We have developed a simple physical model of laser ionization based on resonance saturation that involves the most important collisional and radiative interactions, yet lends itself to analytical solutions that enable the time history of the free electron density to be evaluated. We have been able to demonstrate that in the case of sodium the predictions of this simple model are within 15% of the values calculated by our extensive, LIBQRs computer code. The model has also been used to estimate the ionization time for each of the alkali metals over a wide range of conditions, These results are found to be consistent with several experimental observations.
Articles you may be interested inElectronic processes during ion-beam sputtering of metals studied by resonance laser ionization mass spectrometry AIP Conf.Resonance ionization spectroscopy investigations of electronic processes during ion-beam sputtering of metal atoms AIP Conf.We have been able to show that "laser ionization based on resonance saturation", LIBORS, represents a powerful new form of laser interaction that can be used to efficiently couple laser energy into either a cold gas or a plasma. In essence, the dense population of resonance-state atoms resulting from laser resonance saturation represents both a reservoir of energy that rapidly provides translation energy to the free electrons through superelastic collisions and a large pool of atoms having an ionization energy reduced by the laser photon energy. We show that over a wide range of conditions LIBORS can be superior to both multiphoton ionization and inverse bremsstrahlung by many orders of magnitude. Indeed, LIBORS appears to be particularly well suited to create long plasma channels needed for electronor ion-beam transportation in future inertial fusion schemes. We have estimated that a suitably tuned laser pulse of about 0.7 J and 400-nsec duration should be capable of creating a plasma channel with an electron density of close to 10 15 cm -3 over an area of 0.2 cm 2 for a length of 5 m m sodium vapor of about 0.1 Torr. A similar result should be possible in lithium vapor.
Resonance saturation represents an efficient and rapid method of coupling laser energy into a gaseous medium. In the case of a plasma superelastic collision quenching of the laser maintained resonance state population effectively converts the laser beam energy into translational energy of the free electrons. Subsequently, ionization of the laser pumped species rapidly ensues as a result of both the elevated electron temperature and the effective reduction of the ionization energy for those atoms maintained in the resonance state by the laser radiation. This method of coupling laser energy into a plasma has several advantages over inverse bremsstrahlung and could therefore be applicable to several areas of current interest including plasma channel formation for transportation of electron and ion beams, x-ray laser development, laser fusion, negative ion beam production, and the conversion of laser energy to electricity.
Superelastic laser energy conversion has been shown to be capable of extremely rapid rates of plasma heating with relatively modest values of laser irradiance. In the case of a boron III plasma, dTe /dt≳1013 °K sec−1 has been predicted for a initial boron III ion density of 1017 cm−3. The laser irradiance needed to achieve this rate of change of the free-electron temperature is about 5×108 W cm−2 per cm of path length.
In a heat sandwich oven the metal vapor is confined to be disk shaped and is optically accessible through 360° in the plane of the disk. We have used this feature to optically measure the radial atom density distribution of sodium vapor within this type of oven, under a range of conditions. In particular, we have observed the formation of a donut-shaped atom density distribution when the heat sandwich oven is operated at high temperatures with an under pressure of the argon buffer gas.
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