Academic literature on the topic 'Plasma fysisk'

<strong><span style="font-family: TimesNewRomanPS-BoldMT;"><span style="font-family: TimesNewRomanPS-BoldMT;"><p align="left"> </p></span></span><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;">SAMMENDRAG</span></span></strong><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">Total homocystein (tHcy) er etablert som risikofaktor for hjerte- og karsykdom. Vi har studert determinanter</p><p align="left">av plasma tHcy i et utvalg av den voksne norske befolkning basert på undersøkelse utført av Statens</p><p align="left">helseundersøkelser i samarbeid med Universitetet i Bergen i 1992-1993. Data ble innhentet ved klinisk</p><p align="left">undersøkelse, utfylling av tre spørreskjema og ved blodtester. I alt 18 043 personer i alderen 40-67 år møtte</p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">til undersøkelse og fikk målt plasma tHcy. Plasma folat, plasma kobalamin og 677C</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span><p align="left"><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">→</span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">T mutasjonen i genet<p align="left">for metylentetrahydrofolatreduktase (MTHFR) er bestemt i et underutvalg på 329 personer og på personer</p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">med svært høye tHcy verdier (</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span></p><p align="left"><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">≥ </span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">40 μmol/L). Resultatene fra Homocysteinundersøkelsen i Hordaland har<p align="left">vist at kjønn, alder, folatinntak, røykevaner og kaffeforbruk er de sterkeste determinanter for plasma tHcy</p><p align="left">nivå, mens kobalamininntak, fysisk aktivitet, blodtrykk og kolesterolnivå er mindre sterke determinanter.</p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">Bruk av multivitaminer eller B-vitaminer er forbundet med spesielt lave tHcy nivåer. Personer med tHcy</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span></p><p align="left"><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">≥</span></span></p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">40 μmol/L er karakterisert ved høy forekomst (73%) av homozygositet for 677C</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span><p align="left"><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">→</span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">T mutasjonen i MTHFR<p align="left">genet og lavt folatnivå. Vi konkluderer derfor med at livsstil og etablerte risikofaktorer for hjerte- og</p><p align="left">karsykdom er vesentlige for nivået av plasma tHcy i den generelle voksne norske befolkning.</p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">Nygård O, Refsum H, Ueland PM, Tverdal A, Vollset SE.</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span></p><p align="left"><strong><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;">Homocysteine and lifestyle. The Hordaland<strong><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><p align="left">Homocysteine Study.</p></span></span></strong></span><strong><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><p align="left"> </p></span></strong></span></strong><em><span style="font-size: x-small; font-family: TimesNewRomanPS-ItalicMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-ItalicMT;">Nor J Epidemiol </span></span></em><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">1997; </span></span><strong><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;">7 </span></span></strong><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">(2): 221-224.</span></span></p><p align="left"><strong><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;"><span style="font-size: x-small; font-family: TimesNewRomanPS-BoldMT;">ENGLISH SUMMARY</span></span></strong><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">Total homocysteine (tHcy) concentration is an established cardiovascular risk factor. We have studied</p><p align="left">determinants of plasma tHcy among 18 043 subjects aged 40-67 years from Hordaland county in Western</p><p align="left">Norway who participated in a health screening programme in 1992-1993. Gender, age, folate intake,</p><p align="left">smoking habits and coffee consumption are the strongest determinants of plasma tHcy level, whereas</p><p align="left">cobalamin intake, physical activity, blood pressure and total cholesterol level are weaker determinants. Use</p><p align="left">of multivitamins or B-vitamin supplements are associated with particularly low tHcy levels. In patients with</p><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left">severely elevated tHcy (</p></span></span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><p align="left"> </p></span></span><p align="left"><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">≥ </span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">40 μmol/L, n=67), the combination of homozygosity for the 677C</span></span><span style="font-size: x-small; font-family: Symbol;"><span style="font-size: x-small; font-family: Symbol;">→</span></span><span style="font-size: x-small; font-family: TimesNewRomanPSMT;"><span style="font-size: x-small; font-family: TimesNewRomanPSMT;">T mutation in<p align="left">the methylenetetrahydrofolate gene and low plasma folate levels is a dominant finding whereas a minor</p><p align="left">proportion has overt cobalamin deficiency. We conclude that lifestyle and established cardiovascular risk</p><p>factors are important determinants of the plasma tHcy level in an adult Norwegian population.</p></span></span></p></p>

Electron holes are positively charged nonlinear structures in which trapped electrons are supported by a positive electrostatic potential. These structure are regularly observed in space and laboratory plasmas by means of diverging bipolar electric field signatures. Recent observations and simulations have shown that fast moving electron holes can generate electromagnetic whistler waves via Čerenkov emission. The fast moving positive charge correspond to localised currents which can potentially excite waves. The aim of the project is to study both theoretically and numerically the properties leading to the Čerenkov emission of whistler waves by three-dimensional electron holes. In addition, efforts are dedicated to the derivation of a model providing the properties of emitted whistlers. The model is compared with the observational features of electromagnetic whistler waves generated by electron holes.

This licentiate thesis reports analytical results for the electrostatic response to a test charge moving through dusty plasma. Two particular cases for a slowly moving test charge, namely, grain size distribution and grain charging dynamics are considered. Analytical results for the delayed shielding of a test charge due to dynamical grain charging in dusty plasma are also reported. In the first case, a dusty plasma in thermal equilibrium and with a distribution of grain sizes is considered. A size distribution is assumed which decreases exponentially with the grain mass for large sizes and gives a simple smooth reduction for small sizes. The electrostatic response to a slowly moving test charge, using a second order approximation is found and the effects of collisions are also investigated. It turns out that for this particular size distribution, there is a remarkably simple result that the resulting effective distribution for the electrostatic response is a kappa (generalized Lorentzian) distribution. In the second case, we present an analytical model for the shielding of a slowly moving test charge in a dusty plasma with dynamical grain charging for cases both with and without the collision effects. The response potential is treated as a power series in test charge velocity. Analytical expressions for the response potential are found up to second order in test charge velocity. The first-order dynamical charging term is shown to be the consequence of the delay in the shielding due to the dynamics of the charging process. It is concluded that the dynamical charging of the grains in a dusty plasma enhances the shielding of a test charge. To clarify the physics, a separate study is made where the charging is approximated by using a time delay. The resulting potential shows the delayed shielding effect explicitly. The terms in the potential that depend on the charging dynamics involve a spatial shift given by the test charge velocity and the charging time. This kind of work has relevance both in space and astrophysical plasmas.

This doctoral thesis treats parts of the solar wind interaction with the Martian atmosphere and the water-related features known as gullies. The composition of the escaping plasma at Mars has been investigated in an analysis of data from the IMA sensor, which is a part of the ASPERA-3 instrument suit onboard the European satellite Mars Express. The goal of the investigation is to determine if there are any high abundances of escaping ion species incorporating carbon, such as in CO_2^+ . The most abundant ion species was found to be O^+ and O_2^+, followed by CO_2^+. The following ratios were identifed: CO_2^+/O^+ = 0:2 and O_2^+ /O^+ = 0:9. The escaping plasma, in form of ion beam events, has also been correlated to the magnetic anomalies found on the surface, where no clear association was found. Similar ion beams have also been detected on Venus, which does not have any crustal magnetic fields, and hence the fields are not required for the formation process of the beams. The ion beams' dependence of the direction of the solar wind convection electric field has also been studied, where a correlation was found, suggesting that the ion beams are accelerated by this field. The studies mentioned above are important in order to understand the evolution of Mars and its atmosphere, as well as plasma acceleration processes at non-magnetized planetary bodies. On 5 December 2006 the ASPERA instruments of both Venus Express and Mars Express detected a large enhancement in their respective background count level. These readings are associated with events of SEPs (Solar Energetic Particles), which are believed to be coupled with the CMEs (Coronal Mass Ejection) identified 43 ¡ 67 hours after the SEPs. The CMEs occurred on the far side of the sun (with respect to the locations of Venus and Mars), which indicates that these events can a®ect the space weather in areas situated 90 degrees in both azimuthal directions in the heliosphere with respect to the target. During this event the heavy ion outflow from the atmosphere of Mars increased by one order of magnitude, suggesting that EUV flux levels significantly affect the atmospheric loss from unmagnetized bodies. The gully formations have been investigated with data from the MOC, MOLA and TES instruments onboard the satellite Mars Global Surveyor. The features suggest that there has been fluvial erosion on the surface of Mars. The shallow and deep aquifer models remain the most plausible formation theories. Gully formation processes are important to understand since their eroding agent may be liquid water.
Godkänd; 2008; 20080206 (ysko)

The main topic of this thesis is experimental analysis of material surfaces using scanning probe microscopies. These microscopes are used for characterization through high-resolution topographical imaging, but also for controlled modification of surfaces and molecules. The surface characterization includes evaluation and development of fractal methods for surface roughness determination. The term modification is used for manipulating the structures on a microscale by scraping them with a tiny tip. The major application of this technique in the present work is the analysis of effects induced by plasma-surface interactions. Such studies are fundamental in the understanding of erosion and deposition processes on the first wall in controlled fusion devices. In this work, scanning probe microscopes were for the first time used for studying such plasma-facing materials. Both the surface structure and composition have to be known in order to evaluate new wall-materials for fusion reactors. The materials studied here are graphites, SiC/Al coatings, graphite-silicon mixtures and various silicon carbide based composites. They were all exposed to plasmas, either to lowenergy deuterium plasmas and ions in laboratory experiments, or to the plasma in a socalled tokamak. The results show the usefulness of these high-resolution microscopes in the study of plasma-surface interaction. Several other surface sensitive techniques were also applied, at the home laboratories of our collaborators, the most important ones being Rutherford backscattering spectroscopy and nuclear reaction analysis. The scanning probe microscopy in combination with the ion-beam analysis made it possible to trace fine structural features on the surfaces and to measure the surface roughness. The main results are: (i) the detection of the initial stages of bubble/blister formation on CSi mixtures, SiC/AI coatings and graphites; (ii) the morphological changes and the physical properties of the silicon carbide composites; (iii) the distinction of radiation damages on different phases of multicomponent composites; (iv) the estimation of layer thickness with scanning probe microscopy; (v) the determination of the structure of codeposited layers formed during exposure in a tokamak; (vi) the uptake of deuterium by the materials. The atomic force microscope has also been used to study the human protein spectrin, and we managed to image free spectrins with molecular resolution in an almost natural environment. The elongated spectrin macromolecule was found to be 100 rim long and 5 nm broad. Indications of a substructure were observed. The force between the sensor tip and the molecules was crucial, both for sample movement, manipulation and image resolution. Therefore, the instrument was rebuilt to operate with so called tapping-mode in liquid. Preliminary results with this method on spectrin are presented.
Godkänd; 1995; 20070410 (ysko)

In plasma assisted thin film growth, control over the energy and direction of the incoming species is desired. If the growth species are ionized this can be achieved by the use of a substrate bias or a magnetic field. Ions may be accelerated by an applied potential, whereas neutral particles may not. Thin films grown by ionized physical vapor deposition (I-PVD) have lately shown promising results regarding film structure and adhesion. High power impulse magnetron sputtering (HIPIMS) is a relatively newly developed technique, which relies on the creation of a dense plasma in front of the sputtering target to produce a large fraction of ions of the sputtered material. In HIPIMS, high power pulses with a length of ~100 μs are applied to a conventional planar magnetron. The highly energetic nature of the discharge, which involves power densities of several kW/cm2, creates a dense plasma in front of the target, which allows for a large fraction of the sputtered material to be ionized. The work presented in this thesis involves plasma analysis using electrostatic probes, optical emission spectroscopy (OES), magnetic probes, energy resolved mass spectrometry, and other fundamental observation techniques. These techniques used together are powerful plasma analysis tools, and used together give a good overview of the plasma properties is achieved. from the erosion zone of the magnetron. The peak plasma density during the active cycle of the discharge exceeds 1019 electrons/m3. The expanding plasma is reflected by the chamber wall back into the center part of the chamber, resulting in a second density peak several hundreds of μs after the pulse is turned off. Optical emission spectroscopy (OES) measurements of the plasma indicate that the degree of ionization of sputtered Ti is very high, over 90 % in the peak of the pulse. Even at relatively low applied target power (~200 W/cm2 peak power) the recorded spectrum is totally dominated by radiation from ions. The recorded HIPIMS spectra were compared to a spectrum taken from a DC magnetron discharge, showing a completely different appearance. Magnetic field measurements performed with a coil type probe show significant deformation in the magnetic field of the magnetrons during the pulse. Spatially resolved measurements show evidence of a dense azimuthally E×B drifting current. Circulating currents mainly flow within 2 away cm from the target surface in an early part of the pulse, to later diffuse axially into the chamber and decrease in intensity. We record peak current densities of the E×B drift to be of the order of 105 A/m2. A mass spectrometry (MS) study of the plasma reveals that the HIPIMS discharge contains a larger fraction of highly energetic ions as compared to the continuous DC discharge. Especially ions of the target material are more energetic. Time resolved studies show broad distributions of ion energies in the early stage of the discharge, which quickly narrows down after pulse switch-off. Ti ions with energies up to 100 eV are detected. The time average plasma contains mainly low energy Ar ions, but during the active phase of the discharge, the plasma is highly metallic. Shortly after pulse switch-on, the peak value of the Ti1+/Ar1+ ratio is over 2. The HIPIMS discharge also contains a significant amount of doubly charged ions.

This thesis presents experimental and modelling research on atmospheric pressure hollow cathodes and hollow electrodes. Experiments with the hybrid hollow electrode activated discharge (H-HEAD), which is a combination of a hollow cathode and a microwave plasma source, is presented. The experiments show that this source is able to produce long plasma columns in air and nitrogen at atmospheric pressure and at very low gas flow rates. Measurements of the vibrational temperature of the nitrogen molecules are also presented in this thesis. The vibrational temperature is an indication of the electron temperature in the plasma, an important characteristic of the plasma. Modelling work on the hollow cathode at atmospheric pressure with fluid equations is also presented. It is shown that the inclusion of fast and secondary electrons, characteristic of the hollow cathode plasmas, increases the sheath width. The sheath width was found to be of the order of 100 μm. By modelling the plasma as highly collisional by using the drift-diffusion approximation, it was shown that the increase in sheath thickness was larger at lower pressures than at higher pressures. Still, the sheath width can be of the order of 100 μm. A pulsed atmospheric plasma in a hollow electrode geometry was also modelled by the drift-diffusion fluid equations, with the addition of the energy equation for electrons. Rate and transport coefficients for the electrons were calculated from the solution to the Boltzmann equation as functions of mean electron energy. The dynamics of the plasma at pulse rise time showed large electron density and mean energy peaks at the cathode ends, but also that these quantities were enhanced at the centre of the discharge, between the cathode plates.

The purpose of this project is to study the plasma delay time phenomenon in preparation for the construction of the VERDI spectrometer. To accomplish this, simulations of the spontaneous fissioning process of Cf-252 were created using the fission code GEF, as well as MATLAB. GEF has produced one million fission events, from which the time of flight and kinetic energy of each fission fragment have been calculated with classical mechanics, to replicate the experiment. To imitate the plasma delay time phenomenon, three different models, found in the literature, have been compared. Accounting for other realistic resolution effects and using the first model as the plasma delay time phenomenon, the absolute errors of the mass-yields reaches up to 4 u, whereas the second and third model display absolute errors up to 3 u. Furthermore, it is found that, despite the significant differences in the models' dependencies, the resulting effects are quite similar. All models are found to have a narrowing influence on the pre-neutron emission mass- yield distributions, resulting in an increased peak-to-valley ratio. In the detection of fission fragments, a higher peak-to-valley resolution is often associated with a better mass resolution. This study shows that the plasma delay time could have a misleading influence in regards to estimating an experimental mass resolution.

Neutrons produced in fusion reactions in a magnetically confined plasma carry information about the distributions and densities of the fuel ions in the plasma. This thesis presents work where various theoretical models of different fuel ion distributions in the plasma are used to calculate modeled components of the neutron energy spectrum. The calculated components can then be compared with measured data, either to benchmark and validate the model or to derive various plasma parameters from the experimental data. Neutron spectra measured with the spectrometers TOFOR and the MPR, which are both installed at the JET tokamak in England, are used for this purpose. The thesis is based on three papers. The first paper presents the analysis of TOFOR data from plasmas heated with neutral beams and radio frequency waves tuned to the third harmonic of the deuterium cyclotron frequency, which creates fast (supra thermal) ions in the MeV range. It is found that effects of the finite Larmor radii of the fast ions need to be included in the modeling in order to understand the data. These effects are important for fast ion measurements if there is a gradient in the fast ion distribution function with a scale length that is comparable to - or smaller than - the width of the field of view of the measuring instrument, and if this scale length is comparable to - or smaller than - the Larmor radii of the fast ions. The second paper presents calculations of the neutron energy spectrum from the T(t,n)4He reaction, for JET relevant fuel ion distributions. This is to to form a starting point for the investigation of the possibility to obtain fast ion information from the t-t neutron spectrum, in a possible future deuterium-tritium campaign at JET. The t-t spectrum is more challenging to analyze than the d-d and d-t cases, since this reaction has three (rather than two) particles in the final state, which results in a broad continuum of neutron energies rather than a peak. However, the presence of various final state interactions - in particular between the neutron and the 4He - might still allow for spectrometry analysis. Finally, in Paper III, a method to derive the fuel ion ratio, nt/nd, is presented and applied to MPR data from the JET d-t campaign in 1997. The trend in the results are consistent with Penning trap measurements of the fuel ion ratio at the plasma edge, but the absolute numbers are not the same. Measuring the fuel ion ratio in the core plasma is an important task for fusion research, and also a very complicated one. Future work should aim at measuring this quantity in several independent ways, which should then be cross checked against each other.