Relativistic plasma dynamics
	Short wavelength generation - x-ray laser - XRL

Relativistic plasma dynamics

Light pressure – the route to efficient laser ion acceleration

One of the recent challenges in light-matter interaction consists in the unidirectional acceleration of charged particles by laser light. This can occur through a variety of laser-induced plasma phenomena or, more directly, through transfer of the unidirectional momentum of a propagating laser field, the so-called light pressure.
Utilization of the light pressure requires rather ambitious parameters of laser intensity and temporal pulse shape. In return, theory predicts rather favorable energy conversion efficiencies and narrow ion energy distributions which both are a prerequisites for many applications.
Scientists from the Max Born Institute (MBI) Berlin and from the Max Planck Institute for Quantum Optics (MPQ) Garching and LMU Munich were able to demonstrate this principle in recent experiments (Phys. Rev. Lett. 103 (24), 245003(2009)). The key in the process is to favor the momentum exchange between the laser photons and the target while suppressing unwanted electron heating. Two technologies are essential for this purpose: Ultra-high temporal contrast laser pulses (delivered by the High-Field-Laser at MBI-Berlin) on the one hand and ultimate thin diamond like carbon foils (produced at MPQ/LMU) on the other. The results demonstrate efficient ion beam generation while simultaneously reducing the kinetic energy spread of the ions.
See also Informationsdienst Wissenschaft (MBI) , Informationsdienst Wissenschaft (MPQ/LMU-MAP)

Proton imaging reveals insight to ion beam formation from mass-limited targets

If intense laser pulses interact with mass-limited targets the electron dynamics is confined. This acts back to the temporal and spatial development of strong fields determine ion acceleration at the target surface. Already in previous work (S. Ter Avetisyan et al. PRL 2006 ) we found that laser irradiated water (or heavy water) droplets can generate a quasi-monoenergetic proton (or deuteron) beam. From simulation we could argue that this besides of additional premises might be connected with a specially asymmetric field structure which favors directional emission. Using proton imaging now we found evidence that an advantageous field structure for a directional ion beam can be even set with a micro-sphere which is a versatile target system. The big advantage of such a system is the MHz repetition rate of droplet generation with a liquid jet. But evaporating liquids seem to have also a drawback which we try to puzzle out. In case of evaporating targets the presence of an ambient plasma counteracts the energy transfer between laser and ion beam. The experimental findings are backed with an analytical model and computer simulation.

Results have been obtained in MBI-HHU Transregio18 collaboration.
T.Sokollik et al., Phys. Rev. Lett. 103, 135003 (2009)

Coherent electron dynamics pushes laser driven ion acceleration towards new energy records

Up to now laser-plasma phenomena have been created with solid state targets being thicker than the corresponding skin layer of the laser radiation. The reason in doing so was two-fold: The temporal rise time of the laser intensity starting from the level of ionization up to the peak of the pulse was reasonably long so that heating and disintegration of the skin layer determines the target characteristic. This favors energy transport processes of stochastic nature. Furthermore robust solid state ensembles being extended over micrometers to match the focal spot dimensions of an optical laser but having a thickness of only a few nanometer are hardly available. This situation has changed with the advent of ultra-high temporal contrast laser pulses in combination with a second key technology -- the realization of ultimate thin diamond like carbon foils. We found that the plasma evolution of such a laser excited system is now mainly determined by the coherent electron dynamics in the laser field which drives the kinematics of the whole system. This sets an ultimate implication to the acceleration of ions which is a secondary process due to the charge polarization between the electrons and ions.

Our experimental results show that we can coherently expel a whole electron ensemble from a defined ion bulk which is the whole target. This sets the maximum limit in charge polarization and thus results in a maximum achievable acceleration field. It coincides with a theoretically predicted balance between the normalized laser vector potential and the normalized target thickness. A maximum energy of 13 MeV for protons and 71MeV for carbon ions is observed with a conversion efficiency of 10%.

These are all new record values for a laser driver of only 0.7 J pulse energy.
Results have been obtained in MBI-LMU-MPQ Transregio18 collaboration.

 

Short wavelength generation - X-ray lasers (XRL)

Seeding of x-ray lasers - exploring the crux for application

Nowadays the majority of x-ray ( or EUV) lasers use the amplified spontaneous emission principle where a single trip in a high gain plasma medium drives the signal into saturation. Because this process starts from noise the reproducibility is limited and as a second important point the pulse duration is mainly in a range of several pico seconds. In order to improve the output characteristics of a x-ray laser seeding has been proposed and already some theoretical work and only a few experiments have been carried out world-wide. But this principle, which sounds simple, is rather challenging as the x-ray laser itself. A coherent seed can be realized with a High Harmonic Laser source. In order to keep the experimental effort straightforward stability and efficiency have to be high which is already an own research project. Furthermore also theoretical simulation is ambitious because the x-ray laser transition is narrow which provides a narrow line width but makes shorter pulses difficult to obtain. On the other hand the High Harmonic seed signal has a quite broad frequency distribution.

In order to explore these all tradeoffs a MBI - APRI (Advanced Photon ic Research Institute in South-Korea) collaboration has been launched. First experiments in 2009 were aimed to apply an efficient two-color high harmonic source and seeded x-ray laser emission has been obtained. These very ambitious experiments and accompanying simulation will show to which extend seeding can improve the performance of a x-ray laser in repetitive application experiments. This is especially important for the new x-ray laser station under development at MBI.

Results have been obtained in MBI-APRI BMBF-project collaboration.