Time-averaged Electron Density Research Articles

Time-resolved investigations of pulsed microwave excited plasmas

Pulsed microwave excited (2.45 GHz) argon plasmas generated by a slot antenna type plasma source are investigated by various diagnostic tools. Through the combined use of time-resolved planar optical emission spectroscopy (TPOES), microwave interferometry (MWI) and Langmuir probes the temporal behaviour of the electron density, ne(t), and effective electron temperature, Te(t), for the pulse frequency range of 0.2-20 kHz are measured. Additionally, from TPOES maps of Ar* and Ar+, the qualitative spatially and time-resolved electron temperature distribution is derived. The ne(t) and Te(t) rise and decay times are almost constant throughout the examined frequency range. A ne(t) rise time of 1 ms and a decay time of 0.6 ms is derived from probe and MWI data at 5 Pa. A Te(t) rise time between 5 and 10 µs and a decay time between 50 µs and 80 µs is derived from TPOES and probe measurements at 5 Pa. The maximum time-averaged electron density, e, at 5 Pa is obtained at a pulse frequency f of 200 Hz. With increasing pressure and power the pulse frequency f at which a maximum of e is reached decreases to f50 Hz. The temporal ne(t) and Te(t) behaviour for the investigated pressure range is described by a simple set of equations based on the `Global Model' of pulsed plasmas. It can be concluded that the electron loss rate loss controls both the rise and decay times of ne(t). The loss is in the first order a function of the plasma system dimensions and geometry. The decay of Te(t) depends on loss and the losses due to inelastic scattering.

Non-equilibrium radial and axial transport in RF glow discharges

For the first time, a self-consistent two-dimensional model of RF glow discharges between two parallel plates, assuming a cylindrically symmetric geometry, is simulated by using a Monte Carlo model for electrons and a non-equilibrium fluid model for ions. This simulation is reported in the communication. By considering the evolutions of the charged particles in both the radial and axial directions in a self-consistent manner, the non-equilibrium multidimensional transport phenomenon in RF glow discharges has been investigated. The time-averaged electron densities, ion densities, ionization rate and electric fields in both axial and radial directions in (r,z) space are sketched to present the radial-axial flow dynamics of charged particles. More importantly, significant differences have been observed: the axial-sheath width is smaller and both the electron mean energy and the ionization rate are higher inside the plasma bulk in the two-dimensional model than in the one-dimensional model.

Chromospheric extents predicted by time-dependent acoustic wave models

Theoretical models for chromospheric structures of late-type giant stars are computed, including the time-dependent propagation of acoustic waves. Models with short-period monochromatic shock waves as well as a spectrum of acoustic waves are discussed, and the method is applied to the stars Arcturus, Aldebaran, and Betelgeuse. Chromospheric extent, defined as the monotonic decrease with height of the time-averaged electron densities, are found to be 1.12, 1.13, and 1.22 stellar radii for the three stars, respectively; this corresponds to a time-averaged electron density of 10 to the 7th/cu cm. Predictions of the extended chromospheric obtained using a simple scaling law agree well with those obtained by the time-dependent wave models; thus, the chromospheres of all stars for which the scaling law is valid consist of the same number of pressure scale heights.

Experimental evidence of increased electron temperature, plasma potential, and ion energy near an ICRF antenna Faraday shield

Plasma properties and ion energies have been measured near the surface of the Faraday shield for an ICRF antenna to determine the effect of RF fields on the energy of ions bombarding the shield surface. A resonant loop antenna with a two-tier Faraday shield was used on the RF Test Facility at Oak Ridge National Laboratory. The ECH plasma was produced by a 10.6 GHz klystron and had a central density of ∼10 11 cm −3 and an electron temperature of 5–10 eV. The static magnetic field at the antenna was ∼ 2 kG. A capacitive probe was used to measure the time-varying floating potential and a Langmuir probe was used to measure the time-averaged electron temperature, density, and DC floating potential. Both probes were scanned in front of the antenna. Ion energies were measured with a gridded energy analyzer located next to the antenna. The time-varying floating potential followed the magnetic field pattern of the antenna, indicating that the electromagnetic field is responsible for the potential formation. Potential values of 300 V p-p have been measured. Electron temperatures increased with RF power and reached values ≥60 eV for an RF power of ∼25 kW. The energy of ions hitting a grounded surface also increased with RF power and exceeded 300 eV at ∼25 kW of RF power (compared with energies of 5–15 eV with no RF power). The increase in ion energy can be at least partially explained by an increase in the sheath potential, which in turn is caused by an increase in the electron temperature.

Sheath properties of RF plasmas in a parallel plate etch reactor; the high-frequency regime (ω>ωi)

The time-averaged Poisson equation has been solved in the space charge sheaths of a parallel-plate RF plasma reactor. The frequency of the applied electric field is assumed to be greater than the ion plasma frequency. The time-averaged electron density profile and the stationary ion density profile have been calculated in the space charge sheaths of the RF plasma. The time-averaged sheath thickness and the time-averaged voltage drop across the sheath have been calculated as functions of the relevant discharge parameters. The calculated averaged electron density is compared with experimental data. The model is developed in order to describe the sheath properties for both a collisional and a collisionless sheath.

GENERATION OF EXTENDED CHROMOSPHERES AND MASS LOSS OF LATE-TYPE GIANT STARS DUE TO ACOUSTIC SHOCK WAVES

We investigate the generation of extended chromospheres and mass loss of late-type giant stars due to acoustic shock waves. Effects of both monochromatic waves and acoustic frequency spectra are studied. For our wave computations we use an eulerian time-dependent hydrodynamic code based on the method of characteristics. The possible boundary conditions for the cases of suband supersonic inand outflows are discussed in detail. Ionization of hydrogen is explicitly taken into account. Radiation damping is considered in the thin plasma approximation using a Cox and Tucker type law, fitted on the emission function as given by McWhirter, Thonemann, and Wilson (1975). We obtain three different types of stellar wind solutions: Short-period, acoustic-wave calculations show that a persistent wave energy flux can produce an extended chromosphere. If monochromatic shock waves are used no appreciable mass loss is generated. However, in the case of wave models with stochastically changing wave periods episodic mass loss occurs. This behavior is caused by overtaking and merging of shocks. Occasionally very strong shocks are produced. But the time-averaged mass-loss rates related to these stochastic wave models remain very small. The third type of stellar wind solutions is the generation of continuous mass loss in adiabatic wave models with large wave periods. For these models the wavelengths must be comparable to or larger than a stellar radius. In the case of Arcturus, a mass-loss rate between 10~10 and ΚΓ11 3KO y1 is found. The final flow speed of the wind is larger than 40 km s1. We have applied our method to individual stars, which are: Arcturus (a Bootis), Aldebaran (a Tauri), and Betelgeuse (α Orionis). The chromospheric extents which we find in our time-dependent wave models are 1.12 r*, 1.13 r*, and 1.22 r*, respectively, corresponding to a time-averaged electron density of 107 cm3. These extents are significantly smaller than those derived by Stencel (1981) and Carpenter, Brown, and Stencel (1985), but in the case of Arcturus and Aldebaran, they confirm the results of Judge (1986a, b). The chromospheric temperatures in our wave models increase with height, starting from 6000 Κ up to about 15,000 Κ at the temperature maximum.

Enhancement of the plasma density and deposition rate in rf discharges

The peak and time averaged electron density in rf excited silane-helium mixtures increased significantly above the cw value by square wave modulating the source. The deposition rate of amorphous hydrogenated silicon films is also enhanced and apparently follows the electron density. Attachment to the discharge products appears to be responsible.

Time-Resolved Structural Studies of the Sarcoplasmic Reticulum Membrane

We have used x-ray and neutron diffraction, coupled with the deuteration of selected membrane molecular components, to determine the time-averaged electron density profiles, to 10 A resolution, of the phospholipid bilayer and of the Ca2 ATPase molecule within the total profile of fully functional, isolated sarcoplasmic reticulum membranes (1). Synchrotron x-ray diffraction studies of these membranes with a time resolution of 0.2-0.5 s, coupled with the flash photolysis of caged ATP to initiate synchronously the calcium transport processes of the Ca2+-ATPase molecules in oriented membrane multilayers, have been used to determine the profile structure of the Ca2+-ATPase predominantly in the first phosphorylated intermediate state of the enzyme in the calcium transport process under higher temperature conditions of enzyme turnover (2).

Electron density measurement using the Stark profiles of AlXII lines

Single shot photographic spectra of helium-like resonance lines of AlXII were obtained from a low inductance vacuum spark. The space- and time-averaged electron density determined from the Stark profile analysis of the 1s2−1s4p, 1s2−1s5p, and 1s2−1s6p singlet lines is 4×1021–7×1021 cm−3.

Recent High Spectral and Spatial Resolution Spectroscopy of Laser-Produced Plasmas and Electron-Ion Beam Plasmas

Spectra of plasmas produced by a CO2 laser have recently been obtained using a normal incidence slitless spectrograph and a high spectral resolution (0.028 mÅ) grazing incidence spectrograph. The slitless spectrograph forms images of the plasmas in spectral lines and is similar to the instrument flown by NRL on Skylab. The total wavelength coverage is from about 100 Å to about 600 Å. The shapes of the images depend markedly on the type of atomic transition. Time-averaged electron densities in the expanding plumes are calculated, and the expansion velocity is estimated from the profiles of lines recorded by the grazing incidence spectrograph. In addition, spectra of electron-ion beam plasmas between ∼200 Å and 2000 Å were obtained using a stigmatic normal incidence slit spectrograph. The distribution of plasma emisston between the anode and cathode, and the mass motions in the plasmas are discussed.

Theoretical calculation of the time-averaged electron density distribution for vibrating ethyne molecules in a model crystal structure

Calculations have been made on an ethyne model structure to investigate the influence of internal and external thermal motions of the molecules in a crystal on the time-averaged electron density distribution as obtained by X-ray diffraction. The density distributions in the chemical bonds have been studied from D(r) = q(mol, r)-q(at, r) difference maps, where q(at, r) is the density of the free-atom structure. Vibrations expected to occur at low temperatures reduce the difference densities at the centres of the C-C and C-H bonds by 12 and 23% respectively. Reliable time-averaged difference density distributions can be calculated from the static distributions obtained by theory by use of the root-mean-square deviations (u2)1/2 of the atoms which can be determined by X-ray diffraction. Only negligibly small errors occur in the theoretical difference density maps if librations are accounted for by linear vibrations. In the experimental [Fo-Fc] difference densities errors due to treating librations as linear vibrations are largely, but not completely, compensated by shifts of the atoms.

Nonlinear effects in the positive column of a strongly modulated mercury-rare gas discharge

The properties of the positive column of strongly modulated low-pressure gas discharges in mercury-rare gas mixtures have been studied, both experimentally and theoretically. The current was modulated sinusoidally with a modulation depth of 50%. Calculations have shown that in a limited frequency range nonlinearities in the system have an effect on the time-averaged electron temperature, electron density and excited atom density, and on their waveforms. This behaviour was confirmed by experiments, in which the electron temperature and density were measured by means of microwave techniques and the excited Hg atom densities by measuring the optical absorption and emission. For higher modulation depths differences occur between the results of the calculations and experiments. These discrepancies were found to be more pronounced in pulsed discharges, and are ascribed to changes in the high-energy tail of the electron velocity distribution function.