Strong Non-uniform Field Research Articles

Below-threshold harmonic generation from strong non-uniform fields

Strong-field photoemission below the ionization threshold is a rich/complex region where atomic emission and harmonic generation may coexist. We studied the mechanism of below-threshold harmonics (BTH) from spatially non-uniform local fields near the metallic nanostructures. Discrete harmonics are generated due to the broken inversion symmetry, suggesting enriched coherent emission in the vuv frequency range. Through the numerical solution of the time-dependent Schrödinger equation, we investigate wavelength and intensity dependence of BTH. Wavelength dependence identifies counter-regular resonances; individual contributions from the multi-photon emission and channel-closing effects due to quantum path interferences. In order to understand the underlying mechanism of BTH, we devised a generalized semi-classical model, including the influence of Coulomb and non-uniform field interactions. As in uniform fields, Coulomb potential in non-uniform fields is the determinant of BTH; we observed that the generation of BTH are due to returning trajectories with negative energies. Due to large distance effectiveness of the non-uniformity, only long trajectories are noticeably affected.

Impulse electric breakdown in liquid hydrocarbons. The picture of the phenomenon in a strong nonuniform field with low probability of breakdown

Results are given of research in the field of experimental studies of the physical mechanisms of pulsed spark discharge in liquid hydrocarbons (mineral transformer oils) using present-day integrated diagnostic methods. The emphasis is on the study of the fundamental phenomenon, i.e., the leader process, which accompanies the development of streak lightning and the breakdown of long (laboratory) air gaps and presumably exists in the case of development of breakdown in compressed gases and liquids. The general picture of the phenomenon is formulated for a strong nonuniform field with low probability of breakdown, and further investigation objectives are defined.

Test Particle Energization by Current Sheets and Nonuniform Fields in Magnetohydrodynamic Turbulence

We perform numerical experiments of test particle energization in turbulent magnetic and electric fields obtained from pseudospectral direct numerical solutions of compressible three-dimensional magnetohydrodynamic (MHD) equations with a strong background magnetic field. The natural tendency of turbulent MHD fields is to form current sheets along the magnetic field direction, as well as strong nonuniform fields in the transverse directions. By associating the MHD dissipation length scale with the ion inertial scale, we found differential energization in parallel and perpendicular directions according to the type of particles considered. Electrons develop large parallel velocities, especially in current sheets. Protons instead show higher perpendicular energization due to the nonuniform perpendicular induced electric field produced by the plasma MHD velocity, which varies on proton length scales. Implications for dissipation mechanisms in a coronal heating model are discussed.

Electron-Positron Pair Production By Photons In Nonuniform Strong Fields

A probability of electron-positron pair production by photons in strong nonuniform fields is derived by applying a model trajectory method in the frame of a semiclassical approach. In addition to the well known invariant field parameter chi, a new invariant parameter nu is introduced to characterize the nonuniformity of the field. For nu>>1, the obtained expression is reduced to the uniform-field approximation while it approaches the Bethe-Heitler formula for nu<<1. The pair production is predicted for relatively weak external fields where the uniform-field approximation gives no effect. The theory agrees well with the experimental results of crystal-assisted pair production.

Drilling-induced remanent magnetization in basalt drill cores

SUMMARY Drilling-induced remanent magnetization (DIRM) in drill cores can limit their use for magnetostratigraphic studies and preclude the use of secondary viscous remanence for their azimuthal orientation. DIRM was studied in a drill core of a thick Miocene basalt flow now buried at 0.45 km. Due to zonation of the magnetic properties within the flow, DIRM was observed in specimens whose remanence is controlled by grains ranging from multidomain (MD) to single domain (SD). DIRM in this drill core has the following properties: (1) it is characterized by high intensity and low stability; (2) the DIRM intensity increases by at least a factor of 5 from the centre of the drill core to the drill string's cutting surface, where it appears to have been produced; (3) it is directed down and radially inward towards the centre of the drill core; and (4) it is relatively more dominant and more intense in magnetically less stable MD grains. The observed DIRM can be modelled as a pure IRM acquired in a field of the order of 10 mT. Therefore, the DIRM in this drill core is most easily explained as having been produced during the initial drilling by a strong non-uniform field concentrated near the cutting rim of the drill string. Other processes which might contribute to DIRM production include tearing of grains and possible changes in strain, mechanical shocks and piezo remanent magnetization (PRM). In this drill core, DIRM in the magnetically less stable grains was more effectively cleaned by alternating fields (AF) than by thermal demagnetization, and judicious AF demagnetization was usually successful at defining the primary remanence, especially for specimens from the centre of the drill core, which are less affected by DIRM overprinting. The use of a non-magnetic drill string would further reduce, and might possibly eliminate, DIRM production.

Effect of Prestressing on the ns Range Electrical Breakdown in Polymeric Insulating Films

The electrical impulse breakdown of prestressed polymer films was investigated in the ns range. The effect of prestressing on the breakdown was observed in polyethylene terephthalate (PET) and polycarbonate (PC), but not in polystyrene (PS) or polypropylene (PP). The amount of space charge estimated from the transient currents was large enough to induce electric field distortion in the bulk for all specimens. The effect in PET and PC is attributed to field distortion due to space charge accumulation near the electrodes, which can be estimated as electrons at the cathode and holes at the anode. The reason why the effect was not observed in PP and PS can be explained as the result of their own high breakdown strength in strong non-uniform fields.