On the Capacity of Non-Uniform Phase MIMO Nakagami-m Fading Channels

This letter investigates the ergodic capacity of MIMO Nakagami-m fading channels with both uniformly and non-uniformly distributed phases. We first obtain a tight capacity upper bound for the channel and then derive exact expressions for the low signal-to-noise ratio (SNR) capacity metrics, based on which we examine the impact of fading parameter m on the capacity.

Dose, dose-rate and field size effects on cell survival following exposure to non-uniform radiation fields

For the delivery of intensity-modulated radiation therapy (IMRT), highly modulated fields are used to achieve dose conformity across a target tumour volume. Recent in vitro evidence has demonstrated significant alterations in cell survival occurring out-of-field which cannot be accounted for on the basis of scattered dose. The radiobiological effect of area, dose and dose-rate on out-of-field cell survival responses following exposure to intensity-modulated radiation fields is presented in this study. Cell survival was determined by clonogenic assay in human prostate cancer (DU-145) and primary fibroblast (AG0-1522) cells following exposure to different modulated field configurations delivered using a X-Rad 225 kVp x-ray source. Uniform survival responses were compared to in- and out-of-field responses in which 25-99% of the cell population was shielded. Dose delivered to the out-of-field region was varied from 1.6-37.2% of that delivered to the in-field region using different levels of brass shielding. Dose rate effects were determined for 0.2-4 Gy min⁻¹ for uniform and modulated exposures with no effect seen in- or out-of-field. Survival responses showed little dependence on dose rate and area in- and out-of-field with a trend towards increased survival with decreased in-field area. Out-of-field survival responses were shown to scale in proportion to dose delivered to the in-field region and also local dose delivered out-of-field. Mathematical modelling of these findings has shown survival response to be highly dependent on dose delivered in- and out-of-field but not on area or dose rate. These data provide further insight into the radiobiological parameters impacting on cell survival following exposure to modulated irradiation fields highlighting the need for refinement of existing radiobiological models to incorporate non-targeted effects and modulated dose distributions.

Conventional in vivo irradiation procedures are insufficient to accurately determine tumor responses to non-uniform radiation fields

Purpose: To determine differences in overall tumor responses measured by volumetric assessment and bioluminescence imaging (BLI) following exposure to uniform and non-uniform radiation fields in an ectopic prostate tumor model.

Materials and methods: Bioluminescent human prostate tumor xenografts were established by subcutaneous implantation into male mice. Tumors were irradiated with uniform or non-uniform field configurations using conventional in vivo irradiation procedures performed using a 225 kVp generator with custom lead shielding. Tumor responses were measured using Vernier calipers and by BLI using an in vivo imaging system. Survival was defined as the time to quadroupling of pre-treatment tumor volume. 

Results: The correlation between BLI and tumor volume measurements was found to be different for un-irradiated (R = 0.61), uniformly irradiated (R = 0.34) and partially irradiated (R = 0.30) tumors. Uniformly irradiated tumors resulted in an average tumor growth delay of 60 days with median survival of 75 days, compared to partially irradiated tumors which showed an average growth delay of 24 days and median survival of 38 days. 

Conclusions: Correlation between BLI and tumor volume measurements is lower for partially irradiated tumors than those exposed to uniform dose distributions. The response of partially irradiated tumors suggests non-uniformity in response beyond physical dose distribution within the target volume. Dosimetric uncertainty associated with conventional in vivo irradiation procedures prohibits their ability to accurately determine tumor response to non-uniform radiation fields and stresses the need for image guided small animal radiation research platforms.

Systematic Engineering of Uniform, Highly Efficient, Targeted and Shielded Viral-Mimetic Nanoparticles

In the past decades, numerous types of nanomedicines have been developed for the efficient and safe delivery of nucleic acid-based drugs for cancer therapy. Given that the destination sites for nucleic acid-based drugs are inside cancer cells, delivery systems need to be both targeted and shielded in order to overcome the extracellular and intracellular barriers. One of the major obstacles that has hindered the translation of nanotechnology-based gene-delivery systems into the clinic has been the complexity of the design and assembly processes, resulting in non-uniform nanocarriers with unpredictable surface properties and efficiencies. Consequently, no product has reached the clinic yet. In order to address this shortcoming, a multifunctional targeted biopolymer is genetically engineered in one step, eliminating the need for multiple chemical conjugations. Then, by systematic modulation of the ratios of the targeted recombinant vector to PEGylated peptides of different sizes, a library of targeted-shielded viral-mimetic nanoparticles (VMNs) with diverse surface properties are assembled. Through the use of physicochemical and biological assays, targeted-shielded VMNs with remarkably high transfection efficiencies (>95%) are screened. In addition, the batch-to-batch variability of the assembled targeted-shielded VMNs in terms of uniformity and efficiency is examined and, in both cases, the coefficient of variation is calculated to be below 20%, indicating a highly reproducible and uniform system. These results provide design parameters for engineering uniform, targeted-shielded VMNs with very high cell transfection rates that exhibit the important characteristics for in vivo translation. These design parameters and principles could be used to tailor-make and assemble targeted-shielded VMNs that could deliver any nucleic acid payload to any mammalian cell type.

Tephra-linked peat humification records from Irish ombrotrophic bogs question nature of solar forcing at 850 cal. yr BC.

This paper investigates evidence for palaeoclimatic changes during the period ca. 1500-500 cal. yr BC through peat humification studies on seven Irish ombrotrophic bogs. The sites are well-correlated by the identification of three mid-first millennium BC tephras, which enable the humification records at specific points in time to be directly compared. Phases of temporarily increased wetness are suggested at ca. 1300-1250 cal. yr BC, ca. 1150-1050 cal. yr BC, ca. 940 cal. yr BC and ca. 740 cal. yr BC. The last of these is confirmed to be synchronous at five sites, suggesting external forcing on a regional scale. The timing of this wet-shift is constrained by two closely dated tephras and is demonstrated to be distinct from the widely reported changes to cooler/wetter conditions associated with a solar minimum at 850-760 cal. yr BC, at which time the Irish sites appear instead to experience drier conditions. The results suggest the possibility of either non-uniform responses to solar forcing in northwest Europe at this time, or the existence of unrelated climate events in the early first millennium BC. The findings caution against the correlation of loosely dated palaeoclimate data if the effects of forcing mechanisms are to be understood.

Plane wave and Coulomb asymptotics

A simple plane wave solution of the Schrodinger-Helmholtz equation is a quantum eigenfunction obeying both energy and linear momentum correspondence principles. Inclusion of the outgoing wave with scattering amplitude f asymptotic development of the plane wave, we show that there is a problem with angular momentum when we consider forward scattering at the point of closest approach and at large impact parameter given semiclassically by (l + 1/2)/k where l is the azimuthal quantum number and may be large (J. Leech et al., Phys. Rev. Lett. 88. 257901 (2002)). The problem is resolved via non- uniform, non-standard analysis involving the Heaviside step function, unifying classical, semiclassical and quantum mechanics, and the treatment is extended to the case of pure Coulomb scattering.

Plasma dynamics in an inductively coupled magnetic neutral loop discharge

Magnetic neutral loop discharges (NLDs) can be operated at significantly lower pressures than conventional radio-frequency (rf) inductively coupled plasmas (ICPs). These low pressure conditions are favourable for technological applications, in particular anisotropic etching. An ICP–NLD has been designed providing excellent diagnostics access for detailed investigations of fundamental mechanisms. Spatially resolved Langmuir probe measurements have been performed in the plasma production region (NL region) as well as in the remote application region downstream from the NL region. Depending on the NL gradient two different operation modes have been observed exhibiting different opportunities for control of plasma uniformity. The efficient operation at comparatively low pressures results in ionization degrees exceeding 1%. In this regime neutral dynamics has to be considered and can influence neutral gas and process uniformity. Neutral gas depletion through elevated gas temperatures and high ionization rates have been quantified. At pressures above 0.1 Pa, gas heating is the dominant depletion mechanism. At lower pressures neutral gas is predominantly depleted through high ionization rates and rapid transport of ions by ambipolar diffusion along the magnetic field lines. Non-uniform profiles of the ionization rate can, therefore, result in localized neutral gas depletion and non-uniform processing. We have also investigated the electron dynamics within the radio-frequency cycle using phase resolved optical emission spectroscopy and Thomson scattering. In these measurements electron drift phenomena along the NL torus have been identified.

Neutral gas depletion mechanisms in dense low-temperature argon plasmas

Neutral gas depletion mechanisms are investigated in a dense low-temperature argon plasma-an inductively coupled magnetic neutral loop (NL) discharge. Gas temperatures are deduced from the Doppler profile of the 772.38 nm line absorbed by argon metastable atoms. Electron density and temperature measurements reveal that at pressures below 0.1 Pa, relatively high degrees of ionization (exceeding 1%) result in electron pressures, p(e) = kT(e)n(e), exceeding the neutral gas pressure. In this regime, neutral dynamics has to be taken into account and depletion through comparatively high ionization rates becomes important. This additional depletion mechanism can be spatially separated due to non-uniform electron temperature and density profiles (non-uniform ionization rate), while the gas temperature is rather uniform within the discharge region. Spatial profiles of the depletion of metastable argon atoms in the NL region are observed by laser induced fluorescence spectroscopy. In this region, the depletion of ground state argon atoms is expected to be even more pronounced since in the investigated high electron density regime the ratio of metastable and ground state argon atom densities is governed by the electron temperature, which peaks in the NL region. This neutral gas depletion is attributed to a high ionization rate in the NL zone and fast ion loss through ambipolar diffusion along the magnetic field lines. This is totally different from what is observed at pressures above 10 Pa where the degree of ionization is relatively low (

A study of the biological effects of modulated 6 MV radiation fields

The delivery of spatially modulated radiation fields has been shown to impact on in vitro cell survival responses. To study the effect of modulated fields on cell survival, dose response curves were determined for human DU-145 prostate, T98G glioma tumour cells and normal primary AGO-1552 fibroblast cells exposed to modulated and non-modulated field configurations delivered using a 6 MV Linac with multi-leaf collimator. When exposed to uniform fields delivered as a non-modulated or modulated configuration, no significant differences in survival were observed with the exception of DU-145 cells at a dose of 8 Gy (p = 0.024). Survival responses were determined for exposure to non-uniform-modulated beams in DU-145 and T98G and showed no deviation from the survival response observed following uniform non-modulated exposures. The results of these experiments indicate no major deviation in response to modulated fields compared to uniform exposures.

Modelling the mechanical response of the Reed-mouthpiece-lip system of a clarinet. Part I. A one-dimensional distributed model

The motion of a clarinet reed that is clamped to a mouthpiece and supported by a lip is simulated in the time-domain using finite difference methods. The reed is modelled as a bar with non-uniform cross section, and is described using a one-dimensional, fourth-order partial differential equation. The interactions with the mouthpiece Jay and the player's lip are taken into account by incorporating conditional contact forces in the bar equation. The model is completed by clamped-free boundary conditions for the reed. An implicit finite difference method is used for discretising the system, and values for the physical parameters are chosen both from laboratory measurements and by accurate tuning of the numerical simulations. The accuracy of the numerical system is assessed through analysis of frequency warping effects and of resonance estimation. Finally, the mechanical properties of the system are studied by analysing its response to external driving forces. In particular, the effects of reed curling are investigated.

Single-phase fluid flow distribution and heat transfer in microstructured reactors

Single-phase microreactors and micro-heat-exchangers have been widely used in industrial and scientific applications over the last decade. In several cases, operation of microreactors has shown that their expected efficiency cannot be reached either due to non-uniform distribution of reactants between different channels or due to flow maldistribution between individual microreactors working in parallel. The latter problem can result in substantial temperature deviations between different microreactors resulting in thermal run away which could arise from an exothermicreaction. Thus advances in the understanding of heat transfer and fluid flow distribution continue to be crucial in achieving improved performance, efficiency and safety in microstructured reactors used for different applications. This paper presents a review of the experimental and numerical results on fluid flow distribution, heat transfer and combination thereof, available in the open literature. Heat transfer in microchannels can be suitably described by standard theory and correlations, but scaling effects (entrance effects, conjugate heat transfer, viscous heating, and temperature-dependent properties) have often to be accounted for in microsystems. Experiments with single channels are in good agreement with predictions from the published correlations. The accuracy of multichannel experiments is lower due to flow maldistribution. Special attention is devoted to theoretical and experimental studies on the effect of a flow maldistribution on the thermal and conversion response of catalytic microreactors. There view concludes with a set of design recommendations aimed at improving the reactor performance. (C) 2010 Elsevier Ltd. All rights reserved.

Relativistic laser pulse compression in plasmas with a linear axial density gradient

The self-compression of a relativistic Gaussian laser pulse propagating in a non-uniform plasma is investigated. A linear density inhomogeneity (density ramp) is assumed in the axial direction. The nonlinear Schrodinger equation is first solved within a one-dimensional geometry by using the paraxial formalism to demonstrate the occurrence of longitudinal pulse compression and the associated increase in intensity. Both longitudinal and transverse self-compression in plasma is examined for a finite extent Gaussian laser pulse. A pair of appropriate trial functions, for the beam width parameter (in space) and the pulse width parameter (in time) are defined and the corresponding equations of space and time evolution are derived. A numerical investigation shows that inhomogeneity in the plasma can further boost the compression mechanism and localize the pulse intensity, in comparison with a homogeneous plasma. A 100 fs pulse is compressed in an inhomogeneous plasma medium by more than ten times. Our findings indicate the possibility for the generation of particularly intense and short pulses, with relevance to the future development of tabletop high-power ultrashort laser pulse based particle acceleration devices and associated high harmonic generation. An extension of the model is proposed to investigate relativistic laser pulse compression in magnetized plasmas.

Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/heat-exchanger for optimal performance in platinum catalyzed ammonia oxidation

The present work is focused on the demonstration of the advantages of miniaturized reactor systems which are essential for processes where potential for considerable heat transfer intensification exists as well as for kinetic studies of highly exothermic reactions at near-isothermal conditions. The heat transfer characteristics of four different cross-flow designs of a microstructured reactor/heat-exchanger (MRHE) were studied by CFD simulation using ammonia oxidation on a platinum catalyst as a model reaction. An appropriate distribution of the nitrogen flow used as a coolant can decrease drastically the axial temperature gradient in the reaction channels. In case of a microreactor made of a highly conductive material, the temperature non-uniformity in the reactor is strongly dependent on the distance between the reaction and cooling channels. Appropriate design of a single periodic reactor/heat-exchanger unit, combined with a non-uniform inlet coolant distribution, reduces the temperature gradients in the complete reactor to less than 4degreesC, even at conditions corresponding to an adiabatic temperature rise of about 1400degreesC, which are generally not accessible in conventional reactors because of the danger of runaway reactions. To obtain the required coolant flow distribution, an optimization study was performed to acquire the particular geometry of the inlet and outlet chambers in the microreactor/heat-exchanger. The predicted temperature profiles are in good agreement with experimental data from temperature sensors located along the reactant and coolant flows. The results demonstrate the clear potential of microstructured devices as reliable instruments for kinetic research as well as for proper heat management in the case of highly exothermic reactions. (C) 2002 Elsevier Science B.V. All rights reserved.

NE-LIKE GE SLAB TARGET STUDIES CARRIED OUT AT RAL

The effect of non-uniform target illumination on the soft X-ray lasing output intensity of the J=2-1 Ne-like Ge transitions as a function of length was investigated. As the degree of nonunifonnity increased with length the Ne-like Ge 23.2 and 23.6 nm J=2-1 transitions did not show exponentiation of output intensity. Using an experimentally measured gain-intensity scaling relationship these results were modelled and good qualitative agreement obtained. The model indicates that for Ge targets which are non-uniformly illuminated, even with peak to valley ratios of up to 3 efficient operation can be achieved at between 2-3x threshold intensity. Further studies of the effect of increasing the separation between the two targets of a double target geometry are also presented.