A Laser Power-Build-Up System for H Atom Ionization

Prior to injection into a proton accelerator, GeV energy Hydrogen negative ions must be ionized with good efficiency. Presently used carbon foil strippers become radioactive and produce uncontrolled beam loss which limits the performance of high intensity proton sources. Laser assisted stripping of the electrons is an alternative method which has been demonstrated in proof-of-principle experiments. Laser excitation of the neutral atoms prepares the atoms for subsequent ionization in a magnetic field, and for this megaWatt peak powers are required. 6 This proposal describes a Fabry-Perot cavity which is capable of storing energy (stacking laser pulses) when resonant with the incident laser light. This power recycling multiplies the incident laser power to create fields that are sufficiently intense (MW peak powers) to excite the hydrogen atoms with high probability. The research in Phases 1 and 2 examines the mirror requirements for the extremely high circulating power, the cavity configuration to create a sufficiently large diameter beam, and ultra-high vacuum-compatible design. 7 In phase 1, we made sure that the mirrors would be able to operate with the kW of average circulating light power in the cavity without deformation from absorbed power that would reduce the circulating power. This was done by measuring the fractional power absorption of mirror coatings near 355 nm, and finite element models of the mirror thermal expansion. The large mode size required was modeled and designed, and the cavity design tested and shown to perform well. Mirror damage thresholds were measured. 8 In phase 2 we will build a prototype cavity, and lock a high-power pulsed laser to the cavity to attain MW peak pulse powers. We will quantify the mode distortion from absorbed power, and examine the performance of different mirror coatings. Attention to the materials used and the design will allow ultra-high vacuum cleaning techniques to be used to avoid mirror contamination issues. 9 Power buildup cavities for short-pulse lasers is a rapidly developing field, with applications in commercial spectroscopy systems for gas and molecule detection and recognition in industrial and medical systems, scientific spectroscopy, compact particle accelerators, cavity-dumped systems for short-pulse power increase in scientific uses, as well as part of the process of the stripping of electrons from Hydrogen atoms in particle accelerators. 10 power buildup, Fabry-Perot, 355 nm, mirror absorption 11 Very High power laser beams for multiphoton processes and saturation of weak resonances in atoms and molecules may be attained by stacking pulses in a power buildup resonant Fabry-Perot cavity, taking into consideration the problems posed by very high powers such as mirror damage and distortion, and vacuum contamination by attraction of contaminants into the beam. Use of low absorption mirror coatings on selected substrate materials, along with ultra-high vacuum cleaning processes, allow the pulse stacking to multiply the laser power to megaWatt levels for use in Hydrogen ionization