Who is Stefan A. Halper, the FBI source who assisted the Russia investigation?

Who is Stefan A. Halper, the FBI source who assisted the Russia investigation?  Washington Post Pressure rising on GOP after Trump–DOJ fight's latest turn  The Hill Justice Department To Expand Internal Inquiry After Top-Level Meeting With Trump  NPR FBI Used Informant to Investigate Russia Ties to Campaign, Not to Spy, as Trump Claims  New York Times Why Trump's Latest Twitter Meltdown Is Especially Disturbing  RollingStone.comFull coverage