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Bhavin Patel

Publications and source records attributed to Bhavin Patel.

At least 19 recordsLinked to original sources
arXiv2026

Global Gyrokinetic Simulations of Electromagnetic Turbulence in STEP

This paper presents gradient-driven global electromagnetic gyrokinetic simulations for a conceptual burning flat-top operating point of STEP [1], STEP-EC-HD, and investigates how non-local effects influence the nonlinear saturation and transport of the electromagnetic turbulence at finite ${\beta}$. Local gyrokinetic simulations have shown that including ${\delta}B_{\parallel}$ is essential for the dominant hybrid kinetic ballooning modes, or hKBMs, to be unstable in STEP [2]. Using the long-wavelength ${\delta}B_{\parallel}$ solver [3] implemented in GENE [4], this work demonstrates that the linear mode spectrum can be accurately captured in global geometry, which results in good agreement with an ensemble of local flux-tube simulations. The global framework reproduces the hKBMs identified in [5], while microtearing modes remain challenging to resolve due to their shorter radial scales. Nonlinear simulations reveal clear evidence of an electromagnetic transition to states with extremely large heat fluxes, consistent with local simulations and with the predicted loss of zonal-flow regulation for this proposed operating point [6]. These findings establish the capability of global gyrokinetics to capture finite-${\beta}$ dynamics in STEP-like plasmas and motivate future work to identify the conditions governing this transition.

physics.plasm-ph↗
arXiv2026

MGKDB: An IMAS-aligned multicode gyrokinetic simulation database for reproducible fusion turbulence modeling and data-driven analysis

Expensive fusion simulations are commonly preserved in code-specific formats that limit discovery, comparison, and reuse. We present the Multiscale GyroKinetic DataBase (MGKDB), an open-source software framework and curated archive that converts heterogeneous simulation campaigns into traceable scientific records. Each record links code-native inputs and outputs to provenance and quality metadata, an IMAS-aligned physics representation, and derived diagnostics, preserving model-specific evidence while enabling common-field queries. Production pathways support linear and nonlinear GENE and CGYRO calculations and reduced quasilinear TGLF evaluations. At the September 1, 2026 snapshot, MGKDB contained 1,068,089 records, nearly all of which included a populated gyrokinetics IMAS branch. The software is openly available, while access to the NERSC-hosted production records is managed. Three demonstrations show how these linked representations support scientific reuse. Standardized quantities stored in the Diagnostics branch enable population-scale analysis of archived linear modes; common input coordinates reveal coverage, redundancy, and campaign-driven sampling structure across a multicode collection; and record-level retrieval of native CGYRO inputs drives matched TGLF calculations and produces a traceable dataset for exploratory surrogate modeling. Together, these examples demonstrate how MGKDB supports archive characterization, candidate cross-code and cross-fidelity comparisons, campaign planning, and reproducible data-driven modeling without treating different models as automatically equivalent.

physics.plasm-ph↗
arXiv2026

On the transition to large fluxes and access to second stability in gyrokinetic simulations of electromagnetic turbulence in STEP

This work investigates the nonlinear transition to large heat fluxes observed in local gyrokinetic simulations of electromagnetic turbulence in STEP. Using the stress-balance framework of Zhang et al. (arXiv:2606.04616, arXiv:2607.11789), we confirm that the onset of extreme transport correlates with a critical value of $q^{2}\beta_{e}$, where $q$ is the safety factor and $\beta_{e}$ is the ratio of electron thermal pressure to magnetic pressure, and relate this to a limit on the poloidal beta $\beta_{\mathrm{pol}}$. Crucially, this critical value lies below any relevant linear stability limit in the ($q$, $\beta_{e}$) space (e.g., the onset of ideal or kinetic ballooning modes). Using an extensive set of nonlinear gyrokinetic simulations, we demonstrate that the transition to large fluxes in STEP is governed by a balance between the electrostatic and magnetic-flutter stresses. We argue, and also show numerically, that larger-major-radius tokamaks reach the electromagnetic non-zonal regime at lower $\beta_{e}$, making this MHD-controlled saturation limit more accessible in reactor-scale devices than in small spherical tokamaks. We also demonstrate that access to a second-stable regime enables re-saturation at larger values of $\beta^{\prime}$. We further show that the ideal ballooning mode (IBM) threshold serves as a useful proxy for delineating this second-stable region and also as a qualitative guide for the onset of large fluxes. These results provide a predictive framework for identifying no-go zone predictions from local gyrokinetics and offer new insight into the electromagnetic saturation physics relevant to STEP and other high-$\beta_{e}$ devices.

physics.plasm-ph↗
arXiv2024

Reducing turbulent transport in tokamaks by combining intrinsic rotation and the low momentum diffusivity regime

Based on the analysis of a large number of high-fidelity nonlinear gyrokinetic simulations, we propose a novel strategy to improve confinement in spherical tokamak plasmas by combining up-down asymmetric flux surface shaping with the Low Momentum Diffusivity (LMD) regime. We show that the intrinsic momentum flux driven by up-down asymmetry creates strong flow shear in the LMD regime that can significantly reduce energy transport, increasing the critical gradient by up to $25\%$. In contrast to traditional methods for generating flow shear, such as neutral beam injection, this approach requires no external momentum source and is expected to scale well to large fusion devices. The experimental applicability of this strategy in spherical tokamaks is addressed via simulations by considering actual equilibria from MAST and a preliminary equilibrium from SMART.

physics.plasm-ph↗
arXiv2024

Physics of the low momentum diffusivity regime in tokamaks and its experimental applicability

Strong $E\times B$ plasma flow shear is beneficial for reducing turbulent transport. However, traditional methods of driving flow shear do not scale well to large devices such as future fusion power plants. In this paper, we use a large number of nonlinear gyrokinetic simulations to study a novel approach to increase flow shear: decreasing the momentum diffusivity to make the plasma ``easier to push''. We first use an idealized circular geometry and find that one can obtain low momentum diffusivity at tight aspect ratio, low safety factor, high magnetic shear and low temperature gradient. This is the so-called Low Momentum Diffusivity (LMD) regime. To drive intrinsic momentum flux, we then tilt the flux surface, making it up-down asymmetric. In the LMD regime, this intrinsic momentum flux drives strong flow shear that can significantly reduce the heat flux and increase the critical temperature gradient. We also consider the actual experimental geometry of the MAST tokamak to illustrate that this strategy can be practical and create experimentally significant flow shear. Lastly, a preliminary prediction for the SMART tokamak is made.

physics.plasm-ph↗
arXiv2024

Gaussian process surrogate models for the properties of micro-tearing modes in spherical tokamaks

Spherical tokamaks (STs) have many desirable features that make them a suitable choice for fusion power plants. To understand their confinement properties, accurate calculation of turbulent micro-instabilities is necessary for tokamak design. Presented is a novel surrogate model for Micro-tearing modes (MTMs), the micro-instability thought to be dominant in high beta STs. Direct numerical calculation of micro-instabilities is computationally expensive and is a significant bottleneck in integrated plasma modelling. The considerable number of geometric and thermodynamic parameters, the interactions that influence these coefficients and the resolutions needed to accurately resolve these modes, makes direct numerical simulation for parameter space exploration computationally extremely challenging. However, this and the dearth of accurate reduced physics models for MTMs makes it suitable for surrogate modelling using Gaussian Process Regression, a modern machine learning technique. This paper outlines the further development of a data-driven reduced-order model across a spherical tokamak reactor-relevant parameter space utilising Gaussian Process Regression (GPR) and classification; techniques from machine learning. To build the original simple GP model these two components were used in an active learning loop to maximise the efficiency of data acquisition thus minimising computational cost. The `simple' GP was seen to show a plateau of fidelity with more data and to be under-confident, particular in areas of parameter space close to marginal stability. It is postulated that the presence of multiple sub-types of MTM could be the root cause, with the underlying function being less smooth than expected. An expansion of the model using clustering algorithms to find optimal sub models using a mixture of experts approach is shown to greatly improve the variances in the outputs of the GP model.

physics.plasm-ph↗
arXiv2023

Gaussian Process Regression models for the properties of micro-tearing modes in spherical tokamak

Spherical tokamaks (STs) have many desirable features that make them an attractive choice for a future fusion power plant. Power plant viability is intrinsically related to plasma heat and particle confinement and this is often determined by the level of micro-instability driven turbulence. Accurate calculation of the properties of turbulent micro-instabilities is therefore critical for tokamak design, however, the evaluation of these properties is computationally expensive. The considerable number of geometric and thermodynamic parameters and the high resolutions required to accurately resolve these instabilities makes repeated use of direct numerical simulations in integrated modelling workflows extremely computationally challenging and creates the need for fast, accurate, reduced-order models. This paper outlines the development of a data-driven reduced-order model, often termed a {\it surrogate model} for the properties of micro-tearing modes (MTMs) across a spherical tokamak reactor-relevant parameter space utilising Gaussian Process Regression (GPR) and classification; techniques from machine learning. These two components are used in an active learning loop to maximise the efficiency of data acquisition thus minimising computational cost. The high-fidelity gyrokinetic code GS2 is used to calculate the linear properties of the MTMs: the mode growth rate, frequency and normalised electron heat flux; core components of a quasi-linear transport model. Five-fold cross-validation and direct validation on unseen data is used to ascertain the performance of the resulting surrogate models.

physics.plasm-ph↗
arXiv2015

Mass spectra and decay properties of $D$ Meson in a relativistic Dirac formalism

The mass spectra of $D$ meson states are calculated in the framework of a relativistic independent quark model. For the present study, we have used the martin like potential for the quark confinement. Our predicted states in S-wave, $2\ ^3S_1$ (2605.86 MeV) and $2\ ^1S_0$ (2521.72 MeV) are in very good agreement with experimental result of $2608\pm{2.4}\pm{2.5}$ MeV and $2539.4\pm{4.5}\pm{6.8}$ MeV respectively reported by BABAR Collaboration. The calculated P-wave $D$ meson states, $1^3P_2$ (2468.22 MeV), $1^3P_1$ (2404.94 MeV), $1^3P_0$ (2315.24 MeV) and $1^1P_1$ (2367.94 MeV) are in close agreement with experimental average (Particle Data Group) values of $2462.6 \pm 0.7 $ MeV, $2427 \pm 26 \pm 25$ MeV, $2318 \pm 29 $ MeV and $2421.3 \pm 0.6 $ MeV respectively. The pseudoscalar decay constant ($f_P$= 202.57 MeV) of $D$ meson obtained using this relativistic formalism is in very good agreement with the experiment as well as with the lattice and other available theoretical predictions. The Cabibbo favoured hadronic decay branching ratios, BR$(D^0\rightarrow K^- π^+)$ as $3.835 \%$ and BR $(D^0\rightarrow K^+ π^-)$ as $1.069 \times 10^{-4} $ are also in very good agreement with the respective experimental values of $ 3.91 \pm 0.08\%$ and $(1.48\pm 0.07) \times 10^{-4}$ reported by CLEO Collaboration. Our predicted results in leptonic decay widths of $D$ meson are also in better accord with experiment as well as other theoretical results. The mixing parameters of $D^0 - \bar{D}^0$ oscillation, $x_q$ (5.14 $\times 10^{-3}$), $y_q$ (6.02 $\times 10^{-3}$) and $R_M$ (3.13 $\times 10^{-5}$) are in very good agreement with BaBar and Belle Collaboration results.

hep-ph↗
arXiv2014

Mass spectra and decay properties of $D_s$ Meson in a relativistic Dirac formalism

The mass spectra of $D_s$ meson is obtained in the framework of relativistic independent quark model using Martin like potential for the quark confinement. The predicted excited states are in good agreement with the experimental results as well as with the lattice and other theoretical predictions. The spectroscopic parameters are employed further to compute the decay constant, electromagnetic transition and leptonic decay widths. The present result for its decay constant, $f_P$ (252.82 MeV) is in excellent agreement with the value 252.6 $\pm$ 11.1 MeV reported by CLEO-c and the predicted branching ratios for $(D_s \rightarrow τ\barν_τ, μ\barν_μ)$ ($5.706 \times 10^{-2}, 5.812 \times 10^{-3}$) are in close agreement with the PDG values ($ (5.43 \pm 0.31)\times 10^{-2}, (5.90 \pm 0.33)\times 10^{-3} $) respectively.

hep-ph↗
arXiv2012

Medium effects on the quarkonia states above critical temperature

We present the quarkonia correlators for charmonium and bottomonium systems in the pseudoscalar, vector and scalar channels. For the description of quark-antiquark interaction we adopt the temperature dependant colour screening potential of the power law form. The spectroscopic parameters defined from the model are employed in the spectral functions to compute the quarkonia correlators. We find considerable medium modifications to the effective masses of the quarkonia as well as in the behaviour of the respective radial wave functions. These modifications are then reflected in the computed correlators. The general behaviour of correlators in the vector and scalar channel are in accordance with the latest lattice results while their behaviour in the pseudoscalar channels are found to be different.

hep-ph↗
arXiv2012

Semileptonic decay of $B_c$ meson into $c\bar c$ states in a quark model

The exclusive semileptonic decay form factors and widths of $B_c$ meson in the $c\bar c\ (η_c,J/ψ)$ states based on heavy quark effective theory are computed. The spectroscopic parameters of $B_c$ and the $c\bar c$ states deduced using the phenomenological coulomb plus power form of the $q-\bar q$ potential $(CPP_ν)$ with power index $ν$ have been employed to compute the Isgur wise function as well as the decay form factors. The universal behaviour of the Isgur-wise function at the maximum momentum transfer has been observed for different choices of $ν$ between 0.5 to 2.0. However, such an universality is not observed for the choice of $ν<0.5$. Our predictions for the branching ratio $B_c\rightarrowη_c\ell^+ν_\ell$ lie 0.50 $%$ - 0.56 $%$ while that for $B_c\rightarrow J/ψ\ell^+ν_\ell$ lie 1.8 $%$ - 2.0 $%$ for the choice of the potential index in the range $0.5\leqν\leq2.0$.

hep-ph↗
arXiv2011

Properties of Light Flavour Baryons in Hypercentral quark model

The light flavour baryons are studied within the quark model using the hyper central description of the three-body system. The confinement potential is assumed as hypercentral coulomb plus power potential ($hCPP_ν$) with power index $ν$. The masses and magnetic moments of light flavour baryons are computed for different power index, $ν$ starting from 0.5 to 1.5. The predicted masses and magnetic moments are found to attain a saturated value with respect to variation in $ν$ beyond the power index $ν>$ 1.0. Further we computed transition magnetic moments and radiative decay width of light flavour baryons. The results are in good agreement with known experimental as well as other theoretical models.

hep-ph↗
arXiv2011

Two-Photon, Two-gluon and Radiative Decays of Heavy Flavoured Mesons

Here we present the two-photon and two-gluon decay widths of the S-wave ($η_{Q\in c,b}$) and P-wave ($χ_{Q\in c,bJ}$) charmonium and bottonium states and the radiative transition decay widths of $c\bar c$, $b\bar b$ and $c\bar b$ systems based on Coulomb plus power form of the inter-quark potential ($CPP_ν$) with exponent $ν$. The Schr$\ddot{o}$dinger equation is solved numerically for different choices of the exponent $ν$. We employ the masses of different states and their radial wave functions obtained from the study to compute the two-photon and two-gluon decay widths and the E1 and M1 radiative transitions. It is found that the quarkonia mass spectra and the E1 transition can be described by the same interquark model potential of the $CPP_ν$ with $ν=1.0$ for $c\bar c$ and $ν=0.7$ for $b\bar b$ systems, while the M1 transition (at which the spin of the system changes) and the decay rates in the annihilation channel of quarkonia are better estimated by a shallow potential with $ν<1.0$.

hep-ph↗
arXiv2009

Decay Properties of $D$ and $D_s$ mesons

The decay rates and spectroscopy of the $D$ and $D_s$ mesons are computed in a nonrelativistic phenomenological quark-antiquark potential of the type $V(r)=-{4/3}\frac{α_s}{r}+A r^ν$ with different choices of $ν$. Numerical method to solve the Schrödinger equation has been used to obtain the spectroscopy of $q\bar{Q}$ mesons. The spin hyperfine, spin-orbit and tensor components of the one gluon exchange interactions are employed to compute the spectroscopy of the excited $S$ states, low lying $P$-waves and $D$-waves. The numerically obtained radial solutions are employed to obtain the decay constant and leptonic decay widths. It has been observed that predictions of the spectroscopy and the decay widths are consistent with other model predictions as well as with the known experimental values.

hep-ph↗
arXiv2009

Raditive decay of single charmed baryons

The electromagnetic transitions between ($J^{P}={3/2}^{+}$) and ($J^{P}={1/2}^{+}$) baryons are important decay modes to observe new hadronic states experimentally. For the estimation of these transitions widths, we employ a non-relativistic quark potential model description with color coulomb plus linear confinement potential. Such a description has been employed to compute the ground state masses and magnetic moments of the single heavy flavor baryons. The magnetic moments of the baryons are obtained using the spin-flavor structure of the constituting quark composition of the baryon. Here, we also define an effective constituent mass of the quarks (ecqm) by taking into account the binding effects of the quarks within the baryon. The radiative transition widths are computed in terms of the magnetic moments of the baryon and the photon energy. Our results are compared with other theoretical models.

hep-ph↗
arXiv2008

Mixing and lifetime of B-meson using Coulomb plus power potential

The investigation of mixing phenomena and lifetime in neutral B meson systems provides an important testing ground for standard model flavour dynamics. Spectroscopic parameters has been used to calculate the pseudoscalar decay constant and predictions of mixing mass parameters and lifetimes of the B$_{d}$ and B$_{s}$ mesons.

hep-ph↗
arXiv2008

Properties of $Q\bar Q$ mesons in non-relativistic QCD formalism

The decay rates of $Q \bar Q$ mesons ($Q \varepsilon {c, b}$) are studied in the NRQCD formalism in terms of their short distance and long distance coefficients. The long distance coefficients are obtained through phenomenological potential model description of the mesons. The model parameters that reproduces the mass spectrum of the $c \bar c$, $b \bar b$ and $c \bar b$ mesons are employed to study the decay widths of these mesons. We extract the mass spectrum as well as the reproduces the respective radial wave functions from the different potential models as well as from non-relativistic phenomenological quark antiquark potential of the type $V(r)=-\frac{α_c}{r}+A r^ν$, with $ν$ varying from 0.5 to 2. The spin hyperfine and spin-orbit interactions are employed to obtain the masses of the pseudoscalar and vector mesons. The decay constants with QCD corrections are computed in this model as well as in the case of other potential models for comparison. The digamma and dileptonic decays of $c \bar c$, and $b \bar b$ mesons are investigated using some of the known potential models without and with radiative corrections up to the lowest order. These decay width are also computed within the NRQCD formalism up to $O(v^4)$ by making uses of the respective spectroscopic parameters of the models. Our theoretical predictions of the decays of the $c \bar c$, and $b \bar b$ mesons and the results obtained from some of the other potential schemes are compared with the experimental values. The partial widths and life time of the $B_c$ meson are also computed using the model parameters and are found to be in good accordance with the experimental values.

hep-ph↗