Session Z05: Quantum and Subatomics

Diffusion in Coulomb Plasmas Across Coupling Regimes

We use molecular dynamics simulations to calculate the diffusion coefficients of Coulomb plasmas with percent-level precision.  In previous work we focused on screening lengths typical for white dwarf astrophysics and developed a simple empirical model for diffusion for use in stellar evolution codes.  In this work, we perform additional simulations over a wider range of couplings to further generalize our model, which may have applications in charged soft matter systems.   

Generalized Sheath Criterion for Multi-species Weakly Ionized Plasmas

The problem of sheath formation in glow discharges close to surfaces has been well-studied since 1949 (Bohm, 1949). Since electrons are more mobile than the ions, the surfaces exposed to plasma charge up negatively.   The potential drop from the plasma to the surface is screened over a region determined by the Debye length.    The “Bohm Criterion” determines the sheath formation and its stability for collisionless steady plasmas. This requires that the ions enter the sheath with a directed velocity greater than or equal to the ion acoustic velocity.   This criterion has been consistently used for systems in which the assumption of collisionless plasmas is valid (Riemann, 1991). In the current work, this criterion has been extended to systems with multiple species including effects such as electron inertia and electrical fields.  The analysis further shows that for discharges with multiple species this criteria leads to a symmetric and elegant condition. The resulting conditions simplifies to the "Bohm Criterion” for collisionless plasmas with Boltzmann distribution for electrons.  The generalized criterion is also consistent with experimental results that in the presence of multiple species, the ions do not reach sheath edge as determined by this criterion (Severn, 2003, 2007).

 

Electromagnetic modeling of a plasma-filled resonator for diagnostics

Cold plasmas with temperatures of less than 1000 K are essential to experiments on laser cooling and magneto-optical trapping [1]. Particularly, Cs and Rb plasmas inside microwave cavity resonators are of interest to the experimenters. Analytical perturbative solutions [1] can provide information about the shifts in resonant frequencies in relation to electron concentrations inside a resonator, thus making first-order plasma diagnostics possible.  In this work, the author presents an approach to numerical simulations of plasmas in microwave resonators, including the effects of photo-ionization and multiple resonant modes, as they develop in time. Time-domain electromagnetic (EM) [2] and particle-in-cell (PIC) methods enable more detailed analysis of  plasma evolution in a resonator. Short-time Fourier transform analysis is applied to the simulated electromagnetic fields. The results show how multimodal cavity resonances are affected by the electron concentrations.  The described model may be useful to explain experimental data in laser beam interactions with cold plasmas.

1. M.A.W. van Ninhuijs, Rev. Sci. Instrum., 92, 013506, 2021.

2. A. Semychayevskyy,, PIER M, 73, 197-203, 2018.   

Driven-dissipative electron transport in a quantized conducting array

Nanojunction experiments with single molecules or quantum dots placed between  macroscopic leads allow the exploration of quantum transport at the nanoscale [1]. We study driven-dissipative electron transport in a quantized conducting array of coupled two-level electron sites with a Markovian quantum master equation approach [2] that includes Coulomb interactions, electron tunneling, radiative recombination and non-radiative thermal relaxation [2]. Using literature data [3], we reproduce experimental negative conductance results for a thynylene AH molecular junction at 6 K. By driving a conducting array with resonant incoherent light, photon-induced currents are produced at zero bias voltage, whose current direction depends on the degree of delocalization of  the array electrons. Conditions for current-induced light generation are also discussed.   

Visualization of spatially confined air plasma in macro (mm) tubes

The spatially confined nanosecond pulse laser induced air plasma generated in a rectangular glass tube is visualized through shadowgraphy and self-emission, to understand the role of the radial shock wave compression and re-heating of air plasma. Second harmonic Nd: YAG laser pulses with 10 ns pulse width, peak intensities in the range of 2.5 - 20 ×10⁹ W/cm² corresponding to the laser energies of 50-400 mJ were used to create air plasma at the center of the rectangular tube of dimension 12 mm (L) x 8 mm (D). The shadowgraphy visualization showed that, the strength of the reflected radial shock wave depends on the input laser energy, and cavity aspect ratio (L/D). These parameters were observed to play a crucial role in the enhancement of the plasma properties. Similarly, the self-emission imaging revealed that, due to the compression of radial shock waves, the plasma source split into localized multiple emission centers, that are recombine together and form as a single source at longer time scales. The confined air plasma and shock wave evolution are compared with that of free expansion for different input laser energies. The comparison shows that, the plasma properties and its lifetime are enhanced by 1.5-2 times with the spatial confinement.   

Estimating ionospheric scintillation S4 from ROTI over the Indian region using GPS data

Ionospheric irregularities can be assessed through the rate of change of total electron content (TEC) indicated by ROT, which can be derived from a dual-frequency GNSS receiver at a time interval of 5 min, Pi et al (1997). However, most of the GNSS receivers do not provide scintillation data due to their high sampling rate of at least 50 Hz thus ROTI can be used as a proxy for scintillations. In most of the past studies, it is shown there is some correlation between ROTI and S_4. The significant capturing of the scintillation patch is validated by plotting the temporal variation of S₄ for all the 25 stations under GAGAN for the year 2014.

GPS data has been used to statistically study the ionospheric scintillations, applications of which are beneficial in radio communication. The data used is GAGAN data to make robust conclusions on the scintillation indices.