Generation and application of ultrahigh intensity laser pulses
Prof. S. Karsch, Experimental Physics, LMU & MPI for Quantum Optics
Our group focuses on the generation of novel particle and photon beams (the latter in cose collaboration with F. Grüner) from laser driven sources. This on one hand involves the study and control of ultrahigh-intensity laser interaction with plasmas, on the other hand drives the improvement of the drive laser pulse parameters through continuous laser development.
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.
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.
3. Tools - lasers
a) ATLAS-100 high-power Ti:sapphire laser
The ATLAS-100 Ti:sapphire laser delivers 25 fs, 2J pulses (80 TW) with 5 Hz repetition rate. Its frontend incorporates a modified commercialy available regenerative amplifier with active spectral shaping, allowing to optimize the pulse bandwidth through the whole amplification process, while keeping prepulses and ASE pedestals at a very low level. Beam smoothing and adaptive optics ensure close-to-optimum spatial coherence of the output beam, and a new plasma mirror setup is installed to further improve the temporal contrast.
b) Petawatt Field Synthesizer (PFS)
The development of the PFS is an attempt to overcome the limits of conventional laser technology with regard to shortest pulse duration, high repetition rate, peak power and temporal contrast. We rely on the technique of optical parametric chirped-pulse amplification with the added benefit of pumping the amplifiers with high-intensity, few-ps pump pulses. This strategy allows the use of very short amplifier crystals leading to an intrinsically broader phase-matching bandwidth, a shorter time-window for contrast-reducing parametric fluorescense, and no heat load (= no distortions) in the main amplifiers. The achievable pulse durations should be on the order of 5 fs, ensuring PW-scale peak powers at moderate pulse energy. While this strategy for the laser amplifiers seems to solve a lot of problems at the same time, the main system risk is the necessary high-repetition rate, ps, high-energy pump laser. Its development focuses on diode pumping technology and Yb-doped laser materials.
Lectures for the Max-Planck Research school:
Generation and application of high-intensity laser pulses
Lecturer: Stefan Karsch, lecture, (2+2 hours/week)
The lecture is organized as a two-semester course, covering laser technology in the winter semester and laser-plasma interaction physics in the summer.
1. (winter) Laser technology:
- description of short light pulses Generation of ultra-short laser pulses
- Laser amplification Nonlinear optics: optical parametric amplification
- Limitations for high-power amplification
- Ultra-high power laser pulses: Chirped pulse amplification
- Characterization of ultra-short pulses
2. (summer) Laser-plasma applications:
- electromagnetic description of untense laser fields
- interaction with single electrons
- interaction with plasmas: collective effects
- laser driven electron acceleration
- laser-driven ion acceleration
- X-ray generation: high harmonics, undulator radiation