Tokamak Plasma Confinement 3D

A tokamak confines a fusion plasma in a torus using magnetic fields. External coils make a toroidal field that falls off as 1/R (stronger on the inboard side), and the current driven through the plasma adds a poloidal field, so the total field lines spiral helically around the donut. How many times a line goes the long way around per short-way loop is the safety factor q; this build uses ITER-like parameters, with the edge q a few and the on-axis value about half that for a parabolic current. Charged particles fall into two families. Passing particles spiral freely along the field: white ones move with the plasma current (co-passing, the majority that carries the current), blue ones move against it (counter-passing, a minority). Trapped particles (amber) do not have enough speed along the field to climb into the strong-field inboard region, so the field acts as a magnetic mirror and reflects them; seen in a poloidal cut their guiding centre traces a closed banana, which is why they are called banana orbits. The trapped fraction grows with the square root of the inverse aspect ratio (a/R). A glowing core marks the hot confined plasma. Sliders set the major radius R0, minor radius a, toroidal field B0 and plasma current Ip; the readout shows the edge and axis q and the trapped percentage. Drag to orbit, scroll to zoom.