Shedding light on arc cathode spots I S

Shedding light on arc cathode spots I S Falconer, R Sanginés, O Novak, D R Mc. Kenzie and M M M Bilek School of Physics, University of Sydney, AUSTRALIA Energy of ions ejected from the cathode spot ABSTRACT The characteristics of ions ejected from the cathode spots: We have used a high-speed camera to analyze the motion of the multiple co-existent cathode spots in the University of Sydney's high-current pulsed arc, which move in a direction opposite to that expected from Lorentz force equation, and developed a model to explain our observations. Time resolved optical emission spectroscopy has also been employed to link the emission intensity of the species in the arc plasma with the cathode spot dynamics and to infer trends in the evolution of the charge state distribution. The shape of the spectral lines of the excited species that are ejected from the spots is of relevance to understanding the physics of the spots. As these ions are created towards the edge of the spot and lose their excitation as photons at the boundary of the spot the line shape and shift will give the velocity of the ions near the edge of the spot. A combination of a Fizeau interferometer, an intensified CCD camera and a grating spectrometer have been used to obtain time-resolved spectral line shapes for the pulsed arc. The results of these measurements will be discussed. • Measurements using electrostatic energy analysers indicate that the cathode spots are the source of energetic ions that can reach the anode against the retarding field between the arc electrodes • High-charge-state ions are ejected from the spot. Time-of-flight charge state analysers record multiply charged ions, some of which are very highly charged – charge state 4 and above. These are presumed to come from the spots. • The ion current reaching the anode is a significant fraction of the total arc current These techniques measure the characteristics of the ions at a distance from the spot, whereas appropriate optical techniques will – hopefully - give information about the ions at the surface of the spot. For this reason we are using spectroscopic techniques to investigate ions emitted by the spots. Introduction One aspect of this work is investigation of the temporal and spatial distribution of the emission using standard OES techniques. Some of the results from our temporal distribution studies have been published 3, while the spatial distribution work is still in progress. The generic term – CATHODIC ARCS – is used to describe arcs operating at low gas pressures – and even in vacuum. They are characterised by tiny, extremely bright cathode spots that move erratically over the cathode surface. As the arc current flows from the plasma to the cathode through these spots, an understanding of the physics of these spots is crucial to understanding the properties of cathodic arcs. We have also used a combination of a Fizeau interferometer and a gated, intensified CCD array to measure the spectral line shape of the emissions lines of various charge states of the cathode material. (Aluminium for these experiments. ) The optical system used for these studies is shown in figure 4 and preliminary result in figure 5. Cathode spots have some rather unusual properties 1, 2 that make them difficult to investigate experimentally – and even harder to model. In this poster we discuss some unexpected characteristics of cathode spots that are of technical importance and must be accounted for in any theory of the cathode spots: • Hot – tens of e. V – ions are ejected from the cathode spot with directed energies of tens of e. V. • Fig. 5 The Fizeau interferometer / intensified CCD system used to measure the line shape of ions ejected from the cathode spots of a cathodic arc. Details and the performance of this instrument are discussed in Poster DTP 169 (this poster session) at this conference. In a magnetic field – including the magnetic field generated by the arc current itself - the spots move in a direction OPPOSITE to the Lorentz force direction. Fig. 6. Some spectral line profiles recorded by the Fizeau interferometer. (Uncorrected for the ~0. 013 nm FWHM instrumental function. ) Motion of the cathode spots in a high-current pulsed arc Cathode spots in the University of Sydney’s high-current pulsed cathodic are initiated near a trigger pin at the centre of the circular cathode. We have used a Hamamatsu high-speed camera to observe the motion of the multiple-spots in this arc away from the cathode. (Figure 1. ) 50 e. V Al 1+ Fig. 1 The first frame, taken in the Focus mode, shows the region of the arc cathode viewed by the camera and the centre trigger pin. The remaining frames show the motion of the cathode spot away from the centre trigger pin. The white circle indicates the outer edge of the cathode. Al 2+ Fig. 2 The mean distance of the spots from the centre of the cathode as a function of time, for pulse currents from 800 A to 2, 0 k. A. 50 e. V 2. 4 k. A Sanginés model of spot motion One of the authors has developed a model for the cathode spot motion for the case of a multi-spot arc with no externally applied magnetic field. It assumes that the force the filamentary current through each spot is proportional to where is the current through a cathode spot and is the magnetic field at that spot due to the currents through all the other spots i. e. it’s magnitude is given by the Lorentz force equation, but it is directed in the retrograde direction. As shown in figure 3, this model has successfully explained the observed expansion of the ring of cathode spots shown in figure 2. Al 1+ Fig. 3 Mean spot ring radius vs. time for cathode currents of 2. 4, 2. 0, 1. 6, 1. 2, 0. 8, and 0. 4 k. A. . Theory and experiment fit beautifully with only a single fitting parameter. 50 e. V Al 2+ 50 e. V This model assumes that the arc cathode spots are equally spaced around the trigger pin on a small diameter circle, but can readily be extended to explain the irregularities observed in some of our observations, where not all the spots are equidistant from the trigger pin. This is illustrated in figure 3, where the initial positions of the spots for the model were the positions observed in the first frame of a series of high-speed photographs. The observed expansion of the ring of spots is remarkably similar to that arising from the model. 1. 2 k. A Conclusion Fig. 4 A comparison of the observed expansion of a ring of cathode spots with that predicted by the Sanginés model. Low -mobility spots are indicated by dashed arrows and locations where a spot splits by solid arrows. (The model does no take account of the not-uncomment cathode spot phenomenon of spot splitting. ) Obviously a work in progress: • The breadth of the singly and doubly-charges Al ions suggests the ions are surprisingly energetic, but is it thermal or directed energy? • What causes the splitting of some of the lines? Is it possible some of the ions are moving towards the cathode surface? • Are we indeed looking at what happens adjacent to the surface of the cathode spot? Images of the “plume” of plasma coming away from the cathode surface (Fig. 7) suggest that this may not be the case. • References 1 Some of this work has been recently published in Applied Physics Letters 3 2 3 Juttner, B, Puchkarev, V F, , Hantzche E, and Beilis, I I (1995) Cathode Spots in Handbook of Vacuum Arc Science & Technology eds R L Boxman, D M Sanders, and P J Martin. (Noyes. New Jersey) Juttner, B (2001), J. phys d: Appl Phys 33 2025 -2036 Sanginés, R, Israel, A M, Falconer, I S, Mc. Kenzie, D R, and Bilek, MMM, (2010), Appl. Phys. Lett. 96, 221501 (2010); doi: 10. 1063/1. 3442509 Fig. 7. Images of the “plume” external to the cathode, taken through ~ 10 nm interference filters.