A corona-stabilised plasma closing switch

Corona-stabilised plasma closing switches, filled with electronegative gases such as SF6 and air, have been used in pulsed-power applications as repetitive switching devices for the last 10 years. Their high repetition-rate capabilities coupled with their relatively simple design and construction have made them suitable alternatives to thyratrons and semi-conductor switches. As well as having repetitive switching capabilities, corona-stabilised plasma closing switches have the potential to operate at elevated voltages through the incorporation of multiple electrode sets. This allows high-voltage operation with inherent voltage grading between the electrodes. A further feature of such switches is that they can have relatively low jitter under triggered condition. This paper reports on some of the operational features of a new design of corona-stabilised, cascade switch that utilises air as the insulating gas. At pressures between 0 and 1 bar gauge the switch has be shown to operate over the voltage range of 40 to 100 kV with a jitter below 2 ns

Sintered Hydroxyapatite Ceramic for Wear Studies

A sintered hydroxyapatite (HAP) ceramic for use in wear studies was prepared from a commerical tricalcium phosphate. The sintered HAP had physical properties close to those of human enamel. The coefficient of friction and wear of the sintered HAP ceramic as characterized by tangential force, track width, and surface failure data, approximated those of human enamel.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67948/2/10.1177_00220345780570070401.pd

The uses and abuses of power: teaching school leadership through children's literature

There are relatively few studies of how representations of teachers, schools and educational administrators in popular films and television might be, and are, used in leadership preparation. This paper seeks to add to this small body of work; it reports on an exploratory study of the representation of headteachers in contemporary children's fiction. Thirty-one texts are analysed to ascertain key themes and the major characterisations. The paper draws on children's literature scholars to argue that both the historical school story and its contemporary counterpart focus heavily on the power of the head to control the micro-world of the school. Because these fictional accounts deal with issues of power and justice more openly than many mainstream educational administration texts, this makes them particularly useful in the preparation of potential school leaders

Dexamethasone intravitreal implant in previously treated patients with diabetic macular edema : Subgroup analysis of the MEAD study

Background: Dexamethasone intravitreal implant 0.7 mg (DEX 0.7) was approved for treatment of diabetic macular edema (DME) after demonstration of its efficacy and safety in the MEAD registration trials. We performed subgroup analysis of MEAD study results to evaluate the efficacy and safety of DEX 0.7 treatment in patients with previously treated DME. Methods: Three-year, randomized, sham-controlled phase 3 study in patients with DME, best-corrected visual acuity (BCVA) of 34.68 Early Treatment Diabetic Retinopathy Study letters (20/200.20/50 Snellen equivalent), and central retinal thickness (CRT) 65300 \u3bcm measured by time-domain optical coherence tomography. Patients were randomized to 1 of 2 doses of DEX (0.7 mg or 0.35 mg), or to sham procedure, with retreatment no more than every 6 months. The primary endpoint was 6515-letter gain in BCVA at study end. Average change in BCVA and CRT from baseline during the study (area-under-the-curve approach) and adverse events were also evaluated. The present subgroup analysis evaluated outcomes in patients randomized to DEX 0.7 (marketed dose) or sham based on prior treatment for DME at study entry. Results: Baseline characteristics of previously treated DEX 0.7 (n = 247) and sham (n=261) patients were similar. In the previously treated subgroup, mean number of treatments over 3 years was 4.1 for DEX 0.7 and 3.2 for sham, 21.5 % of DEX 0.7 patients versus 11.1 % of sham had 6515-letter BCVA gain from baseline at study end (P = 0.002), mean average BCVA change from baseline was +3.2 letters with DEX 0.7 versus +1.5 letters with sham (P = 0.024), and mean average CRT change from baseline was -126.1 \u3bcm with DEX 0.7 versus -39.0 \u3bcm with sham(P < 0.001). Cataract-related adverse events were reported in 70.3 % of baseline phakic patients in the previously treated DEX 0.7 subgroup; vision gains were restored following cataract surgery. Conclusions: DEX 0.7 significantly improved visual and anatomic outcomes in patients with DME previously treated with laser, intravitreal anti-vascular endothelial growth factor, intravitreal triamcinolone acetonide, or a combination of these therapies. The safety profile of DEX 0.7 in previously treated patients was similar to its safety profile in the total study population

Dynamic characteristics of corona discharges in point-plane electrode topologies influenced by space charge

This paper was presented at the 2008 International Power Modulator Conference, 27-30 May, Las Vegas

Breakdown initiation fields for surface flashover of dielectric materials immersed in transformer oil

The applied electrical fields required to initiate surface flashover of different types of polymeric insulating material immersed in transformer oil have been investigated. Cylindrical samples of polypropylene, ultra-high molecular weight polyethylene, low-density polyethylene, and Rexolite were tested. Average electrical fields up to ~700 kV/cm were applied to samples via a 10-stage, inverting Marx generator. The samples were held between two electrodes immersed in 20 litres of EOS Ltd. (UK) L10B transformer oil. The stage capacitance of the Marx is 80 nF, and the rise time is around 100 ns with the test cell load. The applied electrical field required to initiate surface flashover was found by firing the system with an increasing charging voltage until a breakdown occurred. Experiments were performed with plane-plane electrodes and two types of point-plane electrode gap. The first point-plane gap was achieved by deploying a high-voltage electrode with a 1-mmdiameter pin protruding perpendicular to the surface, and the second by a high-voltage electrode with a 25-mm-diameter collar centred on the surface. Of the materials tested, polypropylene showed the highest average peak applied fields to initiate the first surface flashover, varying from 325 kV/cm in non-uniform fields to 637 kV/cm in uniform fields