The method 1 wherein {TAIL ~ the step} of applying {HEAD ~ alternating forward and reverse bias potentials} is effected by a bipolar signal having a reverse bias potential of significantly larger magnitude than said forward bias potential 
The method 1 wherein { TAIL ~ the step } of applying { HEAD ~ alternating forward and reverse bias potentials } is effected by a bipolar signal having a reverse bias potential of significantly larger magnitude than said forward bias potential
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH