Nernst effect in Tl-Ba-Ca-Cu-O superconducting films due to infrared laser heating

A Nernst effect has been observed in a high temperature superconductor for the first time. Irradiating superconducting Tl?Ba?Ca?Cu?O thin films by short pulses of a TEA-CO₂ laser, a photovoltaic signal is detected perpendicular to a magnetic field applied parallel to the film surface. The signal is attributed to magnetic flux line depinning and flux line transport driven by the laser induced temperature gradient. The results are described by thermal flux line activation leading to a calculated distribution of pinning energies from 100 K to 4000 K.     

Proposal of a Microwave-Driven Semiconductor Superlattice Oscillator for Generation of THz Radiation

We present the proposal of a microwave-driven semiconductor superlattice oscillator. We show that the interplay of a microwave pump field with a synchronous harmonic field can make a semiconductor superlattice to a gain medium for the harmonic field. Placing the superlattice in a resonator for the harmonic field allows the operation of an oscillator. The gain mechanism is based on Bloch oscillations of miniband electrons. The gain is mediated either by the interaction of the high-frequency field with the single electrons or with space charge domains or with both. The microwave-driven superlattice oscillator should be suitable for generation of coherent radiation up to several THz.     

Infrared detection by Tl−Ba−Ca−Cu−O superconducting films

The response of polycrystalline Tl?Ba?Ca?Cu?O superconducting thin films on short laser pulses has been investigated for radiation between 10 μm and 500 μm wavelength. Fast signals with time constants less than 1 ns were observed for wavelengths longer than about 100 μm whereas for shorter wavelengths only a bolometric signal could be detected.     

Transient-Pulse Nonlinear Spectroscopy with the Radiation of a Multimode THz Gas Laser

We report on transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser. The method is demonstrated for studying the nonlinear response of a current-carrying superlattice to THz radiation; the current through a superlattice can be suppressed by a strong THz field. The method makes use of the pulses of a high power multimode THz gas laser. By splitting the laser beam for selected laser modes into a main beam and a reference beam we monitored with a reference detector the transient power in the main beam. The simultaneous measurement of both the instantaneous response and the instantaneous power allows to obtain the power dependence of the response within a single laser pulse. The method is suitable to study the nonlinear response of matter to THz radiation fields in a large dynamic range.     

An overview of the development of the first wall and other principal components of a laser fusion power plant

This paper introduces the JNM Special Issue on the development of a first wall for the reaction chamber in a laser fusion power plant. In this approach to fusion energy a spherical target is injected into a large chamber and heated to fusion burn by an array of lasers. The target emissions are absorbed by the wall and encapsulating blanket, and the resulting heat converted into electricity. The bulk of the energy deposited in the first wall is in the form of X-rays (1.0-100 keV) and ions (0.1-4 MeV). In order to have a practical power plant, the first wall must be resistant to these emissions and suffer virtually no erosion on each shot. A wall candidate based on tungsten armor bonded to a low activation ferritic steel substrate has been chosen as the initial system to be studied. The choice was based on the vast experience with these materials in a nuclear environment and the ability to address most of the key remaining issues with existing facilities. This overview paper is divided into three parts. The first part summarizes the current state of the development of laser fusion energy. The second part introduces the tungsten armored ferritic steel concept, the three critical development issues (thermo-mechanical fatigue, helium retention, and bonding) and the research to address them. Based on progress to date the latter two appear to be resolvable, but the former remains a challenge. Complete details are presented in the companion papers in this JNM Special Issue. The third part discusses other factors that must be considered in the design of the first wall, including compatibility with blanket concepts, radiological concerns, and structural considerations.     

Tuning characteristics of a uv preionized 20-atmospheres CO2 laser

The tuning characteristics of a uv preionized 20-atmospheres gas discharge CO₂ laser with a novel and simple preionization scheme have been investigated. Continuous tunability in all branches of the 9.4 μm and 10.4 μm band with a total tuning range of more than 60 cm^?1 without frequency pulling is demonstrated. Additionally, a study of the laser pulse behaviour was performed.     

Quantum counter for far-infrared radiation

Two types of three-level quantum counters, based on the Bloembergen and the inverted scheme, are discussed with respect to their application for up-conversion of far-infrared (FIR) radiation to visible light. It is reported on a tunable quantum counter of the inverted type: in optically pumped ruby, photons at 29 cm^-1are absorbed by the transitionbar{E}(²E) →2bar{A}(²E) and lead to an increased R2- fluorescence radiation. By Zeeman splitting of the²Elevels, tuning of the detected frequency from 29 cm^-1to higher and lower frequencies is possible (with 1 cm^-1/Tesla). Using the 891-GHz radiation of an HCN laser we determine for the quantum counter a power conversion efficiency of the order of 10^-6and a frequency bandwidth of 360 MHz. We find the interesting result that phonon bottleneck due to resonant trapping of 29 cm^-1phonons can lead to an increase of the quantum efficiency by orders of magnitude.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Liquid titanium solute diffusion measured by pulsed ion-beam melting

The diffusivities of Sn, Mo, Zr, and Hf in liquid Ti were determined by pulsed ion-beam melting of thin liquid layers. Time-resolved optical reflectance and one-dimensional heat-flow simulations were employed to determine the melt duration. The broadening of nearly Gaussian solute concentration-depth profiles was determined ex situ using Rutherford backscattering spectrometry. Solute diffusivities in the range of 5 to 9×10⁻⁵ cm²/s were determined at temperatures in the range of 2200 to 2500 K. Calculations of buoyancy and Marangoni convection indicate that convective contamination is unlikely.