C1− Continuous crack propagation for mixed-mode fracture problems

In this work a C1− continuous crack propagation algorithm is proposed to improve the numerical simulation of localized deformation patterns, using higher order elements. The algorithm is applied for a standard smeared crack model and is validated by a mixed-mode fracture problem. From the results a reduction of mesh-induced directional bias is observed

Influence of nutrition on feline calcium oxalate urolithiasis with emphasis on endogenous oxalate synthesis

The prevalence of calcium oxalate (CaOx) uroliths detected in cats with lower urinary tract disease has shown a sharp increase over the last decades with a concomitant reciprocal decrease in the occurrence of struvite (magnesium ammonium phosphate) uroliths. CaOx stone-preventative diets are available nowadays, but seem to be marginally effective, as CaOx urolith recurrence occurs in patients fed these diets. In order to improve the preventative measures against CaOx urolithiasis, it is important to understand its aetiopathogenesis. The main research focus in CaOx formation in cats has been on the role of Ca, whereas little research effort has been directed towards the role and origin of urinary oxalates. As in man, the exogenous origin of urinary oxalates in cats is thought to be of minor importance, although the precise contribution of dietary oxalates remains unclear. The generally accepted dietary risk factors for CaOx urolithiasis in cats are discussed and a model for the biosynthetic pathways of oxalate in feline liver is provided. Alanine:glyoxylate aminotransferase 1 (AGT1) in endogenous oxalate metabolism is a liver-specific enzyme targeted in the mitochondria in cats, and allows for efficient conversion of glyoxylate to glycine when fed a carnivorous diet. The low peroxisomal activity of AGT1 in cat liver is compatible with the view that felids utilised a low-carbohydrate diet throughout evolution. Future research should focus on understanding de novo biosynthesis of oxalate in cats and their adaptation(s) in oxalate metabolism, and on dietary oxalate intake and absorption by cats

The physics of photoconductive spark gap switching : pushing the frontiers

Photoconductive switching of an atmospheric, air-¯lled spark gap by a high-power fem- tosecond laser is a novel approach for switching high voltages into pulses with a very fast rise time (order ps) and almost no shot-to-shot time variation (jitter). Such a switch makes it possible to synchronize high-voltage pulses more accurately than presently possi- ble. The goal of this research was to create ultrafast high-voltage pulses in order to develop a new type of electron accelerator and its diagnostics. An interesting future application of photoconductively switched ultrashort pulses is the creation of broadband, high-intensity terahertz radiation, which is a harmless alternative to X-rays for a number of medical and security purposes. Photoconductive spark gap switching in air combines the bene¯ts of two ¯elds of high- voltage switching: First, laser-triggered spark gap switching where the switching medium is either gas or liquid and a laser is used to initiate the breakdown of the gap. Secondly, photoconductive switching where the switching medium is a semiconductor device, which is completely illuminated by a short pulsed laser. If a complete (gas-¯lled) gap is su±ciently ionized by a femtosecond, high-power laser, stochastic breakdown processes (dominating in the laser-triggered switch) no longer determine the actual breakdown-behavior of the gap. The rise time of the photoconductively switched pulse is then determined by the geometry of the gap. The time jitter is limited only by the jitter of the switching laser (as in the semiconductor switch) and, because the switching medium is a gas, high currents can be switched. The principle of photoconductive switching was demonstrated in air and in nitrogen (Chap- ter 3 and 4). A femtosecond Ti:Sapphire laser was cylindrically focused in a 1 mm spark gap biased at 4.5 kV. When su±cient laser power was used (> 0.1 TW) the spark gap switched photoconductively. The measured rise time and jitter of the switched pulses were both below the resolution of the measurement equipment, i.e., better than 100 ps and 15 ps, respectively. Measurements at lower applied voltages but with the same gap distance showed that it was possible to switch voltages as low as 10% of the self-breakdown volt- age. However, a voltage drop over the gap was observed, which became more pronounced when switching lower voltages. Transient-plasma simulations (Chapter 6) explained this behavior by showing that the conductivity of the plasma is a function of the current that runs through the plasma. Together with an interferometric study of the switching plasma (Chapter 5), the simulations also revealed that photoconductive switching does not require full ionization of the switching plasma. The voltage drop can be reduced when more laser power is used to create a switching plasma column that has the same initial electron den- sity but a larger diameter. A three-dimensional electrodynamic model to simulate a photoconductively switched high- voltage spark gap was developed (Chapter 7). This model describes and monitors the elec- tromagnetic ¯eld-propagation in a coaxial spark gap setup after switching. It reveals also the in°uence of discontinuities, such as viewing ports, on the pulse shape and the rise time. The rise time is determined by the time it takes for a stable TEM mode to build up in the gap region. Commonly used zero-dimensional lumped element and one-dimensional trans- mission line models for laser-triggered spark gap optimization are shown to be insu±cient for optimizing the geometry of the photoconductive switch, because the electromagnetic ¯eld-propagation in three dimensions is neglected. We developed an optimization procedure for spark gap geometries based on our fully three-dimensional electrodynamic simulations (Chapter 8). In conclusion, we proved the principle of photoconductive switching of high voltages in air. The shot-to-shot time stability and the voltage working range of a spark gap are greatly enhanced, compared to conventional laser triggering, and high-voltage pulses with a very fast rise time can be made and modelled. New possibilities for compact pulsed DC electron acceleration are opened up by photoconductive switching, as well as numerous applications in other areas of research

Picosecond High Voltage Switching of a Pressurized Spark Gap

Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of (low energy) electrons into a laser wakefield puts extreme demands on the shortness and timing, i.e. a fraction of a plasma period, typically less than 100 fs. In order to meet these requirements, we have revisited the concept of pulsed DC acceleration. Simulations have shown that this concept can be successful if high voltage pulses (of the order MV) can be switched with picosecond precision. As a fust step towards this goal, a IO kV laser triggered pressurized spark gap was designed and built. One of the limitations on risetime and jitter in high voltage laser triggered spark gaps is the initial breakdown process. Since this is a stochastic process it will cause jitter, and the growth rate of the plasma will determine the fastest possible risetime of the pulse. A way to overcome this limitation is to create a line focus between the electrodes, using a high power femtosecond laser. At laser intensities above approximately 10" W/m2 near- threshold or tunneling ionization causes near-instantaneous ionization of. a complete plasma channel between the electrodes, much like a photoconductive semi-conductor switch. Because of the instantaneous ionization and the high degree of ionization in the plasma channel, jitter and risetime are reduced considerably. We will present the fust results from switching of a 10 kV spark gap with 3 mm inter-electrode distance, using a femtosecond Ti:Sapphire laser. A line focus of the laser is created, using cylindrical optics. Folded-wave interferomeny will he described to study the development of the plasma channel on femtosecond timescales

Picosecond high voltage switching for pulsed DC acceleration

Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of a (low energy) electron bunch into a laser wakefield requires acceleration gradients of the order GV/m. In principle DC acceleration can achieve GV/m acceleration gradients. If high voltage pulses of the order MV can be switched with picosecond precision, the performance of such an accelerator would be greatly enhanced and even multistage DC acceleration would become feasible. Presently risetime and jitter of high voltage pulses in high voltage laser triggered spark gaps are limited to the nanosecond regime by the initial stochastic breakdown process in the gap. A way to overcome this limitation is to create a line focus between the electrodes with an intensity above 1018 W/m2 using a high power femtosecond Ti:Sapphire laser. Because of the instantaneous ionization and high degree of ionization in the plasma channel, picosecond switching precision can be achieved and jitter is reduced significantly. A spark gap test setup with 3 mm interelectrode distance has been build and the first measurements have been done. Femtosecond diagnostics for characterization of the laser induced plasma and electro-optic diagnostics for the high voltage pulse have been developed

Fermentation of animal components in strict carnivores: a comparative study with cheetah fecal inoculum

The natural diet of felids contains highly digestible animal tissues but also fractions resistant to small intestinal digestion, which enter the large intestine where they may be fermented by the resident microbial population. Little information exists on the microbial degradability of animal tissues in the large intestine of felids consuming a natural diet. This study aimed to rank animal substrates in their microbial degradability by means of an in vitro study using captive cheetahs fed a strict carnivorous diet as fecal donors. Fresh cheetah fecal samples were collected, pooled, and incubated with various raw animal substrates (chicken cartilage, collagen, glucosamine-chondroitin, glucosamine, rabbit bone, rabbit hair, and rabbit skin; 4 replicates per substrate) for cumulative gas production measurement in a batch culture technique. Negative (cellulose) and positive (casein and fructo-oligosaccharides; FOS) controls were incorporated in the study. Additionally, after 72 h of incubation, short-chain fatty acids (SCFA), including branched-chain fatty acids (BCFA), and ammonia concentrations were determined for each substrate. Glucosamine and glucosamine-chondroitin yielded the greatest OM cumulative gas volume (OMCV) among animal substrates (P < 0.05), whereas total SCFA production was greatest for collagen (P < 0.05). Collagen induced an acetate production comparable to FOS and a markedly high acetate-to-propionate ratio (8.41:1) compared to all other substrates (1.67:1 to 2.97:1)

Risso's dolphins alter daily resting pattern in response to whale watching at the Azores

Peer reviewedPreprin

Koinonia in die stad: Verhoudinge in die groter stadsgemeentes van die Gereformeerde kerke

Koinonia in the city: Relations in the larger urban congregations of the Reformed churchesKoinonia is of vital importance for the church. It is not only a Biblical injunction, but also, sociologically speaking, a prerequisite for the preservation of faith in theChristian community. Therefore it is imperative that the church’s ministry encourages koinonia. This is especially true for the church in an urban context, where relational networks have been severely impaired. The traditional pattern of ministry of churches in the Reformed tradition, the shepherd-flock model, does not succeed in stimulating koinonia under these circumstances. Therefore, the choice is rather made for the body of Christ-model, with complementary koinonia-groupings as theory of practice