Design and production of innovative targets to enhance the laser-driven acceleration is a cornerstone of the ENSURE project. In particular, an innovative target configuration, known as Double Layer Target (DLT) has been studied in details. DLTs are made of two components: a thin substrate on top of which there is a low density material, called nanofoam. In order to enhance the acceleration process, these layers must meet some specific requirements: the substrate should be as uniform as possible over an area of few squared centimeters, while its thickness should be of the order of one micron (one millionth of a meter, roughly a hundredth the width of a hair); the nanofoam, which is the key component of a DLT, should have a density (i.e. mass per unit volume) around 6 mg/cm3, i.e. a few times the density of air. Materials with such extraordinary properties can be produced in dedicated laboratories by the ENSURE team at Politecnico di Milano. First-hand production of the target materials not only comes as a necessity, but also as an opportunity: since the laser-target interaction depends on the material characteristics down to the nanometer (one billionth of a meter!) scale, acting directly on the material synthesis offers additional degrees of freedom to control and improve the acceleration process. To this end, two new laboratories – around 100 squared meters in renovated rooms at Politecnico di Milano, Campus Leonardo – have been established thanks to the ENSURE funding: the “fs-PLD Lab” and the “HiPIMS Lab”. Each laboratory has at its core a specific “deposition system”, i.e. an equipment to produce the desired materials, namely a femtosecond Pulsed Laser Deposition (fs-PLD) and a High Pulse Intensity Magnetron Sputtering (HiPIMS). These two labs work together to obtain a DLT target: the main task of the fs-PLD lab is to produce the low-density foam layer, while the HiPIMS is mostly used for the substrate production.
The new fs-PLD deposition system
A fs-PLD consists of a laser source and an interaction chamber. Laser pulses are sent into the interaction chamber where they ablate (i.e. remove material from) a target made of the elements one wants in the foam layer, for instance carbon. The ablated material undergoes a series of modifications that can be controlled by playing with many process parameters, such as the gas pressure inside the interaction chamber, and is then deposited (hence the name of the technique) as a low-density nanofoam layer unto a substrate. What makes this setup unique is the combination of the laser pulse characteristics and the technical features of the interaction chamber. Indeed, the laser source generates extremely short and intense pulses: time duration is 100 femtosecond (i.e. one tenth of a trillionth of a second) a hundred thousand times shorter than those used in more conventional nanosecond PLDs, while energy per pulse is relatively high (5 mJ, roughly the energy of a table tennis ball moving at 1 meter per second). In addition, the interaction chamber is equipped with a number of peculiar features, such as the possibility of hosting multiple targets or rapidly switching to a configuration where the laser pulses can be sent also toward the substrate. This option can be used to further tailor the foam properties after the deposition has taken place.
The new HiPIMS deposition system
The HiPIMS technique is the last born within the wide family of sputtering deposition systems. It has been developed at the beginning of the century to produce tough and smooth coatings for research and industrial application. The working principle of a sputter deposition is relatively simple: suitable projectiles (such as noble gas atoms) are fired towards a target made of the elements to be deposited. The impact with the projectile atoms cause the erosion, or sputtering, of the target material, which is then collected in the form of a compact, thin film. In order to be accelerated by electric fields, the projectiles atoms are made electrically charged through ionization, thus effectively creating what is known as a plasma. The peculiar characteristic of the HiPIMS technique lies in the strength of the accelerating electric fields, much stronger than in conventional magnetron sputtering systems, which results in a higher kinetic energy for the projectiles and -as a consequence- for the sputtered material. This feature, in turn, leads to more dense and compact materials with excellent mechanical properties, also in the form of an extremely thin (hundreds of nanometers, roughly the size of a flu virus) free standing film: this feature is especially important for the production of high-performance DLT.