ELECTRON-EMISSION FROM CARBON-FIBER TIPS

7 mu m diameter carbon fibres were elecuolytically etched in a 0.1M so dium hydroxide solution to produce micropoint emitters. The behaviour of these tips was tested in a retarding potential spectrometer under a variety of vacuum conditions. At a medium vacuum of similar to 10(-3) mbar, the emission of electrons was very noisy and the lifetime of th e source was relatively short. At pressures of < 10(-6) mbar, it was f ound that the performance of the cathodes, including their lifetime, i s largely independent of the vacuum pressure. The emission characteris tics of carbon fibre tips were recorded. Electron microscopy technique s employed in this investigation were used to study the tip profile. T hey also showed that the inherent instability associated with carbon f ibre emitters resulted from an explosive phenomenon which was encounte red during the initial switch-on of the emission current from freshly prepared cathodes. This process led to the destruction of these cathod es. Pre-conditioning treatment of the tip overcame this problem and pr oduced a large reduction in the noise fluctuation of the total emissio n current. This enabled the recording of energy distributions of field emitted electrons. The spectra obtained demonstrated narrow halfwidth s (FWHM) and an energy shift compared with the Fermi level of tungsten . By increasing the applied electric field on the tip, the spectral sh ift was found to increase. The emission pattern consisted of a random distribution of sub-emission centres, which switch ''on'' and ''off'' randomly with time. The Fowler-Nordheim current-voltage plots of these carbon fibre tips showed that the work function was insensitive to th e vacuum pressure.