Study of the expansion of a plasma generated by electron-beam evaporation

Bésuelle, E.; Nicolaï, Jean-Philippe

doi:10.1063/1.368625

Cited by 16 publications

(10 citation statements)

References 8 publications

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“…It was estimated that at a typical e-beam power of ∼4 kW at 30 kV, each atom is subjected to a high energy electron flux of 0.3 A cm −2 for a beam diameter of ∼7 mm, taking backscattered and secondary yield data from [1]. But the calculated low energy electron flux is found to be ∼3 A cm −2 , taking the atom density of uranium near the source as ∼3 × 10 14 cm −3 corresponding to the source temperature, ionization yield as ∼0.3% (which was measured and is close to typical values reported in [16,17]) and electron temperature the same as the source temperature. Thus, as the high energy electron flux is less by an order of magnitude than the low energy electron flux and its cross section for interaction is also less than that of the low energy group, we have neglected the interaction of high energy electrons.…”

Section: Theorysupporting

confidence: 80%

“…Hence, evapouration from this region is analysed assuming that the density of atoms within the hemisphere of radius r is constant n a0 and it evolves as a point source (obeying the inverse square law) outside this region. If α is the ionization fraction in vapour which is normally ∼0.2% to 0.5% [16,17], the electron density of the plasma generated from vapour will be given by…”

Section: Theorymentioning

confidence: 99%

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

Dikshit¹,

Majumder²,

Bhatia³

et al. 2008

J. Phys. D: Appl. Phys.

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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.

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Section: Theorysupporting

confidence: 80%

Section: Theorymentioning

confidence: 99%

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

Dikshit¹,

Majumder²,

Bhatia³

et al. 2008

J. Phys. D: Appl. Phys.

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“…37 For our case it is estimated that the low energy electron flux ͑ϳ3 A/ cm 2 ͒ is an order of magnitude more than the high-energy flux ͑ϳ0.3 A / cm 2 ͒. In region II ͑Fig.…”

Section: B Characteristics Of the U Atomic Beammentioning

confidence: 62%

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

Majumder

Dikshit

Bhatia

et al. 2008

Review of Scientific Instruments

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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.

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“…The electron temperature T e of the plasma in the vapour was then calculated from the slope of ln(I e ) versus V p curve in the straight line region when the probe voltage is less than the plasma potential [8,9]. The electron density at the measurement point was calculated using relation [10]:…”

Section: Discussionmentioning

confidence: 99%

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

Dikshit¹,

Zende²,

Bhatia³

et al. 2005

J. Phys. D: Appl. Phys.

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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.

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Study of the expansion of a plasma generated by electron-beam evaporation

Cited by 16 publications

References 8 publications

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

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

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

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

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