On the cover of HPL: ‘Targets for high-repetition rate facilities: needs, challenges and perspectives’ by I. Prencipe et al

“An important challenge will be the development of a reliable supply chain of high quality targets,” said Prof. Thomas E. Cowan from Institute of Radiation Physics, Helmholtz–Zentrum Dresden–Rossendorf (HZDR), “which is a central need for the realization of the innovative potential of the new facilities.”

In the 1980s, when the first high power laser came online, fundamental research on laser-matter interaction was driven by laser development and relatively simple targets were used (foils and thick samples). Target fabrication research focused mainly on samples for inertial confinement fusion that required complex targets with very well controlled properties. In the last two decades, targets for fundamental research have become more and more complex. Targets can be designed to access specific regions of the phase diagram, for example extremely high pressures or temperatures in dynamic compression and isochoric heating experiments, to investigate electron dynamics in relativistic plasmas and in the target itself, and to better control laser-driven particle and radiation sources.

The variety of possible target configurations is virtually unlimited and ranges from foils to 3D structures and multi-target assemblies. Target properties need to be reproducible and well controlled to avoid undesired effects (for example plasma instabilities or sample pre-heating), therefore metrology plays a crucial role in target preparation. Depending on the experiment, characterization could be required for target properties such as composition, crystalline structure, morphology, density, and surface quality. Target production and characterization can be challenging and often require a combination of multiple state of the art materials science techniques and research and development programs. The community of target experts from universities, research centers and companies has grown considerably in the last 20 years and makes these competences and capabilities available to experimentalists on a collaborative or commercial basis.
At present, the high power laser community is facing a new challenge that will also affect target fabrication and delivery: the commissioning of high power lasers, which are able to operate with repetition rates of 1-10 Hz. Recent and upcoming advanced user facilities in Europe include not only the HED instrument at European XFEL, the ELI infrastructure, the ESRF HPLF beamline, but also national facilities like Gemini (UK), Apollon (France) and CLPU (Spain). A repetition rate of 1-10 Hz corresponds to a requirement of 60-600 targets/hour, i.e. thousands targets per day. Scaling production of solid targets to large numbers while maintaining costs relatively low is a huge challenge for the target community and could become a bottleneck for the success of high repetition rate facilities.

High repetition rate experiments require mass production of identical targets if the goal is the generation of reproducible laser-driven radiation and particle beams or if a very large number of identical data points are required to collect statistics. On the other hand, fast prototyping is required for exploratory campaigns in which targets with slightly different properties can be used for parametric investigation. The main target fabrication laboratories are developing possible solutions to address these needs. Planar multilayer targets can be produced in large sheets with coating techniques, then cut and assembled on target holders. “Providing large quantities of well-characterized multilayer targets for future high-rep-rate experiments presents a big challenge,” said Dr. Dominik Kraus “but is urgently required for investigating properties of matter at extreme pressure and temperature conditions (100s of GPa and 1000s of K) at future research facilities.”
Micro and nano-structured targets can be produced in large numbers on wafer by lithographic techniques commonly used in the semiconductor industry for microelectronic systems. “These types of targets enable new diagnostic insights on different laser-matter interaction processes,” said Dr. Thomas Kluge, “which is why users often need to establish close bonds with micro and nanofabrication groups. For example, we developed a collaboration with two external groups with experience in electron beam lithography, focused ion-beam cutting and scanning electron microscopy (Leibniz Institute of Photonic Technology Jena and the Institute of Ion Beam Physics and Materials Research at the Helmholtz-Zentrum Dresden-Rossendorf) to produce wide ranges of targets: e.g. nano-gratings, micro wires, nanometer coatings, and perfect steps”.

In addition, efficient precision assembly of large numbers of targets will require automated solutions. Robotic capabilities and mounting jigs have been developed to address this need and are currently used for example at General Atomics (USA) and Central Laser Facility (UK).

Moreover, scaling characterization to large numbers of targets is challenging. Automation can only be applied to characterization techniques with large standoff distances and operating at atmospheric pressure. More likely, thorough characterization will be performed only for a small number of targets for each batch to evaluate statistic deviations from the nominal parameters and systematic variations due to the production process.

Irradiation of solid samples at high repetition rates poses other target-related technical challenges. Target irradiation in this regime requires fast positioning and alignment with precision in the µm range. The target holder should not vibrate, become activated or expand due to thermal loads. In addition, precautions used for the protection of positioning stages from return currents and electromagnetic pulses in low repetition rate experiments are expected to be unreliable in the 1-10 Hz operation regime.
Other crucial issues hindering high repetition rate operation are target debris and target fratricide. Ablated material and high kinetic energy shrapnel from the target can coat and damage unshielded optics, debris shields and diagnostics in the interaction chamber. High repetition rate experiments would require frequent replacement of debris shields, a solution both expensive and time consuming. In addition, the shock launched in the target, as well as re-deposition of ablated material and high intensity light in the pulse wings can damage targets cm away from the focal spot.

In conclusion, target supply and irradiation at high repetition rates pose several challenges to the research community. The development of a sustainable target supply chain requires coordination between target fabrication groups and laser facilities, including common strategic planning, facility support to the development and expansion of the current infrastructure, involvement of target experts in experiment design and proposal evaluation. Collaborations and joint research activities would be helpful to develop technical solutions for high repetition rate experiments by avoiding effort duplication and obtain dedicated laser time.

This review article has been published in High Power Laser Science and Engineering, Vol. 5, No. 3, e17, 2017.

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