At such temperatures atoms are stripped down to their electrically charged components, so the helium created by fusion remains within themagneticbottle.
The energy released in fusion is carried mostly by neutrons, a type of subatomic particle that has no electric charge and hence cannot be confined by themagneticbottle.
Such sculpted magnetic fields that make up themagneticbottle are not particularly strong, so the trick was to make antihydrogen atoms that didn't have much energy - that is, they were slow-moving.
At the same time, that gas must be prevented from touching the walls of the reactor by confining it in a powerful magnetic field known as a magneticbottle.