Mark J. Hagmann

Interstitial microwave hyperthermia applicators having submillimetre diameters

Using microscopic techniques we have fabricated interstitial hyperthermia applicators having diameters of 0.20, 0.33 and 0.58 mm, which will fit through catheters of 30, 26 and 22 gauge, respectively. Existing commercial applicators having a diameter of 1.1 mm required 17 gauge (or larger) catheters. Our new applicators, which operate at 915 MHz, are a smaller version of a design used by others. We have characterized our applicators by determining the energy deposition patterns (SAR) in muscle-simulating phantoms. These patterns were determined by measuring the electric field intensity using a miniature implantable isotropic probe having a diameter of 3 mm. Contours of the SAR data for our applicators, as well as a larger commercial applicator, show that all of these applicators exhibit similar heating patterns. Test results suggest that the durability and power handling capability of our submillimetre applicators are adequate for use in patients. Our new applicators should be useful in the percutaneous treatment of deep-seated tumours, intraoperative treatments, and also permit intraluminal or intravascular access to tumours.

Wide-band-tunable photomixers using resonant laser-assisted field emission

Simulations and experiments show a resonant interaction of tunneling electrons with a radiation field, and photomixing (optical heterodyning) in laser-assisted field emission can cause current oscillations from dc to 100 THz with this effect. Recent simulations by others are shown to be consistent with Fowler–Nordheim theory when the radiation is at low frequencies, and also confirm the existence of the resonance. The relationship of these simulations to photomixing is demonstrated, and estimates are given for the power of signals that could be obtained by this means.

Mechanism for resonance in the interaction of tunneling particles with modulation quanta

Numerical simulations of quantum tunneling with time‐dependent barriers show that there is a resonance, with a marked increase in the tunneling current. For square barriers the resonance occurs when the tunneling particles absorb modulation quanta and the length of the barrier is a multiple of one‐half de Broglie wavelengths. The resonance has a similar mechanism with triangular barriers. However, the relationship is more complex because the absorption and emission of modulation quanta takes place throughout the full length of the barrier, whereas this exchange only occurs at the ends of a square barrier.

A Whole Body Thenmal Model of Man During Hyperthermia

A whole body thermal model of man has been developed to predict the changes in regional temperatures and blood flows during hyperthermia treatments with the miniannular phased array (MAPA) and annular phased array (APA) applicators. A model of the thermo-regulatory response to regional heating based on the experimental and numerical studies of others has been incorporated into this study. Experimentally obtained energy deposition patterns within a human leg exposed to the MAPA were input into the model and the results were compared to those based upon a theoretical deposition pattern. Exposure of the abdomen to the APA was modeled with and without the aberrant energy deposition that has been described previously. Results of the model reveal that therapeutic heating (42°C) of extremity soft tissue sarcomas is possible without significant systemic heating. Very high bone temperatures (50°C) were obtained when the experimental absorption pattern was used. Calculations show that systemic heating due to APA exposure is reduced via evaporative spray cooling techniques coupled with high-velocity ambient air flow.

Simulations of the generation of broadband signals from DC to 100 THz by photomixing in laser-assisted field emission

Photomixing in laser-assisted field emission is simulated using (1) the method of moments to determine the optical fields induced at the tip by the radiation from two lasers, (2) density-functional theory with Floquet methods to determine the effect of these optical fields on the field emission current, and (3) the Drude model with a method first described by Sommerfeld to determine propagation of the current at the difference frequency throughout the length of the metal tip. For metals with high work functions such as tungsten, the magnitude of the current at the difference frequency varies slowly as this frequency is increased up to a roll-off at approximately 100 THz. For difference frequencies up to approximately 100 GHz this current could be carried out of the field emission tube by transmission lines, and at higher frequencies the tip may be used as a long-wire travelling wave antenna to transmit the signal out of the tube.

Resonance due to the interaction of tunneling particles with modulation quanta

Numerical simulations of quantum tunneling with time‐dependent barriers show that there is a resonance, with a marked increase in the transmission coefficient. For a raised cosine potential, and for low energies with square barriers, the resonance occurs when a modulation quantum can take a tunneling particle to the top of the barrier. For energies near the top of a square barrier the resonance may be understood by hypothesizing that a tunneling particle may travel from end to end of the barrier until it is ultimately either transmitted or reflected.

Transit time for quantum tunneling

Abstract A new procedure is presented for evaluating the transit time for tunneling with a one-dimensional barrier. For large barriers the most probable energy fluctuations have a minimum product of their magnitude and the transit time, and the semi-classical time is obtained. There is a separate solution for very small barriers, in which the transit time is indeterminate but bounded.

Determination of barrier traversal time by modulation of the incident wave

The semiclassical expression for barrier traversal time results from a variety of theoretical procedures, but it appears that an opaque barrier is required in each derivation. Following Buttiker and Landauer [IBM J. Res. Dev. 30, 451 (1986)], we use two interfering plane waves as the incident wave and determine the traversal time by attributing differences in the transmitted and incident waves to the ratio of dispersion in the transit time to the modulation period. Calculations are made for rectangular barriers with different sizes. There is essentially agreement with Buttiker and Landauer for opaque barriers which they considered, but the present solution does not diverge for energies near the top of the barrier.

Evaluation of heating patterns of microwave interstitial applicators using miniature electric field and fluoroptic temperature probes

The SAR patterns were determined for four commercially available microwave (915 MHz) interstitial applicators. Values of SAR were determined using a miniature (3 mm diameter) implantable isotropic electric field probe or a custom 0.25 mm diameter fluoroptic temperature probe. These are the smallest such probes that are currently available. Similar radial variation of SAR was found at the axial position of the gap in the outer conductor for each applicator. Electric field probe measurements are much faster and avoid some of the errors caused by the rapid spatial variation of SAR with interstitial applicators. The major limitation on the electric field probe is its size; it is larger than the applicators being tested.

Limitations on the use of Bohm's causal interpretation of quantum mechanics for the computation of tunneling times

Abstract Others have used Bohms causal interpretation to calculate barrier traversal times for plane waves and wavepackets. It is shown that Bohms stream velocity does not represent the motion of any part when there is a superposition of waves. Calculated plane traversal times are upper bounds, and other difficulties are encountered using wavepackets.