Search Results (6 total)

We present an analytical description of free-electron-laser (FEL) oscillations in a perfectly synchronized optical cavity by solving the one-dimensional FEL equations. It is shown that the radiation stored in the cavity eventually evolves into an intense few-cycle optical pulse in the high-gain and low-loss regime despite the lethargy effect. The evolution of the leading slope of the optical pulse, which is defined from the front edge toward the primary peak, is found to play an important role in generating the intense few-cycle optical pulse. The phase space evolution of electrons which interact with the leading slope is solved analytically in a perturbation method, leading to an analytical solution for the optical pulse evolution. The peak amplitude and the pulse length at saturation are found to scale with the electron beam density and optical cavity loss. Those scalings agree well with the intense few-cycle pulses recently observed in a high-power FEL.

I present an analytical solution for the phase space evolution of electrons in a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating in the linear regime before saturation in the resonant case by solving the one dimensional FEL equation together with the solution of the cubic equation, which represents the evolution of the SASE FEL field. The electrons are shown to be bunched around π/6 ahead of a resonant electron every resonant FEL wavelength in the high gain regime. The phase relation is similar to that in a low gain FEL where an electron beam above resonance is injected, explaining the positive FEL gain. The analytical solutions agree well with numerical simulations and are applied to obtain the coherent optical transition radiation (OTR) intensity produced from electron microbunching at FEL wavelength. The coherent OTR intensity is shown to be proportional to FEL intensity.

Analyzing powers A_xx(θ), A_yy(θ), A_zz(θ), and A_y^d(θ) of the H(d→,³He)γ reaction were measured at E_d=17.5 MeV. A hydrogen gas target sealed with thin carbon foils was used and the angular distribution of ³He recoils was observed to measure the analyzing powers. High-statistics data were obtained over a wide c.m. angular range. The results are compared with recent Faddeev calculations based on the realistic two body potential with and without three-nucleon force incorporated.

The first observation of sustained saturation in a free-electron laser (FEL) oscillator at perfect synchronism of an optical cavity is presented. A simultaneous measurement of FEL power and absolute detuning length of the optical cavity ( δL) has clearly shown that the FEL efficiency becomes maximum at δL  =  0.0±1 μm, although it has been considered that only a transient state exists at δL  =  0 due to the well-known laser lethargy effect. The observed efficiency detuning curve is well reproduced by our numerical simulation including a small shot-noise effect.

All the vector and tensor analyzing powers of the p+d scattering have been measured at E_p^lab=2.5 MeV (E_d^lab=5 MeV), where a calculation of the three-nucleon scattering state with an improved treatment of Coulomb force has become available. Measurements have also been made at energies below and slightly above the deuteron breakup threshold at E_p^lab = 3.34 MeV. The experimental data have small statistical errors ranging from ±0.0004 to ±0.0008, and the uncertainties in the scale are less than 1%. The present data together with our previous data on the cross section are compared with several Faddeev calculations based on a separable-form nucleon-nucleon potential or on a realistic potential with Coulomb force being treated nearly correctly or approximately. Disagreements between the calculations and the experiment are found not only in the cross section and the vector analyzing powers A_y and iT₁₁ but also in the tensor analyzing powers T₂₁ and T₂₂.

LaCoO₃ exhibits two magnetic-electronic transitions, one near 90 K and a second near 500 K. A previous study of the paramagnetic scattering using polarized neutrons demonstrated that the low-temperature transition is associated with the thermal excitation of Co³⁺ ions from the low-spin to the high-spin state. In the present work, we extend the paramagnetic-scattering measurements up to a temperature of 700 K. We find that the magnetic-scattering intensity decreases monotonically for temperatures above 300 K, indicating that the high-temperature transition is not dominantly magnetic in origin. Furthermore, the anomalous thermal expansion associated with the low-temperature transition is measured and shown to be consistent with a simple theoretical model for the spin-state transition. For comparison, paramagnetic-scattering measurements for La_0.92Sr_0.08CoO₃ are also presented. In this material the ferromagnetic correlations are substantially stronger than in the undoped compound, and no transition to the low-spin state is observed. Instead, the paramagnetic scattering increases steadily with decreasing temperature until saturating below 24 K, the same temperature at which the magnetization of the zero-field-cooled specimen shows a sharp cusp. These results suggest that the magnetic moments in the doped compound freeze into a spin-glass state at low temperature.