Influence of transient behavior on the determination of the energy confinement time in a tokamak

The disregard of fast processes (e.g., the L-H transition) in a tokamak plasma can lead to large errors in the determination of the energy confinement time τ_E. A major upgrade in the electromagnetic diagnostic system for the analysis of plasma parameters and in the collection system has made it possible to take into account the influence of the transient character of the radial profile and the value of the plasma current I_P as well as the stored energy W on the determination of τ_E from diamagnetic measurements and to investigate fast processes involved in the L-H transition. The energy confinement time τ_E is calculated from the equation τ _E=W/[U_PI_P-d/dt(LI_P²/2)-dW/dt] where U_P is the plasma loop voltage (V), I_P is the plasma current (A), and L is the total internal inductance (H). The total inductance L of the plasma column has been determined from measurements of the quantity β_J+I_i/2, where β_J is the ratio between the gas-kinetic pressure and the pressure of the poloidal magnetic field, and I_i is the internal inductance. The inclusion of transient behavior in the determination of τ_E from diamagnetic measurements gives a correction of up to 50%.