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Statement of problem. The retention of a post is believed to be a major factor in restoration survival. Therefore the cement with the greatest in vitro retention property should give the best performance for a restoration. Purpose. This study compared retentive values of three posts (Flexi-Post, AccessPost, and ParaPost) cemented with five cements (Flexi-Flow, zinc phosphate, Advance, Duet, and Ketac-Cem) plus a control group that consisted of a Flexi-Post No. 2 dowel without cement. Material and methods. A total of 160 recently extracted human single-rooted teeth with crowns removed at the cementoenamel junction were divided into 16 groups of 10 samples. Post holes were prepared according to manufacturers' instructions. Posts were then cemented with one of the five cements. Each sample was placed into a specialized jig and on a tensile testing machine with crosshead speed of 0.638 cm/minute, applied until failure. A two-way analysis of variance and Newman-Keuls multiple range comparison tests were performed for all cemented groups. A one-way analysis of variance and a Student-Newman-Keuls multiple range comparison test were performed only for the groups with the Flexi-Post dowel, this included the no cement condition, with significant results if p < 0.05. Results. Flexi-Post dowel with Flexi-Flow Natural cement obtained the highest retentive value (303.91 pounds). ParaPost dowel with Duet cement exhibited the lowest retention value (21.23 pounds). The Flexi-Post dowel demonstrated higher mean retention than AccessPost or ParaPost dowels; AccessPost dowel was higher than ParaPost dowel. Flexi-Flow cement had the highest overall mean retention followed, in decreasing order, by zinc phosphate, Advance, Ketac-Cem, and Duet cements. Conclusions. Flexi-Post dowel was the most retentive post studied with values ranging from 303.91 pounds with Flexi-Flow Natural cements to 150.93 pounds without cement. Flexi-Flow cements had a higher overall mean retention than other cements studied. (J Prosthet Dent 1998;79:520-5.)

High power free-electron lasers make possible new methods for driving current in toroidal devices with electromagnetic waves. Earlier considerations of beat wave current drive are applied to a hot magnetized plasma and an arbitrary beat wave. Here the beating of two electromagnetic waves resonantly excites a low frequency beat wave that accelerates and heats electrons and leads to a current. The absolute current drive efficiency depends nonlinearly on the two pump wave intensities and is constrained by the Manley-Rowe relations. Accessibility at high plasma densities is not a difficulty, but a degree of frequency tunability of the wave sources is required. Particle simulations indicate that there is good coupling to an electron velocity tail for a Langmuir beat wave with a phase velocity 1.5 to 3 times (Te/me)½, so that all of the high frequency wave source is absorbed and the beat wave damps completely on the electrons. A novel diagnostic, based on an analytical solution for the linearized Fokker-Planck equation describing electron scattering and slowing down, is added to the particle code. This permits the computation of the current drive efficiency, including both the non-linear beat wave coupling and the collisional relaxation of the electron distribution. A realistic scenario for a beat wave current drive experiment in the Livermore Microwave Tokamak Experiment is calculated using the TORAY toroidal ray tracing code, and the scaling to an engineering test reactor plasma is described.

The paper presents a survey of possible parametric instabilities in the upcoming Microwave Tokamak Experiment (MTX) at the Lawrence Livermore National Laboratory in which an intense pulsed freeelectron laser will be used to carry out electron cyclotron heating in the Alcator C tokamak. Potentially the most dangerous instability is stimulated Brillouin backscatter by electrostatic ion cyclotron modes or quasi-modes for ordinary-mode heating at the fundamental harmonic or extraordinary-mode heating at the second electron cyclotron harmonic. Large reflectances may occur for coherent microwave sources. In addition, extraordinary waves at the amplitudes considered here can be non-linearly absorbed by parametric decay processes at the resonance layers where the pump frequency equals the second electron cyclotron harmonic and where it equals twice the local upper hybrid frequency. Finite-bandwidth effects can significantly inhibit parametric instabilities. Density fluctuations induced by drift wave turbulence at the plasma surface, with amplitudes as observed in earlier Alcator C experiments, are expected to spread the electron cyclotron wave packets but may be of insufficient amplitude to suppress Brillouin backscatter instability.

A study of the possible non-linear saturation by mode coupling of the dissipative trappedion mode is presented in which both radial and poloidal variations are considered. The saturation mechanism consists of, the non-linear coupling via convection of energy from linearly unstable modes to stable modes. Stabilization is provided at short poloidal wavelengths by Landau damping from trapped and circulating ions for ?i? d ln(Ti)/d ln(n0) < 2/3, at short radial wavelengths by effects associated with the finite ion banana excursions and at long wavelengths by ion collisions. The authors' analysis applies only to the case ?i< 2/3. A one-dimensional, non-linear partial differential equation for the electrostatic potential derived in earlier work is extended to two dimensions and to third order in amplitude. Included approximately in a systematic way are kinetic effects, e.g. Landau damping and its spatial dependence due to magnetic shear. The stability and accessibility of equilibria are considered for cases far from as well as close to marginal stability. In the first case, three-wave interactions are found to be important when the spectrum of unstable modes is sufficiently narrow. In the latter case, it is found that, for a single unstable mode, a four-wave self-interaction can provide a saturation mechanism. Accessibility of equilibria is determined in both cases. In the case where three-wave interactions are dominant, the stability analysis is inconclusive. However, plasma parameters are found for which equilibria established by means of the four-wave self-interaction are stable to linear perturbation. Cross-field transport is calculated, and the scaling of results is considered for tokamak parameters.

Statement of problem. Recently introduced single-step bonding agents reduce the number of steps involved in the bonding process. Nevertheless, there are few studies on the bond strengths obtained with these new systems. Purpose. This in vitro study evaluated the shear bond strength of a titanium-reinforced cores bonded with 5 multistep bonding systems (ScotchBond Multi-Purpose, OptiBond, All Bond-2, Tenure, and ProBond) and 5 single-step bonding systems (Single-Bond, OptiBond Solo, One-Step, Tenure Quik, and Prime & Bond 2.1). Material and methods. The experiment was divided into 10 groups with 10 specimens per group. The shear bond strengths were evaluated after 24 hours on an MTS universal testing machine with a crosshead speed of 6.35 mm/minute. Results. A 2-way ANOVA showed that All Bond-2 and One-Step recorded the highest means and differed significantly from the Den-Mat systems (Tenure A&B and Tenure Quik), the Kerr systems (OptiBond and Opti-Solo), and the Caulk systems (ProBond and Prime & Bond 2.1). ProBond and Prime & Bond 2.1 bonding systems had the lowest mean and differed from the 3 other brands. Tenure A&B and Tenure Quik bonding systems and OptiBond and Opti-Solo bonding systems did not differ from one another. Conclusion. The single-step bonding agents did not produce an improvement in shear bond strengths. The wide range of shear bond strength reported for the single-step systems appeared to indicate that these bonding systems are technique sensitive. (J Prosthet Dent 1998;80:307-10.)