Fred E. Vermeulen

Ultrafast photoconductive self-switching of subpicosecond electrical pulses

A novel photoconductive switch is proposed. The geometry of this ultrafast switch allows the rising edge of an ultrashort optical pulse to both turn on and turn off a terahertz electrical transient, making the device independent of the substrate material and charge carrier lifetime. A lumped-element model is used to analyze the operation of the switch. The model employed describes the photoexcitation of both a microstrip photoconductive switch layout and a coplanar photoconductive switch layout. It is found that both of the layouts are capable of achieving subpicosecond switching, with the coplanar layout offering greater ease of fabrication and device tunability.

Numerical Simulation Results for the Electrical Heating of Athabasca Oil-Sand Formations

Electrical preheating has been proposed as a method of overcoming many of the problems inherent in using a steamdrive in very viscous reservoirs-such as the Athabasca oil sands of Alberta, Canada. A numerical simulator was developed to study the process of electrically heating oil reservoirs consisting of several layers with different electrical resistivities. This simulator was used to study the effects of electrode placement on the final temperature contours resulting from electrically heating realistic reservoirs. Three cases illustrating the development of a hot-oil communication path flow low in the formation are described.

Free-space detection of terahertz radiation using crystalline and polycrystalline ZnSe electro-optic sensors

We present an experimental investigation demonstrating the use of single-crystal and polycrystalline ZnSe for the detection of freely propagating terahertz (THz) radiation. It is found that polycrystalline ZnSe electro-optic sensors exhibit preferred crystallographic orientations and can provide high bandwidth detection capabilities. The polycrystalline grain size and orientation are shown to be important in detecting distortion-free THz wave forms. By proper choice of the polycrystalline sensor thickness, a THz detection bandwidth comparable to that obtained with a single-crystal sensor is demonstrated. The application of polycrystalline material for free-space electro-optic sampling (FS–EOS) permits the possibility of utilizing nonlattice-matched thin-film integrated THz FS–EOS sensors and generators.

Simple resonating microstructures for gas pressure measurement

The resonance characteristics of an oscillating microcantilever are modified due to the changing damping effects as the ambient air pressure varies, and the device is used as an absolute pressure sensor. Three variations of the microcantilever device were designed, fabricated, and tested for changes in resonant frequency, quality factor, amplitude of oscillation, and actuating current required to maintain the amplitude of oscillation constant as the ambient pressure was varied from 15 to 1450 Torr. The device has been analyzed and a relationship has been derived to aid in the prediction of device behavior. Values for precision comparable to commercial pressure sensors are also presented for each method of detection.

Recombination-independent photogeneration of ultrashort electrical pulses

We report on the operation of a photoconductive (PC) switch, capable of generating ultrashort electrical pulses. The PC switch geometry employed utilizes the rising edge of an ultrafast optical excitation pulse to both turn “on” and later turn “off” an electrical transient. The generation mechanism is, therefore, independent of both the semiconductor material and carrier lifetime. It is found that electrical pulses as short as 2 ps can be formed in bulk GaAs and that the duration of these pulses is limited solely by the time constant constraints of the device. By reducing the device time constant, therefore, this technique can be applied to subpicosecond switching of electrical pulses.

A twisted electrostatic quadrupole for guiding heavy charged particles

Abstract A study is made of the guiding properties of a long electrostatic quadrupole that has been uniformly twisted about its axis. It is shown that stable trajectories exist for particles of arbitrary charge to mass ratio. Possible applications include beam transport elements for heavy particle accelerators, structures for plasma confinement, and devices for guiding the exhaust beams of colloidal propulsion engines in space.

Power Losses in Steel Pipe Deliverring Very Large Currents

This paper presents a finite difference time domain solution for the electromagnetic fields in ferromagnetic conducting steel pipes of the type used to deliver large currents for in-situ heating of heavy oil reservoirs and for in-situ environmental decontamination. A method is described whereby a single measured hysteresis loop can be used to deduce the family of hysteresis loops that governs the variable magnetic behavior throughout the pipe wall. Hysteresis and eddy current losses are calculated, and it is shown that hysteresis effects greatly alter the eddy current distribution and can more than triple the total power losses in the steel pipe when compared to the power losses that would be present if hysteresis effects are ignored and magnetic permeability is assumed constant.

A numerical approach to non‐circular birdcage RF coil optimization: Verification with a fourth‐order coil

It is demonstrated that birdcage resonators, satisfying conditions of quadrature operation and radiofrequency field homogeneity, can be realized in practice on formers of non‐circular cross section described by an equation of the form |x/a|n + |y/b|n = 1 where a and b are constants and n ≥ 2 is an integer. Using a ladder network analogous to that of a conventional circular birdcage, optimization algorithms were employed to determine the elemental current distribution on the non‐circular cylindrical surfaces. A comparison of circular, elliptical, symmetric and asymmetric fourth‐order (n = 4) section birdcage current distributions is presented. A short, asymmetric fourth‐order cage was constructed and tested experimentally at 3 T and compared with a conventional circular‐section head coil. Magn Reson Med 41:1180–1188, 1999.

Analytical approach to noncircular section birdcage coil design: verification with a Cassinian oval coil.

A general analytical framework is presented for the design of birdcage radiofrequency resonators on cylindrical formers having arbitrary cross‐sectional shape. The primary objective of such shapes would be to improve the sensitivity of the NMR experiment to noncircular regions of the human anatomy while maintaining field homogeneity and quadrature polarization comparable to those of standard circular birdcage coils. The shape of the corresponding radiofrequency screen, which is required to decouple the coil from the rest of the NMR system and which is key to the performance, is also provided by this methodology. The theory was tested by constructing a 3‐T, quadrature, proton coil on a shape conforming to the anthropomorphic mean of the human head, namely, the oval of Cassini. Both bench tests (Q) and in vivo spectral and imaging comparisons of the Cassinian coil with an equivalently dimensioned and constructed circular birdcage coil, respectively, predicted and demonstrated in vivo an improvement in SNR of ∼24% over the circular section coil. The experimental RF field homogeneity and quadrature performance were comparable for both coil geometries, with the circular coil being marginally superior. Magn Reson Med 53:201–211, 2005.

Generation of 1.2 ps electrical pulses through parallel gating in ultrathin silicon photoconductive switches

We report on a parallel-gating photoconductive switching technique which is capable of generating ultrashort electrical pulses. Carrier-lifetime-independent pulses as short as 1.2 ps are produced using a long-lifetime intrinsic float-zone (FZ) silicon substrate. The technique utilizes a thin FZ-silicon layer as the light-activated medium, where photoexcitation electrically contacts the switch output to both the bias line and the substrate ground plane. Model calculations based on transmission-line theory and lumped-element analysis accurately describe the experimental results.