Experimental Investigations on Wavefront Distortion of LD-Pumped Neodymium-Doped Silica-Glass Rod with High Thermal Shock Parameter

The characterization of a laser diode (LD)-pumped neodymium-doped silica glass (NDSG) laser is here described. The gain performance and wavefront distortion were measured, and the thermal toughness and uniformity distribution of the material were experimentally observed. At a pumping frequency of 1 Hz and energy 7.79 J, a small-signal gain of 1.16 was measured, and the wavefront distortion reached 2.67 λ (wavelength λ = 1053 nm). At a pumping frequency of 25 Hz with 194 W power, the NDSG was still not cracked, which is consistent with its high thermal shock parameter. However, the material uniformity was relatively poor. These results indicate good prospects for the application of NDSG lasers at high energy and high repetition frequency, but the gain performance, uniformity, and other aspects affected by the manufacturing process need to be improved.

Thermal Shock Resistance of the RE-Oxide and Oxynitride Glasses

Thermal shock resistance of the RE-Si-Mg-O-N glasses (RE = La, Nd, Yb, Lu) with 0 and 20 eq.% of nitrogen was investigated by the indentation-quench method based on propagation of Vickers cracks. Crack growth was measured on the same sample for a test series of different quenching temperatures. Thermal shock resistance of the studied materials was determined as a temperature difference resulting in 10 % growth of the initial cracks (∆T10) and by the thermal shock parameter R calculated from the material properties. Although the comparison of ∆T10 and R values as a function of glass composition revealed some differences between these two approaches, also a common trend was observed. Thermal shock resistance increased with the fractional glass compactness resulting from RE type and N content increase.

Shock parameter calculations at weak interplanetary shock waves

A large set of interplanetary shock waves observed using the Ulysses spacecraft is analysed in order to determine their local parameters. For the first time a detailed analysis is extended to the thermodynamic properties of a large number of events. The intention is to relate the shock parameters to the requirements set by MHD shock theory. A uniform approach is adopted in the selection of up and downstream regions for this analysis and applied to all the shock waves. Initially, the general case of a 3 component adiabatic plasma is considered. However, the calculation of magnetosonic and Alfvénic Mach numbers and the ratio of downstream to upstream entropy produce some unexpected results. In some cases there is no clear increase in entropy across the shock and also the magnetosonic Mach number can be less than 1. It is found that a more discerning use of data along with an empirical value for the polytropic index can raise the distribution of downstream to upstream entropy ratios to a more acceptable level. However, it is also realised that many of these shocks are at the very weakest end of the spectrum and associated phenomena may also contribute to the explanation of these results.