How Kylo Got Vader's Helmet - Star Wars Theory

In Star Wars the Force Awakens, how exactly did Kylo Ren get hold of Vader's helmet from Return of the Jedi, when Luke Skywalker burnt it after defeating Palpatine?

I think some official concept art from JJ Abrams' Episode 7 Art of Star Wars can give us some insight on the possible backstory here.

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