S. Biri2, T. Nakagawa1, M. Kidera1, L. Kenez2, A. Valek2 and Y. Yano1
1) The Institute of Physical and Chemical Research (RIKEN) Hirosawa 2-1,Wako, Saitama 351-01, Japan,
2) Institute of Nuclear Research (ATOMKI), H-4026 Debrecen, Bem ter 18/c,Hungary
One of the most popular ways to obtain higher beam intensities in ECR ionsources is to install an electrode (usually disc) into the plasma chamber.Examined this method in detail we found that majority of the groupsobserved the beam intensity improvement by supplying a suitable biasedvoltage to the electrode and an electron current was injected into theplasma. A few groups observed the enhancement, however, when the electrodeoperated at floating potential - without being an electron donor. Only afew (and sometimes contradictionary) information was found on the optimisedproperties of the electrodes, i.e. position, dimension, shape, material.
In spite of the great success of the "biased-disc" method, the mechanismis still not completely clear. In this contribution, as one step ofunderstanding, we examine what condition we observed the above mentionedtwo modes. The experiments were performed at the 18 GHz RIKEN and at the14.5 GHz ATOMKI ECR ion sources.
It was found that effect of the electrode is strongly depends on the localplasma parameters and on the position of the electrode. At certain mirrorratios and electrode positions we needed to negatively bias the electrodeand inject electrons into the plasma. The electrode operated as an electronsource (Electron Donor ED mode). At higher mirror ratios and other axialpositions the electrode works by directly changing the plasma potential dip(Potential Tuner PT mode). These two modes were checked and successfullyfound both in continuos and in pulsed mode operation. In both (ED and PT)modes we generated higher highly charged ion currents in the RIKEN-ECRISthan without the electrode.
Presenting Author : Biri S.
Presentation : Oral