arXiv subjects

Deepti Sharma

Publications and source records attributed to Deepti Sharma.

10 recordsLinked to original sources

arXiv2026

First Plasma Commissioning and Operational Highlights from India's First Spherical Tokamak at IPR

A compact Spherical Tokamak(ST) is commissioned at Institute for Plasma Research (IPR) to explore low aspect ratio tokamak physics and technologies that complement to the existing high aspect ratio tokamaks namely ADITYA-U and SST-1 by enabling studies on non-inductive startup, current drive in over dense plasmas, and shaped plasma physics on a low cost platform. The device, India's first spherical tokamak has completed major mechanical, magnetic, and electrical integration, and the coil system has been successfully tested with series of integrated commissioning. First plasma experiments have been carried out with a modest Ohmic system assisted by a 2.45GHz microwave system, supported by a centralized control and data acquisition system. An initial diagnostic set comprising visible imaging, spectroscopy, magnetics, and radiation monitors required for machine operation has been installed. This paper presents the integrated commissioning experiences and first plasma experiments of the newly installed machine.

physics.plasm-ph↗

arXiv2026

Relativistic calculations of electron impact excitation cross-sections of neutral tungsten

Reliable tungsten spectroscopy is essential for modelling and diagnosing tungsten-containing plasmas, particularly in edge/divertor conditions where neutral tungsten (WI) is produced by sputtering and contributes to visible/near-UV emission. In this work, we present fine-structure-resolved EIE cross sections for W I computed using the relativistic distorted-wave (RDW) method. Target states are described by multi-configurational Dirac-Fock (MCDF)wave functions with an extensive configuration-interaction (CI) expansion including valence-valence and core-valence correlation through hole and deeper-core configurations. The resulting energies are bench marked against available theoretical results and the recommended values from the NIST database. Fine-structure-resolved cross sections are reported for excitations from the ground level 5d4 6s2 (5 D0) and six metastable levels 5d4 6s2 (5D1 , 5D2 , 5D3, 5D4 , 3P0) and 5d5 (6S)6s (7S3 ) into excited levels belonging primarily to the 5d4 6s(6 D)6p and 5d5 (6S)6p configurations, over incident electron energies from threshold to 500 eV. With the exception of a limited set of previously studied transitions, most of the reported fine-structure-resolved cross sections are presented here for the first time. The results show a strong dependence on the initial level and indicate that excitation from the metastable state yields the largest cross sections among the states considered, highlighting the importance of metastable populations in W I modelling. In addition, radiative transition probabilities for selected prominent transitions are calculated and compared with existing data. The present dataset is expected to be valuable for collisional-radiative modelling and spectroscopic diagnostics of tungsten plasmas in the 1-50 eV range.

physics.atom-ph↗

arXiv2026

Effect of static magnetic island on ITG of ADITYA-U tokamak

Magnetic islands play a crucial role in regulating plasma confinement in tokamaks by interacting with micro-instabilities, such as the ion temperature gradient (ITG) mode. This work presents a detailed investigation of the effects of static magnetic islands on ITG instability, relevant to the ADITYA-U tokamak, using the Global Gyrokinetic Code in Cylindrical Coordinates (G2C3), a particle-in-cell (PIC) framework that employs a neural-network-assisted projection scheme. A two-phase simulation strategy is adopted. In the first phase, static magnetic islands with mode numbers (m, n) = (2, 1) and (3, 1) are introduced by perturbing the equilibrium magnetic flux functions. Particle dynamics within these modified topologies result in the flattening of plasma density profiles in the island regions, confirming island formation and its impact on the equilibrium profiles. In the second phase, the flattened profiles serve as new equilibria for linear electrostatic gyrokinetic simulations with adiabatic electrons, enabling the study of the modified ITG behavior. Magnetic islands significantly restructure the ITG mode, producing a spatial redistribution of potential fluctuations within and around the island region. Moreover, as the island width increases, the growth rates of different toroidal ITG modes converge, suggesting a universal stabilization trend. A comparison between the (2,1) and (3,1) islands indicates that higher-q islands lead to a more spatially extended ITG mode structure, reflecting the longer magnetic connection lengths and weaker curvature drive at outer flux surfaces. These results demonstrate the pivotal role of island-induced equilibrium modifications in determining ITG stability and mode structure in tokamak plasmas.

physics.plasm-ph↗

arXiv2025

Simulation study of neutral tungsten emissions for fusion applications

The article reports electron-impact excitation cross-sections and rate coefficients for neutral tungsten for three transitions (400.87 nm, 429.46 nm, and 430.21 nm) using the relativistic distorted wave approach within the flexible atomic code. Some of these lines are also observed in tokamak plasma. Cross-sections are computed for incident electron energy up to 30 keV. The energy levels in flexible atomic code were corrected to match the NIST database. The electron impact excitation rate coefficients are also provided.

physics.plasm-ph↗

arXiv2025

Effect of convective transport in edge/SOL plasmas of ADITYA-U tokamak

The 2-D edge plasma fluid transport code, UEDGE has been used to simulate the edge region of circular limiter plasmas of ADITYA-U for modelling the measured electron density profile. The limiter geometry of ADITYA-U has been introduced in the UEDGE code, which is primarily developed and used for divertor configuration. The computational mesh defining the limiter geometry is generated by a routine developed in-house, and has successfully been integrated with the UEDGE code to simulate the edge plasma parameters of ADITYA-U. The radial profiles of edge and scrape-off layer (SOL) electron density, ne and temperature are obtained from the simulations and used to model the measured ne profile using Langmuir probe array. It has been found that a convective velocity, vconv. is definitely needed in addition to the constant perpendicular diffusion coefficient, D throughout the edge and SOL regions to model the edge ne profile. The obtained vconv. is inward and radially constant with a value of 1.5 m/s and the radially constant D is ~ 0.2 m 2/s. The value of D ~ 0.2 m2/s is found to be much less than fluctuation induced diffusivities and lies in-between the neoclassical diffusivity and Bohm diffusivity estimated in the edge-SOL region of ADITYA-U tokamak. Furthermore, the transport of radial electron heat flux is found to be maximizing near the limiter tip location in the poloidal plane.

physics.plasm-ph↗

arXiv2022

Enhanced Photon Squeezing in Two-Photon Dicke Model

We explore the phenomena of quadrature squeezing of photons in the Two-Photon Dicke Model under the mean-field approximation in the thermodynamic limit. The strength of photon squeezing is maximized in the region where the coupling strength is of the same order of magnitude as one of the fundamental frequency of the system. This particular region is termed as the unbounded region. The squeezing of the photonic quadratures can be observed only in the superradiant phase where the squeezing is well beyond the standard quantum limit both near the quantum critical point as well as the unbounded region. However, prolonged squeezing is only obtained in the latter case. Furthermore, we explore the critical behavior of photon squeezing near the unbounded region.

quant-ph↗

arXiv2020

Power-law growth of time and strength of squeezing near quantum critical point

The dynamics of squeezing across quantum phase transition in two basic models, viz., the one-axis twisting model in transverse field and the Dicke model, is investigated using Holstein-Primakoff representation in the large spin limit. Near the phase boundary between the disordered (normal) and the ordered (superradiant) phase, the strength of spin and photon squeezing and the duration of time for which the system stays in the highly squeezed state are found to exhibit strong power-law growth with distance from the quantum critical point. The critical exponent for squeezing time is found to be 1/2 in both the models, and for squeezing strength, it is shown to be 1/2 in the one-axis twisting model, and 1 for the Dicke model which in the limit of extreme detuning also becomes 1/2.

quant-ph↗

arXiv2018

Heisenberg Operator Approach for Spin Squeezing Dynamics

We reconsider the one-axis twisting Hamiltonian, which is commonly used for generating spin squeezing, and treat its dynamics within the Heisenberg operator approach. To this end we solve the underlying Heisenberg equations of motion perturbatively and evaluate the expectation values of the resulting time-dependent Heisenberg operators in order to determine approximately the dynamics of spin squeezing. Comparing our results with those originating from exact numerics reveals that they are more accurate than the commonly used frozen spin approximation.

cond-mat.quant-gas↗

arXiv2017

Modeling of eddy current distribution in the SST-1 tokamak

The time varying currents in the Ohmic transformer in the SST-1 tokamak induce voltages that drive large eddy currents in the passive structures like the vacuum vessel and cryostat. Since the vacuum vessel and the cryostat are toroidally continuous without electrical breaks in SST- 1, this leads to a shielding effect on the flux penetrating the vacuum vessel. This reduces the magnitude of the loop voltage seen by the plasma as also delays its buildup. Also the induced currents alter the null location of magnetic field. This will have serious implications on the plasma breakdown and startup and corrective measures may be required in case of an insufficient loop voltage or an improper null. Further, the eddy currents distribution will be vital for the plasma equilibrium and need to be considered while reconstructing the equilibrium. Evolution of the toroidal eddy currents in SST-1 passive structures has been studied using a toroidal-filament model. The model calculations are compared with the measured signals in the magnetic diagnostics like the toroidal flux loops and magnetic pick-up coils installed on the SST-1.

physics.plasm-ph↗

arXiv2016

Spin squeezing and Quantum Fisher Information in the Jaynes-Cummings Dicke Model

We investigate spin squeezing (SS) and the quantum Fisher information (QFI) for the Jaynes-Cummings Dicke (JC-Dicke) model in a two component atomic Bose-Einstein condensate (BEC) inside an optical cavity. Analytical expressions for spin squeezing and the reciprocal of the quantum Fisher information per particle (RMQFI) are derived using the frozen spin approximation. It is shown that in the superradiant phase near the critical point, maximum squeezing and maximum quantum entanglement occurs. The present study is relevant to quantum information processing and precision spectroscopy.

cond-mat.quant-gas↗