A high current hollow cathode source has been developed as a radiometric source standard for the extreme VUV. Over the last few years the interest in VUV spectroscopy has been steadily growing. This holds especially true for fields like basic research, controlled nuclear fusion (both magnetic and inertial confinement), and space based astronomy. In all these areas quantitative spectroscopy is of great importance requiring the availability of radiometric standards.
While the vacuum ultraviolet (VUV) extends from 200 nm (cutoff due to atmospheric absorption) to 0.2 nm (onset of transparency), it is often useful to subdivide this range into a long-wavelength VUV from 200 to 105 nm and an extreme VUV region below 105 nm; for below this wavelength no solid window materials exist and different experimental techniques are required.
If bright continuum sources are required the only solution is a short-duration pulsed source that can operate in vacuum conditions. Laser produced plasmas with pulse durations in the nanosecond regime have been shown to be suitable for transfer source standard application. For many purposes, however, line emission sources are better suited than continuum sources. Such sources can be operated with comparatively low gas pressures requiring, therefore, only modest differential pumping systems and do not suffer seriously from self-absorption if a properly designed differential pumping system is used.
The hollow cathode was jointly developed at the Institute fur Plasmaphysik in Hannover and the PTB in Berlin for use as a line-radiation standard in the VUV. There are two anodes and one cathode, all made of stainless steel and watercooled. The cathode insert is a hollow cylinder with a central bore of 8 mm and a length of 60 mm. The conical shape of the anodes reduces both arcing and short circuiting through metal depositions due to sputtering from the probe cylinder. Central bores of 4 mm in both anodes allow end-on observation of the plasma.
Discharge Parameters and Operation Characteristics:
The spectral emission of the hollow cathode depends basically on the discharge current and voltage drop across the electrodes. The hollow cathode has always been operated with a fixed current, adjusting the voltage drop by regulating the buffer gas pressure.
The radiance of the spectral lines depends approximately linearly on the current in the case of the noble gas lines and approximately quadratically in the case of metal ions sputtered off the hollow cathode. Because of this current dependence, the stabilization of the discharge current has to be better than 1%. The angular distribution of the radiation in a single spectral line was measured within a solid angle of 6 x 10-7 str defined by the imaging aperture in front of a spherical mirror.