|Personal Website Prof. Dr. Florian Grüner|
Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research (biology, chemistry, physics), however, due to their kilometer-scale sizes only a few of these sources exist worldwide. In contrast, ultra-compact, laser-driven plasma-wakefield accelerators provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray facilities to university-laboratory scale.
We have been pioneering this field of „table-top FELs“ ever since laser-plasma accelerators have been emerging. In 2009 we have demonstrated the world’s first laser-driven soft X-ray undulator (LUX) source based on a laser-plasma accelerator (Fuchs et al., Nature Physics, 2009).
Currently, we are setting up the LAOLA lab for realizing a LUX-source reaching the water-window (around 4 nm wavelength) and we will soon also start a setup aiming at an FEL-demonstrator driven by electrons from a laser-plasma accelerator.
Fig. 1: Scheme of LUX-setup. A high-power laser pulse excites a plasma-wakefield inside a cm-scale capillary, accelerating plasma electrons onto energies of few hundreds of MeV, which emit X-ray undulator radiation, with which we will run application experiments. M. Fuchs, et al., Nature Physics (2009)
Fig. 2: How does it all work? The effect of a laser (with relativistic intensities) upon plasma electrons can be compared to the one of a board being moved through a „sea of balloons“. Both, the (so–called ponderomotive force of a) laser/board push electrons/balloons to the side, leaving an electron-/balloon-free cavity behind. In the plasma case this charge separation generates huge electric field gradients strong enough to accelerate electrons onto GeV-scale energies.
In order to reach the goal of a „table-top FEL“ we also set up an experiment at DESY’s REGAE-facility (hier link zu https://regae.desy.de/) to study external injection of REGAE-accelerated electrons into a laser-driven plasma-wakefield. This studies will help understanding the dynamics and growth of the emittance of the electrons – a key to fine-tune the FEL-experiment.
Apart from driving FEL- or LUX-sources, electrons from a laser-plasma accelerator can also drive a Thomson-source, where a part of the driver-laser pulse is scattered off the laser-accelerated electrons to emit undulator-like radiation in the hard X-ray range above the gold K-edge. This allows for novel imaging modalities. Together with the group of Prof. Dr. Christoph Hoeschen our group has initiated a research project in Hamburg dedicated on gold-nanoparticle assisted X-ray fluorescence imaging (XFI). We also closely cooperate with LMU-groups as well as the Center for Translation of Cancer Nanomedicines & Intraoperative Imaging Program and Weill Medical College of Cornell University.
The key goal of our research is to pave the way for future clinical application of XFI as well as a next-generation of pharmacokinetics. Both applications aim at the in-vivo detection of smallest amounts of gold-nanoparticles functionalized with bio-markers. The basic challenge here is to find a way to effectively reduce the intrinsic background in XFI, especially for objects of human size.
We have already performed pilot studies at DESY’s PETRA-III synchrotron aiming at the detection of unprecedented small local amounts of gold-nanoclusters.
Fig. 3: PETRA-III synchrotron experiment for XFI. The brilliant, mono-chromatic pencil beam from a synchrotron is well-suited for localizing gold-nanoclusters by scanning various probes.
We have recently published a paper in >Scientific Reports reporting our experimental as well as conceptual breakthrough for drastically reducing the XFI-background – a key for the future translation of XFI into clinical application.
Recently, within the framework of the Cluster of Excellence "Quantum Universe" we have also initiated a program on astrophysical experiments in the lab. Hence, our research group does not only set up laser-plasma accelerator experiments for driving LUX-/Thomson- and FEL-light sources, but also to study the propagation of the such accelerated electron beams through a plasma in a dedicated, second plasma stage downstream. We have recently published our work on a new variant of a so-called PIC (Particle-in-Cell) code, which allows us designing and interpreting such experiments. Our aim here is to vary relevant parameters such as the electron beam vs ambient plasma densities for studying the scalability of lab results onto astrophysical scales.
Fig. 4: PIC-simulation of relativistic electrons propagating through a plasma (courtesy of S. Jalas). This simulation snapshot, gained with our self-developed PIC-code, shows the “first stage” of the astrophysical lab experiments, in which the electrons are accelerated. This beam would then propage through a homogenous plasma in a second stage downstream, without the laser.