B.N. Breizman

H.L. Berk

Y. Todo

National Institute for Fusion Science (NIFS-Repository)

§ 3. Simulation of Intermittent Beam IonLoss in a Tokamak Fusion Test ReactorExperimentTodo, Y.Berk, H.L., Breizman, B.N. (IFS, Univ. Texas atAustin)The toroidicity-induced Alfven eigenmode (TAE) canbe destabilized by fast ions that have velocities comparableto the Alfven velocity. A decade ago recurrent bursts ofTAEs were observed with drops in neutron emission duringneutral beam· injection (NBI) in the Tokamak Fusion TestReactor (TFTR) [1] and DllI-D [2]. The drops in neutronemission have been recognized as a manifestation of TAE-induced beam ion loss. In the experiments cited multipleTAEs were destabilized during TAE bursts that took place atregular time intervals. The modulation depth of the drop inneutron emission in the TFTR plasma was typically,..,10%.The most important result is that the beam confinement timeis about one-half to one-third of the collisional slowing-down time. This means that TAE activity expels beam ionsbefore their energy is absorbed by the core plasma.We have carried out simulations [3] based on areduced MHD method, for a configuration typical of theTFTR experiment with balanced beam injection. Thesimulation uses a perturbative approach where the TAEspatial profile is assumed fixed, while amplitudes and phasesof the eigenmodes and the fast-ion nonlinear dynamics isfollowed self-consistently. For simplicity we considerconcentric circular magnetic surfaces to describe theequilibrium magnetic field. For the TAE burst simulation theq-profile is taken to vary quadratically with minor radiusfrom a central value of 1.2 to an edge value of 3.0. Themajor and minor radii are Ro =2.4 m and a=0.75 m. Themagnetic field is 1.0 T on axis. Five eigenmodes are takeninto account. The linear damping rate of each mode isassumed to be constant at 4 x 103s-1. Beam ions havebalanced injection with a constant heating power of 10 MWand with a spatial Gaussian profile whose radial scale lengthis O.3m. The injection energy is 110 keY which correspondsroughly to the Alfven velocity parallel to the magnetic field.In the TFTR experiment two types of limiters, toroidal beltlimiter and three poloidal limiters, were used. In the poloidalcross section the limiters roughly defined a circle of radius1m. We model these limiters by removing particles if theyreach a torus of minor radius 1m. Thus the plasma is leaningon the limiter on the strong field side, while on the weakfield side at the midplane there is a 0.5m space between the324plasma edge and the limiter. The slowing-down time ISassumed to be 100 ms. The number of particles used is2.1 x 106 .We start the simulation at an initial time taken as t=Owhen the beam ions are first injected. As time passes,energetic ions gradually accumulate. Synchronized burststake place recurrently at a burst interval that is roughly 2.9ms which closely matches that of experimental value 2.2 msin the TFTR experiment that we are comparing with. Thevolume average beam ion beta value saturates at 0.6%,which corresponds to 0.4 of the classical distribution. Wefind a good agreement in simulation and experiment. Figure1 shows the time evolution of the dominant two modes 2 and5 and the density of the co-injected beam ions. We can seethat the mode 2, which is located at the plasma center, hasprecursory growth before both the modes grow togetherduring each burst. Because the beam injection profile peaksat the plasma center, mode 2 is destabilized before mode 5.We can see a complete flattening of the density at the plasmacore (r/a<0.72) while small increase in the density at theplasma edge (r/a>O.72). The beam ions stored at the plasmacore during the quiescent phases are transported to theplasma edge and lost during the bursts.4.010°3.5 10°0.023.010° 0.01O?2.5 10° 0 0:;to.......,:::i 2.010°-0.01.0I:::1.5 10°-0.021.010°-0.035.010- 1r/a=1.22-0.04r a=1.390.010°61 62 63 64 65 66t [ms]Fig.l. The time evolution of the dominant two modes 2 and5 and the density of the co-injected beam ions at variousminor radius.[1] K. L. Wong et ai., Phys. Rev. Lett. 66,1874 (1991).[2] W. W. Heidbrink et aI., Nucl. Fusion 31, 1635 (1991).[3] Y. Todo et aI., Phys. Plasmas 10, 2888 (2003).

Simulation of Intermittent Beam Ion Loss in a Tokamak Fusion Test Reactor Experiment

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)