Zeke Insepov

Can surface cracks and unipolar arcs explain breakdown and gradient limits

The authors argue that the physics of unipolar arcs and surface cracks can help understand rf breakdown and vacuum arc data. They outline a model of the basic mechanisms involved in breakdown and explore how the physics of unipolar arcs and cracks can simplify the picture of breakdown and gradient limits in accelerators, tokamaks as well as laser ablation, micrometeorites, and other applications. Cracks are commonly seen in SEM images of arc damage and they are produced as the liquid metal cools. They can produce the required field enhancements to explain field emission data and can produce mechanical failure of the surface that would trigger breakdown events. Unipolar arcs can produce currents sufficient to short out rf structures, and can cause the sort of damage seen in SEM images. They should be unstable, and possibly self-quenching, as seen in optical fluctuations and surface damage. The authors describe some details and consider the predictions of this simple model.

Atomistic Simulation of Clustering and Annihilation of Point Defects in Molybdenum

Evolution of a molybdenum system containing self-interstitials and vacancies was studied by molecular dynamics simulation using a new molybdenum interatomic potential. The potential was parameterized by using formation and migration energies of the defects. Clustering and annihilation of the defects were investigated in terms of the defect concentration changes during the calculation. The rate constants were evaluated and compared with the diffusion coefficients. Also investigated was the influence of one-dimensional diffusion on kinetics, as well as the effects of temperature and defect concentrations on the reaction rates.

Sheath parameters for non-Debye plasmas: Simulations and arc damage

This paper describes the surface environment of the dense plasma arcs that damage rf accelerators, tokamaks, and other high gradient structures. We simulate the dense, nonideal plasma sheath near a metallic surface using molecular dynamics (MD) to evaluate sheaths in the non-Debye region for high density, low temperature plasmas. We use direct two-component MD simulations where the interactions between all electrons and ions are computed explicitly. We find that the non-Debye sheath can be extrapolated from the Debye sheath parameters with small corrections. We find that these parameters are roughly consistent with previous particle-in-cell code estimates, pointing to densities in the range 10 24 ‐10 25 m � 3 . The high surface fields implied by these results could produce field emission that would short the sheath and cause an instability in the time evolution of the arc, and this mechanism could limit the maximum density and surface field in the arc. These results also provide a way of understanding how the properties of the arc depend on the properties (sublimation energy, for example) of the metal. Using these results, and equating surface tension and plasma pressure, it is possible to infer a range of plasma densities and sheath potentials from scanning electron microscope images of arc damage. We find that the high density plasma these results imply and the level of plasma pressure they would produce is consistent with arc damage on a scale 100 nm or less, in examples where the liquid metal would cool before this structure would be lost. We find that the submicron component of arc damage, the burn voltage, and fluctuations in thevisible light production of arcs may be the most direct indicators of the parameters of the dense plasma arc, and the most useful diagnostics of the mechanisms limiting gradients in accelerators.

Activation of Nanoflows for Fuel Cells

Propagation of Rayleigh traveling waves on a nanotube surface activates a macroscopic flow of the gas (or gases) that depends critically on the atomic mass of the gas. Our molecular dynamics simulations show that the surface waves are capable of actuating significant macroscopic flows of atomic and molecular hydrogen, helium, and a mixture of both gases both inside and outside carbon nanotubes. In addition, our simulations predict a new “nanoseparation” effect when a nanotube is filled with a mixture of two gases with different masses or placed inside a volume filled with a mixture of several gases with different masses. The mass selectivity of the nanopumping can be used to develop a highly selective filter for various gases. Gas flow rates, pumping, and separation efficiencies were calculated at various wave frequencies and phase velocities of the surface waves. The nanopumping effect was analyzed for its applicability to actuate nanofluids into fuel cells through carbon nanotubes.

Ion Solid Interaction And Surface Modification At RF Breakdown In High‐Gradient Linacs

Ion solid interactions have been shown to be an important new mechanism of unipolar arc formation in high‐gradient rf linear accelerators through surface self‐sputtering by plasma ions, in addition to an intense surface field evaporation. We believe a non‐Debye plasma is formed in close vicinity to the surface and strongly affects surface atomic migration via intense bombardment by ions, strong electric field, and high surface temperature. Scanning electron microscope studies of copper surface of an rf cavity were conducted that show craters, arc pits, and both irregular and regular ripple structures with a characteristic length of 2 microns on the surface. Strong field enhancements are characteristic of the edges, corners, and crack systems at surfaces subjected to rf breakdown.

Simulation Of Ion Implantation Into Nuclear Materials And Comparison With Experiment

A new many‐body potential is proposed for pure molybdenum that consists of using ab initio and atomistic MD simulation methods verified against existing surface erosion experimental data. Mo is an important material for metallic U‐Mo alloys for using them in low‐enriched fuels. Several new Xe‐Mo potentials were also parameterized by comparing the calculated sputtering yield of a Mo‐surface bombarded with Xe ions with experimental data. Calculated results were also compared with defect distributions in CeO2 crystals obtained from experiments by 500 keV Xe implantation at the doses of 1×1017 ions/cm2 at several temperatures.

The Problem of RF Gradient Limits

We describe breakdown in rf accelerator cavities in terms of a number of mechanisms. We divide the breakdown process into three stages: 1) we model surface failure using molecular dynamics of fracture caused by electrostatic tensile stress, 2) the ionization and plasma growth is modeled using a particle in cell code, 3) we model surface damage by assuming unipolar arcing. Although unipolar arcs are strictly defined with equipotential boundaries, we find that the cold, dense plasma in contact with the surface produces very small Debye lengths and very high electric fields over a large area, and these high fields produce strong erosion mechanisms, primarily self sputtering, compatible with crater formation. We compare this model with arcs in tokamaks, plasma ablation, electron beam welding, micrometeorite impacts, and other examples.

Computational problems in modeling arcs

We explore the reasons why there seems to be no common model for vacuum arcs, in spite of the importance of the field and the level of effort expended over more than one hundred years.

Charge relaxation and gain depletion for candidate secondary electron emission materials

Microchannel plates (MCPs) are widely used in photodetectors with a picosecond resolution. Two main characteristics of MCPs, gain and timing resolution, strongly depend on the materials parameters, as well as on the history of electron avalanche evolution. The most important effect that can significantly change the efficiency of an MCP is the effect of saturation of the electronic current, which occurs at high-level input signals. In this paper, the saturation effects are studied numerically, as they are applicable to analysis of large-area, fast photodetectors. It is shown that the saturation effect for short pulses can be reduced by introducing a thin, resistive layer between the bulk material and the emissive coating. The gain and time resolution dependencies on the pore size and voltage are studied numerically. The results are compared with the simulations of other authors and available experimental data.