Negative Ion Source for the Measurement of Plasma Potential by a Heavy Ion Probe

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A Heavy Ion Beam Probe (HIBP) has been used as a reliable method to determine the local plasma potential, and to study the potential and density fluctuations. However, the required beam energy increases up into the MeV region for recent big devices, such as the Large Helical Device. A HIBP based on a negative heavy ion source is attractive because of the possibility of neutral beam injection or Tandem beam acceleration. In this work, we have studied the production mechanism of negative ions in a plasma sputtering type source in order to get a continuous beam with a maximum current density. A new technique to measure the work function of the negative ion production surface has been developed and correlation of the work function to the production efficiency was investigated. We have measured the energy spectrum of the beam at the exit of the ion source, and the spread was less than 10 eV for the sputtering energy less than 300 eV. It is small enough for the high quality fluctuation measurement. A computer code to simulate the beam extraction and beam optic system was developed. A compact ion source was also constructed and the beam optics was studied experimentally.