half

It was as Harry dodged another Bludger, which went spinning dangerously past his head, that it happened. His broom gave a sudden, frightening lurch. For a split second, he thought he was going to fall. He gripped the broom tightly with both his hands and knees. He'd never felt anything like that.
It happened again. It was as though the broom was trying to buck him off. But Nimbus Two Thousands did not suddenly decide to buck their riders off. Harry tried to turn back toward the Gryffindor goalposts -- he had half a mind to ask Wood to call time-out -- and then he realized that his broom was completely out of his control. He couldn't turn it. He couldn't direct it at all. It was zigzagging through the air, and every now and then making violent swishing movements that almost unseated him.

Is the “half” adjective or adverb or something else?