On a recent trip to Charlottesville, Virginia I got hold of a copy of Lyman Spitzer’s Physics of Fully Ionised Gases. Lyman Spitzer invented the stellarator and yes I had a good time and didn’t spend the whole holiday reading plasma physics books. It was maybe 20% of the time tops.

Anyway he has a great introduction to the basics of particle transport in plasmas here’s a quick sketch of one cause in Tokamaks; a magnetic field gradient. Since the toroidal coils are closer together on the inside of the donut that the outside the toroidal field is stronger near the inside wall. This B field gradient causes the electrons and ions to gradually drift away from their intended safety-factor-intended paths. Which way do they go?

To simplify the picture imagine the B field changes suddenly, rather than weakening gradually as you move outwards. It’s strong on the inside of the chamber and weak on the outside. From Faraday’s right hand rule we know the particles experience a force which moves them along helical paths, the radius of the path being inversely proportional to the strength of the field, so we get small circles on the inside and bigger lazier circles outside:

+ and - drift in opposite directions
+ and – drift in opposite directions

Looks like a mad physics pizza but I hope the mechanism is clear, the change in B field strength changes the gyro radius and then it changes back again, 3 steps down 1 step up etc. So drift is normal to the B field gradient.

See you soon!

P.S If you’re passing through Charlottesville don’t miss Daedalus books, their collection is outstanding. Science or otherwise.