• Vinit Kumar

      Articles written in Pramana – Journal of Physics

    • Analysis of pulsed wire method for field integral measurements in undulators

      Vinit Kumar Ganeshwar Mishra

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      Pulsed wire technique is a fast and accurate method for the measurement of first and second field integrals of undulators used in free-electron lasers and synchrotron light sources. In this paper, we present a theoretical analysis of this technique by finding out the analytic solution of the differential equation for the forced vibration of the wire taking dispersion due to stiffness into account. Method of images is used to extend these solutions to include reflections at the ends. For long undulators, the effect of dispersion of the acoustic wave in the wire could be significant and our analysis provides a method for the evaluation of the magnetic field profile even in such cases taking the effect due to dispersion into account in an exact way.

    • Physics design of a 10 MeV, 6 kW travelling wave electron linac for industrial applications


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      We present the physics design of a 10 MeV, 6 kW S-band (2856 MHz) electron linear accelerator (linac), which has been recently built and successfully operated at Raja Ramanna Centre for Advanced Technology, Indore. The accelerating structure is a $2\pi/3$ mode constant impedance travelling wave structure, which comprises travelling wave buncher cells, followed by regular accelerating cells. The structure is designed to accelerate 50 keV electron beam from the electron gun to 10 MeV. This paper describes the details of electromagnetic design simulations to fix the mechanical dimensions and tolerances, as well as heat loss calculations in the structure. Results of design simulations have been compared with those obtained using approximate analytical formulae. The beam dynamics simulation with space charge is performed and the required magnetic field profile for keeping the beam focussed in the linac has been evaluated and discussed. An important feature of a travelling wave linac (in contrast with standing wave linac) is that it accepts the RF power over a band of frequencies. Threedimensional transient simulations of the accelerating structure along with the input and output couplers have beenperformed using the software CST-MWS to explicitly demonstrate this feature.

    • Terahertz radiation source using a high-power industrial electron linear accelerator


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      High-power $(\sim 100 kW)$ industrial electron linear accelerators (linacs) are used for irradiations, e.g., for pasteurization of food products, disinfection of medical waste, etc.We propose that high-power electron beam from such an industrial linac can first pass through an undulator to generate useful terahertz (THz) radiation, and the spent electron beam coming out of the undulator can still be used for the intended industrial applications. This will enhance the utilization of a high-power industrial linac. We have performed calculation of spontaneous emission in the undulator to show that for typical parameters, continuous terahertz radiation having power of the order of $\mu$W can be produced, which may be useful for many scientific applications such as multispectral imaging of biological samples, chemical samples etc.

    • 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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