Gennady V. Miloshevsky

Experimental and computational study of complex shockwave dynamics in laser ablation plumes in argon atmosphere

We investigated spatio-temporal evolution of ns laser ablation plumes at atmospheric pressure, a favored condition for laser-induced breakdown spectroscopy and laser-ablation inductively coupled plasma mass-spectrometry. The 1064 nm, 6 ns pulses from a Nd:YAG laser were focused on to an Al target and the generated plasma was allowed to expand in 1 atm Ar. The hydrodynamic expansion features were studied using focused shadowgraphy and gated 2 ns self-emission visible imaging. Shadowgram images showed material ejection and generation of shock fronts. A secondary shock is observed behind the primary shock during the time window of 100-500 ns with instabilities near the laser cone angle. By comparing the self-emission images obtained using fast photography, it is concluded that the secondary shocks observed in the shadowgraphy were generated by fast moving target material. The plume front estimates using fast photography exhibited reasonable agreement with data obtained from shadowgraphy at early times ≤400 n...

Anion Pathway and Potential Energy Profiles along Curvilinear Bacterial ClC Cl− Pores: Electrostatic Effects of Charged Residues

X-ray structures permit theoretical study of Cl(-) permeation along bacterial ClC Cl(-) pores. We determined the lowest energy curvilinear pathway, identified anion-coordinating amino acids, and calculated the electrostatic potential energy profiles. We find that all four bacterial ClC Cl(-) crystal structures correspond to closed states. E148 and S107 side chains form steric barriers on both sides of the crystal binding site in the StClC wild-type and EcClC wild-type crystals; both the EcClC(E148A) and EcClC(E148Q) mutants are blocked at the S107 site. We studied the effect that mutating the charge of some strongly conserved pore-lining amino acids has on the electrostatic potential energy profiles. When E148 is neutralized, it creates an electrostatic trap, binding the ion near midmembrane. This suggests a possible electrostatic mechanism for controlling anion flow: neutralize E148, displace the side chain of E148 from the pore pathway to relieve the steric barrier, then trap the anion at midmembrane, and finally either deprotonate E148 and block the pore (pore closure) or bring a second Cl(-) into the pore to promote anion flow (pore conductance). Side-chain displacement may arise by competition for the binding site between the oxygens of E148 and the anion moving down the electrostatic energy gradient. We also find that the charge state of E111 and E113 may electrostatically control anion conductance and occupancy of the binding site within the cytoplasmic pore.

Gating Gramicidin Channels in Lipid Bilayers: Reaction Coordinates and the Mechanism of Dissociation

The dissociation of gramicidin A (gA) channels into monomers is the simplest example of a channel gating process. The initial steps in this process are studied via a computational model that simulates the reaction coordinate for dimer-monomer dissociation. The nonbonded interaction energy between the monomers is determined, allowing for their free relative translational and rotational motion. Lowest energy pathways and reaction coordinates of the gating process are determined. Partial rupture of the six hydrogen bonds (6HB) at the dimer junction takes place by coupling monomer rotation and lateral displacement. Coupling rotation with axial separation is far more expensive energetically. The transition state for channel dissociation occurs when monomers are displaced laterally by approximately 4-6 A, separated by approximately 1.6-2 A, and rotated by approximately 120 degrees, breaking two hydrogen bonds. In membranes with significant hydrophobic mismatch there is a much greater likelihood of forming 4HB and possibly even 2HB states. In the 4HB state the pore remains fully open and conductive. However, transitions from the 6HB to 4HB and 4HB to 2HB states take place via intermediates in which the gA pore is closed and nonconductive. These lateral monomer displacements give rise to transitory pore occlusion at the dimer junction, which provides a rationale for fast closure events (flickers). Local dynamics of gA monomers also leads to lateral and rotational diffusion of the whole gA dimer, giving rise to diffusional rotation of the dimer about the channel axis.

Modelling of Kelvin–Helmholtz instability and splashing of melt layers from plasma-facing components in tokamaks under plasma impact

Plasma-facing components (PFCs) in tokamaks are exposed to high-heat loads during abnormal events such as plasma disruptions and edge-localized modes. The most significant erosion and plasma contamination problem is macroscopic melt splashes and losses from metallic divertor plates and wall materials into core plasma. The classical linear stability analysis is used to assess the initial conditions for development and growth of surface waves at the plasma–liquid metal interface. The maximum velocity difference and critical wavelengths are predicted. The effects of plasma density, surface tension and magnetic field on the stability of plasma–liquid tungsten flows are analytically investigated. The numerical modelling predicts that macroscopic motion and melt-layer losses involve the onset of disturbances on the surface of the tungsten melt layer with relatively long wavelengths compared with the melt thickness, the formation of liquid tungsten ligaments at wave crests and their elongation by the plasma stream with splitting of the bulk of the melt, and the development of extremely long, thin threads that eventually break into liquid droplets. Ejection of these droplets in the form of fine spray can lead to significant plasma contamination and enhanced erosion of PFCs. The numerical results advance the current understanding of the physics involved in the mechanism of melt-layer breakdown and droplet generation processes. These findings may also have implications for free surface liquid metal flows considered as the first wall in the design of several types of future fusion reactors.

Application of finite-difference methods to membrane-mediated protein interactions and to heat and magnetic field diffusion in plasmas

A robust finite-difference approach for solving physically distinct cross-disciplinary problems such as membrane-mediated protein-protein interactions and heat and magnetic field diffusion in plasmas is described for rectangular grids. Mathematical models representing these physical phenomena are fourth- and second-order partial differential equations with variable coefficients. The finite-difference coupled harmonic oscillators technique was developed to treat arbitrary aggregates of inclusions in membranes automatically accounting for their non-pairwise interactions. The method was applied to study the stabilization of ion channels in a cluster due to membrane-mediated interactions and to examine the effects of anisotropic membrane slope relaxation on the elastic free energy. To obtain contributions from heat and magnetic field diffusion, the splitting method for the physical processes has been used in the numerical solution of resistive magnetohydrodynamic equations. The fully implicit scheme is outlined, tested and applied to problems of the diffusive redistribution of magnetic field and heat in the plasma.

Dynamics of plasma expansion and shockwave formation in femtosecond laser-ablated aluminum plumes in argon gas at atmospheric pressures

Plasma expansion with shockwave formation during laser ablation of materials in a background gasses is a complex process. The spatial and temporal evolution of pressure, temperature, density, and velocity fields is needed for its complete understanding. We have studied the expansion of femtosecond (fs) laser-ablated aluminum (Al) plumes in Argon (Ar) gas at 0.5 and 1 atmosphere (atm). The expansion of the plume is investigated experimentally using shadowgraphy and fast-gated imaging. The computational fluid dynamics (CFD) modeling is also carried out. The position of the shock front measured by shadowgraphy and fast-gated imaging is then compared to that obtained from the CFD modeling. The results from the three methods are found to be in good agreement, especially during the initial stage of plasma expansion. The computed time- and space-resolved fields of gas-dynamic parameters have provided valuable insights into the dynamics of plasma expansion and shockwave formation in fs-pulse ablated Al plumes in ...

Effects of plasma flow velocity on melt-layer splashing and erosion during plasma instabilities

It is recognized both experimentally and computationally that the main damage of divertor in fusion devices such as ITER could be due to melting of metallic plasma facing components such as tungsten developed during plasma instabilities. Macroscopic melt motion and splashing with ejection of molten droplets into plasma are major concern. The computational modelling of uncoupled/coupled plasma–melt flows is carried out using the developed VoF-MHD model. The goal of this research is to study the effect of viscous plasma flowing with a velocity of 0–5 km s −1 on the melt stability. Development of running waves with large wavelengths is observed on the melt surface in the absence of plasma impact. The magnetic field of 5 T that is parallel to the direction of melt motion completely damps these surface waves. When the magnetic field is perpendicular to the direction of melt motion, the small-amplitude standing waves are formed. The viscous plasma streaming with ∼0.1–5 km s −1 over the melt surface develops waves that are not damped by the magnetic field which is either parallel or normal to the direction of melt motion. It is observed that the surface waves are generated much faster at higher plasma speeds and their wavelength decreases accordingly. The high-speed viscous plasma flowing with ∼ 5k m s −1 produces small melt ripples that break up into droplets carried away by the plasma wind. This is a major concern for magnetic fusion as a reliable source of energy production.

Antiport Mechanism for Cl−/H+ in ClC-ec1 from Normal-Mode Analysis

ClC chloride channels and transporters play major roles in cellular excitability, epithelial salt transport, volume, pH, and blood pressure regulation. One family member, ClC-ec1 from Escherichia coli, has been structurally resolved crystallographically and subjected to intensive mutagenetic, crystallographic, and electrophysiological studies. It functions as a Cl(-)/H(+) antiporter, not a Cl(-) channel; however, the molecular mechanism for Cl(-)/H(+) exchange is largely unknown. Using all-atom normal-mode analysis to explore possible mechanisms for this antiport, we propose that Cl(-)/H(+) exchange involves a conformational cycle of alternating exposure of Cl(-) and H(+) binding sites of both ClC pores to the two sides of the membrane. Both pores switch simultaneously from facing outward to facing inward, reminiscent of the standard alternating-access mechanism, which may have direct implications for eukaryotic Cl(-)/H(+) transporters and Cl(-) channels.

Effects of excitation laser wavelength on Ly-α and He-α line emission from nitrogen plasmas

Laser-produced nitrogen plasmas emitting radiation at 2.48 nm (Ly-α) and 2.88 nm (He-α) are considered potential efficient sources for water-window (WW) microscopy. The atomic and optical properties of nitrogen plasma and influence of the laser wavelength on the line emission in the WW range are investigated. It is found that the optimal temperatures for maximum emission from Ly-α and He-α spectral lines are 40-60 eV and 80-100 eV, respectively. The WW line emission and the conversion efficiency (CE) are estimated for three distinct Nd:YAG laser wavelengths (1064 nm, 532 nm, and 266 nm). The calculated CEs are compared with experimentally observed CE values. It is found that 1064 nm wavelength provides the highest CE from laser to Ly-α and He-α radiation.

Stabilization of ion channels due to membrane-mediated elastic interaction

Recent work shows that linked gramicidin channels may have much longer lifetimes than single channels. We establish that the stabilization of the individual channels can be caused by membrane-mediated elastic interactions between such inclusions. In linear elastic theory, interaction can be rigorously described in terms of coupled harmonic oscillators. We determine the “effective spring constants” for various assemblies using the smectic bilayer model. We consider a range of aggregates; in clusters, channel lifetimes may increase by several orders of magnitude, an effect that is especially pronounced for a channel with many near neighbors.