Operational test of the LaWS weapon. I believe what they're building is meant to be a permanent installation laser weapon.
Wikipedia has this additional information:
"Following a review of several ship classes to determine which had available space, power, and cooling, it was decided that after the Ponce's planned decommissioning in 2018, the LaWS will be moved to the new amphibious transport dock ship USS Portland (LPD-27) for indefinite testing, it will utilise the space and power connections reserved for VLS to house the LaWS power and control modules while the laser itself will be bolted to the deck. As its a trial it will not be integrated with the ships Warfare Centre command & control systems."
"In January 2018 the Navy announced a $150m contract with Lockheed Martin for the production of two more LaWS units to be delivered 2020; one will be fitted to USS Arleigh Burke (DDG-51) while the other will be used for land-based testing. Further contract options could bring its value to $942.8m."
