D. Bhattacharya

Cyclotron resonance heating systems for SST-1

RF systems in the ion cyclotron resonance frequency (ICRF) range and electron cyclotron resonance frequency (ECRF) range are in an advanced stage of commissioning, to carry out pre-ionization, breakdown, heating and current drive experiments on the steady-state superconducting tokamak SST-1. Initially the 1.5 MW continuous wave ICRF system would be used to heat the SST-1 plasma to 1.0 keV during a pulse length of 1000 s. For different heating scenarios at 1.5 and 3.0 T, a wide band of operating frequencies (20–92 MHz) is required. To meet this requirement two CW 1.5 MW rf generators are being developed in-house. A pressurized as well as vacuum transmission line and launcher for the SST-1–ICRF system has been commissioned and tested successfully. A gyrotron for the 82.6 GHz ECRF system has been tested for a 200 kW/1000 s operation on a water dummy load with 17% duty cycle. High power tests of the transmission line have been carried out and the burn pattern at the exit of transmission line shows a gaussian nature. Launchers used to focus and steer the microwave beam in plasma volume are characterized by a low power microwave source and tested for UHV compatibility. Long pulse operation has been made feasible by actively cooling both the systems. In this paper detailed test results and the present status of both the systems are reported.

Local structure and photocatalytic properties of sol–gel derived Mn–Li co-doped ZnO diluted magnetic semiconductor nanocrystals

The structural, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and photocatalytic properties of sol–gel derived Zn1−y−xMnyLixO (y = 0, 0.02 and x = 0, 0.01, 0.02, 0.03, 0.04, 0.06) nanoparticles (NPs) have been investigated. A small linear increase in lattice parameters ‘a’ and ‘c’ has been observed which can be attributed to the small distortion of the Zn tetrahedron. From the Mn K-edge XANES data, it can be inferred that Mn exists as Mn2+ in the 2% Mn-doped ZnO sample. Li doping oxidizes the Mn-cations in the ZnO lattice and hence Mn exists in higher oxidation states (+2 or +3) in (Li, Mn) co-doped ZnO samples. Li doping also favors Mn metal clustering whose signature can be seen in both the Mn K-edge XANES and EXAFS spectra of the (Li, Mn) co-doped samples. UV-light driven degradation of methylene blue (MB) dye aqueous solution has also been demonstrated using pure and doped ZnO and more than 90% dye degradation has been observed within only 90 min of light irradiation. The synthesized materials with visible light emission and dye degradation activity can be used effectively in future optoelectronic devices and in water purification for cleaning of dyes.

Ion cyclotron resonance heating system on Aditya

An ion cyclotron resonance heating (ICRH) system has been designed, fabricated indigenously and commissioned on Tokamak Aditya. The system has been commissioned to operate between 20.0 and 47.0 MHz at a maximum power of 200 kW continuous wave (CW). Duration of 500 ms is sufficient for operation on Aditya, however, the same system feeds the final stage of the 1.5 MW ICRH system being prepared for the steady-state superconducting tokamak (SST-1) for a duration of 1000 s. Radio frequency (RF) power (225 kW) has been generated and successfully tested on a dummy load for 100 s at 30.0 MHz. Lower powers have been coupled to Aditya in a breakdown experiment. We describe the system in detail in this work.

Role of compensating Li/Fe incorporation in Cu0.945Fe0.055−xLixO: structural, vibrational and magnetic properties

Doped transition metal oxides, like CuO, are spintronic materials. An increase of magnetic moment has been reported in Fe-doped CuO. Additional secondary doping elements such as Li may further modify magnetism in transition metal oxides due to changes in valence state and size. Such complex doping may generate secondary impurity phases, restricting the success of such applications. Enhancement of magnetic properties strengthens applicability of the materials in devices. Single phase monoclinic Cu0.945Fe0.055−xLixO crystalline powders without any impurity are synthesized. Structural studies, electronic valence state, fine structure, and local geometry of constituent elements confirm the purity of the materials and provide clues to probable reasons leading to enhanced magnetism. Ferromagnetic coupling between neighboring spins is most likely dependent on the spin and separation between the spin species.

X-ray structural studies on solubility of Fe substituted CuO

CuO is a promising material for the spintronic industry for which lattice distortions/defects play an important role in determining its magnetic and various other physical properties. The ionic radii and charge of Cu2+[VI] (0.73 A) and Fe3+[VI] (0.64 A) are quite different. Hence high Fe substitution in CuO in place of Cu may generate strain/distortions. Fe substitution may enhance magnetic properties, even at room temperature, making such materials interesting for device applications. A detailed structural study on Fe incorporated CuO lattices to confirm phase purity, supported by evidence of the absence of a secondary phase is absolutely essential especially when considering a considerable substitution of up to ∼12.5%. The electronic valence state, fine structure and local neighborhood/geometry of constituent elements need to be investigated using synchrotron based X-ray absorption spectroscopy (XAS). We report, for the first time, such a detailed study on understanding this magnetically and electronically important material: Cu1−xFexO, 0 ≤ x ≤ 0.125.

Physical and magnetic roughness at metal-semiconductor interface using x-ray and neutron reflectometry

A difference in nuclear and magnetic roughness in Fe/Ge thin film was observed by specular polarized neutron reflectometry. The study also shows asymmetric magnetic scattering length density distributions at Fe on Ge and Ge on Fe interfaces. Using specular and diffuse x-ray reflectivity measurements we could estimate the depth of interdiffusion of Fe and Ge at two interfaces. We also observed that the interface magnetic moments are also strongly dependent on the crystalline state of Fe and Ge as well as the deposition sequence. The difference between nuclear and magnetic roughness at magnetic-semiconductor interfaces will play important role in spintronics.

Opto-electronic properties of Zn(1-x)VxO: Green emission enhancement due to V4+ state

Vanadium incorporation in ZnO modifies the lattice structure. The valence state of V plays an important role, controlling the oxygen content and thereby dimensions of the lattice. Both V4+ and V5+ are more electropositive than Zn2+ and reduce oxygen vacancies, resulting in lattice expansion. However, the sizes of both V4+ and V5+ are smaller than Zn2+, thereby resulting in the lattice contraction. The internal competition of increasing oxygen content and reducing effective crystal radius decides the lattice expansion and contraction. This affects the lattice strain and changes electronic levels, which modify absorption and emission processes in between the valence and conduction bands. A strong green emission band not due to oxygen vacancy but due to defects contributed by vanadium is also dependent on the oxidation state of vanadium. Bandgap also increases with the increase in the V4+ content.

Antiferromagnetic coupling between surface and bulk magnetization and anomalous magnetic transport in electro-deposited cobalt film

We report the interesting magnetic behavior of a Co film (thickness ∼350 A) grown on Si/Ti/Cu buffer layer by electro-deposition (ED) technique. Using depth sensitive X-ray reflectivity and polarized neutron reflectivity (PNR) we observed two layer structures for the Co film grown by ED with a surface layer (thickness ∼100 A) of reduced density (∼68% of bulk) compared to rest of the Co film (thickness ∼250 A). The two layer structure is consistent with the histogram profile obtained from atomic force microscope of the film. Interestingly, using PNR, we found that the magnetization in the surface Co layer is inversely (antiferromagnetically) coupled (negative magnetization for surface Co layer) with the rest of the Co layer for the ED grown film. While the PNR results from a Co film of similar layered structure grown by sputtering, showed a uniform magnetization. We also showed that the depth dependent unusual magnetic behavior of ED grown Co film may be responsible for anomalous anisotropic magnetoresistance observed in low field in this film as compared to the Co film grown by sputtering. The study indicates that the interesting surface magnetic property and magneto-transport property of the ED film is caused by its unique surface morphology.

Thermal diffusion in Ni/Al multilayer

Two Ni/Al multilayers deposited by ion beam sputtering of nominal design [Ni(200A)/Al(100A)]×5 and [Ni(50A)/Al(227A)]×5 on Si substrates were annealed at 200°C. As-deposited and annealed samples were characterized by x-ray diffraction (XRD) and x-ray reflectometry (XRR). The effort was to study the path of alloying in the above two multilayers of same elements but of opposite stoichiometric ratio. We find distinct differences in alloying of these samples.

Fe/Au Multilayers: Structure and Magnetoresistance

We have measured the magnetoresistance (MR) in two sets of Fe/Au multilayers, with varying (1) Fe layer thickness, tFe = 3–10 nm, and (2) Au layer thickness tAu = 5–15 nm, grown on Si substrates by sputtering. The multilayer interface structure and magnetic properties were studied by polarized neutron reflectometry (PNR). The study was undertaken to understand the correlation between structure of these multilayers and their magneto‐transport properties.