Maintaining very high temperatures, maintaining a magnetic pinch that is opposite to the magnetic field produced, maintaining a material feed into a strong field that opposes it, and so on. Fusion works in stars because gravity holds the constituents together during the fusion process; and, the high temperatures don't dissipate into the immediate surroundings.
My professors also said that the nuclear furnace would produce all sorts of atoms ranging about the periodic table. Most would be unwanted and would result in dangerous wastes.
Lol. Always find a conspiracy by the big bad corporations to answer any question.
Going back nearly 50 years ago, I worked for a company that was big in nuclear energy… very big. We were building fission plants around the world. At one point, the company decided to give some money and support to Princeton University to support their Tokamac fusion reactor design. One of the Princeton big shots come to our facility to have us produce some drawings for them. We had an excellent illustration department. Going over the drawings, the Princeton expert showed us how their reactor was designed and how it would work.
I asked the expert how long before this design would be a commercial product. He was quite confident the it would be a reality in 10 years and at the most 20 years. That was nearly 50 years ago. LOL. The bottom line is Fusion ain’t easy.