Laser requirements for the design of fast laser-driven semiconductor switches for THz and mm-waves.

A reduced parameter model of fast laser-driven semiconductor switches of THz and mm-waves has been developed. The model predicts peak reflectivity and minimum transmissivity of switches, showing good agreement with experimental data, while requiring fewer inputs than published models. This simplification facilitated a systematic survey of laser parameters required for efficient switching. Laser energy density requirements are presented as a function of laser wavelength, laser pulse width, switched frequency, reflection angle, and semiconductor material (silicon or gallium arsenide). Analytical expressions have been derived to explain the dependence of laser requirements on switch parameters and to derive practical minima of required laser energy density. Diffusion is shown to quickly negate the shallow absorption advantage of laser wavelengths shorter than about 500 nm in silicon or 800 nm in gallium arsenide. Decreasing laser pulse width, to a derived limit, and switching S-polarized THz or mm-wave signals are shown to be means of lowering required laser energy. This is an especially useful result for devices operating at high power levels or THz frequencies, where extended switches are used in quasioptical systems.

Metal-oxide-junction, triple point cathodes in a relativistic magnetron.

Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. To exploit triple point emission, metal-oxide-junction (MOJ) cathodes consisting of dielectric "islands" over stainless steel substrates have been fabricated. The two dielectrics used are hafnium oxide (HfO(x)) for its high dielectric constant and magnesium oxide (MgO) for its high secondary electron emission coefficient. The coatings are deposited by ablation-plasma-ion lithography using a KrF laser (0-600 mJ at 248 nm) and fluence ranging from 3 to 40 J/cm(2). Composition and morphology of deposited films are analyzed by scanning electron microscopy coupled with x-ray energy dispersive spectroscopy, as well as x-ray diffraction. Cathodes are tested on the Michigan Electron Long-Beam Accelerator with a relativistic magnetron, at parameters V=-300 kV, I=1-15 kA, and pulse lengths of 0.3-0.5 micros. Six variations of the MOJ cathode are tested, and are compared against five baseline cases. It is found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back bombardment during magnetron operation, secondary electron emission also may play a significant role. Cathodes exhibit increases in current densities of up to 80 A/cm(2), and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes.

Randomized trial of omeprazole and clarithromycin combined with either metronidazole or amoxycillin in patients with metronidazole-resistant or -susceptible Helicobacter pylori strains.

BACKGROUND: The impact of metronidazole resistance on the efficacy of proton pump inhibitor based triple therapies remains unclear. AIM: To study whether metronidazole resistance affects Helicobacter pylori eradication rates in patients treated for 1 week with either omeprazole 20 mg b.d., metronidazole 400 mg b.d. and clarithromycin 250 mg b.d. (OMC), or omeprazole 20 mg b.d., amoxycillin 1000 mg b.d. and clarithromycin 500 mg b.d. (OAC). METHODS: A randomized, single blind, single centre study with parallel groups was conducted. H. pylori positive patients were enrolled in a metronidazole-resistant (MR; MIC > 8 microgram/mL) or a metronidazole-susceptible group (MS; MIC < 4 microgram/mL), as determined by E-test. Within the strata patients were randomized to either OAC or OMC. RESULTS: One hundred and twenty-two patients were included. The per protocol cure rate for OAC was 52 out of 57 (91%) (MS 23 out of 26 (89%); MR 29 out of 31 (94%)) and for OMC 46 out of 55 (84%) (MS 19 out of 22 (86%); MR 27 out of 33 (82%)). CONCLUSIONS: One-week OAC and OMC are effective therapies. OAC and OMC were equally effective in patients with metronidazole-susceptible strains of H. pylori. Using the OMC regimen, neither equality nor significant differences in treatment outcome could be shown between patients with metronidazole-resistant or -susceptible strains of H. pylori.

Helicobacter pylori Stool Antigen test: a reliable non-invasive test for the diagnosis of Helicobacter pylori infection in children.

OBJECTIVE: To evaluate the Helicobacter pylori Stool Antigen (HpSA) test for the diagnosis of H. pylori infection in children. DESIGN AND SETTING: Prospective cohort study in an academic medical centre. PATIENTS AND METHODS: A total of 106 consecutive children who underwent gastroscopy were included. Biopsy specimens were sampled from the gastric antrum and corpus for the assessment of H. pylori infection by culture and histology. A patient was defined to be H. pylori positive if the results of culture and/or histology proved to be H. pylori positive; a patient was defined to be negative if both test results were negative. All children provided a stool sample within 2 days of gastroscopy. H. pylori antigens in faeces were assessed by an enzyme immunoassay (Premier HpSA, Meridian Diagnostics, Inc., Cincinnati, OH, USA). RESULTS: The mean age of included patients was 8.5 years (range 1-18.5). Thirty patients were H. pylori positive and 76 patients were H. pylori negative. Using the recommended cut-off values of 0.140 optical density (OD) and 0.159 OD, sensitivity and specificity of 100% and 92% were found. The positive and negative predicting values were 83% (30/36) and 100% (70/70), respectively. CONCLUSION: The HpSA test is an accurate test for the diagnosis of H. pylori infection in children, and might therefore be a good alternative for diagnostic tests such as the 13C-urea breath test (UBT).

[Pregnancy during residency]. / Zwanger en in opleiding tot specialist.

The number of female residents in the Netherlands has steadily increased in recent years. Due to the increased time on waiting lists to enter residency programmes and to the increased duration of training, female residents will be older during their residencies. This will probably result in an increased number of pregnancies during residencies. A questionnaire regarding pregnancy during residency was sent to 191 residents in two university hospitals in the Netherlands. The response rate was 74.3%. Fifty percent of the male and only 19% of the female residents had children. No negative effects of a pregnancy on their training were experienced or anticipated by the residents. However, a negative effect on the functioning of the department was expected. No formal provisions, like replacements were available and many solutions to replace pregnant colleagues depended on the flexibility of the colleagues. The wish to have children was high and equally distributed among male and female residents, 92% and 96%, resp. Given the difficulty to seek a permanent position and to have children after residency, the choice of many female residents will be to have their children during residency. This increase in number of pregnancies requires anticipation of the residency programme directors. They should take the lead in proposing adequate regulations.

[Treatment of Helicobacter pylori infections]. / Behandeling van Helicobacter pylori-infectie.

H. pylori infection is a very common infection of the human stomach, related to various diseases such as gastritis, ulcerative disease of the duodenum and stomach, mucosa associated lymphoid tissue lymphomas and gastric cancer. It is generally accepted that the majority of related diseases is curable once H. pylori infection is successfully eradicated. H. pylori eradication will only be achieved after administration of combination therapies including antibiotics and antacids. Resistance to metronidazole and clarithromycin is an emerging problem, leading to impairment of eradication efficacy, irrespective of the regimen administered. Eradication regimens consisting of three or four anti-H. pylori drugs are the most efficacious. Given the easy use and high convenience to the patients and the high efficacy, 1-week proton pump inhibitor (PPI)-triple therapy is preferred as the first-line therapy. Alternatively, bismuth triple therapy or the combination of antacids and bismuth triple therapy can be prescribed; however, these regimens are less convenient to the patients, the course is prolonged or the side effects occur more frequently compared with PPI-triple therapy.

Helicobacter pylori eradication therapy in The Netherlands.

BACKGROUND: Helicobacter pylori cure rates vary in different geographical regions because of differences in hosts as well as in H. pylori strains. OBJECTIVE: To review systematically all available data in the literature to determine H. pylori eradication rates in The Netherlands. METHODS: A search of all published trials on H. pylori eradication therapy performed in The Netherlands was conducted via electronic database search, hand-searching of abstracts from scientific meetings and checking reference lists of pharmaceutical companies. Full papers and abstracts were included. Data on anti-H. pylori therapies were pooled based on duration and combination of drugs. Only triple and quadruple eradication regimens were studied. Dual therapies were excluded, except for ranitidine bismuth citrate-based dual therapies. RESULTS: We analysed 38 study arms, involving 2197 patients. Twenty different pooled regimens were studied with a mean intention-to-treat eradication rate of 83% (range 35-96%). There were no significant differences in the percentage of patients that stopped treatment due to adverse events between the groups. In these pooled regimens only bismuth combined with tetracycline and metronidazole for 1 or 2 weeks was significantly lower in efficacy in metronidazole-resistant strains than in metronidazole-sensitive strains. The prevalence of metronidazole-resistant strains in The Netherlands showed large regional differences (7-50%). CONCLUSIONS: A therapy should be tested in a defined population before becoming standard. Several eradication regimens studied in The Netherlands yield acceptable cure rates of 80% or more on an intention-to-treat basis. We advise taking the local prevalence of metronidazole resistance into account when choosing a first-line eradication regimen.

Microwave window breakdown experiments and simulations on the UM/L-3 relativistic magnetron.

Experiments have been performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. In the baseline case, acrylic microwave windows were mounted between three of the waveguide coupling cavities in the anode block vacuum housing and the output waveguides. Each of the six 3 cm deep coupling cavities is separated from its corresponding anode cavity by a 1.75 cm wide aperture. In the baseline case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. In the new window configuration, three Air Force Research Laboratory-designed, vacuum-rated directional coupler waveguide segments were mounted between the coupling cavities and the microwave windows. The inclusion of the vacuum side power couplers moved the microwave windows an additional 30 cm away from the anode apertures. Additionally, the Lucite microwave windows were replaced with polycarbonate windows and the microwave window mounts were redesigned to better maintain waveguide continuity in the region around the microwave windows. No vacuum side window breakdown was observed in the new window configuration at single waveguide output powers of 120+MW (a factor of 3 increase in measured microwave pulse duration and factor of 3 increase in measured peak power over the baseline case). Simulations were performed to investigate likely causes for the window breakdown in the original configuration. Results from these simulations have shown that in the original configuration, at typical operating voltage and magnetic field ranges, electrons emitted from the anode block microwave apertures strike the windows with a mean kinetic energy of 33 keV with a standard deviation of 14 keV. Calculations performed using electron impact angle and energy data predict a first generation secondary electron yield of 65% of the primary electron population. The effects of the primary aperture electron impacts, combined with multiplication of the secondary populations, were determined to be the likely causes of the poor microwave window performance in the original configuration.