Generation of ultra-high intensity laser pulses, laser-based particle acceleration
1. Laser-wakefield electron acceleration
An intense laser pulse propagating through a tenuous plasma medium produces - analoguous to a boat on the water - a travelling plasma wave in its wake. This "wakefield" exhibits strong quasistatic longitudinal (and transverse) field components, and electrons with some initial energy can "surf" on the wake and gain energy. The fields can achieve many orders of magnitude higher values than in conventional RF accelerators, affording a strong reduction in accelerator size and significantly higher beams densities. In order to turn that basic principle into a practical accelerator, the follwing stages in the process are studied in detail:
- generation and diagnostics of laser-driven wakefield in plasmas
- injection of electrons into the wakefield and acceleration dynamics
- extraction of accelerated bunches from the plasma
All these stages have to work together reproducibly to generate electron beams with truly novel properties suitable for driving a brilliant X-ray source.
Multi-GeV Electron Acceleration
With the ATLAS multi-PW laser we are aiming to accelerate electrons to energies of multiple GeV.
We use laser-accelerated electron beams to drive a plasma wakefield of their own, potentially providing electron beams with extremely high beam quality.
We develop advanced diagnostic to improve our understanding of the physics of laser wakefield acceleration and the generated electron beams.
2. Laser development
In order to ensure competitiveness of our research in plasma physics, we continue to improve our main tools, the driving laser pulses. Not only a constant increase in the achievable output power qualifies a longterm-successful laser infrastructure, but also the constant improvement of not-so-obvious laser pulse parameters such as temporal contrast, focusability, shot-to-shot repeatability and availability for user experiments.
a) ATLAS-3000 high-power Ti:sapphire laser
ATLAS-3000, the backbone of CALA’s high-intensity laser infrastructure, is a 2.5 PW, 1 Hz Ti:Sa ultrafast laser. It comprises the MPQ’s former ATLAS laser as a frontend, which has been developed by our group over the past 2.5 decades into a 300 TW laser system and complements it with a commercial 90-J amplifier from Thales LAS and a home-built final compressor. The final compressed energy of 70 J and a pulse duration of 27 fs makes it one of the leading Ti:Sa laser systems world-wide, and the world’s highest average-power PW laser system. Its pulses are intended to drive a variety of applications from the acceleration of ions for radiobiology and radiation oncology studies, over ultralow-emittance electronbeams as drivers for medically relevant high-brilliance x-ray sources all the way to studies of nuclear processes and high-field QED experiments. However, in order to drive all these applications on an internationally competitive level, not only raw power is key, but also the pulse-to-pulse stability, beam quality and spatiotemporal contrast of the system have to satisfy extreme demands.
b) PFS-Pro
PFS-Pro is a state of the art Yb:YAG laser system that provides a unique combination of average power, pulse energy and pulse duration and will be the driving laser for new high repetition rate X-ray sources. It tackles the challenge of third generation laser systems to provide pulses with multi-terawatt peak powers and kilohertz repetition rate using established thin-disk technology and a novel nonlinear broadening scheme. PFS-Pro consists of a chirped pulse amplifier that increases the pulse energy of a commercial fiber laser by a factor of more than 30 million. To this end the pulses originating from the fiber oscillator are stretched in time and coupled into a thin-disk regenerative amplifier to finally obtain an output energy of more than 100 mJ with a kHz-scale repetition rate. Two 4 kW 969nm diode laser stacks provide the pump power for the thin-disks making this amplifier one of the strongest amplifiers of its kind. For more details please refer to our publication. The pulses originating from the amplifier system are then recompressed in time by a grating compressor to a pulse duration of about 1.3 ps. To further shorten the pulse duration, a novel nonlinear broadening concept based on multipass cells was adapted for high energy pulses. PFS-pro is optically synchronized to the pump laser of the former PFS system at MPQ, which currently is the world’s highest peak power Yb:YAG laser system. It delivers up to 7J in 700 fs at 3 Hz repetition rate and is used to provide energetic pulses for joint PFS/PFS-pro experiments, such as the plasma-modulated plasma accelerator (P-MoPA) experiment, an joint effort with Oxford university to use commercially available thin-disk laser technology for driving kHz laser-wakefield electron acceleration and to transform the average power of such sources.