I understand that the nuclear cross section determines the probability of fusion taking place. When I was talking to my friend he said that the cross section doesn't depend on anything but the particles and their energies. But I fail to understand this. For example, let us consider ionization. I have a hot cathode, and an anode placed at distance from each other. And between them, I have a gas. So the electrons would ionize the gas and move to the anode. But this is rare as most of them miss. So if I have a magnetic field, the electrons spiral and there is a higher probability of them colliding with neutral atoms (This is used in ion thrusters). Now, since the probability increased, shoudn't the cross section increase too? So the cross section depends on a lot of factors, right? But the confusion arises when almost all websites say that the collision cross section is basically pi(2r)^2 . But isn't it dependent on other aspects? like the magnetic field we just talked about? Then why is it pi(2r)^2? Doesn't the same principle apply to fusion? For a certain energy, the gamows factor determines the probability of fusion. Indirectly, the more collisions we have, the higher the fusion cross section. Since, the collisions or collision cross section are higher the probability of fusion increases. And because the collision cross section depends on external factors like magnetic fields, nuclear cross section does too, doesn't it? The D-T fusion graphs only work for plasmas that are confined and follow a maxwellian distribution don't they? In other words, they only work for devices that similar to the tokamak. So, is the cross section really dependent on various factors like magnetic fields? electric fields? Am i right?