Ingvar Axnäs

An ionization region model for high-power impulse magnetron sputtering discharges

A time-dependent plasma discharge model has been developed for the ionization region in a high-power impulse magnetron sputtering (HiPIMS) discharge. It provides a flexible modeling tool to explore ...

Experimental investigation of the critical ionization velocity in gas mixtures

The critical ionization velocity which is of cosmogonic and astrophysical interest has hitherto mainly been investigated for pure gases. Since in space we always have gas mixtures, it is of interest also to study gas mixtures. The present report, which is a summary of a more detailed report (Axnäs, 1976), summarizes the results of systematic experiments on the critical ionization velocity as a function of the mixing ratio for binary gas mixtures of H2, He, N2, O2, Ne and Ar. The apparatus used is a coaxial plasma gun with an azimuthal magnetic field. The discharge parameters are chosen so that the plasma is weakly ionized. In some of the mixtures it is found that one of the components tends to dominate in the sense that only a small amount (regarding volume) of that component is needed for the discharge to adopt a limiting velocity close to that for the pure component. Thus in a mixture between a heavy and a light component having nearly equal ionization potentials, the heavy component dominates. Also, if there is a considerable difference in ionization potential between the components, the component with the lowest ionization potential tends to dominate.

Measurement of the magnetic field change in a pulsed high current magnetron discharge

In this paper we present a study of how the magnetic field of a circular planar magnetron is affected when it is exposed to a pulsed high current discharge. Spatially resolved magnetic field measur ...

A bulk plasma model for dc and HiPIMS magnetrons

A plasma discharge model has been developed for the bulk plasma (also called the extended presheath) in sputtering magnetrons. It can be used both for high power impulse magnetron sputtering (HiPIM ...

Experiments on the Magnetic Field and Neutral Density Limits on CIV Interaction

Abstract A series of experiments on the critical ionization velocity (CIV) interaction in the impact configuration are reported, where the plasma density, the neutral density and the magnetic field strength have been varied. A combination of microwave interferometry and spectroscopy has been used to measure the plasma density and to study the electron energy distribution. The efficiency of the CIV process is evaluated as function of (1) the ratio between the Alfven velocity and the plasma stream velocity, and (2) the ratio between the ionization frequency and the ion gyro frequency. In the parameter range studied so far, efficient electron heating is found only when both these parameters exceed unity.

The bifurcation structure of periodically forced current disruptions

The bifurcation structure of periodically driven current oscillations in the central chamber of a magnetized triple plasma device is investigated experimentally. The target chamber of the triple plasma device is positively biased with respect to the source chamber and the bias voltage mainly drops in a strong double layer formed in the central chamber. At the low potential side of the double layer, a variable negative potential forms that gives rise to a region of negative resistance in the static current-voltage characteristic of the device. In this regime, if a sufficiently high inductance is added to the external circuit, strong nonlinear low-frequency oscillations both in the plasma current and the voltage drop over the plasma occur. These oscillations are interpreted as the periodically repeated exchange between magnetic energy and particle motion in the double layer. The dynamics of the current circuit is described by a van der Pol-type equation where the nonlinearity is given by the derivative of the current-voltage characteristic of the plasma. An additional periodic driver signal, added to the bias voltage, gives rise to a considerably enriched dynamical behaviour as predicted by the theory of driven nonlinear oscillators, including frequency entrainment, quasiperiodicity, periodic pulling and period doubling bifurcations. The experimental observations are well explained by means of the known bifurcation structure of the the periodically driven van der Pol oscillator model.

A fluid simulation of the AMPTE solar wind lithium release

Abstract The effect of the AMPTE Li release in the solar wind on the plasma dynamics and magnetic field structure is studied on the basis of both a one-fluid and a three-fluid model which include expansion and ionization of the Li cloud as well as lateral flow effects in a first-order approximation. In spite of its limited applicability (assumption of strong momentum coupling between solar wind protons and Li ions) the one-fluid model is found to be able to reproduce details of both the magnetic field and the plasma density variations. An alternative multi-fluid model which treates electrons, protons and Li ions as separate fluids coupled by macroscopic fields, predicts strong momentum coupling to occur only immediately after the release while subsequently protons are allowed to penetrate almost freely the Li ion cloud.

Critical Ionization Velocity Interaction: Some Unsolved Problems

Different problems of current interest regarding the critical ionization velocity (Civ) phenomenon are discussed. The article is divided into five sections corresponding to different aspects of the interaction: velocity, magnetic field strength, geometry, neutral gas density, and time duration. In each section, experiments and theories — microscopic and macroscopic — are discussed.

Voyager Saturnian ring measurements and the early history of the solar system

Abstract The mass distribution in the Saturnian ring system is investigated and compared with predictions from the plasma cosmogony. According to this theory, the matter in the rings has once been in the form of a magnetized plasma, in which the gravitation is balanced partly by the centrifugal force and partly by the electromagnetic forces. As the plasma is neutralized, the electromagnetic forces disappear and the matter can be shown to fall in to 2 3 of the original saturnocentric distance. This causes the so called “cosmogonic shadow effect”, which has been demonstrated earlier for the asteroidal belt and in the large scale structure of the Saturnian ring system. The relevance of the cosmogonic shadow effect is investigated for parts of the fine structures of the Saturnian ring system. It is shown that many structures of the present ring system can be understood as shadows and antishadows of cosmogonic origin. These appear in the form of double rings centered around a position a factor 0.64 (slightly less than 2 3 ) closer to Saturn than the causing feature. Voyager data agree with an accuracy better than 1%.

Anomalous cross-field currents in a CIV laboratory experiment

Abstract The axial and radial ion velocities and the electron radial velocity are determined in coaxial plasma gun operated under critical velocity conditions. The particle velocities are determined from probe measurements together with He I 3889 A absolute intensity measurements and the consideration of the total momentum balance of the current sheet. The ions are found to move axially and the electrons radially much faster than predicted by the E/B drift in the macroscopic fields. These results agree with what can be expected from the instability processes, which has earlier been proposed to operate in these experiments. The experiment is therefore a direct experimental demonstration that instability processes have to be invoked not only for the electron heating, but also to explain the macroscopic velocities and currents.