We propose a new technique for evaluating the RF voltage across the
electrodes of low-pressure capacitively-coupled plasma reactors when direct
measurements are not possible. It is based on determining the coordinates of
the turning point in the RF breakdown curve and using known values of the
electron drift velocity for the gas. The results are in good agreement with
those obtained by direct measurements at the driven electrode. Furthermore
it allows RF breakdown curves to be determined for different frequencies,
giving results that are physically reasonable (coincidence of right-hand
branches) and in agreement with other published results. The technique for
determining RF voltage we proposed is valid when there is no discharge
plasma between electrodes (e.g., before gas breakdown), as well as for
negligibly small discharge currents (before extinction of the weak-current
discharge mode).