achieved [4]. From the long runs large data sets can be collected to provide reliable statistics. To extend the long-term operation abilities of the laser system, and therefore the runtime of the measurement campaigns, we are currently developing a new front end for the ANGUS laser system.With the design approach of the ANGUS laser system in mind we focus on the long-term stability in the development of the new front-end. The target is to reach 35 μJ pulse energy with 20 fs transform limit at 1 kHz close to 800 nm central wavelength. For these parameters and application optical parametric chirped pulse amplification (OPCPA) is a suitable candidate. It has been demonstrated that such systems can be operated for more than a week with stable generation of high contrast ultrafast pulses reaching down to the few cycle regime [5].A measure to get to long-term stability is to use an industrial grade Yb:KGW femto second laser as a common source for white light generation(WLG)and pump for the subsequent OPCPA stages. As only a fraction of the pulse energy emitted by the drive laser is needed for WLG in a bulk YAG crystal to provide the seed, complicated synchronization schemes between two lasers can be avoided.The major part of the pulse energy is converted in a second harmonic generation (SHG)stage to be used as a pump for the OPCPA stages. With headroom to spare the SHG stages can be optimized for stable operation instead of maximum conversion efficiency.Furthermore, without the necessity to reach the few cycle regime, the OPCPA stages can be designed for pulse energy stability instead of maximum amplification bandwidth. Also the fact that no additional laser amplifier stages are needed in the pump arm allows for a compact setup that should be less sensitive on environmental influences. Our approach is unique in the sense, that we focus on maximum stability in parameters, rather than optimizing the setup for minimum pulse lengths or efficiency.Currently we are optimizing the SHG and the OPCPA stages for the stability goals with a 3D+1 split step code.In parallel we are setting up the first prototype OPCPA stage.We will report on simulation as well as experimental results regarding the SHG pump and the firstOPCPA stage with focus on the achievable stability in pulse energy and spectral parameters.[1] W.P. Leemanns et al. “GeV electron beams from a centimetre-scale accelerator”, in Nature Physics 2, 696–699 (2006)[2] A-L. Calendron et al. “Laser system design for table-top X-ray light source”,in High Power Laser Science and Engineering 6: e12 (2018)[3] E. A. Peralta et al.“Demonstration of Electron Acceleration in a Laser-Driven Dielectric Micro-Structure”,Nature 503, 7474 (2013)[4] N. Delbos et al."LUX --A Laser-Plasma Driven Undulator Beamline", Nucl. Instr. Meth. Phys. Res. A 909, 318 (2018)[5]R. Budriūnas et al."53 W average power CEP-stabilized OPCPA system delivering 5.5 TW few cycle pulses at 1 kHz repetition rate”, Opt. Express 25, 5797-5806 (2017

Onset of the optical damage in CaF2 optics caused by deep-UV lasers

The exterior sides of calcium fluoride (CaF2) outcoupling mirrors are damaged by ArF laser light irradiation with high pulse-energy densities (80 mJ/cm2). The damage is generated by a partial alteration of the CaF2 substrate to calcite. The CaF2 decomposition is driven by photochemical processes due to the UV light and the presence of water vapor and is supported by elevated temperatures within the laser beam transmitting area. Small filaments act as starting points for the decomposition process, where kerogenous carbon and calcite can occur

Long-Term Stabilization of Temporal and Spectral Drifts of a Burst-Mode OPCPA System

We demonstrate a stabilization system for temporal and spectral drifts of an OPCPA pump-probe laser at the FLASH soft-x-ray FEL-facility. We achieve drifts of 5.7fs rms and 3.2nm rms, respectively over two days

Stability Demands for Laser-systems in Plasma Acceleration

The LUX laser-plasma-accelerator [1], built in close collaboration of the University of Hamburg and DESY, isdesigned to provide plasma electron beams with enhanced stability as a driver for future compact light sources.After significant in-house development of the driving 200 TW ANGUS laser system, the laser has reached anoperational stability, that enables us to repeatedly demonstrate day-long operation of the laser-plasma acceleratorwith several 10k consecutive electron beams and high availability. The electron beam quality is sufficient to drivea miniature undulator, generating synchrotron-type undulator radiation at wavelengths well below 9 nm.We report on experimental results of the long-term stability of the ANGUS laser system with sub 2% energystability over a full day, and pulse duration stability on the 1% level, that have been enabled through activefeedback loops at various stages within the amplification chain. Coupled with a comprehensive diagnostics andcontrol system, that contributes to operational necessities such as short daily startup times and optimizedmaintenance procedures, the system has enabled us to gather statistical data of the accelerator performance, whichis a crucial step for the accelerator community.These day-long experimental campaigns have indicated, that the stability of the accelerator performance islimited by both fast fluctuations and slow drifts, of the spectral and spatial properties of the laser pulses. Theselimitations are inherent to the complex amplification chains and system architecture of most PW class lasersystems, with long beam-paths and numerous optical components, that each contribute to the degradation of laserstability [2]. To further enhance the capabilities of laser-based plasma accelerators it is therefore crucial toovercome these inherent challenges of the technology and to investigate new approaches combining high gain andthe passive stability. We are therefore investigating OPCPA-based pulses as seeds for our main amplification chainin order to generate ultrafast laser pulses tailored for stable long-term plasma accelerator operation as the primaryfocus.In this contribution we will discuss the strict stability demands of laser systems for laser-plasma particleacceleration with the example of the performance of the 200TW ANGUS laser system. Besides reviewing thecurrent capabilities of the laser and their influence on the accelerator, we will suggest new approaches forovercoming the inherent limitations of Ti:Sapphire technology, with a focus on the potential capabilities of whitelight seeded optical parametric amplifiers driven by highly reliable industrial femtosecond pump lasers.[1] N. Delbos, C. Werle, I. Dornmair, T. Eichner, L. Hübner, S. Jalas, S. W. Jolly, M. Kirchen, V. Leroux, P. Messner, M. Schnepp, M. Trunk, P.A. Walker, P. Winkler, A. R. Maier, "LUX -- A Laser-Plasma Driven Undulator Beamline", Nucl. Instr. Meth. Phys. Res. A 909, 318 (2018)[2] V. Leroux, S. Jolly, M. Schnepp, T. Eichner, S. Jalas, M. Kirchen, P. Messner, C. Werle, P. Winkler, and A. Maier, "Wavefront degradationof a 200 TW laser from heat-induced deformation of in-vacuum compressor gratings", Opt. Express 26, 13061-13071 (2018