COMPARISON OF ELECTROPLATED AND HAND PAINTED CATHODES

ON SELF MAGNETIC PINCH RADIOGRAPHIC DIODE OPERATION

James Macdonald, Jim Threadgold, Kevin Webb, Sav Chima Pulsed Power Group, AWE Aldermaston, Berkshire, RG7 4PR, UK

Abstract

The repeatability of the radiographic performance of the Self Magnetic Pinch (SMP) diode is crucial to its use in Hydrodynamic experiments [1]. Years of experience of operation at AWE have shown that its repeatability is dependant upon the build quality of the diode, this includes component initial design, manufacture, storage, preparation and installation.

A program of work is being undertaken to enhance repeatability of the diode preparation process. As part of this program an electroplating method to prepare the cathode is being investigated. Reported here is work carried out to investigate the surface finish that can be achieved from both traditionally hand painted and electroplated cathodes as well as the results of an experimental series carried out to compare the performance of cathodes prepared using the two techniques.

I. INTRODUCTION

The repeatability of the radiographic performance of the Self Magnetic Pinch (SMP) diode is crucial to its use in hydrodynamic experiments. A program of work is being undertaken to enhance repeatability of the diode preparation process. As part of this program an electroplating method to prepare the cathode tip is being investigated. It is felt that this is a useful investigation as the cathode tip is the source of the intense electron beam that is vital to a pinch.

The Self Magnetic Pinch diode is well-established established in the radiographic community. The diode consists primarily of a small cathode tip set at a small distance from a foil anode, (Figure 1 shows the basic construction of the diode). These components must be replaced after each shot. Good shot-to-shot repeatability is crucial, but the continued renewal of the diode leads to many potential sources of variation. A program of work is underway at AWE to introduce better control to the configuration of the diode.

Figure 1. A Schematic of the Self Magnetic Pinch diode

One aspect of this study is to understand the variations that can arise from the process followed to prepare the cathode tip.

Traditionally, cathode tips for SMP shots have been prepared by hand painting a colloidal silver solution onto the end area of the tip. This method works well, and has done so for some years now. The process, due to its manual nature, is prone to variation. Varying temperature and humidity also play a part in the process, and it is desirable to rule out the effects that these factors may have. Figure 2 shows a hand-painted tip. The silver can be also sprayed onto the tip instead of painted. Electroplating the silver onto the tip is being investigated as an alternative process.

Figure 2. A hand-painted cathode tip, 8mm outer diameter

978-1-4673-5168-3/13/$31.00 ©2013 Crown

II. SURFACE FINISH ANALYSIS

Figure 3, below, shows white light interferometry images, and highlights the significant differences between the processes.

Figure 3. Showing the surface thickness of (top row, l-r) hand-painted tips prepared by different operatives (a and b), and (bottom row, l-r) a hand-sprayed tip (c) and an electroplated tip (d)

Figure 3a shows a hand-painted tip surface, exhibiting reasonably even coverage, but with many areas where there is a much thinner coating (These show up in the image as small regions of zero signal intensity as the probe signal was not able to reflect back out of the depressions to the detector.). Figure 3b shows some banding, as a result of the tip being prepared by a less experienced operative. Figure 3c shows a hand-sprayed tip coating, the image below shows a variation in surface coating thickness quite clearly, whereas a visual inspection did not. Figure 3d shows the result for an electroplated tip, demonstrating significantly improved uniformity.

The differences between the results of each process become even more evident under the scanning electron microscope. Here, we see clearly the areas where the coating is very thin to non-existent (Figure 4a shows the hand-painted result). The sprayed coating looks very sparse indeed (Figure 4b). In stark contrast, the electroplated coating (Figure 4c) is very smooth, but does show some “blisters” across the surface. The electroplating process used to obtain the finish shown in Figure 4c used a low voltage for a relatively long time. If the process parameters are changed, the final finish can be varied, too. Figure 4d shows the results of a two-stage process using two different voltages, both of which were much higher than that used in Figure 4c, for much shorter times. (Please note the slightly different scale.)

This analysis has enabled us to understand the different surface morphologies obtained by the different processes, and this can then be tallied against SMP shot results.

Figure 4. Showing the surface structure of (top row, l-r) hand-painted tip and a hand-sprayed tip (a and b) and (bottom row, l-r) electroplated tips using different parameters (c and d)

The main aim of this study is to gain a measure of the consistency of SMP shots once the tip preparation process has been finalised and applied identically across a batch of tips. An electroplating process roughly halfway between the processes used to obtain the finishes shown in Figure 3c and d was used to prepare a batch of tips.

III. RESULTS

A short series of shots were fired on one of AWE’s X-ray machines, and the results gained by the electroplated tips were very similar to those obtained by the traditional hand painted tips, Figure 5.

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Figure 5: A simple graph showing the initial results for electroplated cathode tip shots, and how they compare with traditional methods

This shows us that the electroplating process is a potentially viable alternative to manually painting the tips. It is felt that a more thorough investigation is needed, in order to fully understand the effects at play, but the

process certainly shows good promise. The diode parameters for the test were chosen to be where the pinch is known to be very reliable, with a diode voltage of 4.5 MV and the a-k gap set to 7 mm. This configuration was able to pinch using an unplated tip.

Further tests will be performed at different voltages and a-k gaps in order to gain a better understanding of the behaviour of electroplated tips. It is currently felt that larger differences may be seen when the pinch is operated near the edges of its envelope, as this is when minor variances would have greatest effects. The primary aim of this work will be to quantify any possible improvements in shot consistency resulting from electroplating the tips. A secondary aspect of this investigation is to help understanding of how the surface finish affects the performance of the pinch. One school of thought is that a smooth surface is better than a rough one, as it would hold off the rising voltage for a little longer, and thus start emitting electrons at a higher energy. An opposing view is that a rough surface, such as that shown in Figure 3d, with a high spatial frequency, will encourage uniform electron emission, and so assist in setting up the conditions for the pinch to occur, earlier on in the pulse. Other thoughts include the fact that sharp surface features will encourage electron emission, but this will lessen the fields experienced in neighbouring locations. This could lead to uneven emission, giving rise to uneven plasma distribution, which could potentially skew the pinch. Uneven emission could also cause more rapid erosion of the feature, leading to the restoration of a more even field distribution after a short time. It could simply be that, in the conditions of the pinch diode, all of these effects are too subtle to be affecting the overall behaviour of the pinch. AWE is looking forward to working with Sandia National Labs to better characterize the electroplating process and its relationship with the SMP diode.

IV. SUMMARY The results from an initial investigation into an alternative cathode tip surface preparation process have been presented. They show that electroplating is a potential alternative to the traditional method of hand-painting the tip ends. Silver has been used throughout, but other materials may also be suitable. Only one point in the parameter space for Self magnetic Pinch operation has been investigated thus far, but further work is planned to investigate other operating points.

V. REFERENCES

1. J. Threadgold et al, “Development of the self

magnetic pinch diode as a high brightness

radiographic source”, 16th IEEE Pulsed Power Conference, Albuquerque, NM, USA, June 2007.