N." "Mizuguchi

"Mizuguchi,  N."

NIFS-Repository (National Institute for Fusion Science)

Nonlinear MHD Simulation of CollapseEvent in LHDMIZUGUCHI, NaokiTo model the large-scale collapse event termed CDC(core density collapse) observed in the vicinity of themarginally stable condition for the outward-shifted LHDoperations, we have executed nonlinear magnetohydro-dynamic(MHD) simulations in a realistic LHD conﬁgu-ration with a large pressure gradient. The simulationresults reproduce the basic characteristics of the exper-imental observation on the CDC events in LHD. Thespatio-temporal development of the core pressure struc-ture is investigated in detail1).The initial condition of the simulation is given by a nu-merical equilibrium obtained by using the HINT2 code.It roughly models the LHD plasma for the time justprior to a CDC, in which the pressure proﬁle has alarge gradient with the central and volume-averaged betaβ0 = 6.6% and < β >= 1.8%. The simulation codeused in this study is the non-axisymmetric version ofthe three-dimensional ﬁnite diﬀerence code (MEGA-D)developed at NIFS, in which the time development ofthe nonlinear resistive compressive MHD equations aresolved explicitly.The simulation result shows the linear growth of the re-sistive instability which has the nature of the ballooningmode under the assumption of large value of the resis-tivity. The growth of the modes are saturated soon, andthe system experiences the energy relaxation three timesin about 500τA (< 1 msec). It should be noted thatthe linear mode structures are localized in the edge re-gion, whereas the core pressure rapidly falls as the systemreaches the ﬁnally relaxed state. The co-existence of theedge perturbation and the core collapse is comparable tothe experimental observations. The lost pressure formsa wider tail in the peripheral region (see Fig. 1).This result shows that the core collapse can be inducedonly by the convective process rather than the conduc-tive one. The core pressure is remarkably reduced att = 550τA, while it had withstood the disturbance duringthe former period. The most salient feature on that pe-riod is the disordering of the magnetic ﬁeld structure(seeFig. 2). The system keeps well the structure of thenested ﬂux surfaces in the core region in the beginning,whereas they abruptly diminish at around t = 300τA.The simulation result shows a deformation of the pres-sure proﬁle due to the instabilities. In Fig. 2(b), onecan ﬁnd that the magnetiv surfaces diminish in the coreregion, implying that the plasma ﬂows are driven by thepressure gradient along the magnetic ﬁeld lines whichmight be formed by the reconnections of the ﬁeld lines.Thus, the core collapse can be caused by the disturbanceof the magnetic ﬁeld due to the growth of the edge in-stabilities.In summary, we have modeled the nonlinear dynamicsof the collapse event like CDC in LHD by means of thenonlinear MHD simulation. Comparative analysis withexperimental operation parameters is our ongoing study.Fig. 1: Time development of the radial pressure proﬁleon the horizontally elongated equator.Fig. 2: Temporal change in the magnetic structure.Puncture plots of the magnetic ﬁeld for (a) t=0 and (b)300 τA are plotted.1) N. Mizuguchi, Y. Suzuki, and N. Ohyabu, “Nonlin-ear dynamics of a collapse phenomenon in heliotronplasma with large pressure gradient”, Nuclear Fusion, (2009) vol.49 095023197§2. o  MHD Simulation of Collapse Event in LHDMizuguchi, N.

Nonlinear MHD Simulation of Collapse Event in LHD

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