Plasma-filled rippled wall rectangular backward wave oscillator driven by sheet electron beam

Performance of the backward wave oscillator (BWO) is greatly enhanced with the introduction of plasma. Linear theory of the dispersion relation and the growth rate have been derived and analysed numerically for plasma-filled rippled wall rectangular waveguide driven by sheet electron beam. To see the effect of plasma on the TM₀₁ cold wave structure mode and on the generated frequency, the parameters used are: relativistic factor γ = 1.5 (i.e. v/c = 0.741), average waveguide height y ₀ = 1.445 cm, axial corrugation period z ₀ = 1.67 cm, and corrugation amplitude ε = 0.225 cm. The plasma density is varied from zero to 2 ×10¹² cm^− 3. The presence of plasma tends to raise the TM₀₁ mode cut-off frequency (14 GHz at 2 ×10¹² cm^− 3 plasma density) relative to the vacuum cut-off frequency (5 GHz) which also causes a decrease in the group velocity everywhere, resulting in a flattening of the dispersion relation. With the introduction of plasma, an enhancement in absolute instability was observed.     

High-Order Finite-Differences Schemes to Solve Poisson's Equation in Polar Coordinates

In the present work finite difference schemes of second andfourth order are derived for the solution of Poisson's equationin polar coordinates. To solve the resulting system of linearequations, a direct method similar to Hockney's method is developed.The schemes are tested on six test problems whose exact solutionsare known. The numerical results obtained by these finite-differenceschemes show that they produce very accurate results.     

Thermal expansion behaviour and phase stability of AFe<Subscript>2</Subscript>As<Subscript>2</Subscript> (A = Ca,Sr and Eu) using powder diffraction technique

The thermal expansibilities and phase stabilities of AFe ₂As ₂ (A = Ca, Sr and Eu) have been investigated by powder diffraction techniques in the temperature range 5–600 K. We found the anisotropic thermal expansivities with temperature for all the compounds. The lattice parameter in the tetragonal phase ( A_T) of CaFe ₂As ₂ contracts with increasing temperature, whereas C_T expands. The rate of contraction in A_T is lower than the rate of expansion in C_T. Other compounds show normal thermal expansion behaviour along both a- and c-axes. In-plane expansion (i.e., along the a-axis) is found to be the smallest for EuFe ₂As ₂ and the highest for BaFe ₂As ₂. However, the rate of change of thermal expansivities along out-of-plane (i.e., along the c-axis) is higher as we go from Ba, Sr, Eu and Ca, respectively. Above 600 K, we notice the appearance /disappearance of certain reflections which suggest that tetragonal phase is not stable above this temperature for these compounds.     

Planar electron beams in a wiggler magnet array

Transport of high current (～kA range with particle energy ～1 MeV) planar electron beams is a topic of increasing interest for applications in high-power (1–10 GW) and high-frequency (10–20 GHz) microwave devices such as backward wave oscillator (BWO), klystrons, gyro-BWOs, etc. In this paper, we give a simulated result for transport of electron beams with velocity $V_{\rm b} = 5.23 \times 10^{8}$  cm s^???1, relativistic factor γ?=?1.16, and beam voltage = ～80 kV in notched wiggler magnet array. The calculation includes self-consistent effects of beam-generated fields. Our results show that the notched wiggler configuration with ～6.97 kG magnetic field strength can provide vertical and horizontal confinements for a sheet electron beam with 0.3 cm thickness and 2 cm width. The feasibility calculation addresses to a system expected to drive for 13–20 GHz BWO with rippled waveguide parameters as width w?=?3.0 cm, thickness t?=?1.0 cm, corrugation depth h?=?0.225 cm, and spatial periodicity d?=?1.67 cm.     

High Accuracy Difference Schemes for the Cylindrical Heat Conduction Equation

High accuracy implicit difference methods are derived for thecylindrical heat conduction equation. Some unconditionally stableimplicit formulas are derived. The utility of the new schemesare shown by testing the schemes on two examples.     

Thermal conductivity of large-grain niobium and its effect on trapped vortices in the temperature range 1.8–5 K

Experimental investigation of the thermal conductivity of large grain and its dependence on the trapped vortices in parallel magnetic field with respect to the temperature gradient \(\nabla T\) was carried out on four large-grain niobium samples from four different ingots. The zero-field thermal conductivity measurements are in good agreement with the measurements based on the theory of Bardeen–Rickayzen–Tewordt (BRT). The change in thermal conductivity with trapped vortices is analysed with the field dependence of the conductivity results of Vinen et al for low inductions and low-temperature situation. Finally, the dependence of thermal conductivity on the applied magnetic field in the vicinity of the upper critical field H _c2 is fitted with the theory of pure type-II superconductor of Houghton and Maki. Initial remnant magnetization in the sample shows a departure from the Houghton–Maki curve whereas the sample with zero trapped flux qualitatively agrees with the theory. A qualitative discussion is presented explaining the reason for such deviation from the theory. It has also been observed that if the sample with the trapped vortices is cycled through T _c, the subsequent measurement of the thermal conductivity coincides with the zero trapped flux results.     

L subshell fluorescent X-ray measurements to study CK transitions in the 66<Emphasis Type="Italic">≤</Emphasis> <Emphasis Type="Italic">Z</Emphasis> <Emphasis Type="Italic">≤</Emphasis>83 region

L subshell fluorescent X-rays in Dy, Ho, Er, Lu, Ta, W, Pt, Au, Hg, Pb and Bi have been measured using synchrotron with selective creation of electron vacancies in individual subshells. Coster–Kronig (CK) yields were derived from the measured intensities. Present measurements have been made at photon energies above the edges where differences between measured and theoretical attenuation coefficients are almost negligible. Parametric trends for the results with Z were developed to cover all Zs in the range of 66–83. The trends predict the switching-off of L 1–L 2, N₁ transition at Z = 67. The extent of fall /rise of f_Lij values corresponding to off /on of certain transitions is found inversely proportional to the difference in binding energies of two consecutive subshells involved in the transition. For Zs above /below these rises /falls, f_L13 and f_L12 values are almost constants. f_L23 values involving no break at Zs follow the general photoionization behaviour that ionization probability is highest at the edge energy and decreases with photon energy. Yield measurements with synchrotron radiation for Dy, Ho, Lu, Hg and Bi and experimental values for f_L23, f_L12 in Lu and for f_L13 in Ta are being quoted for the first time.     

Computation of Internal and External Compressible Flows Using EDICT

Results are presented for finite element computations involving high speed, viscous compressible internal and external flows. The stabilized finite-element formulations for the Navier-Stokes equations in the conservation law form are solved using the conservation variables. To improve the accuracy of the base method, especially in the regions of flow that are associated with shocks, boundary-layers and their interactions, the Enhanced-Discretization Interface-Capturing Technique (EDICT) is utilized. An error indicator is employed to identify the regions in the computational domain that need enhanced discretization for increased accuracy. The method is implemented on a shared-memory parallel computer and is used to study complex flows, that involve shock-wave/boundary-layer interactions, in supersonic diffusers and wind-tunnels. The start-up problem in supersonic wind-tunnels, caused by a narrow second throat in the diffuser section, is simulated. This computation brings out some of the very interesting features of the unsteady dynamics of the start-up shock.