{HEAD ~ A sufficiently large electrical potential} is applied between {TAIL ~ the anode} and cathode to cause a discharge current therebetween which results in the production of a pair of counter propagating laser beams within the block 
is applied between
{ HEAD ~ A sufficiently large electrical potential } is applied between { TAIL ~ the anode } and cathode to cause a discharge current therebetween which results in the production of a pair of counter propagating laser beams within the block
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH