Mitsubishi Corp.’s Diamond Generating energy subsidiary is doubling down on Boston. Diamond completed its $70 million acquisition of Boston Energy Trading and Marketing from energy giant NRG on Aug. 1. Then Diamond acquired a controlling stake in Nexamp Inc., a Boston-based solar company, after first investing in the company in 2016. Diamond CEO Satoshi Hamada said he also recently moved his office from Los Angeles to Boston. Nexamp plans to add 75 workers to its current staff of 105 over the next 18 months, and is relocating from Liberty Square to 101 Summer St.