Absorbed Power Density Research Articles

Tumour-treating fields (TTFields): Investigations on the mechanism of action by electromagnetic exposure of cells in telophase/cytokinesis

Tumour-treating fields (TTFields) use alternating electric fields which interfere with dividing cells, thereby reducing tumour growth. Previous reports suggest that electrical forces on cell structure proteins interfered with the chromosome separation during mitosis and induced apoptosis. In the present report we evaluate electromagnetic exposure of cells in telophase/cytokinesis in order to further analyse the mechanism of action on cells. We performed numerical electromagnetic simulations to analyse the field distribution in a cell during different mitotic phases. Based thereon, we developed an electric lumped element model of the mitotic cell. Both the electromagnetic simulation and the lumped element model predict a local increase of the specific absorption rate (SAR) as a measure of the electromagnetically induced power absorption density at the mitotic furrow which may help to explain the anti-proliferative effect. In accordance with other reports, cell culture experiments confirmed that TTFields reduce the proliferation of different glioma cell lines in a field strength- and frequency-dependent manner. Furthermore, we found an additional dependence on the commutation time of the electrical fields. The report gives new insights into TTFields’ anti-proliferative effect on tumours, which could help to improve future TTFields application systems.

Thermo-mechanical response of FG tungsten/EUROFER multilayer under high thermal loads

With the aim to evaluate the performance of functionally graded (FG) tungsten/EUROFER multilayer under fusion relevant transient heat loads, this study summarizes the experimental results on type-I edge localized mode (ELM)-like thermal shock exposures of a FG tungsten/EUROFER multilayer. Absorbed power densities of 0.19 and 0.38 GW/m2 with a pulse duration of 1 ms, and base temperatures of room temperature and 550 °C were chosen. We assessed the effect of variable FG-layers that consisted of three designed thicknesses and two kinds of layer numbers. The conclusion is that the introduction of FG-layers improves the thermal shock resistance. The thermal shock crack thresholds at RT and 550 °C of five multilayers lie between 0.19 and 0.38 GW/m2.

Effect of High-Energy-Rate Forging on Microstructure and Properties of W-TaC Alloys

The effects of the addition of TaC and hot deformation on the properties of tungsten have been studied. The tungsten and tantalum carbide (W-TaC) alloy was prepared by hot pressing followed by the high-energy-rate forging (HERF) process. The density and Vickers hardness of W-TaC alloy were tested. The microstructure of the composite was characterized by metallographic microscope, scanning electron microscopy, and electron backscattering diffraction (EBSD) imaging. The transient thermal shock resistance of forged W-TaC alloy was also investigated by applying 100 pulses edge-localized mode-like loads in an electron beam facility. The results show that the HERF processing can improve the density and hardness of W-TaC alloy. Preferred orientations of W grains and the subgrain boundaries were observed from the EBSD images. The crack threshold of the developed W-TaC alloys was located between 0.22 and 0.33 GW/m2 at room temperature. When the base temperature increased to 100 °C, no cracks can be observed on the loaded surface at absorbed power density 1 GW/m2.

Peculiarities of light absorption by spherical microcapsules

We address the peculiarities of optical radiation absorption in a poly-layer spherical microparticle which simulates an inorganic/organic poly-shell absorbing microcapsule with a water-based cargo. By means of the finite-difference time-domain technique, the spatial distribution of absorbed light power in microcapsules of various sizes and internal structure is numerically simulated. We have engineered the optimal structure of a capsule that provides for light absorption enhancement and consists of a strong refracting transparent coating and an absorbing layer which covers a liquid core. The proposed multi-layer microcapsule prototype provides for a manifold increase in the absorbed light power density in comparison with usual single-layer absorbing capsule. We show that for wavelength-scale microcapsules it is optimal to use a material with the refractive index larger than 2 as an outer shell, for example, titanium dioxide. The highest values of the absorbed power density can be obtained in microcapsules with absorbing shell thickness of approximately λ/10. When laser radiation is scattered by a dimer constituted by two identical absorbing microcapsules the absorbed power density can be maximized by choosing proper dimer spatial configuration. In the case of strongly absorbing particles, the absorption density is maximal if the distance between capsules is about one capsule diameter. For weakly absorbing particles, light absorption increases when particles in the capsule are in geometrical shades of each other.

Thermal shock induced oxidation of beryllium

Beryllium has been chosen as a plasma facing material for the first wall of the experimental fusion reactor ITER, mainly because of its low atomic number and oxygen getter capabilities, which are favorable for a high plasma performance. While the steady state operational temperature of 250 °C has no deteriorating effect on the beryllium surface, transient plasma events can deposit power densities of up to 1 GW m−2 on the beryllium armor tiles. Previous research has shown that the oxidation of beryllium can occur under these thermal shock events. In the present study, S-65 grade beryllium specimens were exposed to 100 thermal shocks with an absorbed power density of 0.6 GW m−2 and a pulse duration of 1 ms, leading to a peak surface temperature of ∼800 °C. The induced surface morphology changes were compared to a steady state heated specimen at the same surface temperature with a holding time of 150 s. As a result, a pitting structure with an average pit diameter of ∼0.45 μm was observed on the thermal shock loaded surface, which was caused by beryllium oxide grain nucleation and subsequent erosion of the weakly bound beryllium oxide particles. In contrast, the steady state heated surface exhibited a more homogeneous beryllium oxide layer featuring small pits with diameters of tens of nm and showed the beryllium oxide grain nucleation in a beginning stage. The experiment demonstrated that thermal shock loading conditions can significantly accelerate the beryllium oxide grain nucleation. The resulting surface morphology change can potentially alter the fusion application relevant erosion, absorption, and retention characteristics of beryllium.

Mechanical properties and thermal shock performance of W-Y2O3 composite prepared by high-energy-rate forging

The effects of the addition of Y2O3 and hot-deformation on the mechanical properties of tungsten (W) have been studied. The processing route comprises a doping technique for the distribution of Y2O3 particles in a tungsten matrix, conventional sintering in a hydrogen environment, and high-energy-rate forging (HERF). The microstructure of the composite was characterized by using transmission electron microscopy and electron backscattering diffraction imaging technique, and its mechanical properties were studied by means of tensile testing. The thermal shock response of the HERF processed W-Y2O3 was evaluated by applying edge-localized mode-like loads (100 pulses) with a pulse duration of 1 ms and an absorbed power density of up to 1 GW m−2 at various temperatures between room temperature and 200 °C. HERF processing has produced elongated W grains with preferred orientations and a high density of structure defects in the composite. The composite material exhibits high tensile strength and good ductility, and a thermal shock cracking threshold lower than 100 °C.

Extending microwave plasma assisted CVD SCD growth to pressures of 400 Torr

The fundamental operational behavior of a microwave cavity plasma reactor was studied for pressures between 120 and 400Torr, i.e. the plasma dimensions, the absorbed power density and the operational field map were measured and the reactor growth performance was evaluated by growing single crystalline diamond (SCD). For pressures that exceeded 300Torr the reactors' behavior differed from previous observations at lower pressures. As pressures were increased up to 400Torr, the discharge became visually intense and absorbed power densities increased at a higher rate versus pressure up to 670W/cm3 at 400Torr. The higher discharge absorbed power densities were necessary for reactor operation at these higher pressures. The intense plasma shrunk and pulled away from the substrate holder as pressure increased. Efficient reactor operation required repositioning of the discharge with respect to the substrate holder by retuning of the reactor geometry. Successful growth of SCD was demonstrated over the entire pressure range. A designed pocket holder suppressed the formation of a polycrystalline rim for moderate methane concentrations and the SCD top surface expanded up to 40%. Freestanding SCD plates showed high crystalline quality without birefringence and had nitrogen incorporation levels below 160ppb. The SCD growth rate tripled with increasing pressure and was as high as 28μm/h for 380Torr and increased to 51μm/h after retuning the reactor geometry. Increasing the methane concentration from 5 to 9% increased the growth rate by 40%, but the crystalline quality deteriorated, i.e. the grown SCD material contained birefringence and visible defects. Additionally, the polycrystalline rim reappeared for methane concentrations of 7% and above.

Features of light absorption by an ensemble of microcapsules

Features of laser radiation scattering and absorption by an ensemble of two identical microcapsules are theoretically studied. Each capsule is modeled by a two-layer spherical micron-size particle consisting of water core and light absorbing polymer shell. By the numerical electrodynamics techniques, it is shown that the internal structure and mutual arrangement of particles affect the spatial distribution and amplitude characteristics of the power absorbed. The highest values of the absorbed power density in model microcapsules could be attained if the particles are shifted relative to each other by a distance of about their diameter (strongly absorbing particles) or if the particles are in the geometric shadow of each other (weak absorption).

A high-resolution frequency variable experimental setup for studying ferrofluids used in magnetic hyperthermia.

A scanning system for specific absorption rate of ferrofluids with superparamagnetic nanoparticles is presented in this study. The system contains an induction heating device designed and built with a resonant inverter in order to generate magnetic field amplitudes up to 38 mT, over the frequency band 180-525 kHz. Its resonant circuit involves a variable capacitor with 1 nF of capacitance steps to easily select the desired frequency, reaching from 0.3 kHz/nF up to 5 kHz/nF of resolution. The device performance is characterized in order to compare with the theoretical predictions of frequency and amplitude, showing a good agreement with the resonant inverters theory. Additionally, the setup is tested using a synthetic iron oxide with 10 ± 1 nm diameter suspended in liquid glycerol, with concentrations at 1%. Meanwhile, the temperature rise is measured to determine the specific absorption rate and calculate the dissipated power density for each f. This device is a suitable alternative to studying ferrofluids and analyzes the dependence of the power absorption density with the magnetic field intensity and frequency.

Engineering polyshell spherical microcapsules for optimal localized light absorption.

Optical radiation absorption in a polylayer spherical microparticle simulating an inorganic/organic polyshell absorbing microcapsule is considered. With the aim of the finite-difference time-domain technique, the spatial distribution of the absorbed light power in microcapsules of various sizes and internal structure is numerically calculated. For the purpose of light absorption enhancement, we have engineered the optimal structure of a capsule consisting of a strong-refracting transparent outer coating and an absorbing layer which covers a liquid core. The proposed microcapsule prototype provides for a manifold increase in the absorbed light power density in comparison with the usual single-layer absorbing capsule. We show that for light-wavelengths-scaled microcapsules it is optimal to use a material with the refractive index larger than two as an outer shell, for example, titanium dioxide (TiO2). The highest values of the absorbed power density can be obtained in microcapsules with absorbing shell thickness of approximately a tenth of a laser wavelength.

A Low-cost Beam Profiler Based On Cerium-doped Silica Fibers

A beam profiler called the Universal Beam Monitor (UniBEaM) has been developed by D-Pace Inc. (Canada) and the Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, University of Bern (Switzerland). The device is based on passing 100 to 600 micron cerium-doped optical fibers through a particle beam. Visible scintillation light from the sensor fibers is transmitted over distances of tens of meters to the light sensors with minimal signal loss and no susceptibility to electromagnetic fields. The probe has an insertion length of only 70mm. The software plots the beam intensity distribution in the horizontal and vertical planes, and calculates the beam location and integrated profile area, which correlates well with total beam current. UniBEaM has a large dynamic range, operating with beam currents of ∼pA to mA, and a large range of particle kinetic energies of ∼keV to GeV, depending on the absorbed power density. Test data are presented for H- beams at 25keV for 500μA, and H+ beams at 18MeV for 50pA to 10μA. Maximum absorbed power density of the optical fiber before thermal damage is discussed in relation to dE/dx energy deposition as a function of particle type and kinetic energy. UniBEaM is well suited for a wide variety of beamlines including discovery science applications, radio-pharmaceutical production, hadron therapy, industrial ion beam applications including ion implantation, industrial electron beams, and ion source testing.

Extraction of bioactives from Orthosiphon stamineus using microwave and ultrasound-assisted techniques: Process optimization and scale up.

This work demonstrated the optimization and scale up of microwave-assisted extraction (MAE) and ultrasonic-assisted extraction (UAE) of bioactive compounds from Orthosiphon stamineus using energy-based parameters such as absorbed power density and absorbed energy density (APD-AED) and response surface methodology (RSM). The intensive optimum conditions of MAE obtained at 80% EtOH, 50mL/g, APD of 0.35W/mL, AED of 250J/mL can be used to determine the optimum conditions of the scale-dependent parameters i.e. microwave power and treatment time at various extraction scales (100-300mL solvent loading). The yields of the up scaled conditions were consistent with less than 8% discrepancy and they were about 91-98% of the Soxhlet extraction yield. By adapting APD-AED method in the case of UAE, the intensive optimum conditions of the extraction, i.e. 70% EtOH, 30mL/g, APD of 0.22W/mL, AED of 450J/mL are able to achieve similar scale up results.

Influence of surface morphology and microstructure on performance of CVD tungsten coating under fusion transient thermal loads

Thick tungsten coatings have been deposited by chemical vapor deposition (CVD) at a rapid growth rate. A series of tungsten coatings with different thickness and surface morphology were prepared. The surface morphology, microstructure and preferred orientation of the CVD tungsten coatings were investigated. Thermal shock analyses were performed by using an electron beam facility to study the influence of the surface morphology and the microstructure on the thermal shock resistance of the CVD tungsten coatings. Repetitive (100 pulses) ELMs-like thermal shock loads were applied at various temperatures between room temperature and 600°C with pulse duration of 1ms and an absorbed power density of up to 1GW/m2. The results of the tests demonstrated that the specific surface morphology and columnar crystal structure of the CVD tungsten have significant influence on the surface cracking threshold and crack propagation of the materials. The CVD tungsten coatings with a polished surface show superior thermal shock resistance as compared with that of the as-deposited coatings with a rough surface.

Oxide segregation and melting behavior of transient heat load exposed beryllium

In the experimental fusion reactor ITER, beryllium will be applied as first wall armor material. However, the ITER-like wall project at JET already experienced that the relatively low melting temperature of beryllium can easily be exceeded during plasma operation. Therefore, a detailed study was carried out on S-65 beryllium under various transient, ITER-relevant heat loads that were simulated in the electron beam facility JUDITH 1. Hereby, the absorbed power densities were in the range of 0.15–1.0 GW m−2 in combination with pulse durations of 1–10 ms and pulse numbers of 1–1000.In metallographic cross sections, the emergence of a transition region in a depth of ~70–120 µm was revealed. This transition region was characterized by a strong segregation of oxygen at the grain boundaries, determined with energy dispersive x-ray spectroscopy element mappings. The oxide segregation strongly depended on the maximum temperature reached at the end of the transient heat pulse in combination with the pulse duration. A threshold for this process was found at 936 °C for a pulse duration of 10 ms. Further transient heat pulses applied to specimens that had already formed this transition region resulted in the overheating and melting of the material. The latter occurred between the surface and the transition region and was associated with a strong decrease of the thermal conductivity due to the weakly bound grains across the transition region. Additionally, the transition region caused a partial separation of the melt layer from the bulk material, which could ultimately result in a full detachment of the solidified beryllium layers from the bulk armor. Furthermore, solidified beryllium filaments evolved in several locations of the loaded area and are related to the thermally induced crack formation. However, these filaments are not expected to account for an increase of the beryllium net erosion.

Thermal shock behaviour of H and H/He-exposed tungsten at high temperature

Polycrystalline tungsten samples were characterized and exposed to a pure H beam or mixed H/He beam containing 6% He in GLADIS at a surface temperature of 600 °C, 1000 °C, or 1500 °C. After 5400 s of exposure time with a heat flux of 10.5 MW m−2, the total accumulated fluence of 2 × 1025 m−2 was reached. Thereafter, edge localized mode (ELM)-like thermal shocks with a duration of 1 ms and an absorbed power density of 190 MW m−2 and 380 MW m−2 were applied on the samples in JUDITH 1. During the thermal shocks, the base temperature was kept at 1000 °C. The ELM-experiments with the lowest transient power density did not result in any detected damage. The other tests showed the beginning of crack formation for every sample, except the sample pre-exposed with the pure H-beam at 1500 °C in GLADIS. This sample was roughened, but did not show any crack initiation. With exception to the roughened sample, the category of ELM-induced damage for the pre-exposed samples is identical to the reference tests without pre-exposure to a particle flux.

Experimental study of ELM-like heat loading on beryllium under ITER operational conditions

The experimental fusion reactor ITER, currently under construction in Cadarache, France, is transferring the nuclear fusion research to the power plant scale. ITER’s first wall (FW), armoured by beryllium, is subjected to high steady state and transient power loads. Transient events like edge localized modes not only deposit power densities of up to 1.0 GW m−2 for 0.2–0.5 ms in the divertor of the machine, but also affect the FW to a considerable extent. Therefore, a detailed study was performed, in which transient power loads with absorbed power densities of up to 1.0 GW m−2 were applied by the electron beam facility JUDITH 1 on beryllium specimens at base temperatures of up to 300 °C. The induced damage was evaluated by means of scanning electron microscopy and laser profilometry. As a result, the observed damage was highly dependent on the base temperatures and absorbed power densities. In addition, five different classes of damage, ranging from ‘no damage’ to ‘crack network plus melting’, were defined and used to locate the damage, cracking, and melting thresholds within the tested parameter space.

Thermal shock tests to qualify different tungsten grades as plasma facing material

The electron beam device JUDITH 1 was used to establish a testing procedure for the qualification of tungsten as plasma facing material. Absorbed power densities of 0.19 and 0.38 GW m−2 for an edge localized mode-like pulse duration of 1 ms were chosen. Furthermore, base temperatures of room temperature, 400 °C and 1000 °C allow investigating the thermal shock performance in the brittle, ductile and high temperature regime. Finally, applying 100 pulses under all mentioned conditions helps qualifying the general damage behaviour while with 1000 pulses for the higher power density the influence of thermal fatigue is addressed. The investigated reference material is a tungsten product produced according to the ITER material specifications. The obtained results provide a general overview of the damage behaviour with quantified damage characteristics and thresholds. In particular, it is shown that the damage strongly depends on the microstructure and related thermo-mechanical properties.

Achieving an intense enough maintenance electric field in a low-pressure discharge sustained by a microwave field under ambipolar diffusion regime such that periodic parametric instabilities are generated33This paper is dedicated to the memory of Prof Z Zakrzewski (1935–2014), a microwave discharge specialist.

The intensity of the maintenance electric field of a given discharge is one of its internal parameters. Under ambipolar diffusion conditions, it is almost exclusively set by particle losses, which are related to the dimensions of the discharge vessel and to the gas pressure, and ultimately are determined by the electron energy distribution function. For instance, raising the density of microwave power absorbed in a discharge tube essentially increases the electron density without much increasing the amplitude of the maintenance E-field. To raise the intensity of this E-field in such a case, one needs to reduce the volume into which electromagnetic power is absorbed relative to the diffusion volume, i.e. the volume within which electrons transfer their power through collisions with heavy particles.To show this point, we consider a power balance based on the power lost per electron through collisions with heavy particles, θL, to the power absorbed (over a period of the microwave field) per electron in the discharge, θA. The power θA, which depends on , the square of the amplitude (intensity) of the maintenance electric field, adjusts to compensate for the power lost θL. The analysis presented is achieved for a particular microwave discharge configuration that is known to provide an intense E0-field, which means x ⩾ λDe, where x is the oscillation amplitude of electrons in the E0-field and λDe the electron Debye length. Such a condition allows one to observe periodic parametric instabilities at, or close to, the electron-plasma frequency f pe and at their corresponding ion-plasma frequency f pi, these oscillations being caused by the simultaneous propagation of an electron-plasma wave and an ion-plasma wave in the discharge as a result of an applied ‘pump’ power, which also sustains the discharge.A 2D hydrodynamic calculation of the specific plasma discharge system is performed, which yields the value of the x/λDe ratio in particular through a global energy balance equation. This equation requires that the volume over which power is absorbed in the discharge multiplied by θA must be equal to the discharge diffusion volume multiplied by θL, to which must be added the power lost in the ambipolar field Ea and in the plasma sheath. By comparison, in electromagnetic surface-wave discharges these two volumes are equal. The present considerations could possibly be extended to obtain a better insight into dc, RF and microwave micro-discharges when their discharge volume appears to be smaller than the volume over which particles are lost. Such a situation could explain their unusually high level of absorbed power density, as a result of which highly excited and ionized plasmas are generated.

Frequency considerations for deep ablation with high-intensity focused ultrasound: A simulation study.

The objective of this study was to explore frequency considerations for large-volume, deep thermal ablations with focused ultrasound. Though focal patterns, focal steering rate, and the size of focal clusters have all been explored in this context, frequency studies have generally explored shallower depths and hyperthermia applications. This study examines both treatment efficiency and near-field heating rate as functions of frequency and depth. Flat, 150 mm transducer arrays were simulated to operate at frequencies of 250, 500, 750, 1000, 1250, and 1500 kHz. Each array had λ2 interelement spacing yielding arrays of 2000-70 000 piston-shaped elements arranged in concentric rings. Depths of 50, 100, and 150 mm were explored, with attenuation (α) values of 2.5-10 (Np/m)/MHz. Ultrasound propagation was simulated with the Rayleigh-Sommerfeld integral over a volume of homogeneous simulated tissue. Absorbed power density was determined from the acoustic pressure which, in turn, was modeled with the Pennes bioheat transfer equation. Using this knowledge of temperature over time, thermal dose function of Sapareto and Dewey was used to model the resulting bioeffect of each simulated sonication. Initially, single foci at each depth, frequency, and α were examined with either fixed peak temperatures or fixed powers. Based on the size of the resulting, single foci lesions, larger compound sonications were designed with foci packed together in multiple layers and rings. For each depth, focal patterns were chosen to produce a similar total ablated volume for each frequency. These compound sonications were performed with a fixed peak temperature at each focus. The resulting energy efficiency (volume ablated per acoustic energy applied), near-field heating rate (temperature increase in the anterior third of the simulation space per unit volume ablated), and near- and far-field margins were assessed. Lesions of comparable volume were created with different frequencies at different depths. The results reflect the interconnected nature of frequency as it effects focal size (decreasing with frequency), peak pressure (generally increasing with frequency), and attenuation (also increasing with frequency). The ablation efficiency was the highest for α = 5 (Np/m)/MHz at a frequency of 750 kHz at each depth. For α = 10 (Np/m)/MHz, efficiency was the highest at 750 kHz for a depth of 50 mm, and 500 kHz at depths of 100 and 150 mm. At all sonication depths, near-field heating was minimized with lower frequencies of 250 and 500 kHz. Large-volume ablations are most efficient at frequencies of 500-750 kHz at depths of 100-150 mm. When one considers that near-field heat accumulation tends to be the rate limiting factor in large-volume ablations like uterine fibroid surgery, the results show that frequencies as low as 500 kHz are favored for their ability to reduce heating in the near-field.

Study on hairpin-shaped argon plasma jets resonantly excited by microwave pulses at atmospheric pressure

In the present study, atmospheric pressure argon plasma jets driven by lower-power pulsed microwaves have been proposed with a type of hairpin resonator. The plasma jet plume demonstrates distinctive characteristics, like arched plasma pattern and local plasma bullets. In order to understand how the hairpin resonator works, electromagnetic simulation of the electric field distribution and self-consistent fluid simulation of the interaction between the enhanced electric field and the pulse plasma plume are studied. Simulated spatio-temporal distributions of the electric field, the electron temperature, the electron density, and the absorbed power density have been sampled, respectively. The experimental and simulated results together suggest that the driving mechanism of the hairpin resonator works in the multiple electromagnetic modes of transmission line and microwave resonator, while the local plasma bullets are resonantly generated by local enhanced electric field of surface plasmon polaritons. Moreover, it should be noticed that the radian of the arched plasma plume is mainly affected by the input power and gas flow rate, respectively.