4 - Hollow cathode, penning, and electron-beam excitation sources

Abstract

This chapter focuses on windowless high-current hollow-cathode sources that emit electrons below 100 nm. Because no solid window material is available for wavelengths shorter than about 105 nm (cutoff of LiF), plasma vacuum-ultraviolet radiation sources below 105 nm have to be differentially pumped. Typical examples of such plasma sources are wall stabilized arcs that operate at about atmospheric pressures and need powerful and, therefore, expensive differential pumping systems. At such high pressure, continuum emission is available from the dense plasma. To reduce the size and the cost of the differential pumping system, plasma sources operating at lower pressures are very desirable. While high-pressure sources emit mainly continuum radiation, the radiation emission of low-pressure discharges is limited to atomic and ionic emission lines from the components of the plasma. Typical electron energies of these beam electrons are in the range of a few hundred electron volts. This energy is sufficient to ionize the atoms of the buffer gas (mostly noble gases) that fills the interior of the hollow cathode. The ions are accelerated toward the cathode surface and obtain sufficient kinetic energy to sputter the cathode material from the surface into the discharge. By direct collision with the electrons and by charge-transfer collisions with the buffer gas ions, the sputtered atoms are also ionized.