Greenwald density limit: n̄_G = I_p/(π a²) · 10²⁰ m⁻³ (I_p in MA, a in m)

Greenwald empirical density limit (1988): in a circular tokamak with plasma current I_p (MA) and minor radius a (m), the line-averaged density n̄ (10²⁰ m⁻³) cannot stably exceed n̄_G = I_p/(πa²). Above this limit the plasma enters a…