NEWBURYPORT – A Lynn man was sentenced to serve 11-14 years in state prison after being convicted of holding a knife to his girlfriend’s throat as he raped her in their Newhall Street home in 2009.Luis Mercado, 33, formerly of 45 Newhall St., #1, went on trial earlier this week in Newburyport Superior Court on seven indictments.Jurors deliberated just three hours on Thursday before convicting him of all the indictments of rape, two counts of assault and battery with a dangerous weapon, assault with a dangerous weapon and three counts of violation of a court order.In addition Judge Richard E. Welch III ordered Mercado to be on probation for another six years when he is released at which time he is to stay away from his accuser and her family.