Project description
Objective O1
Design and execution of crystal irradiation experiments
In a laser-plasma acceleration experiment, after the laser interaction with the thin target, a secondary source of particles is formed consisting of an electron cloud that accelerates ions (especially protons), which exhibit a heterogeneous energy spectrum that varies with the solid angle. As was previously discussed, this particular type of radiation poses great interest in condensed matter irradiation experiments. Therefore, such an irradiation experiment will be designed for the ELI-NP Technical Design Report. At the CETAL facility at INFLPR (which is a partner in ELI-NP project) such experiments will be performed for testing purposes and acquiring experience for the future ELI-NP irradiation experiments. Therefore, in order to design a laser-plasma accelerated particle bunch irradiation of materials experiment, one PhD student will visit (for 3 months) the CETAL facility at INFLPR in order to determine the ideal positioning of the samples that are to be irradiated and to design the experimental set-up. For the successful irradiation of our proposed materials the development of an experimental set-up is paramount. To be able to do this, an understanding of the particle beam spectrum is very important, in order to determine which are the best possible experimental conditions to use. In this respect, there are several options for possible detection methods, such as micro-channel plate (MCP) detectors and impressionable photographic films. Once the experimental set-up is put in place, measurements on the energy spectrum of the particle bunches will be performed for different solid angles.
Depending on the distance between the secondary source position with respect to the sample to be irradiated, the spot where the particle beam hits the target differs in size, but also in energy distribution on the surface, because the particles from the center of the bunch have higher energies than the peripheral ones. Different particle energy ranges and distributions will be obtained and characterized for use in irradiation experiments. Afterwards, the prepared crystal samples and solar cells will be irradiated for different particle energy distributions. A part of the budget will be designed for these irradiation experiments as costs of services performed by third parties.
Objective O2
Investigation of radiation effects on the rare earth doped fluoride crystals
Using a Bridgman crystal growth equipment, various YbF3 concentration doped (Ca,Ba)F2 crystals will be grown. In order to study the morphology of the etch pits and to estimate the dislocations density the chemical etching method will be used. Freshly cleaved surfaces will be the subject to chemical etching with HCl solution for few minutes at a fixed temperature using a thermostat. The cleavage surfaces of crystals will be examined with an optical microscope equipped with an image capture and image processing software, type Buehler, OMNIMET. This system allows to determine area of etch pits and dislocation density. The dielectric measurements will be performed using a RLC Meter ZM 2355, over the temperature range of 150–300 K, at audio-frequencies in (1–100) kHz domain. The real part of the dielectric constant, ε1,can be calculated from the measured capacitance. The ε2 will be then calculated from the measured loss tangent D= ε2/ ε1. The activation energy and the reciprocal frequency factor will be calculated in order to characterize the observed relaxations. The number of dipoles that contribute to the dielectric relaxation peak can be calculated from the dielectric spectra by plotting Ttanθ versus 1000/T, where T is the temperature and tanθ is the tangent of the loss angle θ. For the spectroscopic investigations the samples will be cleaved or cut from the obtained crystals and optically polished (the samples thickness will be between 2-3mm). After that, room temperature optical absorption spectra will be recorded by Shimadzu 1650PC and FTIR Nexux 470 spectrophotometer. These types of characterization will be performed both before and after irradiation. Concerning the optical properties of the fluoride crystals, as a result of irradiation, modification of doping ionization states may occur, or color centers may form.
Objective O3
Investigation of radiation effects on the semiconductor crystals
The novel aspect for the laser–plasma particle accelerated irradiation of the silicon crystals and of the solar cells used in modern space applications, will be the potential to assess the effects of the cosmic-like radiation for a whole range of controlled conditions. Ten space solar cells of different technology (silicon – suited for low-power (100 W) and short mission duration (3-5 years), triple-junction (GaInP/GaAs/Ge, GaInP/GaAs/Ge) – the highest efficiency solar cells currently available for space use, thin-film (Cu2S/CdS) – with lowest mass) will be exposed to a ionizing radiation flux generated by ELI. The cells will be fully characterized before exposure and after exposure in the same environmental conditions. Measurements will watch to both microscopic (structural defects) by optical microscopy and macroscopic effects (cells efficiency, fill factor).
Directionally solidified multicrystalline silicon is widely used in terrestrial photovoltaic applications because of its lower cost, but is currently not used in space applications. However, because of a richer distribution of crystalline defects and impurities, the multicrystalline silicon would pose a more interesting candidate - from the material science point of view - for investigation under exposure to cosmic-like radiation, due to the multitude of physical phenomena that may occur. Multicrystalline silicon samples under different growth condition (2 types of crucible material and 3 pulling rates) will be produced in the Crystal Growth Laboratory using a Bridgman equipment. The major types of radiation damage phenomena in solids, which are of interest for solar cells, are ionization and atomic displacement of the silicon matrix. This may result in changes of conductivity. Changes in lifetime of minority carriers, which is limited by recombination on energy levels in the band gap of silicon introduced by some impurities, may also occur through their interaction with the particle radiation. Silicon atoms dangling bonds at crystalline extended defects may be satisfied by protons or ions from the particle accelerated radiation. In order to study the impact of the irradiation on the electrical properties of silicon, resistivity and lifetime of minority carriers spectroscopy measurements will be performed on the grown multicrystalline silicon samples.
Objective O4
Numerical modeling of the laser accelerated proton and electron radiation through interaction with a thin film
Currently, there are several simulation tools for running computationally intensive electromagnetic and electrostatic problems. The code of these tools uses the particle-in-cell (PIC) method to track individual particles in a continuous phase space while simultaneously computing moments of the distribution such as densities and currents on Eulerian (stationary) mesh points. Having these computational tools, an assessment has to be made on which one is the most suitable for our purposes taking into consideration existent infrastructure, applicability on the specific conditions dictated by the laser beam driven proton and electron radiation and the software efficiency in these specific conditions. Based on the fact that the computational power needed to run codes using the PIC method is substantial, the efficiency of the software will be evaluated on one of the most powerful supercomputers in Romania (IBM BlueGene/P, 11.7 Tflops), installed at West University of Timisoara. Using the parallel version of the software on the supercomputer mentioned above the computation time should be substantially reduced. Having already assessed and adapted the most efficient and compatible computational tool, a first step will be to run previously studied experimental conditions which have already been published in literature. The comparison of our numerical simulations with the existent ones is a good validation of the selected software and also a good way to engage our team into new research fields available at the upcoming ELI- NP facility. Once having gauged the reliability of the numerical computations performed, these will be applied on the experimental conditions (Objective O1) in order to compare and contrast the numerical results with the ones provided by the existent detection equipments at the CETAL site at INFLPR-Magurele of the laser-target interaction.