Miscellaneous publications

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In modern laser-based ion acceleration systems, the field distribution of the focused laser beam at the position of the target strongly influences the overall characteristics of the resulting ion beam. To obtain an unidirectional and quasi mono-energetic ion beam, a flat-top field distribution of the focused laser beam is optimal. This can only be achieved, by using a beam-profiling system that reshapes the incident laser beam into an Airy-shaped field distribution in the far field. Here, we present an extensive design study of such a beam-profiling system based on two free-form mirrors. In order to realize the rings of zero intensity, corresponding to the roots of the Airy-function, strong curvature peaks on the first mirror are necessary. Additionally, the alternating phase in between these rings can only be achieved with grooves on the second mirror. These aspects actually raise the question, if the used purely geometric optical modeling approach is still valid. Therefore, our design study is entirely accompanied with wave-optical simulations to identify influences of diffraction within the beam profiling system. We find that especially the grooves on the second mirror are mandatory, not only to ensure the alternating phase, but also to realize the roots of zero intensity of the Airy-function. On the other hand, these grooves cause diffraction effects in the beam-profiling system that slightly degrade the at-top focal field. These influences are in the range of a few percent and cannot be further avoided.

The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model, where light particles (photons) must have a minimum energy (frequency) to ionize atoms. The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength - the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field. Isolated attosecond pulses recently enabled studies of the dynamics of tunneling ionization of atoms in gases. On page 1348 of this issue, Schultze et al. experimentally show that atoms in a solid are also excited via the tunneling process.

Hochleistungslaser ermöglichen es inzwischen, relativistische Elektronenpulse mit bemerkenswerten Eigenschaften zu erzeugen. Neben der äußerst kurzen Beschleunigungslänge sind vor allem die kleine Quellgröße und auch die kurze Pulsdauer interessant. Mit diesen Elektronenpulsen lässt sich zudem elektromagnetische Sekundärstrahlung im Kiloelektronenvolt-Bereich erzeugen. Mit dieser Röntgenstrahlung würden auch Universitäten relativ kompakte Röntgenquellen zur Verfügung stehen, mit denen sich Effekte beobachten lassen, die auf äußerst kurzen räumlichen und zeitlichen Skalen ablaufen. Momentan ist diese Art von Forschung nur an großen, konventionellen Synchrotron-Beschleunigern möglich.