Plasma Loss Mechanisms Research Articles

Plasma loss mechanisms from Saturn's magnetosphere

Since the first up‐close observations of Saturn, made by the Pioneer 11 probe in 1979, a great deal has been learned about the dynamics of the gas giant's magnetosphere. In‐depth observations made by the Cassini orbiter, which has been circling Saturn since 2004, have revealed fundamental differences between the behavior of Saturn's magnetosphere and that of the Earth's magnetosphere. Earth's magnetospheric plasma is largely populated by ions captured from the solar wind, whereas Saturn's plasma comes predominantly from water vapor that spews from massive geysers on the southern end of its icy moon Enceladus. Ionized water vapor from Enceladus streams out at 12 to 250 kilograms per second, yet observations show that the concentration of plasma in Saturn's magnetosphere is at a relatively steady level. This discrepancy has left researchers searching for potential plasma loss mechanisms. In a review, Thomsen highlights the progress made in recent years in understanding this question.

Transport reduction by current profile control in the reversed-field pinch

An auxiliary poloidal inductive electric field applied to a reversed-field pinch (RFP) plasma reduces the current density gradient, slows the growth of m=1 tearing fluctuations, suppresses their associated sawteeth, and doubles the energy confinement time. This experiment attacks the dominant RFP plasma loss mechanism of parallel streaming in a stochastic magnetic field. The auxiliary electric field flattens the current profile and reduces the magnetic fluctuation level. Since a toroidal flux change linking the plasma is required to generate the inductive poloidal electric field, the current drive is transient to avoid excessive perturbation of the equilibrium. To sustain and enhance the improved state, noninductive current drivers are being developed. A novel electrostatic current drive scheme uses a plasma source for electron injection, and the lower-hybrid wave is a good candidate for radio-frequency current drive.

Voyager 2 electron observations in the magnetosphere of Neptune

This paper describes electron data obtained during the Neptune encounter by the Voyager 2 plasma science experiment. We derive the densities and temperatures of low‐energy (10–5950 eV) electrons and the electrostatic potential of the spacecraft near Neptune. The data indicate that the escape of charged particles from Triton and the local ionization of atoms in the neutral torus originating from Triton are the major plasma sources. We infer that this neutral torus of hydrogen atoms has a density of about 300 cm‐3 and an inner boundary at 8RN. The data near Neptune exhibit signatures suggesting that both precipitation into Neptune's atmosphere and ring absorption are important plasma loss mechanisms. Plasma transport in the magnetosphere appears to be very fast; the diffusion coefficient is xl0−7L3 RN2 s−1.

Venus' nighttime horizontal plasma flow, ‘magnetic congestion’, and ionospheric hole production

A persistent feature of the nightside Venus ionosphere is the existence of localized magnetic field aligned columns of depleted ionization—i.e., ‘holes’—in which the radial component of the field is enhanced. By analogy to the earth's auroral regions it has been inferred that parallel electric fields can rapidly deplete the ionization. Another, complementary plasma loss mechanism at night is downward transport into the lower ionosphere where chemical depletion occurs. A simple rectilinear two dimensional MHD model is used to explore the effects of field aligned plasma loss and cooling on a dense plasma convecting across a weak magnetic field. By parameterizing field aligned variations and explicitly solving for cross variations, it is shown that the abrupt horizontal enhancements of the vertical magnetic field as well as abrupt decreases of the plasma density to very low values—the two characteristics of a hole—can be produced in the presence of field aligned losses. The buildup of the magnetic field occurs in conjunction with a deceleration of the horizontal flow. The horizontal decrease in plasma density occurs abruptly when the magnetic field intensifies to such a level that the associated horizontal transport time becomes smaller than the field aligned loss time; this process is referred to as ‘magnetic congestion’.

Major radius compression in a large Tokamak

In the presence of certain plasma loss mechanisms, major radius compression of a plasma of reduced size may give higher temperatures than those obtainable in full aperture Tokamak operation. The authors give compression scaling laws of temperature for various loss mechanisms and identify situations where major radius compression may be usefully applied.

Analysis profiles of oxide films on chrome steel by auger emission and x-ray photoelectron spectroscopies

Abstract Samples of steel from two different sources were examined. The materials had nominally the same bulk composition but different samples from each batch had been hardened under two slightly different conditions. The surface oxide films were analysed by both Auger emission spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) after a number of ion bombardments, and the variation in composition with depth was established. In general the outermost oxide layers were chromium deficient and XPS results suggested the presence of Fe 2 O 3 at the surface, together with atmospheric contamination. After ion bombardment the proportion of chromium oxide increased and both iron metal and oxide (Fe 3 O 4 ) were present. The amount of chromium oxide reached a maximum at the steel oxide interface and on further bombardment was replaced by chromium metal. AES and XPS results were in agreement qualitatively and also quantitatively after measurement with a curve analyser of the areas under the peaks of certain elements. The present investigation has shown that AES and XPS can give very similar analyses provided some simple corrections are applied. However, the use of the higher resolution of XPS can provide additional information which cannot be obtained by AES. Thus, using expanded XPS scans, together with suitable curve analysis techniques, it is relatively simple to separate signals from oxide and from free metal regions of the sample, and to study in detail the changes in composition of the oxide from the oxide-air to the oxide-metal interfaces. The study of the four steel samples in this manner has shown that there is a thin region rich in chromium oxide adjacent to the metal. The high level of chromium falls off fairly rapidly nearer the surface and appears to stabilize at a very low level. This is particularly obvious in the thicker oxides (PH), where there is a thick surface layer which is principally iron oxide. It would appear that under the conditions of the heat treatment given to the present samples the thickening of the oxide is due almost entirely to iron oxide. Other work has shown that the low temperature air-formed oxide on these steels is chromium rich and it is suggested that the thickening is due to diffusion of iron through the original film with little movement of the chromium. In view of the high chromium deficiency in the outer layers of the oxides examined it is apparent that the supposedly protective chromium oxide film on chrome steel is situated at the oxide-metal interface and not on the outer surface. The present investigation has shown the existence of two unreported iron satellites. One would appear to be associated with the mixed valence oxide Fe 3 O 4 and another with the purely trivalent oxide Fe 2 O 3 . The satellites could be associated with a strong plasma loss mechanism, or be due to shake-up phenomena as in the case of copper and nickel oxides 14 As might be expected from the normal oxidation behaviour of iron, the present oxide films appear to consist of a thin layer of Fe 2 O 3 overlying the main Fe 3 O 4 oxide. The removal of this outermost Fe 2 O 3 layer is rapid and is accompanied by the changes in the satellites mentioned above, together with changes in the position of the Fe 2p 3 2 oxide peak. In conclusion, it is clear that a detailed XPS examination can not only provide information on overall compositional changes, as can be obtained by AES, but can also provide a comprehensive picture of changes in oxide composition, including those due to oxides of one metal in different valency states.