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Biomedical subjects

K Ronald

Publications and source records attributed to K Ronald.

At least 19 recordsLinked to original sources
PubMed2006

Experimental study of coaxial free-electron maser based on two-dimensional distributed feedback.

The first experimental study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback is presented. A new type of cavity formed with coaxial 2D surface photonic band gap structures was used. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam of energy 475 keV. By tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The -band coaxial FEM generated 15 MW of radiation with a 6% conversion efficiency, in excellent agreement with theory.

Journal Article↗
PubMed2004

Dispersion of helically corrugated waveguides: analytical, numerical, and experimental study.

Helically corrugated waveguides have recently been studied for use in various applications such as interaction regions in gyrotron traveling-wave tubes and gyrotron backward-wave oscillators and as a dispersive medium for passive microwave pulse compression. The paper presents a summary of various methods that can be used for analysis of the wave dispersion of such waveguides. The results obtained from an analytical approach, simulations with the three-dimensional numerical code MAGIC, and cold microwave measurements are analyzed and compared.

Journal Article↗
PubMed2004

Compression of frequency-modulated pulses using helically corrugated waveguides and its potential for generating multigigawatt rf radiation.

A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory.

Journal Article↗
PubMed2003

Experimental studies of the influence of distributed power losses on the transparency of two-dimensional surface photonic band-gap structures.

Two-dimensional (2D) surface photonic band-gap (SPBG) structures have been suggested to realize 2D distributed feedback. The 2D SPBG structures can be obtained by providing 2D periodic perturbations of the waveguide surface. Such a structure can be used in a wide variety of applications including microwave electronics and integrated optics. The theoretically predicted effect of the transparency of the 2D SPBG structure when distributed Ohmic losses inside the structure are relatively high in comparison with the wave coupling coefficient has been observed in a series of experiments. The results obtained are in good agreement with theoretical predictions.

Journal Article↗
PubMed2000

High-gain wide-band gyrotron traveling wave amplifier with a helically corrugated waveguide

First bandwidth measurements of a novel gyrotron amplifier are presented. The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz ( 21% relative bandwidth), and an efficiency of 29%, in good agreement with theory.

Journal Article↗
PubMed2000

Self-amplification of coherent spontaneous emission in a cherenkov free-electron maser

Ultrashort pulses of microwave radiation have been produced in a dielectric-lined Cherenkov free-electron maser (FEM) amplifier. An intense initial seed pulse, due to coherent spontaneous emission (CSE), arises at the leading edge of the electron pulse. There is evidence to show that 3-4 cycle spikes are produced through the amplification of these seed pulses. A strong dependence of the start-up power on the rise time of the electron pulse has been found. The experimental results are verified by a theoretical analysis. Our study shows that amplification in a FEM amplifier is always initiated by CSE arising from the edge of the electron pulse when the rise time is comparable to the electromagnetic wave period.

Journal Article↗
PubMed1999

Generation of powerful subnanosecond microwave pulses by intense electron bunches moving in a periodic backward wave structure in the superradiative regime.

Experimental results of the observation of coherent stimulated radiation from subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating wave are presented. The subnanosecond microwave pulses in Ka and W bands were generated with repetition frequencies of up to 25 Hz. The mechanism of microwave pulse generation was associated with self-bunching, and the mutual influence of different parts of the electron pulse due to slippage of the wave with respect to the electrons; this can be interpreted as superradiance. The illumination of a panel of neon bulbs resulted in a finely structured pattern corresponding to the excitation of the TM01 mode. Observation of rf breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the absolute peak power as high as 60 MW for the 300-ps pulses at 38 GHz. These results are compared with numerical simulations. The initial observation of 75-GHz, 10-15-MW radiation pulses with a duration of less than 150 ps is also reported.

Journal Article↗
PubMed1992

Demonstration of antibodies in archival sera from Canadian seals reactive with a European isolate of phocine distemper virus.

Sera from seals infected during the 1988 European epizootic of phocine distemper virus and sera from Canadian seals collected since 1972 have been tested for the presence of antibodies to morbillivirus. Approximately one third of the Canadian sera have been shown to contain anti-morbillivirus antibodies; the possibility that these populations of seals provided a source of infection for European seals is discussed.

Animals↗
PubMed1989

The phospholipid and fatty acid compositions of seal platelets: a comparison with human platelets.

1. Platelet phospholipid compositions were studied in four species of phocid seals consuming herring or herring and shrimp and in human subjects consuming a normal mixed diet. 2. There were no major differences in platelet phospholipid, cholesterol and protein levels between different species of seal nor between seals and human subjects, nor in the relative abundance of the individual types of phospholipid. 3. The seal platelet phospholipids (phosphatidylcholine (PC) and phosphatidylethanolamine (PE), were greatly enriched in the omega 3 fatty acid, eicosapentaenoic acid (EPA) and depressed in arachidonic acid (AA) relative to the corresponding human platelet phospholipids. 4. Much less accumulation of EPA in phosphatidylserine (PS) and phosphatidylinositol (PI) was found. 5. The EPA contents of the individual seal platelet phospholipids exhibited considerable differences (including EPA discrimination from PI) but gave patterns which were generally similar to those reported for human volunteers consuming fish/fish oils enriched in EPA. 6. These results suggest that the seal platelet may be a useful model for studying the metabolism and function of the omega 3 fatty acids, such as EPA, in relation to platelet reactivity, phospholipid turnover and the formation of AA- and EPA-derived eicosanoids.

Animals↗
PubMed1988

Vitamin D metabolism in the hooded seal (Cystophora cristata).

Fish-eating mammals, such as seals, appear to ingest levels of vitamin D that are toxic to most mammals. To determine how seals cope with high vitamin D intakes, the metabolism of tritiated cholecalciferol ([3H]D3) was investigated in hooded seal (Cystophora cristata) pups during their postweaning fast and pups and adults consuming herring alone or supplemented with 400,000 iu D3 daily. [3H]D3 was metabolized to 25-[3H]OHD3 and 24,25-[3H](OH)2D3. 1,25-[3H](OH)2D3 was not detected, but plasma levels of 1,25-(OH)2D were similar to those in other mammals and were not affected by vitamin D intake. Plasma vitamin D, 25-OHD and 24,25-(OH)2D increased with vitamin D intake, but 25-OHD did not increase to the extent seen in other mammals. The supplemented seals showed no evidence of toxicity. Levels of 24,25-(OH)2D were higher in the unsupplemented seals (4 to 33 ng/mL) than reported in other mammals with similar 25-OHD levels and did not decrease with 25-OHD. High levels of 24,25-(OH)2D relative to 25-OHD have also been found in hooded seals in the wild. The half-lives of vitamin D, 25-OHD and 24,25-(OH)2D were shorter than those reported for most other mammals. Increased conversion of 25-OHD to 24,25-(OH)2D and a high capacity for vitamin D storage in their large blubber mass appeared to be factors in the resistance of seals to vitamin D toxicity.

Animals↗
PubMed1988

Kinematics and estimated thrust production of swimming harp and ringed seals.

The propulsive motions of swimming harp seals (Phoca groenlandica Erxleben) and ringed seals (Phoca hispida Schreber) were studied by filming individuals in a flume. The seals swam at velocities ranging from 0.6 to 1.42 m s-1. Locomotion was accomplished with alternate lateral sweeps of the hind flippers generated by lateral flexions of the axial body in conjunction with flexion of the flippers. The frequency of the propulsive cycle increased linearly with the swimming velocity, and the maximum angle of attack of the flipper decreased, but the amplitude remained constant. The kinematics and morphology of this hind flipper motion indicated that phocid seals do not swim in the carangiform mode as categorized by Lighthill (1969), but in a distinct mode that mimics swimming by thunniform propulsors. The hind flippers acted as hydrofoils, and the efficiency, thrust power and coefficient of thrust were calculated from unsteady wing theory. The propulsive efficiency was high at approximately 0.85. The thrust power increased curvilinearly with velocity. The drag coefficient ranged from 0.012 to 0.028 and was found to be 2.8-7.0 times higher than the theoretical minimum. The drag coefficient was high compared with that of phocid seals examined during gliding or towing experiments, indicating an increased drag encumbered by actively swimming seals. It was determined that phocid seals are capable of generating sufficient power for swimming with turbulent boundary layer conditions.

Animals↗
PubMed1987

Blood levels of thyroid hormones and certain metabolites in relation to moult in the harp seal (Phoca groenlandica).

1. Changes in blood levels of thyroxine (T4), triiodothyronine (T3), free fatty acids (FFA), glucose, pyruvic acid and lactic acid in the harp seal during moult, were studied. 2. Serum levels of both T4 and T3 showed significant increase in the moult phase from that in the pre-moult phase. While T4 level continued to remain high during the early and late post-moult phases, T3 level dropped in the latter two phases to the same low pre-moult level. 3. The T3/T4 ratio was significantly higher during the pre-moult phase than that in all the other phases. It is suggested that the high pre-moult T3/T4 ratio marks the initiation of moult. 4. There were no significant changes in the levels of the metabolites studied except that of FFA which was highest in the moult phase indicating the hormonal basis of lipid mobilization.

Animals↗
PubMed1985

Weight loss and catabolic adaptations to starvation in grey seal pups.

Five grey seal pups lost from 18 to 32% of their initial body weight during a 21 day starvation period. Blubber fat mass density was 0.93 +/- 0.03 g/cm3. A considerable loss of blubber fat was recorded, but analysis of the weight loss and body size data indicated that blubber fat was retained for thermoregulatory reasons, particularly in the lean, smaller seals. It is possible that phocid seals during periods of negative energy balance have a higher rate of protein catabolism than normal for terrestrial mammals.

Adaptation, Physiological↗
PubMed1985

Potentially conflicting metabolic demands of diving and exercise in seals.

Metabolic replacement rates (Ra) for glucose and free fatty acids (FFA) were determined during rest, exercise, and diving conditions in the gray seal using bolus injections of radiotracers. In the exercise experiments the seal swam at a metabolic rate elevated twofold over resting Ra for glucose and FFA while resting were similar to values found in terrestrial mammals and other marine mammal species. During exercise periods glucose turnover increased slightly while FFA turnover changes were variable. However, the energetic demands of exercise could not be met by the increase in the replacement rates of glucose or FFA even if both were completely oxidized. Under diving conditions the tracer pool displayed radically different specific activity curves indicative of the changes in perfusion and metabolic rate associated with a strong dive response. Since the radiotracer curves during exercise and diving differed qualitatively and quantitatively, it is possible that similar studies on freely diving animals can be used to assess the role of the diving response during underwater swimming in nature.

Animals↗
PubMed1984

Pollutants in harp seals (Phoca groenlandica). I. Organochlorines.

Between 1976 and 1978, 248 harp seals were sampled from 5 locations in the Northwest Atlantic and Arctic for organochlorine (OC) residue analysis in tissue. Blood, kidney, brain, muscle and blubber samples were analysed for PCB, DDT, dieldrin, hexachlorobenzene (HCB), and heptachlor epoxide. Levels were generally greatest in blubber tissue. Overall, mean levels of DDT and PCB were greater than those of other OC. A barrier prevented OC compounds from accumulating in the brain to the level one might expect in lipid tissues. Positive correlations were found between 1: DDT and PCB, 2: DDT and dieldrin, and 3: PCB and deildrin. Male harp seals bioaccumulated DDT, PCB, heptachlor epoxide and dieldrin with age. As a group, adult males sampled in the Gulf of St. Lawrence carried the highest concentrations of all OC, except that chlordane did not appear to bioaccumulate and levels of hexachlorobenzene were minimal. Females generally ceased exhibiting significant bioaccumulation once breeding age was reached, due to transplacental and transmammary residue transfer of OC from mother to pup. There were some significant differences in OC levels between locations with DDT: Gulf greater than Front; PCB: Gulf greater than Front; dieldrin: Gulf greater than Front; heptachlor epoxide: Front greater than Gulf. No significant differences were found, however, when northern locations (Grise Fiord, Northwest Greenland, Pangnirtung) were compared to southern (Gulf, Front) on a group basis. The latter is not surprising in view of the harp seals extensive cyclical annual migration between the southern breeding grounds and the High Arctic.

Animals↗
PubMed1984

Pollutants in harp seals (Phoca groenlandica). II. Heavy metals and selenium.

Between 1976 and 1978, 249 harp seals were sampled from five locations in the Northwest Atlantic and Arctic for heavy metal and selenium residue analyses in tissue. Significant loading was apparent only in blood, brain, kidney, liver and muscle. Samples were analysed for mercury (Hg), selenium (Se), copper (Cu), cadmium (Cd) and lead (Pb). The seals carried higher levels of Cd than of the other metals. Residue levels of Cd were highest in kidney tissue; levels of other metals were highest in liver. Both males and females bioaccumulated Cd, Hg and Se. Cd, Hg, Se and Cu residues were detected in tissue from neonatal seals, indicating that transplacental and transmammary transfer of these elements had occurred. Despite the passage of residues from mother to pup, females bore significantly higher levels of Hg and Cd than males. Conversely, levels of Cu, Se or Pb did not appear to differ significantly between sexes. There was considerable individual variation in residue levels. This, coupled with the extensive annual migration undergone by these animals, made it difficult to arrive at definite conclusions regarding geographic accumulation patterns.

Animals↗
PubMed1981

Plasma concentrations of oestrone, progesterone and corticosteroids during late pregnancy and after parturition in the harbour seal, Phoca vitulina.

Blood samples were taken at weekly intervals during the last 2 months of pregnancy from a mature (7-year-old) harbour seal which had conceived and was kept in captivity. The concentrations of oestrone, oestrone sulphate, progesterone and glucocorticoids were determined by radioassays. The plasma levels of unconjugated oestrone were slightly greater than those of oestrone sulphate. Total oestrone declined steadily over the last month from a peak of 2.3 ng/ml at 30 days before parturition. Plasma progesterone concentrations rose to 61 ng/ml at 2 weeks before parturition and fell to about half that value by 2 days before birth of a normal male pup. Plasma glucocorticosteroids reached a peak of 164 ng/ml, also at 2 weeks, but showed only a slight decline thereafter. A higher value (392 ng/ml) was recorded 5 days after parturition.

Animals↗