2.1 Laser Plasma Dynamics and Particle Acceleration
Project coordinator(s): M. Schnürer

Tracing dynamics of laser-induced fields on ultrathin foils using complementary imaging with streak deflectometry

We present a detailed study of the electric and magnetic fields, which are created on plasma vacuum interfaces as a result of highly intense laser-matter interactions. For the field generation ultrathin polymer foils (30–50 nm) were irradiated with high intensity femtosecond (~ 100 EW/cm^2 and picosecond (100 PW/cm^2) laser pulses with ultrahigh contrast. To determine the temporal evolution and the spatial distribution of these fields the proton streak deflectometry method has been developed further and applied in two different imaging configurations. It enabled us to gather complementary information about the investigated field structure, in particular about the influence of different field components (parallel and normal to the target surface) and the impact of a moving ion front. A modification and combination of two known models allowed for an extensive and accurate reproduction of the experimental results in both imaging configurations. The controlled change of the laser pulse duration from 50 femtoseconds to 2.7 picoseconds led to a transition of the dominating force acting on the probing proton beam at the rear side of the polymer foil. The applied proton deflectometry method allowed for an unambiguous determination of the polarity of the detected magnetic field at the rear side of the ultrathin foil.

Results have been obtained within Transregio18 and Laserlab CHARPAC programms.

F. Abicht et al. , Phys. Rev. Accel. Beams 19, 091302 (2016)

Coulomb driven energy boost in laser – heavy ion acceleration

While the laser plasma driven acceleration of light ions (H,C,O) was intensively investigated during the last years, little was reported on the acceleration of heavier ions. The challenge of heavy ion acceleration results directly from some basic principles: Ions are accelerated proportional to their charge to mass ratio Z/A leading to higher velocity (~MeV/u) for the lighter material. This ratio is limited as even ultra-strong laser pulses do not fully ionize the heavy material. So far the acceleration of the heavy material was demonstrated with high laser energy (>10J) and demanding target preparation. In our latest research we overcame these restrictions, enabling for the first time, the acceleration of heavy ions towards > 1 MeV/u in presence of the contamination layer by comparable little laser energy on target (~1.3J). The combination of the ultra-thin heavy material targets and our laser parameters - 70 TW @ 35 fs at ultra-high temporal laser contrast, made the difference. Thehigh degree of ionization of the heavy material (Z>40+ for Gold ions) especially at the boundary of the target – which acts as a enormous repelling space charge – leading to acceleration by a so called Coloumb explosion for the heavy ions.

Results have been obtained within Transregio18 and Laserlab CHARPAC programms.

J. Bränzel et al., Phys. Rev. Lett. 114, 124801(2015)

Tracing ultrafast dynamics with proton probing

We investigated how proton probing of localized and strong fields in a longitudinal geometry (that is probe beam propagation along the field vector of interest) can trace field dynamics at ultrafast timescale. During probing at a timescale of about hundred femtoseconds the kinetic energy of the probing protons is redistributed and this redistribution is measured and used to trace the fast evolving field. Analysis of the findings need appropriate analytical models being justified with numerical simulation. Our results show possibilities for beam manipulation in cascaded acceleration schemes.

Results have been obtained within Transregio18 and Laserlab CHARPAC collaboration.

F. Abicht et al., Appl. Phys. Lett. 105, 034101 (2014)

The beat in laser accelerated ion beams

It is comprehensible that the cyclic and ponderomotively driven movement of electrons in the laser field imprints as well onto the fields they produce. When thinking about such a modulation there are at least two circumstances which have to be considered concerning ion acceleration. Already the lightest ion – the proton – has considerable mass and nonlinear changes in its velocity during a few femtoseconds need fields with strong modulation depth. Therefore it was assumed that the ion acceleration is mainly determined by the temporal field envelope and reflects averaging over a timescale of the laser pulse duration. We have found for the first time that at high intensities also protons will be influenced by this oscillating laser field showing faint density modulations in their velocity distribution when accelerated by the intense laser pulse in a plasma. This phenomenon becomes detectable if specific prerequisites are fulfilled. Concerning our present data one has to realise an ASE background contrast of about 10 orders. In order to measure the effect with a mass-spectrometer (Thomson type) a reasonable resolution is needed. Also an additional spatial resolution in order to discriminate oscillations with relatively high frequency against noise is crucial. The observed effect demonstrates the very low longitudinal emittance of the finally accelerated ion bunches and allows studying source dynamics in TNSA experiments with a temporal time scale of the laser cycle.

Results have been obtained in Transregio18 collaboration and Laserlab CHARPAC support .

M. Schnürer et al., Phys. of Plasmas 20, 113105 (2013)

Stable laser ion acceleration - Ions sailing in light winds

Laser driven ion acceleration focuses on a mechanism called “light sail” where electrons from a laser-induced plasma are pushed forward in front of the laser pulse through ponderomotive forces, pulling ions behind them. For the light sail phenomenon to occur in a laser irradiated thin foil its thickness and density must be matched with the laser intensity and pulse duration. In accordance with theory we found the optimum pulse parameters at 35 fs duration and 5*10^19 W/cm^2 intensity when irradiating a 25 nm thin CH-foil. But in case of enormous pressure (here the light pressure) instabilities arise in the dynamical process of acceleration. As predicted in simulation we demonstrated with specific laser and target parameter in our experiment that a stable ion interface up to the end of acceleration can be established. This finding is important because the light sail mechanism becomes increasingly dominant with more powerful lasers as they are presently constructed in various laboratories.

Results have been obtained in MBI-LMU/MPQ Transregio18 collaboration.

S. Steinke et al., Phys. Rev. ST Accel. Beams 16, 011303 (2013)

Negative ions from a laser accelerator

Negative ions, fragile atomic systems, are important in accelerator-based physics. Their generation from a laser driven plasma seems highly unlikely because of the huge electromagnetic fields inside the plasma. Nevertheless, we detected a high number of negative oxygen ions O^1- with MeV energies and, more recently C^1- and H^1- ions after irradiation of a spray target, a MBI-patent, by intense (5×10^19 W/cm^2 ) and ultrashort (35 fs) laser pulses. Negative ions are created when fast positive ions, laser accelerated inside a cylindrical interaction zone, capture electrons from neutral target atoms in the surrounding spray cloud.Thereby, the quality (emittance) of the laser accelerated ions remains nearly preserved.The mechanism implies also the existence of a large number of neutral atoms with MeV energies which could be proven in a recent (2011/12) LASERLAB-EUROPE access experiment.
S.Ter-Avetisyan et al., Appl.Phys.Lett.99 (2011) 051501

P.S. This project has been successfully continued in 2013 and fast neutral atom as well as fast negative ion generation has been studied in detail.


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. cf. S. Steinke et al., LPB 28(2010)215