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Jordan Palmer was under the influence of synthetic cannabis when he battered Terrence Ojuederie with a flat-screen TV.
The 42-year-old had the bottom bunk in their cell at HMP Peterborough, and is thought to have been smoking the former "Legal-High" called Spice.
A jury at Peterborough Crown Court accepted that involuntary intoxication had happened to Palmer, and that he had not intended to take the drug on December 9 last year.
Palmer was found guilty of manslaughter by diminished responsibility following a two-and-a-half week trial.
Judge David Farrell QC, sentencing Palmer to 14 years in prison, said: "The killing was particularly brutal.
"You subjected Mr Ojuederie to a totally unprovoked and ferocious attack, punching him, kicking him, stamping on him and using a television set as a weapon to inflict serious and fatal injuries to his head and face."
Palmer and Mr Ojuederie had shared a cell for around three months.
Mr Ojuederie's family gave this statement: "There was never going to be a happy outcome of this trial for us. Nothing can bring Terry back to us or take away the pain and devastation we are living with each day.
We are left instead with an emptiness and the horror of his final moments as heard in detail.
"The bitter feeling at how this could happen in prison, a place we thought Terry would be safe, will remain with us forever, as will the torment of not knowing for sure the truth of what really happened in his cell that night."