Search

A diamagnetic diagnostic system is designed, prepared and installed at Aditya Upgrade Tokamak. Also, the raw data is acquired in a specified acquisition system with suitable front-end electronics. The entire system consists of separate loops and is first calibrated in the presence of all coils, which conduct electricity during the plasma discharge. Then, the change in the toroidal magnetic field due to the plasma is measured using the diamagnetic system, and hence, several plasma parameters including poloidal beta, stored energy, etc. are estimated. The measurements were then repeated during various tests of the tokamak plasma, and several interesting facts were observed. The present paper covers the entire concept of designing magnetic diagnostics – from basic considerations to final data interpretation. [ABSTRACT FROM AUTHOR]

Spectroscopy in vacuum ultraviolet (VUV) and visible ranges plays an important role in the investigation and diagnosis of tokamak plasmas. However, under harsh environmental conditions of fusion grade devices, such as ITER, VUV–visible systems encounter many issues due to the degradation of optical components used in such systems. Here, near-infrared (NIR) spectroscopy has become an effective tool in understanding the edge plasma dynamics. Considering its importance, a NIR spectroscopic diagnostic has been developed and installed on the ADITYA-U tokamak. The system consists of a 0.5 m spectrometer having three gratings with different groove densities, and it is coupled with a linear InGaAs photodiode array. Radiation from the ADITYA-U edge plasma has been collected using a collimating lens and optical fiber combination and transported to the spectrometer. The spectrum in the NIR range from the ADITYA-U plasma has been recorded using this system, in which Paβ and Paγ along with many spectral lines from neutral and singly ionized impurities have been observed. The influxes of H and C have been estimated from measurements. The H influx value is found to be 2.8 × 1016 and 1.9 × 1016 particles cm−2 s−1 from neutral hydrogen lines Hα and Paβ, respectively, and the C influx value is found to be 3.5 × 1015 and 2.9 × 1015 particles cm−2 s−1 from the neutral carbon and singly ionized carbon, respectively. A good agreement is seen between these results and the results obtained by using a routine photomultiplier tube based diagnostic. [ABSTRACT FROM AUTHOR]

The initial magnetization processes in melt-spun Sm-Co alloys with remanent ratios above 0.7 have been studied. Alloys of [([Sm.sub.(1/(6+x)][Co.sub.(5+x)/(6+x)]).sub.94][Nb.sub.3][C.sub.3] (x = 0.67, 1.7, 2.3 and 3) alloys modified with Nb and C were melt spun at wheel speeds of 20 and 40 m/s, and the compositions span the range between the [Sm.sub.2][Co.sub.17] and Sm[Co.sub.5] stoichiometries. Structurally, all alloys formed in the Tb[Cu.sub.7]-type structure, although a small amount of [Sm.sub.2][Co.sub.7] was observed in Sm-rich samples, and a small amount of fcc Co formed in Co-rich samples. From transmission electron microscopy analysis, grain sizes typically ranged from 100 to 500 nm, and the grains were randomly oriented. The as-spun ribbons had remanence ratios of 0.7 and coercivity values ranging from 3 to 18 kOe. The initial magnetization curves showed a steep linear response to the applied field for all samples, suggesting that nucleation-controlled processes dominate the magnetization process. In addition, the initial susceptibility decreased with increasing Sm content, reflecting the anisotropy differences between Sm[Co.sub.5] and [Sm.sub.2][Co.sub.17] compounds. The initial susceptibility also increased with increasing wheel speed, suggesting that the microstructure has important ramifications on the magnetization process as well. Index Terms--Anisotropy, permanent magnet, rapid solidification, remanence enhancement, SmCo alloys.

Rapidly solidified [Ni.sub.54][Fe.sub.27-2x][Ga.sub.19+2x] ferromagnetic shape memory alloys are made by melt-spinning with variation of Fe and Ga contents in order to describe the martensitic phase transformation, microstructures and magnetic properties. The microstructures of the as-spun ribbons have displayed a gradual transition from cellular to dendritic structure with increasing Ga concentration and with the presence of some internal martensitic twin bands at higher Ga content.

High coercive, high remanent permanent magnets are achieved by rapid solidification of binary Sm-Co alloys and Sm-Co alloys modified with Nb and C.The role of Sm/Co ratio on the magnetic properties is investigated and also significant improvements in coercivity with only minor alloying additions are reported.

The magnetization reversal in single-phase [RE.sub.2][Fe.sub.14]B and two-phase [alpha]-Fe/[RE.sub.2][Fe.sub.14]B with varying nanoscale grain structures and intergranular exchange interactions produced via controlled segregation during crystallization is investigated. The loss of coercivity arises because domain-wall processes dominate the magnetic reversal as the exchange interactions increase.

The structural transformation from the metastable disordered TbCu7-type SmCo7 structure to the equilibrium ordered Th2Zn17-type Sm2Co17 structure is revealed by x-ray diffraction analysis using Reitveld refinement. It is found that the magnetic properties are dependent strongly on the stage of the transformation, as the coercivity strongly depends on the annealing temperature.