This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
Basic Answer:
Fusion researchers come to the field from a number
of different disciplines, because the field is small and young
and no school has a major in "fusioneering" or "plasma physics".
For undergraduates, a major in physics, astrophysics, or
electrical engineering would provide a perhaps the best
background for studying plasma physics. Nuclear and mechanical
engineering are also viable options, particularly if your interest
lies more in reactor design and engineering. At this point the
majority of graduate opportunities are on the plasma physics side,
though this may change as the science evolves and (we hope) more
reactor engineers are needed.
My opinion is that it is more important to look for research
opportunities relevant to the field, and the choice of major
is a little less important.
 
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