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