M. S. Bhatia

Measurement of photoionization cross section in atomic uranium using simultaneous observation of laser-induced photoionization and fluorescence signals

Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.

Convection in molten pool created by a concentrated energy flux on a solid metal target

During surface evaporation of metals by use of a concentrated energy flux such as electron beam or lasers, a liquid metal pool having a very high temperature gradient is formed around the hot zone created by the beam. Due to temperature dependence of surface tension, density, and depression of the evaporating surface caused by back pressure of the emitted vapor in this molten pool, a strong convective current sets in the molten pool. A proposition is made that this convection may pass through three different stages during increase in the electron beam power depending upon dominance of the various driving forces. To confirm this, convective heat transfer is quantified in terms of dimensionless Nusselt number and its evolution with power is studied in an experiment using aluminum, copper, and zirconium as targets. These experimentally determined values are also compared to the theoretical values predicted by earlier researchers to test the validity of their assumptions and to know about the type of flow in the melt pool. Thus, conclusion about the physical characteristics of flow in the molten pool of metals could be drawn by considering the roles of surface tension and curvature of the evaporating surface on the evolution of convective heat transfer.

Evolution of a Two-Temperature Plasma Expanding With Metal Vapor Generated by Electron-Beam Heating

During the electron-beam evaporation of metals, a weakly ionized plasma is formed, which consists of two different groups of electrons characterized by different energy spreads (or temperature). While this plasma expands along with the metal vapor, a thermodynamic equilibrium between these two groups of electrons is gradually established by electron-electron Coulomb collisions and electron-atom inelastic collisions. The evolution of this two-temperature plasma was experimentally observed by a Langmuir probe during an electron-beam evaporation of zirconium. Mathematical expressions for the effect of different interactions on the evolution of the electron temperatures of the plasma were derived and applied to our experimental observations. Taking the initial temperature of the plasma at the source of vapor, the total cross section for electron-atom inelastic collisions was calculated, the order of which agreed well with the reported values. Finally, the contributions of each type of interaction (electron-electron and electron-atom) on the cooling of the high-temperature group of electrons in the plasma are quantified.

Use of multiwavelength emission from hollow cathode lamp for measurement of state resolved atom density of metal vapor produced by electron beam evaporation

State resolved atom population of metal vapor having low-lying metastable states departs from equilibrium value. It needs to be experimentally investigated. This paper reports the use of hollow cathode lamp based atomic absorption spectroscopy technique to measure online the state resolved atom density (ground and metastable) of metal vapor in an atomic beam produced by a high power electron gun. In particular, the advantage of availability of multiwavelength emission in hollow cathode lamp is used to determine the atom density in different states. Here, several transitions pertaining to a given state have also been invoked to obtain the mean value of atom density thereby providing an opportunity for in situ averaging. It is observed that at higher source temperatures the atoms from metastable state relax to the ground state. This is ascribed to competing processes of atom-atom and electron-atom collisions. The formation of collision induced virtual source is inferred from measurement of atom density distribution profile along the width of the atomic beam. The total line-of-sight average atom density measured by absorption technique using hollow cathode lamp is compared to that measured by atomic vapor deposition method. The presence of collisions is further supported by determination of beaming exponent by numerically fitting the data.

Effect of periscope reflecting mirror on uncertainty of measured temperature of an electron beam heated metal vapor source

The temperature of the hot zone created by impact of a high power density electron beam (e-beam) needs to be monitored for reasons concerning process evaluation and safety. In high throughput e-beam evaporators, direct line of sight viewing of the hot zone using an optical pyrometer on a continuous basis is ruled out due to opacity introduced by rapid coating of the vacuum windows within a few seconds. An alternative that permits continuous visual monitoring relies on a periscopic arrangement that makes use of process generated thin film mirror formed by deposition of evaporating metal atoms. This paper discusses the additional error introduced by the thin film mirror during this type of monitoring arrangement for the case when a two-color pyrometer is used as temperature sensor. The dominant factors, namely temperature, pressure and reactivity of metal being evaporated, which affect the measured temperature are found and their effects are confirmed through our experimental data. It is shown that due to dynamically changing spectral reflectivity of the thin film mirror, the additional error in measured temperature could be in the range of ~0–35% of the value recorded by direct viewing method depending upon the above parameters. Finally, as an alternative, we propose a novel method and show through calculations that this method avoids errors experienced in the periscopic method and yet allows extended continuous monitoring time.

Note: Design of transverse electron gun for electron beam based reactive evaporation system

In this paper design of a 10 kV, 10 kW transverse electron gun, suitable for reactive evaporation, supported by simulation and modeling, is presented. Simulation of the electron beam trajectory helps in locating the emergence aperture after 90° bend and also in designing the crucible on which the beam is finally incident after 270° bend. The dimension of emergence aperture plays a vital role in designing the differential pumping system between the gun chamber and the substrate chamber. Experimental validation is done for beam trajectory by piercing a stainless steel plate at 90° position which is kept above the crucible.

Collisional effects on metastable atom population in vapour generated by electron beam heating

The metastable atom population distribution in a free expanding uranium vapour generated by electron beam (e-beam) heating is expected to depart from its original value near the source due to atom–atom collisions and interaction with electrons of the e-beam generated plasma co-expanding with the vapour. To investigate the dynamics of the electron–atom and atom–atom interactions at different e-beam powers (or source temperatures), probing of the atomic population in ground (0 cm−1) and 620 cm−1 metastable states of uranium was carried out by the absorption technique using a hollow cathode discharge lamp. The excitation temperature of vapour at a distance ~30 cm from the source was calculated on the basis of the measured ratio of populations in 620 to 0 cm−1 states and it was found to be much lower than both the source temperature and estimated translational temperature of the vapour that is cooled by adiabatic free expansion. This indicated relaxation of the metastable atoms by collisions with low energy plasma electrons was so significant that it brings the excitation temperature below the translational temperature of the vapour. So, with increase in e-beam power and hence atom density, frequent atom–atom collisions are expected to establish equilibrium between the excitation and translational temperatures, resulting in an increase in the excitation temperature (i.e. heating of vapour). This has been confirmed by analysing the experimentally observed growth pattern of the curve for excitation temperature with e-beam power. From the observed excitation temperature at low e-beam power when atom–atom collisions can be neglected, the total de-excitation cross section for relaxation of the 620 cm−1 state by interaction with low energy electrons was estimated and was found to be ~10−14 cm2. Finally using this value of cross section, the extent of excitational cooling and heating by electron–atom and atom–atom collisions are described at higher e-beam powers.

Electron beam evaporation of aluminium with a porous tantalum rod in melt pool

For most metals, evaporation owing to electron beam heating proceeds in an efficient manner at temperatures substantially higher than the melting point. This is particularly true for aluminium, which has a large separation of melting and boiling point (>1000 K). This leads to situations where convective heat transfer plays an increasingly dominant role with increase in incident e-beam power and puts a limit on the surface temperature (and consequently the atomic flux). To mitigate this heat drain, a porous tantalum rod (~35% porosity) was placed at the point of e-beam impact to act as a convection arrestor and a wick within the aluminium melt pool. The molten aluminium around the porous rod was found to ooze through the capillaries of the porous rod to emerge as a vapour stream. On measuring the atomic flux and surface temperature at e-beam powers (below a value where tantalum softens), it was found that ~50% higher temperature was reached with the tantalum rod than without it. Also, for generating the same atomic flux, the required e-beam power in the case of the porous rod was about one-sixth of the e-beam power required without the rod. Such an efficient evaporation has been reported earlier but without any qualification on the ion fraction of the vapour stream. In our experiment, the ion content in the vapour stream was measured. It was found that the ionization yield in the case of the porous rod was about five times the yield without the rod. Estimation of ionization yields owing to various processes led to the conclusion that higher ionization in the case of the porous rod can be attributed to a higher emission of secondary electrons from the porous rod causing enhanced electron impact ionization.

Evaluation of SAR Reduction for Dipole Antenna Using RF Shield

In this paper we propose to use radio frequency (RF) shield on dipole antenna to reduce Specific Absorption Rate (SAR) in the spherical head model. RF Shields made of Ferrimagnetic material are used to suppress surface current on antenna. Many kinds of simulation are performed to investigate the effect of various parameters like thickness, size, location and type of the shield on the SAR using CST-Microwave studio, the field simulation software. Paper includes numerical evaluation of the SAR reduction and also analyzes the SAR data for shielding effectiveness. Drastic reduction in SAR was observed in case of Ferrite3 material with increase in shielding size and thickness. However there was no appreciable change in SAR for change in shielding location. Simulation results will be useful for compliance testing of wireless communication devices.