Electrical properties of He-induced W "fuzz" within the pre-breakdown and breakdown regimes

Abstract

The investigation of the He-induced W "fuzz" electrical properties was carried out. For the research, an automated experimental setup was designed. The setup was based on a vacuum chamber operated under high vacuum condition (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> Pa). The vacuum diode under investigation comprised of a flat W "fuzz" cathode with an area of about 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a 2 mm radius cylindrical copper anode rounded with a 2 mm radius hemisphere. The cathode-anode distance was ~100 µm. The voltage applied was up to 10 kV. An HV power supply was controlled employing the DAC/ADC module "LCard <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> E20-10". This module registered automatically currents and voltages of interest also. As the result, the data on the dielectric strength of the diode with the W "fuzz" cathode were obtained. The effect of the cathode conditioning was demonstrated. An increase in the breakdown electric field strength from about 4•10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> V/cm for the very first breakdown up to 9•10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> V/cm for the 20th breakdown was registered. In addition, the emissive characteristics of the W "fuzz" cathode were studied in accordance with the Fowler-Nordheim theory. The estimated value of the electric field enhancement factor β near the cathode surface was found to be ~60 before the breakdown and ~80 after the breakdown. The obtained β values were somewhat smaller than the ones obtained by other research groups for the nanostructured W cathodes. This fact should be investigated additionally.