arXiv subjects

P. Raj

Publications and source records attributed to P. Raj.

6 recordsLinked to original sources

arXiv2026

Characterization of a prototype parallel-plate $^{238}$U fission chamber with DD and DT fusion neutron sources

The SPARC tokamak will employ $^{238}$U-based fission chambers (FCs) to monitor high-performance deuterium-tritium (DT) plasma operations, spanning neutron yield rates from ${\sim}10^{15}$ to ${>}10^{19}$ n/s. This work validates the $^{238}$U FC design, which utilizes a parallel-plate detector geometry and borated polyethylene collimation to prioritize unscattered DD and DT fusion neutrons. Experimental testing with both DD and DT neutron generators corroborates vendor-specified efficiencies and demonstrates excellent detector linearity, with measured count rates showing good agreement with OpenMC neutronics simulations. Further characterization confirms the $^{238}$U FC's robustness against SPARC-relevant environmental challenges, including stray magnetic fields up to 14 mT and possible signal degradation risks associated with $\sim$30 m long cable runs. These results confirm that the $^{238}$U FC, supported by indirect neutron shielding as well as appropriate pulse height thresholds, provides a reliable solution for fusion power measurements as part of the SPARC neutron diagnostics suite.

physics.plasm-ph↗

arXiv2026

Synthetic model of gamma-ray emission during DT experiments on the SPARC tokamak

In thermonuclear plasmas, plasma ions undergoing nuclear reactions emit gamma-rays with energies in the MeV range. Their spectroscopy can convey much plasma information, such as the DT fusion power, the spatial and velocity distributions of the fast ions, and the plasma heating performance. In the present work, we simulate the gamma-ray emission expected in the SPARC tokamak during a primary reference discharge, when the tokamak is expected to generate $140$ MW of fusion power and reach an energy gain factor of $Q\approx11$. We focus particularly T(D, $\gamma$)He-5, B-10(He-4, p $\gamma$)C-13 and D(He-3, $\gamma$)Li-5 reactions. We use realistic plasma profiles calculated with the TRANSP code and simulate radiofrequency heating of the plasma with CQL3D and TORIC. Possible locations for gamma spectrometers based on lanthanum bromide inorganic scintillators are suggested. For each, the signal-to-noise ratio of gamma-rays over neutrons is evaluated using the ray-tracing code ToFu and high fidelity Monte Carlo models (MCNP and OpenMC) to solve radiation transport in SPARC. A dedicated neutron attenuator made of high density polyethylene is scoped to allow gamma-spectroscopy during high neutron yield experiments. And finally, the performance of LaBr$_3$ detectors in reconstructing the fusion power generated by SPARC is discussed.

physics.plasm-ph↗

arXiv2026

Effects of 3D printed capsule material on activation thin foil irradiation and counting for fusion neutron yield measurements

Activation foils are used to independently measure the time integrated neutron yield and total fusion energy produced in both inertial and magnetic confinement fusion, making them crucial in the neutron diagnostic suite. The activated foils must be remotely transported from the neutron source to the detector inside of a small capsule, which can impact both the foil irradiation and the associated activation measurement. The aim of this paper is to evaluate the performance of various activation foils and to characterize the effects of different capsule materials to inform the design choices for future systems, such as the SPARC tokamak. Through a combination of FISPACT simulations and irradiation experiments with a deuterium-tritium neutron generator, we tested several different material choices for foils, capsules, and gamma-ray spectrometers. Aluminum and copper foils are found to be suitable for a multi-foil irradiation configuration. The use of 3D-printed thermoplastic capsules reduces the number of measured decay-photon counts, yet the reduction is smaller than the associated measurement uncertainty. Finally, lanthanum-based detectors are shown to be viable alternatives to the standard high-purity germanium spectrometer, although with poorer energy resolution.

physics.ins-det↗

arXiv2004

High Pressure X-Ray Diffraction Study of UMn2Ge2

Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered tetragonal ThCr2Si2 structure with space group I4/mmm, a = 3.993A and c = 10.809A under ambient conditions. Energy dispersive X-ray diffraction was used to study the compression behaviour of UMn2Ge2 in a diamond anvil cell. The sample was studied up to static pressure of 26 GPa and a reversible structural phase transition was observed at a pressure of ~ 16.1 GPa. Unit cell parameters were determined up to 12.4 GPa and the calculated cell volumes were found to be well reproduced by a Murnaghan equation of state with K0 = 73.5 GPa and K' = 11.4. The structure of the high pressure phase above 16.0 GPa is quite complicated with very broad lines and could not be unambiguously determined with the available instrument resolution.

cond-mat.mtrl-sci↗

arXiv2001

Magnetization studies on superconducting MgB$_2$ - lower and upper critical fields and critical current density

Magnetization studies have been carried out on superconducting MgB$_2$ (T$_c$=37K) in the temperature range of 2-50K and in magnetic field up to 5 Tesla. From these measurements, values of the lower critical field H$_c1$(0), upper critical field H$_c2$(0) at zero K are estimated to be ~300 Oe and ~12.5 Tesla, respectively, which yield a value of ~26 for the Ginzburg-Landau parameter, k. Magnetization hysteresis loops have been obtained at various temperatures from which the magnetization critical current density is estimated using Bean's critical state model.

cond-mat.supr-con↗

arXiv1997

Inelastic Neutron scattering in CeSi_{2-x}Ga_x ferromagnetic Kondo lattice compounds

Inelastic neutron scattering investigation on ferromagnetic Kondo lattice compounds belonging to CeSi_{2-x}Ga_{x}, x = 0.7, 1.0 and 1.3, system is reported. The thermal evolution of the quasielastic response shows that the Kondo interactions dominate over the RKKY interactions with increase in Ga concentration from 0.7 to 1.3. This is related to the increase in k-f hybridization with increasing Ga concentration. The high energy response indicates the ground state to be split by crystal field in all three compounds. Using the experimental results we have calculated the crystal field parameters in all three compounds studied here.

cond-mat.str-el↗