Initiation and Sustainment of Unipolar Arc Discharges by a Microsecond Pulse Plasma

Unipolar arcs have been produced by contacting metal surfaces with microsecond pulse plasmas. Plasma temperature T_e, density n_e and potential (with respect to ground) were controlled in the limits 7–12 eV, 10¹⁸–10¹⁹ m^?3, 20–40 V, respectively, and the influence of these parameters on arc current amplitude (50–500 A) and ignition probability has been investigated. It was found that the ignition is the most limiting process requiring surface contaminations as well as the transport of net currents to the surface. The amplitude of the current was proportional to n_eT_e^1/2.     

Influence of Surface Contaminations on Cathode Processes of Vacuum Discharges

The influence of surface contaminations on the ignition and maintenance of vacuum discharges is discussed qualitatively. Surface analysis of the electrodes and gas analysis during the discharges demonstrate that fresh electrodes contain always impurities within the upper surface layers, which affect the behaviour of vacuum arcs and vacuum breakdowns. The most effective way for cleaning the surfaces are the discharges themselves, if they burn in UHV. During that cleaning the following variations have been found: Are cathodes spots change from rapid moving ones with small erosion (type 1) to slow ones with strong erosion (type 2). The ignition of nanosecond discharges needs higher field strength and field emission current density. Polishing effects by short discharges (< 5 ns) become more pronounced. Erosion craters in nanosecond discharges increase.     

Jodometrische Sulfitbestimmung in Gelatine mit biamperometrischer Endpunktanzeige

Zusammenfassung Die jodometrische Titration des Sulfitgehaltes von Fotogelatinen mit biamperometrischer Indikation (Dead-Stop-Methode) wurde untersucht. Die Methode führt zu einer einfachen und schnellen Bestimmung des Sulfitgehaltes von Fotogelatinen auch in Gegenwart von Nitrit. Die Standardabweichungen der beschriebenen Methode wurden ermittelt.

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Jodometric determination of sulphite in gelatine with biamperometric indication

Summary The jodometric titration of the sulphite content of fotogelatines with biamperometric indication (Dead-Stop-Method) was studied, leading to a simple and rapid procedure of determination of sulphite in gelatines. This method is suitable in the presence of nitrite too. The standard deviation was estimated.

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Herrn Prof. Dr.K. Issleib, Direktor der Sektion Chemie der Martin-Luther-Universität, möchten wir für die Bereitstellung von Mitteln sowie für sein förderndes Interesse an dieser Arbeit danken.     

Film deposition by laser-induced vacuum arc evaporation

Recent results of a study of the deposition process and the technological development of laser-arc are presented. Studies of the influence of arc current on film deposition were carried out for Ti, TiC, and C. On the basis of these results, specific multilayered structures have been prepared. As an example, a Ti/TiC multilayer system with 25-nm single layers is described. Results of structural and chemical analysis by means of Auger electron spectroscopy (AES) are presented. They show that diamond-like carbon film with a refractive index in the range of between 2.05 and 2.5 can be deposited effectively     

Über den Gleichspannungsdurchschlag im Ultrahochvakuum

DC-breakdowns with arc-conditioned electrodes in vapour-free vacuum have been investigated with high temporal resolution. The breakdowns started always with a cathode flare, the ignition voltage of which was strongly correlated with prebreakdown field emission. The temporal development of gap plasma, discharge current and gap voltage during the breakdowns could be satisfactorily explained by the models of explosive electron emission and space-charge limited diode conduction. No change of mechanism was observed with gap distances up to 6 mm and voltages up to 120 kV. Generally the resistance of the discharge channel became low enough for a transition to the arc stage. However, in a few cases a self-limitation of the discharge occurred due to starvation of the material transfer from the cathode. At higher gap distances the surrounding walls had a distinct influence on the ignition of the discharges.     

Microscopic high speed investigations of vacuum arc cathode spots

The main parameters and dimensions of cathode spots have been under discussion for years. To solve these current questions, a new system was specially designed. The image converting high speed framing camera (HSFC) combines a micrometer lateral resolution with a nanosecond time resolution and a very high optical sensitivity. This camera was used to study the microscopic behavior of vacuum arc cathode spots in a pulsed high current arc discharge on copper. The direct observation of these spots with high resolution revealed that one single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 10 μm and a mean distance of 30-50 μm between them. The mean existence time of these subspots on copper was found to be about 3 μs, where the position of a subspot remains unchanged (with an upper limit of about 5 μm) during its existence time. The lower limit of the current density in the cathode spots was estimated to be on the order of 10¹⁰-10¹¹ A/m². An upper limit of the crater surface temperature was estimated by a comparison between the brightnesses of a cathode spot and of a black body radiation lamp to about 3000 K     

Investigations of the Current Density in the Cathode Spot of a Vacuum Arc

A short review is given on the problem of the current density in the cathode spot of a vacuum arc, theoretical models and experimental results are discussed. A new measuring method is presented which is directly related to the current carrying area of the spot. It leads to current densities of 10¹² A/m² at clean surfaces (comparable with results obtained from crater measurements) and 10⁹?10¹⁰ A/m² with contaminated surfaces.     

The method of laser-sustained arc ignition

The method of laser-assisted vacuum arc deposition (laser-arc) was developed to effectively produce high-quality coatings, especially multilayer coatings with a reduced number of droplets. The efficiency of this method depends mainly on the ignition probability of the vacuum arc discharges. In the present work, the process of arc initiation by laser irradiation is studied and the influencing factors such as power density of laser irradiation, target material, and electrode configuration are discussed. It is shown that a high ignition probability (ρ>50%) on such coating materials at Ti, TiC, TiN, and C can be guaranteed with a laser power density of about 10⁸ W/cm², which can be achieved with a low-cost Nd-YAG pulse laser. Measurements of the plasma current preceding the arc ignition and microscopic investigations of the erosion pattern caused by the laser irradiation suggest that the process of plasma-sustained are ignition (unipolar arcing) plays an important role during the buildup phase of the vacuum arc discharge     

Pulsed arc deposition of super-hard amorphous carbon films

Hydrogen-free amorphous carbon films with hardness up to 75 GPa have been deposited by special pulsed arc techniques. The influence of plasma and deposition conditions on the film properties is discussed and some applications are shown. PACS 52.77.Dq; 68.60.-p; 81.15.Aa; 81.15.Ef; 81.70.Ex