Ultrashort microbunch electron source

We demonstrate that extremely thin (<1 \ensuremath{\mu}m thick) samples of the charge-density-wave (CDW) conductor ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ display high-quality narrow-band noise spectra when driven by a dc electric field. In the presence of combined dc and large-amplitude ac fields, the CDW becomes mode locked to the external signal, giving rise to Shapiro steps in the dc I-V characteristics. In the presence of combined dc and small-amplitude ac fields, distinct resonance features in the complex ac conductivity \ensuremath{\sigma}(\ensuremath{\omega}) are observed whenever the internal and external frequencies coincide. Analysis of the Shapiro-step interference suggests that the ac-dc interference arises from a direct coupling between the driving fields and the low-frequency dielectric relaxation mode in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$. Resonances in the ac conductivity are shown to be analogous to those present in a phase-shifted resonant harmonic oscillator. Both noise and interference experiments yield a ratio between the narrow-band noise frequency and the CDW current-density of ${f}_{\mathrm{NBN}/{J}_{\mathrm{CDW}}}$=12\ifmmode\pm\else\textpm\fi{}3 kHz ${\mathrm{cm}}^{2}$/A, suggesting that the intrinsic pinning potential in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ has a periodicity equal to the CDW wavelength.

Power consumption measurements for ac and pulsed dc for electrostatic coalescence of water-in-oil emulsions

Thin-films of gold-metalized polypropylene submitted to DC and AC electric fields in conditions where electrical discharges in ambient are avoided emit light in the visible part of the optical domain. The origin of the light is discussed with the help of light intensity vs. voltage characteristics and spectral analysis. Two components of the light with different features have been unraveled being due to an interfacial phenomenon and to the polymer itself. The first one is attributed to the formation of surface plasmons excited at the metal/dielectric interfaces by charge injection/extraction. These coherent collective excitations of the electron gas at the metal surface decay by emitting a typical optical signature in the red part of the visible domain. The second component of the electroluminescence spectrum is attributed to the formation of excitonic states in polypropylene excited upon charge recombination or hot electron effects. A strong evidence is provided by the spectral shape analysis which shows the spectrum exhibits identical features with the cathodo-luminescence spectrum of the material. This main component of the emission spectrum under AC field would also be involved in the emission under DC stress, the latter being so weak preventing a spectral analysis. These excitonic states provide a route for a series of reactions associated with chemical degradation of the polymer during its ageing under high field. Finally, it is shown that similar excited species are generated in polyethylene as well pointing towards a generic origin of these excited states in polyolefins.

Electron accelerators have a wide range of applications in basic research, medicine and industry. In order to reduce costs and size of future devices and facilities, advanced compact accelerator concepts are subject to extensive research and development efforts in accelerator physics. A promising approach is the dielectric laser accelerator (DLA). The electromagnetic near-fields of an ultra-short laser pulse scattered by a dielectric structure are used to accelerate electrons. Dielectrics increases the achievable accelerating gradient by a factor of at least 10 in comparison to metallic structures. Recent progress in the development of femtosecond lasers and lithographic techniques in semiconductor fabrication has paved the way to experimental demonstration of DLAs in 2013. Possible applications of DLAs are in attosecond science, in ultrafast electron-microscopy and -diffraction, and in providing relativistic electrons for lithography and radiotherapy. The use of near-fields in structures with period lengths in the order of micrometers leads to complicated particle dynamics during acceleration. Theoretical descriptions of the electromagnetic fields and the resulting particle dynamics are presented in this thesis. In order to confine the particles in the accelerating structures, a focusing scheme is introduced which is adapted from ion accelerators. This allows to transport particles through a DLA structure of arbitrary length. Due to non-linear dynamics of the accelerated electrons, the particle trajectories are calculated numerically. A simulation tool based on the analytical description of the electromagnetic fields in the structures is implemented. Since DLA structures are quasiperiodic, the description simplifies to one complex number per structure period, i.e., the spatial harmonic of the synchronous accelerating mode. The required computational effort is thus significantly reduced as compared to the full electromagnetic fields. Many applications of DLAs require particle distributions of high charge. The beam dynamics are additionally affected by wake fields, which are generated by the interaction of the charge distribution ensemble with the dielectric structures. Detailed studies of wake fields in DLA structures are discussed in this thesis. Moreover, first measurements of energy spectrum modulations caused by wake fields are presented. The wake field effects can lead to instabilities. For the analysis of possible instabilities, wake fields are integrated in the aforementioned numerical simulation tool. This extended tool is applied to compare particle dynamics with simplified analytical models of longitudinal and transverse wake effects and instabilities. These studies provide estimates for the maximum attainable charges of an accelerated particle bunch. For higher bunch charges, the effects from the wake fields exceed the accelerating and focusing laser fields and thus lead to beam loss. Finally, a possible damping mechanism is presented in order to counteract the instabilities and increase the bunch charge limits.

This Working Group (WG) focused on EM Structure‐Based Accelerators, which covers a broad area of mechanisms and experiments. Topics covered included dielectric wakefield accelerators (DWA), photonic bandgap accelerators (PBGA), inverse free electron lasers (IFEL), vacuum laser accelerators (VLA), other novel schemes, and supporting analysis and modeling. In addition, this WG was tasked at the Workshop with developing conceptual (strawman) designs for a 1‐GeV accelerator system based upon any of the experimentally‐proven approaches covered in this WG. Two strawmen designs were developed based upon IFELs and DWAs. The presentations given and strawmen designs indicate great progress has been made in many areas. Proof‐of‐principle experiments will occur shortly in PBGA and VLA. Other well‐proven devices, such as IFELs, are becoming accepted as “workhorse” providers of microbunches.

After conventional incubation (n = 2) or incubation in a 60‐Hz modulated 2450‐MHz microwave field (n = 2), four Leghorn chickens (Gallus gallus) of both genders were assessed for ability to perceive magnetic fields (dc at 4.0 milliteslas; 60‐Hz ac at 1.7 milliteslas rms) by the method of conditional suppression. Each bird operantly responded for rewards of grain while being subjected to a Pavlovian conditioning procedure in which a 90‐s presentation of an H field served as a conditional stimulus (CS) that terminated in a brief faradic shock to the feet, which served as the unconditional stimulus (US). Five to seven pairings of CS and US occurred aperiodically during daily, 60‐min sessions. A total of six tensession blocks of formal testing was conducted, two blocks with the ac field, two with the dc field, and two with a sham field in which the US alone was presented to provide control baselines of responding. Neither mode of incubation nor gender was a reliable source of variation, but highly reliable changes (Ps &lt; 0.01) in the character of operant responding—increased variability—were observed during presentations of the ac or dc field. The domestic fowl may therefore share with several other avian species a sensitivity to H fields, although artifactual sensory cueing borne of vibration or heating of the H‐field source was not definitively ruled out. If real, the birds' sensitivity to the nominal dc field was probably associated with behavioral modulation – with incessant movement in the field by a chicken, especially of its head, during the operant‐respondent measures of conditioning. On the basis of our findings and those of other investigators, we believe that detection of H fields by avian species has its locus in the retina or in other neuroanatomical substrates of the visual system.

Controlled delivery of drugs adsorbed onto porous Fe3O4 structures by application of AC/DC magnetic fields

Dye indicators of membrane potential have been available for the past 15 years and have been employed in numerous studies of cell physiology. Since the cell membrane is a likely primary site for the cascade of events resulting in a biological response to electromagnetic fields, methodologies for monitoring the membrane voltage will be critical. This laboratory has developed a series of dyes by using theoretical molecular orbital calculations to predict electrochromic structures. The spectra of electrochromic dyes are altered via a direct coupling of the molecular electronic states with an electric field. This mechanism has the advantage of providing both high temporal and high spatial resolution because the effect is instantaneous and is localized to the level of individual indicator molecules. It therefore can have significant advantages over traditional microelectrode techniques because fast changes in potential can be monitored simultaneously over many different regions of a biological preparation. We have used these dyes to monitor membrane potentials induced by both DC and AC electric fields. In a series of studies with a model membrane system, a spherical lipid bilayer, we showed that the potential develops on the membrane in good agreement with a time-dependent solution to Laplace's equation. Cell membranes can also be stained with voltage sensitive dyes. In experiments with dc fields, we are able to map the variation of the induced membrane potential along the surface of the cell by employing digital video fluorescence microscopy. We can also use the fluorescence microscope to detect membrane potential induced by ac fields by using a phase-sensitive detection scheme to extract the corresponding change in the light intensity from the fluorescent indicator. The technology can be extended to more complex biological preparations and can be used in conjunction with other optical techniques such as those which monitor intracellular ion concentrations. It may, therefore, prove highly valuable for the elucidation of biological responses to electromagnetic fields.

Temperature profiles of an opposed-jet diffusion flame subjected to an electric field

The successful application of superconductivity to motors and other power system components depends on the characterization and subsequent minimization of the ac loss in the superconductor used for fabrication of the component. The superconductive component, excited by an ac power source, may be exposed to large dc magnetic fields and/or ac fields. To further complicate the situation, the transport properties of the tapes are strongly dependent on the angle between the applied field and the YBCO c‐axis (normal to the tape surface). We report on measurements of the transport ac loss of a YBaCuO tape at 65 K, at several frequencies, in applied dc fields of 1–3 T with the field parallel and perpendicular to the tape normal.

A theory of dissipative nonlinear conductivity, σ(1)(ω,H), of s-wave superconductors under strong electromagnetic fields at low temperatures is proposed. Closed-form expressions for σ(1)(H) and the surface resistance R(s)(ω,H) are obtained in the nonequilibrium dirty limit for which σ(1)(H) has a significant minimum as a function of a low-frequency (ħω ≪ k(B)T) magnetic field H. The calculated microwave suppression of R(s)(H) is in good agreement with recent experiments on alloyed Nb resonator cavities. It is shown that superimposed dc and ac fields, H = H(0) + H(a)cosωt, can be used to reduce ac dissipation in thin film nanostructures by tuning σ(1)(H(0)) with the dc field.

The possibility for DC and AC power lines running parallel to each other, sharing the same right‐of‐way or even the same tower has been increased in China. In this work, the coupled DC and AC electric field effects are analysed with a reduced‐scale model. Improved measuring methods are proposed to obtain both the DC and AC components of the ground level electric field and ion current density. Depressing effects of the AC field on the ground level DC field and of the DC field on the ground level AC field are observed when the applied DC voltage exceeds the onset value. The mechanism of the influences of AC power lines on the nearby DC power lines is analysed with experiments and simulations. Both the influence of the AC field on the corona onset on the surfaces of DC conductors and the influence of the AC field on the motion of the space charges are proved to be important in the field effects under the hybrid lines.

Simulations of a Dielectric Laser Accelerator (DLA) are presented. Where in an electron beam is passed through a double grating structure of a dielectric material, whilst laser light polarized in the direction of travel of the electron beam is applied to both sides. This results in acceleration of the electron beam. The LSP (Large Scale Plasma) code is a particle in cell code which is applied in a novel way to model the DLA. Ultimately answers are needed to the questions: what is the acceleration gradient, how does this depend on the starting energy of the electrons and what is the current that can be accelerated through the DLA? DLA's have great potential in many fields, from providing compact linear accelerators, x-ray light sources and medical devices to high energy particle research.

A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio εI/εf. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/εf, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of εI/εf. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists between the measured shear yield stress, current density, and the applied breakdown field for oxidized aluminum particles suspended in silicone oil and those predicted by the present model. Recommendations are given for the design of ER fluids with high yield strength and reasonable current density.

A bipolar charge transport model has been developed to simulate the dynamics of space charge under dc fields. The modeling shows space charge build-up and decay comparable to experiments. In this paper, this bipolar charge transport model is taken further to simulate the space charge in polyethylene under ac fields. The results show that there is small quantity of phase-dependent bipolar charge accumulation mostly in the vicinity of the electrodes instead of moving into the bulk under ac fields. This causes a slight distortion of the local field in the bulk. However at lower frequencies (less than 1Hz) there is increased charge accumulation and penetration. The conduction current is quite small and hence the displacement component dominates the total current. Bipolar charge recombination is going to reach a saturation level after a certain ac stressing time. Increasing the ac voltage will normally increase the amount of total charge and the charge recombination rate.