Articles written in Pramana – Journal of Physics

    • Ionospheric plasma by VHF waves

      R P Patel Abhay Kumar Singh R P Singh

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      The amplitude scintillations of very high frequency electromagnetic wave transmitted from geo-stationary satellite at 244.168 MHz have been recorded at Varanasi (geom. lat. 14′ 55′N) during 1991 to 1999. The data are analyzed to determine the statistical features of overhead ionospheric plasma irregularities which are mostly of small duration <30 minutes and are predominant during pre-midnight period. The increase of solar activity generally increases the depth of scintillation. The auto-correlation functions and power spectra of scintillations predict that the scale length of these irregularities varies from 200–500 m having velocity of movement between 75 m/sec to 200 m/sec. These results agree well with the results obtained by other workers.

    • Influence of material parameters on the performance of niobium-based superconducting radiofrequency cavities


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      A detailed thermal analysis of a niobium (Nb)-based superconducting radio-frequency (SRF) cavity in a liquid helium bath is presented, by taking into account the temperature and magnetic field dependence of surface resistance and thermal conductivity in the superconducting state of the starting Nb material (for SRF cavity fabrication) with different impurity levels. The drop in SRF cavity quality factor ($\mathcal{Q}_{0}$) in the high acceleration gradient regime (before the ultimate breakdown of the SRF cavity) is studied in detail. It is argued that the highfield $\mathcal{Q}_{0}$-drop in SRF cavity is considerably influenced by the intrinsic material parameters such as electrical conductivity and thermal diffusivity. The detailed analysis reveals that the current specification on the purity of Nb material for SRF cavity fabrication is somewhat over-specified, as also inferred by the experimental work reported by some of the laboratories in the recent past. In line with these encouraging experimental results, in this paper, based on a rigorous calculation, we show that the Nb material with relatively low purity can very well serve the purpose for the accelerators dedicated for spallation neutron source (SNS) or accelerator-driven sub-critical system(ADSS) applications, where the required accelerating gradient is typically up to $\rm{20 MV m^{−1}}$. This information will have important implication towards the cost reduction of superconducting technology-based particle accelerators for various applications. We think this theoretical work will be complementary to the experimental efforts performed in various laboratories at different corners of the globe.

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