[en] Based on careful design of composition, a two-glassy-phase Cu_47.2Zr_46.5Al_5.5Nb_0.8 bulk metallic glass with a large plasticity (∼16%) was successfully prepared. Energy-dispersive spectroscopy and electron energy loss spectroscopy confirm that the bright matrix phase is rich in Zr/Nb while the dark particle phase is rich in Cu/Al. Theoretical calculations suggest that a small difference in shear modulus between phases may be necessary before pronounced plastic deformation can occur

No abstract available

[en] Aluminium alloy powder having a nominal composition of Al₉₃Fe₃Cr₂Ti₂ (at.%) and a mean size of 50 μm has been prepared using high pressure gas atomisation. The powder was sieved to provide starting materials with well defined size distributions and was consolidated using extrusion. Processing parameters including temperature and extrusion ratio have been investigated as a function of both particle size and initial structure. The influence of process variables on the mechanical properties of the extruded material has been investigated using microhardness and tensile tests at room temperature. The result of these preliminary investigations are presented and discussed

[en] Aluminium alloy powder having a nominal composition of Al_93Fe_3Cr_2Ti_2 (at%) has been prepared using gas atomisation. The atomised powder present a microstructure of an aluminium matrix reinforced with a spherical quasicrystalline icosahedral phase, in the range of nano metre in size. The powder was consolidated into bars using warm extrusion. The microstructure of the extruded bars retains the quasicrystalline microstructure and the bars present outstanding mechanical properties, i.e. proof stress of 280 MPa at 300 degree centigrade. Upon heating the microstructure evolves towards the equilibrium. The thermal evolution was investigated by means of x-ray diffraction, differential scanning calorimeter, scanning electron microscopy and transmission electron microscopy. According to these observations a transformation in two steps is proposed. A first step consists in the decomposition of the supersaturated solid solution of the matrix and the quasicrystals, and a second step in the transformation of the quasicrystals into the equilibrium phases. (Author)

[en] We study experimentally and theoretically, the effects of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and the minimum micropulse length can be significantly shorter than that obtained without modulation. The FEL can operate during part of the modulation cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity loss

[en] The nanocrystalline Fe-Si-B-Nb-Cu alloy, known as FINEMET, is now a well established commercial soft magnetic material exhibiting excellent permeability while maintaining a high saturation magnetization. It is produced by melt-spinning to form a ribbon with an amorphous structure. The purpose of this investigation was to use 3-dimensional atom probe microanalysis to experimentally quantify the local phase chemistries of this nanocomposite microstructure, with particular reference to the partitioning behavior of the Al

[en] Working Group 4 at the 8th Advanced Accelerator Concepts Workshop (ACC'98), held July 5-11, 1998 in Baltimore, Maryland hosted more than fifteen scheduled or impromptu talks (all punctuated with lively discussion) on the general topic of 'Beam Monitoring, Conditioning, and Control at High Frequencies and Ultrafast Timescales'. This report is a summary of these talks and discussions

[en] The effects of up to 10 at% substitution of Fe by Al on the microstructure and DC and AC magnetic properties of nanocrystalline FeSiBCuNb alloy ribbon are summarised and analysed. The minimum DC H_c developed during annealing decreases by 40% for 2 at% Al (to 0.3 A/m) and remains roughly constant for larger Al contents. The largest peak value of μ_0.4 at 50 Hz also corresponds to 2 at% Al. The best frequency response for μ_0.4 occurs for 6 at% Al while there was no improvement in AC power loss behaviour over the 0% Al alloy. The improvements in DC H_c and AC μ_0.4 are ascribed to a reduction in K₁ of the Fe-Si-based nanocrystallites by the introduction of Al

[en] It is well known that the multipass, multibunch beam breakup (BBU) instability imposes a potentially severe limitation to the average current that can be accelerated in an energy recovery linac (ERL). Simulation results for Jefferson Lab's FEL Upgrade Driver are presented which predict the occurrence of BBU below the nominal operating current of the machine. In agreement with simulation, BBU was observed and preliminary measurements to identify the higher-order mode (HOM) causing the instability are shown. In addition, measurements performed to experimentally determine the threshold current are described. Using a newly developed two-dimensional BBU simulation code, we study the effect of optical suppression techniques, first proposed by Rand and Smith in 1980 [1], on the threshold current of the FEL. Specifically we consider the effect of (1) reflecting the betatron planes about 45 degrees and (2) rotating the betatron planes by 90 degrees. In two pass recirculators, a 90 degrees rotation significantly increases the threshold current of BBU. The successful installation of a five skew-quadrupole reflector in the backleg of the FEL has been shown to be effective at suppressing the instability and comments on preliminary operational experience will be given

[en] In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW Free Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed