K Wiesemann

Kinetics of ozone and nitric oxides in dielectric barrier discharges in O2/NOx and N2/O2/NOx mixtures

Concentrations of NO, NO2, NO3, N2O5, and O3 were measured by classical absorption spectroscopy in dielectric barrier discharges in flowing O2/NOx and N2/O2/NOx mixtures. The results of measurements in different parts of the discharge chamber and in its exhaust are compared to a numerical zero-dimensional kinetic model and good agreement is found. The experimentally found upper limit of the NOx concentration allowing ozone production is confirmed by the kinetic calculations for both gas mixtures. The rotational temperature of different nitrogen bands was measured by high-resolution emission spectroscopy. The results are explained on the basis of a simplified model and related to the gas temperature in the microdischarge channel and the surrounding gas.

X-ray bremsstrahlung measurements on an ECR-discharge in a magnetic mirror

The hot electron component of a microwave heated electron cyclotron resonance (ECR)-discharge is investigated. The plasma formed from different noble gases is contained in a simple magnetic mirror configuration. Improved X-ray diagnostic methods and an improved numerical deconvolution scheme are used to measure the volume bremsstrahlung spectrum emitted by the high energy electron and to derive their energy distribution function. For kinetic energies below 350 keV the electron energy distribution obeys a power law, if a cold plasma component (kTe<100 eV) is present. The density of electrons with energies above 10 keV turns out to be smaller than 1014 m-3. These results, together with the observed dependence of the power-law distribution function both on the pressure and on the nuclear charge number Z of the neutral gas, provide new insights into the electron acceleration and loss mechanisms of an ECR-discharge.

Simon short circuit effect in ECRIS

The plasma confinement within an electron cyclotron resonance ion source significantly influences the charge state distribution and hence the performance of the source. The different axial and radial diffusion processes govern the confinement time. In many experiments it has been shown that negatively biasing the end plate in the injection region improves the charge state distribution. In a few x-ray and vacuum ultraviolet spectroscopy experiments to clarify the mechanism it is observed that the biasing improves the confinement of the plasma. It is estimated that the effect cannot be explained solely by secondary electron emission from the plate into the plasma. We propose that by biasing, the overall balance between radial ion losses and axial electron losses will change, resulting in a different diffusional mode of the entire plasma. Hence, the plasma potential and the average charge state of ions in the plasma are significantly influenced. Usually, the ion flux is dominating radial diffusion while the ...

Absolute calibration of the efficiency of a VUV-monochromator/detector system in the range 110 - 450 nm

For determining the efficiency of a vacuum-UV monochromator with a multiplier type PMT-142 in the range 110 - 450 nm two different methods for absolute and six methods for relative calibrations were used. The results of all relative calibrations except for one agree to within an accuracy close to the experimental error in the range of their mutual overlap. The accuracies of the relative efficiency curves were found to be 18, 14, 7, 3, 6 and 10% for the ranges 110 - 120, 120 - 140, 140 - 250, 250 - 350, 350 - 400, and 400 - 450 nm, respectively. Absolute calibrations were performed at 147 and nm with an accuracy of 8% in both cases. The differences in the grating illuminations at the monochromator applications and in the calibration were taken into account. The influences of the accuracies of all operations necessary for the absolute calibration on the overall calibration error were thoroughly analysed. If there are significant differences between the geometries of the system illumination during calibration and during application, they may be the main source of error.

Collisions of doubly charged nitrogen molecules with rare gas atoms

The authors report on single electron capture in slow collisions of doubly charged nitrogen molecules with the rare gases He, Ne and Ar. The vertical double ionization energy of N2 as well as vertical excitation energies of N22+ have been calculated by means of the multiconfiguration coupled electron pair approximation (MC-CEPA) method. Furthermore, experiments have been carried out applying the technique of translational energy spectroscopy. The energy gain spectra of the resulting N2+ molecules have been analysed, showing the single electron capture to be dominated by N22+ (X 1 Sigma +g) projectile ions, as well as by N22+ ions in the low-lying metastable c 3 Sigma +u state. The position of the experimentally observed reaction window is discussed in terms of Franck-Condon transitions and is compared with predictions of classical descriptions.

Spectroscopic determination of the cold electron population in very low pressure ECR discharges in N2/He mixtures

We describe a spectroscopic study of the electron distribution functions (EDFs) in two types of electron cyclotron resonance (ECR) discharges at very low pressure: a discharge in a simple magnetic mirror trap (ECRD) and a discharge in a sheared minimum-B-trap used as ion source for multiply charged ions (ECRIS). The discharges were run in mixtures of helium and nitrogen at about 7?mPa. Special attention was paid to electrons with kinetic energies below 10?eV. Their contributions were attained by a quantitative analysis of the vibrational structures in the emission spectra of nitrogen, the analysis of processes of step-wise excitation or an analysis of the rotational populations of nitrogen. The models for evaluating the EDFs are discussed in detail. Besides collision processes in the volume of the discharge processes occurring at the chamber walls contribute significantly to the particle balance and are included in the model. In all cases considered low energy electrons contribute more than 90% to the electron population of the discharges. For every discharge EDFs are measured for two benchmark conditions at low and high discharge power. For the ECRIS other conditions are covered by a measurement of the plasma potential, which turned out to be below 30?V for the conditions considered. This low value is a good indicator for the presence of low energy electrons. The plasma potential was obtained from a retarding field analysis of the energy of ion beams extracted from the plasma.Besides the EDFs, our study yields insight into the important collision and wall processes in these low-pressure discharges, including the first observation of rotational cooling of desorbing N2 molecules in an ECRIS.

A comparative study of the electron distribution function in the positive columns in and /He dc glow discharges by optical spectroscopy and probes

We compare to the probe method a spectroscopic method for determining in plasmas the electron distribution function (EDF) over a wide energy range. For a test of the radiative-collisional model we use to describe the plasma radiation, the measured vibrational distributions of (C-B) and (B-X) were compared with calculated ones using our model and EDFs measured by Langmuir probes. From this comparison we obtain a value for the rate constant for vibrational relaxation at the walls. In a second step we invert the system of model equations for obtaining the EDF from measured line intensities. From the vibrational structure of the emission spectra of the nitrogen molecule the EDF is obtained in the energy range of 1.5-4.5 eV. From the relative intensities of the emission of nitrogen molecules and helium atoms the EDF for electron energies above 11 eV is derived. In the region between these ranges the EDF is interpolated. The results agree within the limits of the experimental errors with the EDF measured directly by the probe.

Study of plasma polymerization from acetylene in pulsed r.f. discharges

Plasma polymer films were deposited from an argon and acetylene mixture by plasma enhanced chemical vapor deposition in pulsed radio frequency discharges. The discharge on-time varied from 50 to 150 ms and the off-time was kept constant at 1900 ms. The kinetics of the film growth was studied by in-situ reflectance measurements. The films were further investigated ex-situ by spectroscopic ellipsometry, %in the ultraviolet and the visible (UV/VIS) atomic force microscopy, scanning and transmission electron microscopes. Damages in the films caused by residual stress were investigated with an optical microscope. The hydrogen content and film densities were measured by nuclear resonance reaction analyses. Some film properties namely the residual stress, deposition rate, optical properties and surface roughness were significantly influenced by the duration of the discharge pulses. We found refractive indices of the films in the visible in the range 1.60-1.73. The hydrogen-to-carbon ratio in the films and the film density were about 3:2 and 0.6 g/cm3, respectively.

Studies of electron heating and multiply charged ion production in an electron cyclotron resonance plasma

Electron heating and production of multiply charged heavy ions were measured in an electron cyclotron resonance plasma heated by microwaves in a magnetic mirror trap. Dependence of ion yield and heating upon independent machine input parameters of power, magnetic field configuration, pressure, heating duty factor were measured. The ion yields are compared to a theoretical model.

Partial and state-selective cross sections for multiple ionisation of rare-gas atoms by electron impact

Partial ionisation cross sections for single electron impact on rare-gas atoms (Ne, Kr and Xe) have been measured for charge states up to q=8. In addition, by using the translational energy spectroscopy technique (TES) the authors have determined state-selective cross sections for the production of doubly and triply charged neon ions in specific electronic states. The contributions of direct and inner-shell processes are discussed.