LASER SYSTEM FOR POSITRONIUM EXCITATION TO RYDBERG LEVELS FOR AEGIS EXPERIMENT
Creators: name: VILLA, FABIO
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: This work was developed in the framework of the Aegis (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment. Aegis goals are to perform experimental fundamental research on antihydrogen. Its first goal is a direct measurement of gravity acceleration g of the Earth on antimatter with a precision of 1%. This experiment is being developing at Antiproton Decelerator (AD) at CERN. In order to measure g on antihydrogen, we measure the free fall of a bunch of atoms for some decimeters of propagation using a Moir� deflectometer. The antihydrogen is produced from antiprotons, accumulated from the beam of AD, via a charge-exchange reaction with positronium excited to Rydberg levels. The cross section of this reaction is a function of the fourth power of positronium principal quantum number. The positronium atoms are generated in a porous silica target from a bunch of positrons generated by a 22Na radioactive source and accumulated in a Surko trap. The positronium is then excited to Rydberg levels by the laser system we developed in this thesis. We proposed an incoherent two step laser excitation: the first step from ground level to n=3 at 205 nm, the second from n=3 to a Rydberg level between n=20 and 30, with a wavelength of 1650-1700 nm. We present the research and development of this laser system, including the optical transport line from the laser table to the positronium chamber inside Aegis cryostat, and a detailed theoretical study on the first step of the excitation. Both wavelengths are produced from a Nd:YAG source using nonlinear optics. The first transition is realized using a sum frequency crystal made of beta - Barium Borate (BBO) that add the fourth harmonic of the source (266 nm) to a parametrically generated pulse at 894 nm in a noncollinear configuration. The infrared pulse is produced by an Optical Parametric Generator (OPG) based on a Periodically Poled Potassium Titanyl Phosphate (PPKTP) crystal that generates pulses at 894 nm (signal) and 1312 nm (idler, for energy conservation) from a 532 nm second harmonic pump. The infrared pulse is then amplified in a couple of BBO crystals using 532 nm pump. We can generate about 300 microJ of 205 nm energy per pulse, while we calculated that the saturation level of the transition is about at 10 microJ per pulse. The second transition is realized using an OPG pumped by the fundamental Nd:YAG harmonic at 1064 nm, producing pulses at 1650-1700 nm (signal) and 2844-2996 nm (idler). We tested a Periodically Poled Lithium Niobate (PPLN) and a PPKTP, finding the latter more suitable for its greater signal energy production capability. The generated signal is then amplified with a couple of bulk KTP crystals up to 3 mJ per pulse, pumping them with 1064 nm fundamental harmonic. The estimated saturation energy for this second transition is 0.17 mJ per pulse. We studied and tested an optical transport line from the laser table to Aegis apparatus. This transport line must carry the ultraviolet and infrared pulses for the two transitions from standard pressure and temperature to ultrahigh vacuum (10^-13 mbar) and cryogenic (4 to 0.1 K) temperature, inside a very uniform magnetic field of 1 Tesla and controlled electric fields. We studied a solution based on mirrors and windows, but it present a strong issue on angular misalignment during the cooling down of the apparatus and on costs of its components. We developed an alternative solution based on the transmission of the propagation with optical fiber, inserted in the cryostat with feedthrough. We found that this solution is very effective for infrared transmission, but it presents losses for ultraviolet pulses. We characterized the system and found a suitable fiber for ultraviolet transmission that allow us to carry enough energy to excite positronium inside Aegis apparatus. We have done a theoretical study of the incoherent excitation from ground state of the positronium to n=2 or 3 in external magnetic field. We studied the sublevel structure of the first three levels of positronium in small magnetic field (with perturbation theory) or high magnetic fields (with the numerical diagonalization of the interaction matrix) and we calculated the perturbed energy levels and the dipole elements between states in this two conditions. We developed a rate equations model of the excitation and studied the excitation dynamics as a function of the positronium velocity, of the magnetic field and of the laser energy and spectrum.