In this paper, ignition curve for deuterium/helium-3 fusion reaction is studied. Four
fusion reactions are considered. Zero-dimensional model for the power balance equation has been
used. The closed ignition curves for ρ = constant (ratio of particle to energy confinement time) have
been derived. The results of our calculations show that ignited equilibria for deuterium/helium-
3 fuel in a spherical tokamak is only possible for ρ = 5.5 and 6. Then, by using the energy
confinement scaling and parameters of the spherical tokamak reactor, the plasma stability limits
have been obtained in ne, T plane and, to determine the thermal instability of plasma, the timedependent
transport equations have been solved.