| In the deuterium-tritium(D-T)magnetic confinement fusion reactor,after slowing down and thermalization,the 3.52 Me V alpha particle from the fusion reaction will become helium ash,which inevitably exists in the burning plasmas.If helium ash accumulates in the plasma core,it will cause radiation enhancement and fuel ion dilution,decrease the plasma temperature and fusion power and even induce the plasma disruption,and thus endanger the safe operation of the fusion reactor.In order to achieve the efficient and safe operation of future reactors,the concentration of helium ash in the core must be controlled.At present,it is generally believed that the microturbulence is the main reason for the anomalous transport of charged particles,including impurities.The energetic alpha particles mainly heat electrons,then the collisionless trapped electron mode(CTEM)turbulence,which is driven by electron temperature gradient and/or electron density gradient,may be one of the turbulence mode that causes the transport of helium ash in D-T plasma.In the fusion reactor,attention should be paid to the transport of D-T fuel ions as well as that of helium ash,because we do not want to exhaust fuel ions(especially the expensive T ions).The effect of CTEM turbulence on the anomalous transport of helium ash and deuteriumtritium(D-T)ions in tokamak plasma with weak magnetic shear are simultaneously studied by kinetic theory.Firstly,the effects of helium ash on linear instability of CTEM are investigated.The perturbed densities of D-T ions and helium ash(electrons)are obtained by using the linear gyro-kinetic(bounce-kinetic)theory,respectively,and the dispersion equation of CTEM instability is gained through the quasi-neutrality condition.By solving the dispersion equation,the analytical expressions of real frequency and linear growth rate are derived,then the parametric dependence of CTEM instability in the presence of helium ash is analyzed.We find that when the temperature of helium ash is higher(lower),the helium ash destabilizes(stabilizes)the CTEM instability.Moreover,as the helium ash concentration is increased,the helium ash density profile becomes steeper,and more significant effects from helium ash are observed.The critical temperature for distinguishing the stabilization and destabilization of helium ash is given.Increasing the electron temperature and flattening the electron density profile can stabilize the CTEM instability,which is helpful for understanding the effects of CTEM turbulence on helium ash transport in the plasmas with electron cyclotron resonant heating.In addition,it is found that increasing the fraction of T ions in the DT plasma(i.e.,enhancing the isotopic effects)can also destabilize the CTEM instability.The physical mechanism of destabilization(stabilization)from these parameters on the CTEM instability can be attributed to the strengthening(weakening)of resonance between drift wave and electrons,thus more(less)energy can be transferred from the resonant electrons to the wave,which enhances(weakens)the CTEM instability.Furthermore,utilizing the quasi-linear theory,we calculate the quasi-linear fluxes of the helium ash,D and T ions,respectively,and obtain the corresponding diffusion coefficient and convection velocity.Then,the effects of CTEM turbulence on the transport of helium ash and D-T fuel ions are analyzed simultaneously.The results show that the normalized diffusion coefficient is much larger than the normalized convection velocity for the parameter range we used,and the transport flux is always positive,which is very beneficial to the removal of helium ash from the core of the burning plasma.Moreover,we also found that the loss of fuel ions is inevitable when removing helium ash,but increasing the electron temperature,flattening electron density profile or enhancing isotope effects is favorable for removing more helium ash than D-T ions.These results provide a theoretical reference for future burning plasma to reduce the loss of fuel ions when expelling helium ash. |
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