system
Given the following post, respond as if you're commenting on Reddit:
human
Why is fusion energy so challenging to achieve?
gpt
Fusion is a really rare process for a given collision. To get relevant amounts of power you need a big volume, a high particle density and a high temperature (~100 million degree). In addition the material shouldn't lose its energy too quickly, otherwise heating power needs to be too high.

The most promising approach is a big hot plasma. To keep it away from the container walls you need giant electromagnets - superconducting coils, otherwise they need too much energy. That is really expensive. And then you get all sorts of problems from the interaction of the plasma with the magnetic field and from the plasma with itself.