An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed.

This article presents a new theoretical model of ion collection by a cylindrical Langmuir probe at medium and higher pressures which we call the "modified TALBOT and CHOU model". The model makes use of the following theories; (a) the kinetic theory by CHOU, TALBOT and WILLIS [6, 71 and (b) the theory by ZAKRZEWSKI and KOPICZYNSKI [lo, 1 I]. The basic idea is to calculate the decrease of probe ion current due to collisions with neutrals according [6, 71 and the increase of the ion current due to destruction of an orbital motion according [lo, I I]. The computed results are presented in the form of graphs suitable for probe data interpretation at medium and higher pressures. The applicability of the results at particular plasma conditions is also discussed.

No abstract

The seeds of Lamb's Quarters (Chenopodium album agg.) were stimulated by lowpressure discharge. The tested seeds were exposed to plasma discharge for different time durations (from 6 minutes to 48 minutes). Germination tests were performed under specified laboratory conditions during seven days in five identical and completely independent experiments. Significant differences between the control and plasma-treated seeds were observed. The treated seeds showed structural changes on the surface of the seat coat. They germinated faster and their sprout accretion on the first day of seed germination was longer. Germination rate for the untreated seeds was 15% while it increased approximately three times (max 55%) for seeds treated by plasma from 12 minutes to 48 minutes.

A novel probe and approach to the direct measurements of the plasma potential in a strong magnetic field is suggested. The principle of this method is to reduce the electron saturation current to the same magnitude as that of the ion saturation current. In this case, the floating potential of the probe becomes identical to the plasma potential. This goal is attained by a shield, which screens off an adjustable part of the electron current from the probe collector due to the much smaller gyro-radius of the electrons. First systematic measurements have been performed in the CASTOR tokamak.PACS : 52.70.Ds