Probe measurements in a nonstationary plasma

Abstract

Single and double probes are simple and common tools for plasma measurements. In the case of nonstationary plasmas, the values of the plasma density obtained with these tools may differ significantly from the correct values measured, e.g., by microwave methods. The reason for such discrepancy could be the Bohm criterion failure during the plasma transition to the steady state. Indeed, the Bohm criterion, which is commonly used as a boundary condition at the plasma-sheath edge, directly determines the ion saturation current to the probe surface. The transition-time duration is studied and explained quantitatively for various plasmas produced by a version of a ferroinductor-coupled plasma source, which has its magnetic core fully immersed in the plasma. Corresponding conversion factors for probe measurements have been evaluated. Also, the influence of a certain amount of “hot” non-Maxwellian electrons on probe characteristics has been investigated.

Physical limitations in ferromagnetic inductively coupled plasma sources

“…Comparing the voltage required to get the saturation and the evaluated values of T eff (tens of volt vs volts), one might expect that intensive electron beams exist in the "cold" bulk plasma. The evaluated T eff was about 2-3 times smaller than the one obtained formerly in these plasma sources [4][5][6][7][8][9] . To verify this point we compared the decay time of the afterglow plasma t dec at high and low plasma current I pl .…”

“…The routine usage of the probe current-voltage characteristics to derive the electron temperature was not relevant in our case. When the probe bias was below a few tens of volts, as needed for ion current saturation, the probe current became affected by high energy electron beams, related to the voltage induced in the plasma and typically present in such plasma sources 9 . As a result, the probe current became very sensitive to the voltage falls and currents in the plasma, i.e.…”

“…8 . Oppositely, when the probe bias is sufficiently high, the ion saturation current I p is proportional to the plasma density n and almost insensitive to the above mentioned factors 9,11 . Comparing the voltage required to get the saturation and the evaluated values of T eff (tens of volt vs volts), one might expect that intensive electron beams exist in the "cold" bulk plasma.…”

“…The experiments were carried out in a glass vacuum vessel of 32 cm diameter and 50 cm height, similar to our earlier work 4,9 (see Fig. 1).…”

“…by discharging a preliminary charged capacitor via the FICP primary winding 4 . However, almost all studies of this device were performed with various rf drivers [5][6][7][8][9] because of a clear reason: in this case the FICP devices produce stationary plasmas where the plasma exists during the whole operation time of the rf oscillator, including CW regime. On the other hand these powerful (10-15 kW) oscillators could be rather sophisticated and expensive units.…”