Plasma Target Design & Characterization

The technique of creating ultra-relativistic electron bunches from wakefields of relativistically intense ultra-short laser pulses or high-current particle beams, which plow through dilute plasma environments, may trigger a revolution in particle-accelerator engineering. By harnessing the extreme electric-potential gradients along the propagation direction of these wakes, this technology permits the generation of GeV-energy electrons on just a centimeter-scale. Our group is dedicated to advance and apply plasma-based particle accelerators and further the understanding of the underlying physics.

Plasma-target fabrication and characterization

Simulated gas-density (top) and velocity (bottom) distribution of molecular hydrogen inside a capillary with two gas inlets. The capillary features a diameter of 250 µm and length of 15 mm. A pressure of 100 mbar is applied to the inlets of 600 µm diameter.

Improving performance and beam quality of plasma-wakefield accelerators with respect to charge, mean energy, energy spread, transverse emittance, pointing stability and shot-to-shot reproducibility requires the design and refinement of specialized and tailored plasma targets.
The most common plasma target in laser-driven wakefield acceleration is a supersonic gas jet. Inside such a jet, the leading edge of the incoming laser beam creates an ionized plasma plume, in which a plasma wake can be excited by the intense laser pulse. This setup, although easy to operate, suffers from notable shortcomings. Usually gas jets are fired by opening solenoid valves, which feature finite opening times and can fluctuate in their mechanical opening motion. This may lead to the creation of shock waves and density fluctuations in the plasma profile, which in turn causes variations in the wake and hence electron-beam parameters. Furthermore, external laser guiding beyond the laser defraction length is difficult to achieve and to control in such a setup.
Hence, our goal is to develop plasma targets operating in a stable equilibrium regime, with geometries that allow for steady-state gas flows and fully tailored gas-density profiles thus providing laser-beam guiding and electron-injection control. This is realized by utilizing density-tailored capillaries with diameters on the order of a few hundred microns and lengths of up to a few centimeters. These capillaries, the gas in- and outlets, and more complex geometries are produced in our target fabrication and characterization laboratory by laser machining microstructures out of robust sapphire crystals. This method offers great flexibility.
For the design and manufacturing of such capillaries it is crucial to understand the gas flow through these complex structures for different parameters like gas type or pressure. This can be modeled with an appropriate solver for the Navier-Stokes fluid equations under the given boundary and initial conditions. We are deploying the software kit OpenFOAM for this task. It is an open source CFD (computational fluid dynamics) toolkit specifically designed for modeling, meshing, solving, post-processing and analyzing fluid-dynamics problems. It contains fluid-dynamics solvers covering a wide range of scenarios such as compressible or incompressible fluids, turbulence and supersonic flows.
The figure depicts a capillary with two gas inlets. Here, the ends of the capillary act as the gas outlets into vacuum. Depending on backing pressure, controllable stationary density profiles will evolve in the central channel after a certain amount of time (usually a few microseconds). The figure shows the stationary gas density (top) and velocity distribution (bottom) in a simulated 2D model. The gas exiting into vacuum is simulated by means of boxes with absorbing boundary conditions at the capillary ends.
As can be seen, the gas density in between the two gas inlets is constant at about 5∙1018 cm-3 and dropping to vacuum pressure towards the capillary outlets. This density can be easily changed by altering the pressure at the inlets, so that constant densities in a range from ~1015 to 1019 cm-3 are possible. At the same time, the gas is stationary in the central part. This is important for stability, since fluctuations in this geometry do not develop due to a lack of turbulent flow. As a consequence, this leads to stable and reproducible conditions for plasma-acceleration applications.

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