XGC-0 User Manual Scenarios for Running XGC Simulations
Contents
1. XGC 0O User Manual 1 June 2006 Scenarios for Running XGC Simulations There are basically three ways to run XGC simulations to compute the shape of the density and temperature pedestal at the edge of H mode discharges in tokamaks The physics and results associated with the three scenarios are described in detail in the paper by C S Chang Seunghoe Ku and H Weitzner in Physics of Plasmas 11 2649 2004 1 Neoclassical Ion Solution This scenario produces a quick solution to demonstrate neoclassical ion effects without the effect of neutrals and without following the time development of the pedestal build up First turn off the influx of neutrals neu col mode 0 and start with a very narrow pedestal small values of eq_den_ped_width eq_tempi_ped_width and eq_den_ tempe_width The pedestal will then relax to a steady state solution The results of this scenario are described in Section 5 of Physics of Plasmas 11 2649 2004 During the relaxation a radial electric field will build up in the pedestal due to a radial current density When the plasma reaches steady state the flux surface averaged radial current density will go to zero Jiotale hiona GJelectionr 0 Since the ion radial current density is much much larger than the electron radial current density driven by neoclassical effects we have Jionr 0 The ion radial current density consists of three parts Jion Jaarn T Jolassicalpolatizationt T Jnesel2. assicalpolarization t The term Jelassicalpolarization r 1s computed using textbook analytic expressions and it is needed to recover the geodesic acoustic mode structure The Jarivenx and Jneoclassicalpolarization r terms which are generally much larger than the Jclassicalpolarization r term are computed numerically within the XGC 0 code The ion loss mechanism that produced the radial electric field is described in detail in Reference 2 Effect of Neutrals The second scenario produces a more physically realistic solution by starting with a very broad low pedestal large values of eq_den_ped_width eq_tempi_ped_width and large values of eq_den_tempe_width and using an influx of neutrals typically neu_col_mode 2 to build up the density pedestal The results of this scenario are described in Section 6 of Physics of Plasmas 11 2649 2004 The influx of neutrals can be specified by either a source strength or an edge neu tral density or an edge recycling rate If a recycling rate is used set the recycling rate 1 neu_recycle_rate less than 1 0 If the recycling rate is set greater than or equal to 1 0 the code is likely to run out of memory and crash WARNING The user must specify enough memory for neutrals at least 10 MB greater than the charged particle memory or the code can crash During the course of a simulation using this scenario the flux surface averaged radial current density will rapidly decrease to smal
3. l values and then hover around zero Fluctua tions around zero will be smaller when more charged particles are used in the simulation The radial electric field will increase to its nearly steady state value and then hover with fluctuations around that value Usually about 50 ion transit times around the tokamak are sufficient to reach nearly steady state conditions Note that electrons are produced by neutral ionization events and on closed flux surfaces each electron remains on the flux surface where it was born WARNING If too few neutrals are used in the simulation the statistics will be poor for energy conserving ion neutral collisions Effect of Turbulence A simple model for turbulent transport is available in the XGC 0 code and a more complete 3 D turbulence simulation will be available in the XGC 1 code For the third scenario the simple turbulent transport model is used in the XGC 0 code The results of this scenario are described in Section 7 of Physics of Plasmas 11 2649 2004 In the XGC 0 code it is assumed that the particle transport driven by turbulence in ambipolar The turbulent diffusion coefficient is written in the form 2 Drurbulence A Tdecorrelation where the user specifies Deurbulence and sets A much smaller than the pedestal width typically A pi and the code computes the corresponding value of T ecorrelation In response to this turbulent particle diffusion electrons and ions carry out a random walk a
4. long the radius with a net flux down the density gradient WARNING It is found that the XGC 0 code is numerically unstable when Diurbulence 1S a function of the radius In effect a radially uniform value of Diurbulence can be thought of as a spatially averaged flow shear rate General Comments Charged particles are initially loaded uniformly in space The density profile is produced in the XGC O code by giving lower weight to particles in regions of space where the density is lower In the absence of collisions the weight for each particle is fixed in time The neutral influx can be adjusted to maintain the overall charged particle number even when some charged particles are lost to the walls A linear interpolation is used to compute the charged particle density at zone centers Consequently the effective size of each particle cloud is determined by the spatial zone dimensions Since a four point averaging technique is used is used around each gyro orbit the charged particle size perpendicular to the magnetic field is related to the gyro radius
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