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The time decay of electron density in pulsed helium afterglow discharges is studied experimentally employing both conventional and modified microwave-cavity techniques. The two methods permit density measurements from w^lO 11 cm" 3 to as low as n e^2 X 10 4 cm" 3 in helium at pressures of 0.4 and 4.0 Torr. For Debye lengths less than ^1% of the characteristic diffusion length A, the electron loss rate is through ambipolar diffusion controlled by the atomic ion He + at 0.4 Torr and by the molecular ion He 2 + at 4.0 Torr. At lower electron densities, the electrons diffuse more rapidly than ambipolar diffusion, and in the limit of Debye lengths much greater than the characteristic diffusion length, the electrons diffuse freely to the walls. The experimentally observed effective diffusion coefficient D 8 in the transition region is compared with a mathematical expression proposed by Phelps relating D 8 to n e . Satisfactory agreement is obtained at 4.0 Torr where the electron mean free path X<3CA. By using this expression to calculate theoretically expected decays of electron density, a computer optimization procedure produced an even better fit to experimental data by slightly altering a numerical parameter in the formula. At 0.4 Torr, where X«A and the theoretical treatment is not expected to be valid, diffusion in the transition region is observed to occur at a rate substantially less than that predicted theoretically.

It has been established that the ability of erbium lasers to ablate hard dental tissue is due primarily to the laserinitiated subsurface expansion ofthe interstitial water trapped within the enamel and that by maintaining a thin film of water on the surface of the tooth, the efficiency of the laser ablation is enhanced. It has recently been suggested that a more aggressive ablative mechanism, designated as a hydrokinetic effect, occurs when atomized water droplets, introduced between the erbium laser and the surface of the tooth, are accelerated in the laser's field and impact the tooth's surface. It is the objective ofthis study to determine ifthe proposed hydrokinetic effect exists and to establish its contribution to the dental hard tissue ablation process. Two commercially available dental laser systems were employed in the hard tissue ablation studies. One system employed a water irrigation system in which the water was applied directly to the tooth, forming a thin film of water on the tooth's surface. The other system employed pressurized air and water to create an atomized mist of water droplets between the laser hand piece and the tooth. The ablative properties of the two lasers were studied upon hard inorganic materials, which were void of any water content, as well as dental enamel, which contained interstitial water within its cystalline structure. In each case the erbium laser beam was moved across the surface of the target material at a constant velocity. When exposing material void of any water content, no ablation of the surfaces was observed with either laser system. In contrast, when the irrigated dental enamel was exposed to the laser radiation, a linear groove was formed in the enamel surface. The volume of ablated dental tissue associated with each irrigation method was measured and plotted as a function of the energy within the laser pulse. Both dental laser systems exhibited similar enamel ablation rates and comparable ablated surface characteristics. The results of the study suggest that, although the manner in which the water irrigation was introduced differed, the mechanism by which the enamel was removed appeared basically the same for both dental laser systems, namely rapid subsurface expansion of the interstitially trapped water. It is the conclusion of this study that if the proposed hydrokinetic effect exists, it is not effective on hard materials, which are void of water, and it does not contribute in any significant degree in the ablation of dental enamel.

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A study has been conducted to determine the trade-offs among beam divergence, output power, and mirror alignment tolerances of high power argon ion lasers operating in various low order transverse modes. The radial intensity distribution and angular divergence of the actual laser modes from the active cavity have been measured and are compared with the theoretical passive cavity modes. The intensity distributions of higher order laser modes are found to differ markedly from those predicted by theory. The differences consist primarily of enhancement of the intensity of the off-axis peaks relative to that of the center peaks. These differences are caused by the relative high small-signal gain and the saturable gain characteristics of the medium. The beam divergence in free space of different transverse modes agrees very closely with that predicted for the passive cavity modes. The three lowest order transverse modes (i.e., TEM(00), TEM(01*), and TEM(10)) diverge at 1.1, 2.4, and 3.6 times the rate of a diffraction limited gaussian beam having the same diameter at the 1/e(2) intensity points. The mirror alignment tolerances associated with different transverse modes have been measured for both double concave and plano concave cavity configurations using a wide variety of spherical mirrors. For a given cavity configuration, as the order of the transverse mode increases, the alignment requirements become progressively less stringent. In accordance with theory, the alignment tolerances are found to be less severe for double concave cavities than for plano concave cavities. For a given cavity configuration, higher order transverse modes have a larger beam diameter and consequently couple to a larger volume of the laser medium. Therefore, a greater power output may be obtained in a given length by using a higher order transverse mode. Data on the relative power available in different transverse modes as a function of cavity configuration are given. A knowledge of the trade-offs among beam divergence, cavity stability, and power output for different transverse modes allows the optimum transverse mode and cavity configuration for a given laser application to be selected.