Project description
Objective O1
Investigation of gamma radiation effects on the rare earth doped fluoride crystals
In order to establish the optimal growth conditions for (Ca,Ba)F2:(YbF3, ErF3) both numerical simulations and experimental investigations of the growth setup will be performed at UVT (activity 1.1). Finding the optimal growth conditions is important in order to obtain good quality crystals, without structural defects, such as gaseous inclusions, high dislocation density and non-uniform dopant distribution. Optimum growth conditions are defined by: the position of the crucible at the start of the growth process, the electrical power on the resistance in order to obtain the best temperature gradient and the optimum growth rate. In order to establish the better growth conditions (different for the two types of crystals) we have to growth some MeF2 (Me=Ca, Ba) crystals using various growth conditions [UVT activity 1.3]. After this step, using a Bridgman crystal growth equipment, pure and various YbF3 and ErF3 concentration doped (Ca,Ba)F2 crystals will be grown [UVT activities 1.3, 1.6 and 1.9].
Ionizing radiation can modify the physical and chemical properties of the materials. For each type of material, a certain amount of radiation energy is needed to obtain the desired effect; the useful dose being estimated through research. The most common radiation used in radiation hardness testing of materials is gamma. Gamma radiation has many advantages over other radiation types such as high penetrability (good uniformity of dose distribution in material) and minimal rise in material temperature during irradiation. A very important parameter is the dose uniformity ratio (DUR) defined as the ratio of the maximum and minimum dose in the sample. This ratio increases with the density of the sample and its size. If the research objective is to correlate radiation effect in the sample to the dose, DUR should be close to unity (commonly lower than 1.05). We will establish gamma irradiation set-ups by dose mapping of gamma radiation field in different geometries in order to obtain low DURs for different dose rates (IFIN-HH activity 1.2). Because we expect saturation effects of gamma irradiation on the optical and dielectric properties of the crystals, different irradiation doses will be used. In order to establish the useful dose range, we will irradiate pure (Activity 1.4) and doped crystals (IFIN-HH activity 1.7, 1.10) at two screening doses, 1 and 10 kGy, using a dose rate of 4.5 kGy/h. Based on the measurements after irradiation, dose range will be expanded so that the dose response functions (effects vs. dose) will cover both linear and saturation regimes. For the evaluation of dose rate effects, irradiations of pure and doped crystals (IFIN-HH activity 1.4, 1.7, 1.10) will be redone using the same dose range, but at a different dose rate, of 0.5 kGy/h.
In order to investigate the effects of gamma irradiation on the optical and dielectric properties of the crystals different irradiation doses will be used. Using the dielectric relaxation process, information about charge compensating defect type can be obtained (the F- ion, in various positions together with the Yb3+ or Er3+ ion is an electric dipole, considering as a defect). Using the information obtained from the optical absorption measurement the type of charge compensating defects will be determined. In addition, the dielectric properties of these types of crystals will be studied in order to see the effect of the gamma irradiation on the charge compensating defects formation in these crystals. 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, e1, can be calculated from the measured capacitance. The e2 will be then calculated from the measured loss tangent D= e2/ e1. 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 Ttan? versus 1000/T, where T is the temperature and tan? is the tangent of the loss angle ?. 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 [UVT activities 1.5, 1.8 and 1.11].
Objective O2
Optimization of the gamma flux production obtained via laser interaction through numerical simulations
In order to assess the effectiveness of the PIC codes, a comparison between the two chosen codes will be done, namely between the PICLS code and the EPOCH code after a previous stage of adapting the EPOCH code on the UVT BlueGene/P supercomputer (activity 2.1). A further scaling analysis will be carried out for the EPOCH code on two machines, the IBM BlueGene/P at UVT and the IBM BladeCenter at IFIN-HH. (activity 2.2). After having conducted the code optimizations for the specific machines, several numerical studies will be performed in order to fine the effect of several key parameters, such as the type of gas (CO2, N2, He, O2 and Ar), laser spot size, plasma density etc., on the accelerated electron bunch (IFIN-HH activities 2.3, 2.4). For each type of gas studied a probing of the optimal pulse parameters will be done and then implemented in the numerical modeling. The pulse spot size will be varied from 50 µm to 150 µm to determine if a compromise can be achieved between the number of accelerated particles and their energy. For the goals proposed in this project, the pulse power will be set at 1PW. Furthermore, some tests will be performed to determine if a mixture of some of the gases produces improved results when comparing to the single gas results (IFIN-HH activity 2.5). Each gas or gas mixture target, respectively, will have densities in accordance with values than can be obtained experimentally. In addition, a numerical study on the influence over the gamma production of the gas jet geometry will be conducted. The target geometrical density profile will be written to match the profiles that can be achieved experimentally with gas jets.
Objective O3
Optimization of large-flux proton beams generated via laser interaction through numerical simulations
Numerical studies with the goal of optimizing the particle beam resulted from laser-matter interaction are proposed. The first set of optimization will be conducted on gas and foam-like targets (UVT activity 3.1), which have several key advantages, such as: high rep-rate, low debris relaxed laser contrast operation. The next step will consist in a study intended for the optimization of particle beams produced from microstructured targets, which had promising results in the past [Nature 439 (2006) 445] (UVT activity 3.2). To conclude, once the optimal parameters have been identified, a further optimization numerical study will focus on the laser parameters that will be available with the introduction of the ELI-NP facility (UVT activity 3.3).
Objective O4
Investigation of proton irradiation effect on solar cells operation
This objective is defined for acquiring technical and scientific skills in using the ELI-NP facilities for accelerated testing the solar cells performance in space-like environment. Numerical experiments will have a crucial role in designing experimental procedures at ELI-NP, in minimizing the number of irradiation experiments and, therefore, in reducing the testing costs. The proposed activities target to the development of a numerical tool for evaluating the 3J-solar cell performance in a well defined environment (protons energy, flux, cell temperature, level of solar irradiance).
The following methodology is proposed for developing a numerical tool devoted to the study of the 3J-solar cell operating in space-like environment. It starts from the Schockley equation of the illuminated solar cells and includes specific equations to describe the influence of protons irradiation on the PV conversion efficiency. Firstly, the modeling process will be focused on developing numerical algorithms for describing in terms of Shockley equation the current-voltage characteristics of 1J-solar cell operating in different irradiation conditions and at different temperatures. In order to understanding the physics of irradiated solar cells, a special attention will be paid for numerical modeling spectral dependence of the solar cells parameters as function of the protons energy and fluence. The model will be tested and validated against experimental data. For this, current-voltage characteristics of twelve samples from different crystalline solar cells will be measured before and after irradiation with different fluxes of protons. The samples will be irradiated at TANDEM linear accelerator from IFIN-HH (activities 4.1, 4.3, 4.5). The main attributes of the irradiation experiments are: ions energy will be varied between 30 keV and 6 MeV; at least six levels of the proton fluence will be considered: from 109 cm-2 to 1014 cm-2. Secondly, the models developed for 1J-solar cell will be extended to 3J-solar cells. The designing of the irradiation experiments of 3J-solar cell will be based on the experience accumulated in the first stage, aiming to minimize their number.
The activities planned to be conducted at UVT are briefly described next. In the framework of E13/2014-RO-CERN-Programme, at the PV Laboratory of UVT an experimental stand for measuring the solar cells I-V characteristics under AM0 spectrum was developed. Activity 4.2 was tailored in order to extend the capability of the experimental setup to measure the spectral characteristics of the solar cell. An USB fiberized microspectrometer will be purchased and integrated into the experimental setup. The acquisition data system and the LabView application will be adapted for spectral measurements. Electronic circuits for conditioning the signal will be designed and implemented. An application for processing data will be written. Using the experimental data, a mathematical model for irradiated 1J-solar cell will be devised (UVT activity 4.4). The model will be implemented in a MathCAD numerical application. Samples from commercial crystalline 1J-solar cells will be prepared. The samples will be fully characterized before and after irradiation with protons. “Fully characterization” means: (1) measurement of I-V characteristics under AM0 and AM1.5G spectra, at different irradiance levels (200 to 1360 Wm-2) and at different temperatures (20 to 100 Celsius); (2) Measurements of quantum efficiency; (3) Measurements of the spectral dependence of the following parameters: short circuit current, open circuit voltage, serial and parallel resistances, photocurrent, dark current, diode ideality factor, fill factor. The comparison of numerical and experimental results will provide a feedback control loop for tuning the model. Using a similar procedure, a numerical model for irradiated 3J-solar cell will be devised (UVT activity 4.6) Activity 4.7 is dedicated to calibrate the 3J-solar cell model, in order to accurately reproduce the measured parameters of commercial spatial solar cells provided by manufactures in product datasheet. The calibrated model will be implemented in a computer code with a friendly interface. The tool will be run in order to explore ways to extend the cell lifetime and to design experiments for characterizing solar cells in space-like conditions at ELI-NP.