Study Of Reducing The Magnetic Field Required For Ohmic Ignition

Thermonuclear fusion involves heating the plasma to a high enough temperature and maintaining a certain density for fusion reaction to sustain itself and continue releasing nuclear energy.In tokamaks,ohmic heating is the most basic and simplest heating method.Because the ohmic heating power decreases when the plasma temperature increases,more expensive and complicated auxiliary heating methods,such as neutral beam heating and wave heating were also developed to continue heating the plasma to achieving higher temperature.However,the inherent plasma current plays an important role in maintaining the equilibrium and stability of the tokamak.The Joule heating from the current could also contribute substantially to the input power to the plasma.Therefore,the study of using ohmic heating alone to achieve ignition has obvious practical significance.As the efficiency of ohmic heating decreases sharply with the rise of plasma temperature,for the tokamak with circular cross section,achieving ohmic ignition requires a magnetic field of about 20 T,which is not realistic in engineering.However,Chu et al.showed that if the cross section of the tokamak is extended longitudinally,the circumferential magnetic field strength required for the tokamak ignition can be reduced to about 6 T(Chu MS,et al.1985,Nuclear Fusion.25),which greatly relieves the problem that the tokamak ohmic ignition requires a strong magnetic field.However,this study did not consider the effect of natural transport barriers on ignition conditions observed by recent magnetic constraint experiments.In this work,we studied the effects of different plasma densities and temperature profiles on the critical magnetic field of ohmic ignition.It is found that for the plasma density and temperature distribution caused by the existence of external transport barriers,the magnetic field required by ohmic ignition increases.This is because the fusion power of the plasma is mainly concentrated in the core of the plasma.The plasma core distribution tends to change gently due to external transport barriers or other factors,which will reduce the fusion power.Therefore,a stronger magnetic field is needed to reduce the power loss of the plasma.Our results show that under the same parameters,considering the external transport barrier effect,the magnetic field required for ohmic ignition will increase by about 20%,which will not have a disastrous impact on the ignition conditions of ohmic.The research in this paper is helpful to explore the possibility of ohmic ignition and the optimization of ohmic heating of tokamak thermonuclear fusion reactor in the future.