Failure of electromagnetic launch (EML) solid armatures can be caused by various mechanisms, but a critical performance limit is due to arcing at the rail/armature contact surface above the transition velocity. A previous theoretical transition model based on velocity skin effect and current wave (VSE/CW) phenomena is reviewed and further data of the changing current distribution at the contact surface is derived as the vaporization current wave moves forward until the arcing transition occurs. This VSE/CW model predicts experimental transition velocities with reasonable accuracy over a wide range of parameters (bore size from 15 to 90 mm). To achieve the required contact pressure distribution with the optimum contact region geometry, monolithic armatures are considered to be necessary, which is consistent with the highest transition velocities being achieved with such aluminum alloy armatures. Possible improvements in the performance of solid armatures are evaluated for monolithic armatures with copper rails by optimizing armature geometry, dimensions, contact pressure distribution, and the use of resistive contact region materials. An increase of >40% in transition velocity to >2.5 km/s is predicted above reported experimental results.

Previous theoretical predictions of the transition velocity of solid armatures based on the velocity skin-effect (VSE) usually assume that adequate rail/armature contact pressure exists at the rear armature surface. It has also been shown that the rear angle (phi) of a triangular armature contact region should be minimized but that for phi < 30 it is probably not possible to achieve the required contact pressure distribution at "start-up" when magnetic forces are small. Transition velocities are derived for conditions where contact pressure does not initially exist (or is inadequate) at the armature rear for axial distances from 0 to >10 mm and for values of phi from 12 degrees to 90 degrees,It is shown that the absence of adequate contact pressure at the rear armature surface results in significant reductions in the transition velocity in reasonable agreement with experimental results reported previously. An armature concept to overcome these reductions in transition velocity is discussed in which the rear armature contact regions (with phi < 15) are supported by the addition of a rear support attached to but insulated from the armature trailing arms. It is estimated that the transition velocity of such an armature, of 90 mm diameter and 4 kg launch package mass, would be about 2.0 km/s.

Hybrid armatures have potential advantages over solid and plasma armatures because they could have a reduced axial length and consequently a lower mass than the former and lower resistive losses than the latter. However it is expected that their use could result in excessive arcing erosion damage to the rails at low velocity and increased forces in the rail/armature gap due to partial confinement of the arcing energy losses in this region. In this transitioning hybrid armature concept, it is proposed to overcome these problems by having initially a solid armature contact region, armature arcing surfaces of a low erosion rate metal and an "open" armature geometry. The latter allows the rapid expansion of arcing and erosion products radially inwards from the small rail/armature gap to the much larger bore volume. The armature will be mainly of aluminum alloy with the solid contact region being located at the armature rear with dimensions limited to achieve a predetermined transition velocity of about 1.0 km/s. The armature surface forward of the solid contact region is not in contact with the rails and is "clad" with a high temperature low erosion rate metal such as a tungsten/copper/nickel alloy to minimize the armature arcing erosion depth and consequently the are voltage and energy losses. The rail launcher electrical efficiency and performance limits are assessed for launch packages of 4 kg mass, 90 mm diameter, kinetic energy of 10 MJ and exit velocity of 2.3 km/s. It is estimated that this hybrid armature could result in an effective launcher efficiency about 4% higher than expected for a similar transitioning solid armature including the effect of the reduction in armature parasitic mass.

The fundamental parameters that influence the performance and efficiency of Rail Launchers are shown to be: mechanical energy/bore volume (p(0)), effective magnetic flux density (B-0) and the total mass/effective bore area (m). To achieve a high electrical efficiency these parameters should be minimized by increasing the bore diameter and reducing the bore length, subject to other design constraints such as increasing launch package mass.The efficiency of Rail Launchers is limited by energy losses in the rail and armature, arcing losses following transition and the barrel residual magnetic energy at exit. These are derived analytically including the effect of field diffusion, velocity skin-effect (VSE), predicted transition velocities and arcing phenomena A range of aluminium allay armatures with breech-fed copper rails are considered with kinetic energies up to 20MJ and bore diameters from 40 to 180mm. Electrical efficiencies for 10 and 20MJ kinetic energy launchers with minimum diameters are estimated to be 52 and 62% respectively. An increase in efficiency to 67% is predicted for larger diameter launchers with transition velocities increased to > 2.0km/s due to lower energy density and electrical action.

OBJECTIVE Behçet's disease (BD) is a rare multisystem disorder characterized by vasculitis. At present, there are no laboratory markers that correlate well with the clinical activity in BD. This has led to the development of an instrument (BD Current Activity Form) to measure activity. Scoring is based on the history of new clinical features present over the preceding 4 weeks prior to assessment. Standardized questions were developed for all parts of the form. The face validity of the proforma was determined following worldwide collaboration with physicians and ophthalmologists managing patients with BD. The aim of this study was to evaluate the interobserver reliability of this form. METHODS Nineteen patients fulfilling the International Study Group criteria for BD were randomly allocated, questioned and examined independently on the same day by five physicians experienced in BD. RESULTS There was good agreement between the physicians' rating of oral [intraclass correlation coefficient (ICC) = 0.87] and genital (ICC = 0.95) ulceration, skin involvement (ICC = 0.62 for pustules and ICC = 0.66 for erythema nodosum), arthritis (ICC = 0.62), headache (ICC = 0.80), large vessel (kappa = 0.53), nervous system (kappa = 0.61) and eye involvement (kappa = 0.77). There was poor agreement for the question relating to the presence of bloody diarrhoea (ICC = 0.28). There was significant bias in the rating of fatigue by one of the physicians (F = 5.2, P = 0.001). CONCLUSION Overall, this instrument has good interobserver reliability for assessing general disease activity. We therefore suggest that this proforma has a place in routine clinical monitoring of patients with BD, as well as assessing outcome in therapeutic trials.

Previous theoretical predictions of the transition velocity of solid armatures based on the velocity skin-effect (VSE) usually assume a fixed geometry and that adequate rail/armature contact pressure exists. In this paper the effect of contact region geometry on the transition velocity is investigated and the optimum geometry derived. The critical geometrical parameters are considered to be the contact region angle /spl phi/ and rear radial depth. These parameters need to be as small as possible to maximise the transition velocity but large enough to maintain sufficient contact pressure to avoid the early initiation of a current wave which would reduce the transition velocity. Comparison of predicted and experimental results for 15 to 90 mm aluminium alloy armatures shows reasonable agreement. It is considered that the VSE is the dominant mechanism limiting the transition velocity of solid armatures provided their contact region geometry and structural design is such that adequate contact pressure is maintained.

In this study of arcing phenomena in transitioned solid armatures it is proposed that following forward acceleration of the are plasma, re-strikes occur towards the rear of the contact surface resulting in commutation of current to the re-strike arcs, Thus a repetitive sequence of current conduction is predicted at a frequency dependent on the are transit time along the armature surface.An analytical solution of the proposed theoretical model for the arcing phase evaluates the transient rail/armature gap voltages due to are voltage, rail and armature resistances and magnetic flux changes, Comparison of the predicted rail/armature gap voltages at the front of the armature with measured muzzle voltage waveforms for a typical 90mm armature shows sufficiently dose agreement to support the validity of the theoretical model. The additional resistive losses as a result of transition are estimated to be >30% of the exit kinetic energy of 10MJ, The deposition of most of this as thermal energy in the rail/armature gaps is expected to have a significant impact on structural design requirements.

Knapp was the first person to report using local anaesthesia for eye surgery in 1884 when he carried out an enucleation following a retrobulbar injection of cocaine.' Retrobulbar and peribulbar injections are now routine ophthalmic procedures that are commonly performed to induce anaesthesia and akinesia. In institutions carrying out day case surgery this is often the principal technique of anaesthesia. Complications of local anaesthesia for ophthalmic surgery are well recognised.2 Hay et al described 23 cases of globe perforation, 12 after retrobulbar and 11 after peribulbar injections.3 Although the incidences of globe penetration or perforation are rare, figures vary from three in 4000 retrobulbar blocks4 to one in 16 224 peribulbar blocks,5 there are preoperative measures that, if adopted, can further reduce this risk. Establishing a rapport with patients reduces their anxiety and stress, thereby lowering arterial blood pressure and improving patient cooperation. The use of warmed anaesthetic solutions has been shown to be more comfortable for the patient. There appears to be no significant difference in the complication rate whether a sharp or a blunt needle is used for the injection6 although it is generally agreed that the incidence of globe perforation is reduced if the needle length is less than 3 cm.2 A good knowledge of orbital anatomy is important whether it is the anaesthetist or ophthalmologist giving the anaesthetic. Globes with an axial length greater than 26 mm are particularly at risk. These tend to be myopic eyes which are not only larger but also have thinner sclera.4 7 Preoperative axial length measurements can identify these patients and alert both the anaesthetist and surgeon. Ultrasound biometry is routine for patients with cataracts as part of the intraocular lens implant power calculation but we have not read any recommendation that this should be performed for patients undergoing other forms of ocular surgery under local anaesthesia. Myopes undergoing trabeculectomy or vitrectomy, for instance, are presumably exposed to the same risks of globe perforation. Perhaps the umbrella should be further extended to include myopes receiving retrobulbar injections for other procedures in view of reports of needle penetration following retrobulbar steroid injections.8 9We are not aware ofany publications concerning this issue and suggest that there may be a rationale for axial length measurements in all myopes undergoing peribulbar or retrobulbar injections whether for the administration of medications or for anaesthetic purposes.

A theoretical model of the complete arcing transition sequence of solid armatures with resistive contact regions is proposed which includes evaluation of contact region melting, current wave, magnetic 'saw-effect' phenomena and current distribution at the contact surface, It is concluded that the velocity skin-effect (VSE) is the dominant mechanism influencing the arcing transition velocity though it is estimated that the latter could be increased to > 2.5km/s (as limited by the VSE) with resistive armature contact regions add the optimum combination of the critical parameters, For applications with bore sizes of about 90mm previous theoretical predictions of the unsuitability of aluminium alloy as a contact region material, because of its relatively low resistivity, are confirmed.

A general method for the electro-thermal design optimisation of nontransitioning solid armatures with resistive contact regions is presented which predicts the maximum velocity at which transition to an arcing contact occurs. Comparison with a wide range of published experimental results supports the validity of the velocity skin-effect (VSE) model used. It is shown that the optimum combination of armature design parameters with copper rails requires a contact region resistivity within the range of molybdenum to titanium and that transition velocities, as limited by the VSE, of >2.5 km/s should be achieved with 90 mm bore armatures and these optimum parameters.<>

Velocity limitations of EM Rail Launchers with solid armatures due to the velocity skin-effect are assessed for a range of homogeneous materials (copper, aluminium, molydenum and tungsten). Performance criteria as limited by armature melting in the region adjacent to the rail/armature contact surface and by rail surface melting are deduced analytically based on defined electro-thermal reference conditions. The performance criteria predict the reference transition velocity at which the solid contact surface changes to mainly a melted contact surface which is shown to be a fundamental property of the materials.The analysis shows that the critical factors in obtaining a high transition velocity are a relatively high resistivity armature material combined with high temperature rail surface coatings. For example a transition velocity of > 2.0 km/s for a molydenum armature and a tungsten rail surface coating on a copper rail are predicted compared with < 0.5 km/s for the aluminium/copper combination frequently used. Such increase in transition velocity could be important in the development of a solid armature as the first stage of a multi-stage launcher with transition, hybrid and possibly plasma armatures as the later stages.

The necessary conditions for obtaining consistent multiple-arc breakdowns in a 60 kV pressurised-air spark gap of 1.7 nH effective inductance have been investigated. Theoretical conclusions, which are confirmed by the experimental results, show that the important factors influencing multiple-arc breakdown are the gap-voltage decay time and the ratio of initial arc-channel impedance to that of the complete gap. The spark gap has also been used with a single 15 kJ capacitor at peak currents of 1.0 MA, the combined inductance of the unit being 14 nH.

Problems arising during the development and operation of a 100 kV multiple-arc solid-dielectric closing switch for use in fast low-inductance switching applications are discussed. The switch has been operated at peak currents up to 2.0 MA and for a pulse length of 70 μs. The arc voltage is about 200 V and the switch inductance is 5 nH. Breakdown is achieved at any voltage using a high-current trig...

Click to increase image sizeClick to decrease image size Notes 1 The author, and the Foundation wish to acknowledge their indebtedness to Barbara H. Carlton, formerly on the Foundation staff, who was responsible for the preparation of the data on which this report is based Additional informationNotes on contributorsT. E. JamesFootnote 1 1 The author, and the Foundation wish to acknowledge their indebtedness to Barbara H. Carlton, formerly on the Foundation staff, who was responsible for the preparation of the data on which this report is based

Pulse-breakdown data in the nanosecond range are given for 3–5 mm nonuniform-field air spark gaps at a pressure of 1–6atm. The 3-electrode gap has a central disc trigger electrode having an edge radius of 0.5 mm. A negative voltage pulse, whose rate of rise increases linearly from 10 to 100kV/ns, is applied to the trigger electrode with various voltages previously applied to the outer main electrodes. Breakdown is initiated with a very low jitter by field emission from the trigger electrode. Comparison with similar data for pressurised uniform-field gaps is made.

Unagitated cultures of Acanthamoeba in the logarithmic growth phase have been observed to consist of over 99% mononucleates and less than 1% multinucleates. A small increase in the relative numbers of multinucleates occurs as the culture age and conditions for growth become limiting. A large increase in the proportion of multinucleates occurs during logarithmic growth, if the cultures are agitated by shaking or by the bubbling from a stream of air. After an initial increase and until the end of logarithmic growth, the numbers of multinucleates in the agitated cultures are maintained at a constant fraction of the total population. Proportions of multinucleates as high as 25% are observed during this period. A second increase in the relative numbers occurs as conditions for growth become limiting. Under these conditions, the multinucleates may exceed 50% of the total population. The factors inducing multinuclearity as well as the kinetics of the mononucleate and multinucleate accumulation curves are discussed.