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STELLADescription | Purpose | Algorithms | Results | Publications Short Description3D time-dependent Fokker Planck solver for electron and ion distributions f(v,theta,z;t) parallel to a z-dependent magnetic field. It is not bounce-averaged, and retains the v_par*df/dz streaming term in the FP equation. It thus applies in all collisionality regimes. There can be an arbitrary number of magnetic wells. A parallel electric field maintains ambipolar flow of the electrons and ions.Date/Active Use1994.AuthorsO. Sauter, R.W. Harvey, F.L. Hinton, K. Kupfer, Yu.V. PetrovLanguageF77/F90 + MPIPurpose/Function/Special FeaturesThe aim is to model kinetic effects on the transport of electrons and ions in the direction parallel to a spatially varying magnetic field. Particular applications are the plasma scrape-off-layer(SOL) in tokamak plasmas, and parallel transport in open magnetic systems, for example, mirrors. At present, 2D fluid codes are widely used in describing the plasma transport in the tokamak edge region. But in the plasma SOL the fluid approach is only marginally valid since relevant collision lengths are of the order of the scale lenths for variation of the plasma parameters. Similarly, in magnetic mirrors, including tandem mirror devices, there occur regions of mean free path both greater and less that the parallel magnetic field spatial scale lengths. Bounce averaging, as in CQL3D is not sufficient for accurate modeling; it is necessary to retain the parallel streaming term.STELLA solves for the fully kinetic electron and ion distributions, including new effects such a trapping due to variation of magnetic field strength along the field lines. The code has been parallelized using MPI. This enables simultaneeous calculation in an acceptable execution time of the distorted electron and ion distributions, along with the self-consistent ambipolar electric field. The STELLA code is derived from CQL3D. The initial work was by Sauter, Harvey, and Hinton [1] as the "CQLP" option in the CQL3D code. This line of work was further developed in Sauter, Angioni and Lin-Liu[2] giving widely used, all-collisionality, neoclassical formulas for electrical conductivity and bootstrap current. A stand-alone parallel transport code, FPET, was developed by Kupfer and Harvey, et al.[3], targeting tokamak scrape-off-layer applications [4,5] but initially without the -mu*grad(B) magnetic trapping term. The physics of localized electron cyclotron and lower hybrid current drive efficiency as compared to flux-surface-averaged CD efficiency was examined in [6]. The code was renamed to STELLA, for application to stellar wind modelling. The -mu*grad(B) force was added. Basic AlgorithmsThe time-integration of the electron or ion Fokker-Planck equations is by Alternating-Direction-Implicit (ADI), alternating between time advancement of the 2d velocity Fokker-Planck equation and the spatial advection equation. Determination of the ambipolar electric field is by a semi-linear iteration algorithm.Coupled DiagnosticsGenerally, the diagnostics available with CQL3D are incorporated into STELLA, including the SXR and neutron diagnostics.Key ResultsThe code has been benchmarked by calculation of classical electrical conductivity and parallel thermal transport. Parallelization, seperate determination of the electron and ion distributions with strong kinetic effects, and determination of the ambipolar electric field, have been demonstrated. General agreement has been shown between calculated nonthermal effects on Boron IV line radiation and experimental observations[4,5].Selected Publications
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