Cryogenically-cooled two-micron solid-state lasers: recent results and future prospects

Efficient powerful laser sources in the two-micron regime are in demand for many applications in the areas of remote-sensing, defence, medicine, and materials interactions. Recently, dramatic progress has been shown in scaling cw output power from 2-micron fibre lasers

Children As Researchers: Exploring Issues And Barriers In English Primary Schools

This thesis identifies and explores the issues and barriers that appear to influence children's and adults' experiences of children's engagement in self-directed empirical research in five English primary schools associated with the Children's Research Centre at The Open University. As far as is known, this is the first in-depth study of children as independent researchers in the context of English primary schools. A flexible, multimethod research design was adopted. Predominantly qualitative data was generated through focus groups held with, and questionnaires distributed to, the young researchers and their peers and through individual unstructured interviews with adults. The qualitative data generated through these methods was analysed in the style of Grounded Theory (Glaser and Strauss, 1967; Strauss and Corbin, 1998). Quantitative data was subject to exploratory data analysis. This complemented and informed the qualitative analyses. The central categories which emerged from the data and, in particular, the identification of important issues by the children, have together informed the staged construction of a new model. This model illustrates the factors and processes that had an impact on both the children's experiences of research training and the research process and outcomes. The model demonstrates that these are inextricably interrelated. It is hoped that consideration of the issues and barriers identified will provide a basis for the further implementation and evaluation of young researcher initiatives in schools. The findings of the study have been drawn on to make recommendations for policy, practice and future research, particularly in those areas which are identified as significant by, and to, the children involved. It is also hoped that this study will address a gap in our knowledge and understanding of children as researchers and inform critical debate concerning children's voice and participation, adult-child power relationships and children's rights in English primary schools and more widely

Tm:fiber laser in-band pumping cryogenically-cooled Ho:YAG laser

Cryogenically-cooled diode-pumped lasers have received significant interest in recent years for their demonstrated orders of magnitude improvement in output radiance using simple laser resonator configurations. Here we present a technique that offers the potential for a further order of magnitude improvement utilising the in-band-pumping hybrid-laser architecture to excite cryogenically-cooled bulk gain media. Preliminary results will be discussed, for a narrow linewidth Tm:fiber MOPA system operating in the 100W regime, pumping a cryogenically cooled Ho:YAG gain element, and employing a simple cavity configuration. Low quantum defect operation and power-scaling potential will be discussed

Transport IV characterisation of MgB2 conductor at a bend radius of 50mm

Performance of state of the art MgB2 multifilamentary conductor at a required bend radius is essential for many applications including but not limited to magnets and motors. The characterisation is generally done with benchmark transport Ic but further detail can be seen in IV characteristics which are undertaken in this paper. Two conductors with the same architecture but different diameters, 0.89 and 0.45 mm were measured from 32 K to 20 K in self-field in conditions of as received and deformed to a 50 mm bend diameter, corresponding to strains of 1.4 % and 0.7 % respectively. The qualifying 0.45mm conductor was further measured in background fields up to 3 T. The smaller diameter wire was found to have no signs of degradation of critical behaviour in Ic or IV characteristic

The thermal conductivity of perlite insulation under varying conditions of temperature, humidity and packing density

Measurements have been made of the thermal conductivity of expanded perlite under varying conditions of density and humidity over the temperature range 113 K to 298 K. A thermal conductivity test cell was constructed according to the requirements of ASTM C177. The measurement system and test methods are described. The new test data shows that perlite conditioned in standard conditions has a thermal conductivity of 0.0153 to 0.0193 W/mK at 113 K. Varying the relative humidity (R.H) of the environmental chamber used to prepare some samples from 40% to 80%, made little to no change to the thermal conductivity values. However, increasing the density of the perlite packed into the cavity of test cell, by 15% to 25% resulted in moderate increases in the thermal conductivity at the maximum density of 2.0% and 7.7% and was dependent on the source

Temperature-dependent analytical thermal model for endpumped solid-state lasers

Analytical expressions for the temperature distribution and thermal-lens power in end-pumped solid-state lasers are reported. Enabled by including a temperature-dependent thermal conductivity, applicable from cryogenic to elevated temperatures, these prove insightful for practical systems

First electrical characterization of prototype 600 A HTS twisted-pair cables at different temperatures

Following the development of twisted-pair cables prepared with High Temperature Superconducting (HTS) tapes andtheir initial tests at 4.2 K in liquid helium at CERN, the cable samples of 2 m lengths were subsequently tested inflowing helium gas at temperatures between 10 K and 77 K at University of Southampton. A cryostat with optimizedhybrid HTS current leads was purposely built for the tests up to 2.5 kA. The cryostat has two separate helium flowconduits, each accommodating a twisted pair and allowing independent temperature control. With the completion ofthe tests on the twisted-pair cables, a 5 m long semi-flexible Nexans cryostat was also set up for the testing ofprototype HTS links assembled at CERN. The link, which is optimized for application to the remote powering ofLHC 600 A electrical circuits, consists of a compact multi-cable assembly with up to 25 twisted-pair 600 A HTStapes. The cables are cooled by a forced-flow of helium gas the inlet temperature of which can be changed in order to compare the electrical performance over a range of temperatures. The paper reports on the results of powering tests performed on the individual cables and the integration process for the forthcoming tests of the prototype links

The Design of a Lightweight HTS Synchronous Generator Cooled by Subcooled Liquid Nitrogen

A final design of a dasiacorelesspsila 100 kW HTS synchronous generator, to be built at the University of Southampton is under way. The new generator will use the same conventional 2-pole, 3 phase stator used by for the dasiairon coredpsila generator. The new HTS rotor has no central core and the rotor winding is built with 15 double pancake coils. Each coil has 38 turns of BiPb2223 superconducting tape, with nominal current of 180 A at 77 K. In this design, only two flux diverters are used at the top and bottom of the winding to help shape the magnetic field. The coils are located on the inner wall of the cryostat. The construction of the cryostat poses our greatest mechanical challenge. Although we have avoided problems with differential thermal contractions by making the cryostat entirely from stainless steel, the cryostat requires considerable reinforcements. Additional supports and stiffening ribs must be welded to the thin-walled structure. Distortion/rippling during welding must be eliminated to ensure the final welds to the ring flanges can be made. The pole-pieces, which help to improve the waveform characteristics of the machine, are located in the warm space between the cold flange of the cryostat and the inner wall of the vacuum vessel. Unfortunately, the overall magnetic performance has been affected by the parallel stacking of the coils. Some suggestions for altering the current stator have been presented which show that improvements to the voltage waveforms can be achieved<br/

Critical current and stability of MgB2 twisted-pair DC cable assembly cooled by helium gas

Long length superconducting cables/bus-bars cooled by cryogenic gases such as helium operating over a wider temperature range are a challenging but exciting technical development prospects, with applications ranging from super-grid transmission to future accelerator systems. With limited existing knowledge and previous experiences, the cryogenic stability and quench protection of such cables are crucial research areas because the heat transfer is reduced and temperature gradient increased compared to liquid cryogen cooled cables. V-I measurements on gas-cooled cables over a significant length are an essential step towards a fully cryogenic stabilized cable with adequate quench protection. Prototype twisted-pair cables using high-temperature superconductor and MgB2 tapes have been under development at CERN within the FP7 EuCARD project. Experimental studies have been carried out on a 5-m-long multiple MgB2 cable assembly at different temperatures between 20 and 30 K. The subcables of the assembly showed similar critical current, consistent with the Ic of single twisted-pair MgB2 cable tested previously. The assembly was shown to operate stably with flowing helium gas cooling in a moderate vicinity of the critical current. A low and reproducible specific contact resistance of ~200 n? cm2 was obtained

Two-micron cryogenically-cooled solid-state lasers: recent progress and future prospects

Efficient powerful laser sources in the two-micron regime are in demand for many applications in the areas of remote-sensing, defence, medicine, and materials interactions. Dramatic progress has been demonstrated in cw-power scaling of 2-micron fibre lasers; however, power-scaling in a pulsed mode of operation is limited by nonlinear effects and a relatively low damage-threshold-power. To fully capitalise on the potential advantage for high pulse-energies of the conventional 'bulk' 2-micron solid-state laser, extreme measures have to be taken to mitigate the three-level character and thermal effects in the laser medium resulting from heat generated during the pump cycle. Alleviation of these detrimental effects can be achieved by simply cooling the gain medium to cryogenic temperatures, benefitting from lower population in the terminal laser levels, and a large increase in the thermal conductivity, with a proportional decrease in the thermo-optic coefficient (dn/dT) and expansion coefficient. Combined these result in a massive reduction in thermo-optic aberrations. In this paper, we report on improved measurements of the spectroscopic properties of Ho:YAG at various temperatures between room and liquid nitrogen (LN) temperatures, utilising a multi-Watt Tm:fibre ASE source we have been able to properly identify the absorption features of interest with an accuracy better than 0.2nm. Results for other Ho-doped gain media will be discussed and the latest performance of a cooled 2-micron Ho:YAG laser in-band pumped by a narrow-linewidth Tm fibre laser presented