Q, break-even and the nt_E diagram are well defined and understood for steady-state fusion plasma conditions. Since many fusion experiments are transient, it is necessary to clarify the definitions for instantaneous Q values and break-even so that the nt_E diagram can be interpreted for transient plasma conditions. This discussion shows that there are two mathematically correct methods to describe the nt_E diargram for a transient plasma. The TFTR method which is consistent with previous analyses of the Lawson cycle, and prior definitions for Q and break-even describes a transient fusion plasma in terms of Q = P_fusion/P_aux with the plasma energy confinement time for the nt diagram given by t_E* = W_p / P_heat where W_p is the total plasma kinetic energy and P_heat = P_aux + P_alpha - P_brem is the net power heating the plasma. In the TFTR definition break-even (P_fusion = P_aux) occurs at Q=1, ignition occurs at Q = infinity and the nt_E* values required to achieve a given Q are the same in transient and steady-state plasmas. The JET/JT-60 method uses the definitions of Q* = P_fusion/(P_aux - dW_p/dt) and t_E = W_p /(P_heat - dW_p/dt). This method produces the confusing result that break-even requires Q* = P_aux/(P_aux - dW_p/dt) which is >1 for many cases of interest. In addition, the nt_E value required to achieve break-even depends on dW_p/dt and therefore experimental data points with different dW_p/dt must be compared to different Q* curves on the Lawson diagram. For a pulsed plasma, this issue can be avoided by using the definition of fusion gain first introduced by Lawson, namely Q = fusion energy per pulse divided by auxiliary plasma heating energy supplied per pulse.

*Work supported by U.S. DOE Contract No. DE-AC02-76-CHO3073.