Spot Lifetime Research Articles

A spectroscopic and photometric study of 12 BM Camelopardalis

Radial velocities from 1916.95 to 1991.95 and photometry from l979.25, both published and new in this paper, are presented and analyzed. A new solution of the radial velocity curve reveals a new period of 80.90 days and an eccentricity of e = 0.05 +/- 0.02, both very different from the 80.17 days and 0.35 found by Abt et al. (1969). An alternative solution with e = 0 is given because we cannot decide firmly whether or not the small eccentricity is real. We find V sin i = 11.3 +/- 0.3 km/s from Maidanak and 10 unequal depth. 0.048 mins and 0.026 mins. The orbital ephemeris for conjunction (K gisnt behind) is JD(hel.) 2,448,111.1 (+/- 0.4 days ) + 80.898 days (+/- 0.004 days ) E, consistent with both the radial velocities and the photometry. With the ellipticity effect removed, the light curve shows residual variability which we fit with a two-spot model. During the 13 years covered by photometry there were nine different starspots, the largest one producing a light loss of 0.19 mins. Rotation periods for the nine spots ranged from 78.6 +/- 0.5 days to 83.7 +/- 0.4 days from which we concluded that the K giant does rotate synchronously but with a differential rotation coefficient of k = 0.06 +/- 0.01. Lifetimes for the nine spots ranged from 1.1 to greater than 4.2 yr and were consistent with the empirical spot lifetime laws of Hall & Henry (1994). Use of the mass function, the orbital period, the V sin i, the two different ellipticity effect amplitudes, and various logical constraints led to ranges of possible masses, radii and inclinations. The most believable solution was around i = 90 deg, R(sub 1) = 24 solar radii, M(sub 1) = 1.1 solar mass, and M(sub 2) = 0.6 solar mass. THe Rossby number for the K giant is 0.48, small enough compared to the critical value of 0.65 to explain why, though rotating 'slowly', it does have large spots.

Characterization of d.c. plasma torch voltage fluctuations

The arc root fluctuations at the anode-nozzle of a d.c. plasma spray torch with a special configuration of the electrodes allowing to work with the same gas flowrate with nozzle diameters ranging from 6 to 10 mm were systematically studied. The plasma gas was Ar/H2 (25 vol % H2), the current was varied between 200 and 600 A and the plasma gas flowrate between 24 and 80 slm. After 30–60 mn working the nozzle wall started to be sufficiently eroded to have a stagnant arc spot which lived until arcing created another one. It was shown that the life time of the upstream arc spots were 30–40 % longer than the downstream ones which could play an important role in the electrode erosion. Dimensional analysis allowed to find a relationship between the nozzle diameter D, the arc current I and gas flow rate G and the mean spot lifetime which is closely connected with the difference between D and the electrical diameter of the arc column. The comparison of voltage signal and light emission at a point of the plasma jet close to the nozzle exit on its axis allowed to determine the mean electrical field within the plasma column and the mean position of the arc root. The comparison with the electrode erosion area for well defined conditions showed a good correlation with the calculated arc root position.

Time-resolved description of cathode spot development in vacuum arcs

A spherical, one-dimensional model describing the time-resolved development of non-stationary vacuum are cathode spots is solved numerically. The model includes electron emission, the Nottingham effect, ion current, sheath effects, evaporation, melt ejection, ohmic heating and heat conduction. Using two different termination conditions for spot development, a lower and an upper bound of the spot lifetime as well as of the final crater radius can be calculated. The results are in good agreement with experimental values. A new feature of our work is that at small crater radii, the ohmic heating cannot be compensated by the cooling effects. Because of thermal run-away, critical temperature is then reached below a thin liquid surface layer. It is assumed that these surface layers are then pushed off in a phase explosion. Several subsequent phase explosions can occur during crater development, so continuous crater expansion due to melt ejection is superimposed on a discontinuous component. This mechanism produces a broad deterministic distribution in final crater radii and spot lifetimes if the crater current or the surface roughness vary only slightly. This is in agreement with the fact that a broad variation in the crater radii can often be found on cathode surfaces despite nearly constant experimental conditions.

Scaling laws for sonoluminescence

The extremely nonlinear dynamics of bubble motion is described by the Navier–Stokes equations of fluid mechanics, as coupled to the laws of heat conduction and mass diffusion. In order to gain insight into the limitations of the hydrodynamic theory of bubble collapse, scaling laws have been derived for the maximum radius, collapse temperature, hot spot lifetime, collapse pressure, and ambient radius in terms of the applied sound field and the assumption that the bubble is a gas-filled cavity. In order to match the scaling laws to the experimental observations of collapse temperature and flash width requires that the Mach number for bubble motion exceed unity. In conclusion, a description of sonoluminescence must include the properties of imploding shock waves. [Work supported by the US DOE Division of Engineering and Geophysics; R. L. is supported by an A.T.&T. fellowship.]

Evolution of starspots in the long-period RS CVN binary V1817 Cygni = HR 7428

Photometry between 1982 and 1989, published and unpublished, is analyzed. The ellipticity effect produces variability with a full amplitude of 0.033 m in V. A recent time of light minimum (JD 2445988.0 + or - 0.3 d) combined with an old spectroscopic time of conjunction from the 1920's yields a much improved orbital period (108.854 + or - 0.003). Removal of the ellipticity effect reveals starspot variability. Four different spots were observed at various times, two of them present simultaneously in the light curve during 1985. Mean spot lifetimes were around 2 years and the largest amplitude attributed to starspots was 0.04 m in V during 1986. Derived rotation periods for two spots were 5.3 + or - 1.2 percent slower than synchronous and 3.0 + or - 0.4 percent faster. The differential rotation coefficient for the K2 giant is k = 0.25 + or - 0.04, compared to k = 0.186 for the sun. V1817 Cygni has the longest orbital period of any binary known to execute synchronous rotation.

Current density and the cathode spot lifetime in a vacuum arc at threshold currents

The autograph method was used to investigate the arc spot motion over a virgin tungsten cathode under a magnetic field of H=0.1-0.7 T in the current range 2-50 A. The cathode spot (CS) has been found to start dividing at a current of a few amperes. The current density measured at the moment of CS death is (1.2+or-0.5)*1011 A m-2 for a cold cathode and (3+or-1)*1010 A m-2 for a hot one. The CS lifetime is 25-50 ns for a cold cathode and it increases to 150-200 ns for a heated one. Neither the current density nor the CS lifetime depends on current. Estimates have shown that the erosion rate decreases with current and increases with temperature. The backflow electron current from the CS plasma is invoked to account for the experimental evidence.

Characterization of the Cathode Spot

A general description of arc cathode spots is given with respect to structure, size, and temporal behavior. The density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes. Extrapolation to the region within the spot indicates plasma densities in excess of 1025 m-3. Recent studies of spot current density suggest values of about 1012 A/m2. In this connection, the problem of the electrical conductivity of the spot plasma is discussed. Finally, experiments on spot lifetime and models of crater formation are reviewed.

Concerning the Types of Cathode Spots

The kinds of electrical-arc cathode spots described in literature are analyzed. Division of spots into two types qualitatively different in nature-explosive spots and thermal spots-is proposed. The observed microsecond oscillations of the brightness of a rapidly moving spot are interpreted in terms of heat accumulation in the cathode and random motion of the spot. The transition of the spot to a thermal region is analyzed and the lifetime of nonstationary thermal spots is shown to be proportional to their size squared.

Analysis of the Cathode Spot of Metal Vapor Arcs

This paper presents an analysis on the statics and dynamics of a current carrying plasma channel under the influence of an applied magnetic field and under very general flow conditions. Based on this analysis a stability criterion for a discharge channel which emanates from a crater of a low-pressure arc cathode attachment wil be derived. As a consequence of this stability theory one may explain arc spot motion by a kink instability of the discharge channel causing the channel to bend more and more and eventually contacting the cathode surface at a nearby site thereby creating a new emitting spot. By means of this criterion, the dissipation mechanisms during a spot lifetime are determined for pure copper and molybdenum cathodes under the assumption of least possible voltage requirement.

Cathodic and Anodic Spot Phenomena on Rotating Electrode Disks and Plasmatron Electrodes

AbstractThe arc spot life time and velocity before reaching melting temperature have been calculated for different electrode materials. They vary between 10−10 to 10−14 s and 102 to 106 m/s for heat flux densities of 109 to 1012 W/m2. The experimental examination of such results was carried out by means of fast rotating electrode disks and tubular electrodes in plasmatrons. The measurements resulted in life times of more than 10−5 s with erosion rates from 10−10 to 10−2 g/As and electrode spot diameters ranging from 0,01 to 5 mm while the heat flux density varied between 107 to 1012 W/m2. Ten different types of continuous traces, single spots, and craters have been found. Some spot phenomena, the high erosion rates of rotating disks and the similarity of ignition events have been clarified for electrodes made from good conductive materials and less conductive. But the melting and boiling of the spot surfaces was inevitable in all cases.

The flux-rope-fibre theory of solar magnetic fields

The flux-rope theory of solar magnetic fields is reviewed briefly and, together with the dynamo theory, compared with various observational results. Dynamo and related theories are based on fields controlled by the plasma, and it is shown that such fields cannot account for the strong surface fields or even emerge without becoming tangled. Observations which appear uniquely explicable in terms of powerful (⪝4000 G), helically twisted flux ropes and their many twisted flux fibres (≈3×1018 Mx) are listed as follows. (i) Emerging magnetic flux is seen first as pairs of small, closely spaced flux concentrations whose motions suggest magnetic control to provide bipolar regions of extent⪞105 km. The associated system of arch filaments rotates on the disk as would a series of emerging flux fibres twisted into a rope. (ii) Sunspots form by the accretion of pores and magnetic knots of like polarity, sometimes moving along curved paths between stationary elements of opposite polarity. (iii) Fluxes of⪞1022 Mx in large sunspots must have been concentrated to strengths of⪞4000 G before emerging, and also strongly helically twisted to avoid the flute instability. (iv) The trumpet-shaped flux-rope-fibre sunspot model (Figure 6) accounts readily for the phenomena of the moat convection, the sunspot energy deficit, the complex Evershed flow, penumbral filaments (flux ≈3×1018 Mx) and temporary light bridges. (v) Asymmetries in sunspot groups (in spot size, lifetime and proper motion) show that the spot fields are extensions of two submerged magnetic structures comprising strong fields. (vi) Sunspots decay by the loss of magnetic knots with strong fields and flux ≈5×1018 Mx. These must be isolated flux tubes, twisted to account for their stability. (vii) Flux fibres leaving a spot are prone to the kink instability, thus accounting for their sudden appearance in pairs, the transport of total flux several times that of the spot and net flux equal to that of the spot. (viii) Ephemeral active regions and X-ray bright points are explained similarly without invoking improbably huge quantities of new flux. (ix) Atmospheric structures show a high prevalence of helical twists (force-free fields) and rotary motions on all scales from spicules to large prominences. It is difficult to account for these twists unless they are present in emerging flux. (x) In and above the photosphere the flux fibres (≈3×1018 Mx) fray into loose associations of flux threads (≈3×1017 Mx) to provide a simple, selfconsistent model of the solar filigree and the chromospheric rosette (bush) with its group of mottles (spicules). (xi) Global patterns of surface and coronal magnetic fields reveal puzzling features such as the migration of large unipolar regions and the freedom from differential rotation of some structures. Submerged flux ropes ‘peeling’ out of the Sun provide a starting point for explaining these effects. These results provide a strong case for the flux-rope theory against the entrenched dynamo theory, and suggest that more observations should be made of the above ten phenomena. Where possible, simultaneous observations should be made of Zeeman effects and of plasma distributions and velocity field seen in white light and spectral lines.

A thermal-diffusion model of the erosion of electrodes in an MHD generator

We propose a diffusion-heat-conduction model of erosion at a cathode spot, on the basis of which we estimate the characteristic lifetimes of cathode spots and the erosion of electrodes made of graphite, copper, iron, and chromium under the conditions of an MHD generator. We show that the rate of erosion depends substantially on the geometric shape of the cavity.

On the Retrograde Motion of Arcs in Magnetic Fields

Experiments are reported which allow the determination of retrograde velocity of individual cathode spots and of the plasma flow in a pulsed discharge using various metals and carbon as cathode materials. For discharge currents from 10 to 40 amps, pressures of 3 mmHg and magnetic field strengths of 6.103G retrograde spot velocities from 30 to approximately 300 m/sec are observed and the corresponding plasma flow velocities are in the range from 4,400 to 8,600 m/sec. On cathode materials with low melting points, the splitting rate of spots and the motion of individual spots is small, whereas under identical conditions the spots on refractory materials are highly mobile, the splitting rate is large, and the lifetime of individual spots is short.