Study On High Energy Particle Loss Channel In Quasi-Axisymmetric Stellarator

The rotating transformation of the magnetic field in the stellarator is formed by the current in the external field coil,which can make the magnetic confinement fusion plasma run stably.Although stellarators run stably for a long time,it faces the problems of complex 3D structure and large neoclassical transport.A Novel kind of Tokamak-like stellarator jointly proposed by Southwest Jiaotong University and the National Institute of fusion science（NIFS）.It is the first quasi-axisymmetric Stellarator in China（CFQS,Chinese First Quasi-axisymmetric Stellarator）.CFQS combines the advantages of tokamaks and stellarators.Thus,it has the advantages of low neoclassical transport and high MHD stability.However,for all quasi-axisymmetric symmetric stellarators,it is difficult to achieve a perfect quasi-axisymmetric when considering multi-objective optimization.The axisymmetry-breaking will reduce the confinement of plasma in stellarators.Meanwhile,high energy particle transport in fusion reactor will be of great concern in the future.Because the confinement ofα-particles is important for the self-sustaining of plasma（high-energy particles play a role on heating plasma）.In addition,ifα-particles escape from the last closed flux surface of the plasma and bombard the vessel of the stellarator device,it will cause serious damage to the wall.Based on CFQS device,in this paper we enlarged its size to ITER（International Thermonuclear Experimental Reactor）reactor device.After enlargement,the average magnetic field is 5.3 T,the volume is about 800 m³,the major radius is 9.0 m,the minor radius is 2.2 m,and the toroidal period N=2.In order to simplify our discussion,this paper only studies theα-particles with 3.5 Me V energy birthed in DT fusion reaction,and only considers the particle behavior before the slowing-down time.Because of the high kinetic energy,the collision of particles can be ignored.In order to discuss the influence of axisymmetry-breaking on the behavior and losses of collisionlessα-particles,a semi-analytic representation of radial and poloidal drifts in Boozer coordinates is given in this paper,by which we found two effective routes to mitigateα-particle losses.The semi-analytic method is a set of radial and polar drift functions describing high-energy particles,and particles’losses can be analyzed directly.In this way,the optimizing of the stellarator can be accelerated.One of the two effective routes is adjusting the location of the quasi-axisymmetric radial position,by which the radial drift near the loss channel reduce and the poloidal drift enhance in the whole plasma region.At the same time,theα-particles confinement near the magnetic surface at the quasi-axisymmetric position can be improved.The other method is to add a large radial electric field.Such route enables the enhancement of the poloidal drift in peripheral regions of the identified loss channels.Thus,loss ofα-particles falling in these loss channels can be reduced.It is noteworthy that the confinement ofα-particles is improved only in strong radial electric field.In order to verify the feasibility of the above semi-analytic method and the methods of reducing the loss fraction,we obtained magnetic field via VMEC code,and then we simulated particles’motion through MATLAB program based on the guiding center orbit equation.In this program,particles’trajectories iteratively calculated by fourth order Runge-Kutta method,the time step was set asΔt=6.25×10^-8 s,the total time was 10 ms and 413664 particles were estimated.The particles were evenly distributed in a flux surface.The number of particles on each flux surface conforms to the DT fusion reaction model.The particles’pitch angle covered passing and trapped particles.According to the MATLAB simulation results,the correctness of the semi-analytic method and the method of reducing the loss fraction were confirmed.