Reinforcement of relativistic electron pulses by direct acceleration in the laser field

A targeted and direct acceleration of electrons in extremely strong laser fields would make it possible to realize novel, ultra-compact accelerators. To achieve this goal, however, the intrinsic motion of electrons in the alternating electromagnetic field of a laser pulse must be rectified and decoupled from the field. This fundamental challenge is being intensively researched worldwide. In experiments at the Max Born Institute, researchers have now succeeded in realizing a concept of direct laser acceleration and theoretically understanding it in detail. This concept opens up the possibility of producing relativistic and ultra-short electron pulses on extremely small acceleration distances of less than one millimeter. Such electrons and x-ray sources based thereon have a variety of applications in spectroscopy and structural analysis, in medical-biological research and in nanotechnology.

As electrons in very strong laser fields can be accelerated to relativistic energies, a fundamental question of the physics of light matter interacts. Although a free, stationary electron is driven by the electric and magnetic fields of a laser pulse to oscillations at extremely high speeds, but with the decay of the light field, the electron comes to rest and a net energy transfer by the direct acceleration in a laser field does not take place. However, this fundamental principle, which is often discussed in physics examinations, is bound to certain prerequisites of the spatial extent and intensity of the laser pulse. But if these conditions are violated, e.g. By focusing the laser or the presence of strong electrostatic fields in a plasma, electrons can actually be accelerated by the interaction with a laser pulse.

Many research groups around the world are currently working on the question of how fast electrons can be extracted from an extremely strong laser field and how short electron pulses of high charge density can be generated with ultra-short laser pulses.

Fig. 1a: Schematic representation of the realized principle of direct electron acceleration in the laser field and its implementation in the experiment.

In a light field of "relativistic" intensity (I> 10¹⁸ W / cm²), the electrons oscillate at speeds close to the speed of light and their kinetic energy ranges from megaelectronvolt (MeV) to gigaelelectronvolt (GeV at I> 10²² W / cm²). These strong fields of light can be achieved by focusing very short laser pulses with high pulse energies on room areas of a few micrometers. The resulting spatial intensity distribution already allows an acceleration of electrons to high kinetic energies. The principle is known as "ponderomotive" acceleration and represents an elementary process in the interaction of strong light fields and matter. Various theoretical studies have predicted that, in addition, the number and energy of the electrons can be significantly increased by an additional direct acceleration in the laser field but only if the electron-light interaction is deliberately interrupted. These considerations were the starting point for the experiments of Julia Braenzel and her colleagues at the Max Born Institute.

In the experiments at the MBI, the electrons were decoupled from the light pulse at a specific point in time by means of a laser light-impermeable separator film. Thus it could be shown that this can increase the number of electrons with high speeds. With a 70 TW Ti: sapphire laser (2 J @ 35 fs) and 30 to 100 nm thin target films of PVF plastic, <10 9 electrons could be generated with kinetic energies in the MeV range, due to the ponderomotive force in the propagation direction of the Lasers were emitted. During the actual interaction, the film is in an almost fully ionized state, that is, it has become a plasma.

Fig. 1b: Electrons detected in the laser propagation direction of a single (F1) and double-foil target (F1F2), in which the second foil acts as a separator. The plastic films used have a layer thickness of F1 = 35 nm and F2 = 85 nm. Ne indicates the integrated number of electrons for the entire detection range (0.2-7.5 MeV) with respect to the spectrometer aperture.

For sufficiently low film thicknesses below 100 nm, a part of the laser light can pass through this plasma and thereby the electrons already emitted behind the film are overtaken by the transmitted light pulse. Quasi intrinsically synchronized, the "slow" electrons are injected into the transmitted, still relativistic laser field (<8 x 10¹⁸W / cm2). Now, if a second thin film is placed as a separator at a suitable distance behind the first film, an amplification of the electron signal can be found in a very specific energy range. Figure 1a schematically shows the time course in the experiment and Figure 1b compares the resulting electron distribution obtained with and without additional separator foil. The separator film is impermeable to the transmitted laser light but permeable to the fast electrons, therefore the electrons can be decoupled from the light field. The point in time at which the interaction of the electrons with the transmitted laser pulse is interrupted is predetermined by the distance between the films.

The experiments carried out in the group of Matthias Schnürer show that the amplification of the electron signal becomes maximal at a certain distance and completely disappears for very large distances. The theoretical concept of leaving electrons at high kinetic energies by timely decoupling from the laser pulse after acceleration has been confirmed by numerous series of measurements and numerical simulations. The experiments and the analytical model show that slow electrons with kinetic energies below 100 keV are accelerated to an order of magnitude higher energy by the presence of the second foil. This effect leads to a compression of the electrons in a narrow energy range. Unlike the so-called "keel wave acceleration", which has already demonstrated the generation of GeV electrons with a laser-driven plasma wave, the direct laser acceleration can be scaled to very high laser intensities and plasma densities. In addition to fundamental physical insights, future applications in the field of laser-based sources of relativistic electrons are therefore based on this concept.

Original publication

Amplification of relativistic electron bunches by acceleration in laser fields

J. Braenzel, A. A. Andreev, F. Abicht, L. Ehrentraut,, K. Platonov, M. Schnürer

Physical Review Letters 118 (2017) 014801/1-5

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