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.

    • Electromagnetic design of 325 MHz superconducting single-spoke resonators for Indian Facility for Spallation Research


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      We present an electromagnetic design of 325 MHz superconducting single-spoke resonators (SSRs) to accelerate the H$^−$ ions from 3 to ∼160 MeV for the proposed Indian Facility for Spallation Research project. The geometrical parameters of SSRs have been optimised to maximise the achievable acceleration gradient, and to minimise the multipacting growth rate, keeping a reasonably low value of heat dissipation on the cavity surface. For the geometrical optimisation of SSR cavities, we followed a one-dimensional procedure, where we vary and fix the geometrical parameters one by one in a sequential manner. We explain the behaviour of various RF parameters of the SSR with respect to its geometrical parameters, based on which the optimum value of each of the geometricalparameters has been chosen. For the optimised geometry of SSRs, we performed the higher-order mode (HOM) analysis and estimated the transverse and longitudinal kicks imparted to the beam by HOMs. The effect of azimuthala symmetry of the electromagnetic field is also analysed and presented for the adopted design. A holistic design approach is described in the paper that addresses the interlinked optimisation requirements.

    • Numerical study on the impact of errors in a 325 MHz radiofrequency quadrupole and assessing the validity of quasistatic approximation in the analysis


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      The performance of a radiofrequency quadrupole (RFQ) is known to be sensitive to its geometric errors, as well as to the amplitude and phase errors of the input RF. Extensive statistical simulations are typically performed using the available computer codes, for assessing the effect of various errors, to fix the tolerances on these errors. Each of the statistical simulation requires beam dynamics calculations to be performed with a modified electromagnetic(EM) field configuration corresponding to the particular set of geometric errors. Although modified EM field due to geometrical errors should be accurately obtained using EM codes such as CST-MWS, such an approach is time consuming. It is therefore common practice to obtain the EM fields, under quasistatic approximation, which is faster. Such an approach is routinely followed in codes such as TRACEWIN. In this paper, we first present extensive errorstudies, using this approach, for a 325 MHz, 3MeV RFQ, which has been designed as a front-end accelerator for the envisaged Indian Facility for Spallation Research. The validity of quasistatic approximation for such error analysisis explicitly checked in this paper by comparing the results obtained using TRACEWIN, with those obtained using the EM fields derived from the CST-MWS code. To the best of our knowledge, such a comparison has not been reported before. Also, since the available computer codes do not give the option to study the effect of RF phase jitter in an RFQ, techniques evolved for this study are also described in this paper.

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