Theory Weekly Highlights for September 2012

September 28, 2012

A more efficient calculation of the turbulent exchange (also called the anomalous energy transfer) between ions and electrons has been implemented in prototype form in GYRO. This method reduces the large statistical error normally encountered in a direct time-averaging by using a time-symmetrized form of the exchange moments. This symmetrization also ensures that the species-sum of the exchanges is zero to approximately five significant figures. Comparison with TGLF shows reasonable agreement, and preliminary simulations with TGYRO/TGLF confirm previous estimates that the exchange physics (at least for a sample DIII-D discharge) does not alter the temperature profile prediction by more than a few percent.

Lang Lao attended and chaired the Fourth ITER Integrated Modeling Expert Group (IMEG) Annual Meeting in Chateau de Cadarache, France September 18-20, 2012. The two main goals of the meeting are to discuss progress in the ITER partners domestic integrated modeling programs and to review progress and to advice ITER on its Integrated Modeling Program to develop an Integrated Modeling Analysis Suite (IMAS) and an infrastructure to support ITER plasma operation and plasma research.

September 21, 2012

As a part of the RF SciDAC project for simulation of wave-plasma interactions, the 5-D Monte-Carlo code ORBIT-RF was coupled with the 2-D linear full wave code TORIC. Previous self-consistent simulations were done with ORBIT-RF coupled with the 2-D linear full wave code AORSA. In both, fast wave solutions for the electric field and wave vector components (E+, E-, θk, k⊥) are passed to ORBIT-RF to compute quasi-linear diffusion heating for wave-plasma interactions. Here, θk is defined as the direction of the fast wave in the poloidal plane such that cos(θk) = kr/ k⊥, with r the major radius. AORSA does not provide θk, and in the ORBIT-RF/AORSA simulations, kr = k⊥ was assumed. TORIC computes θk and the solution for a typical DIII-D FW discharge with neutral beam injection indicates that the assumption kr = k⊥ is reasonable. The computed local power absorption profile from ORBIT-RF/TORIC for a background Maxwellian distribution is within 10 % of that computed from ORBIT-RF/AORSA. For iterative simulations of ORBIT-RF with TORIC, the interface module developed for ORBIT-RF/AORSA is being modified through collaborations with PPPL and ORNL.

September 14, 2012

In recent analytic work on the Bessel function heat flux model for pellet ablation, a new semi-analytic transonic flow solution was derived, revealing explicit Z dependence in the various hydrodynamic flow quantities. Comparison with recently published numerical simulations on deuterium pellets found remarkably good agreement. The model predicts that the erosion rates of argon and neon pellets, as measured by drp/dt where rp is the pellet radius, are factors of 10 and 8 respectively, slower than that for deuterium pellets. The model is based on an earlier model (see highlight from October 23 2009 at Theory Weekly Highlights for October 2009) for the attenuation of the energy flux carried by hot incident Maxwellian plasma electrons slowing down and pitch angle scattering on ablation pellet clouds. The numerical solutions had the same structure as the exact Modified Bessel function solution for hydrogenic media, where the pitch angle diffusion term can be neglected and the argument of the Bessel function contained a scalable numerical factor. This argument has a nice physical interpretation as being proportional to an effective mean free path ~ 1/(Z(Z+1)1/2], reflecting the coupling of the slowing down and pitch angle scattering processes. The new ablation model is being modified to include the refractory pellets like beryllium. The non-hydrogenic (Z 1) pellets are being considered for disruption mitigation in ITER.

September 07, 2012

The capability to import data from the “2008 International Multi-tokamak Confinement Profile Database” (so-called UFILE format) has been added to the PROFILES_GEN tool. PROFILES_GEN converts data in multiple formats to a standard format (input.profiles) for use by GYRO, NEO and TGYRO. The new code to read UFILEs is written in python and Fortran (2003) based on the current 2008 UFILE specification with no reference to antiquated, non-portable legacy data readers. This includes diagnostic and control capability for ease of use by “ordinary” users. With this new functionality, PROFILES_GEN can import ITERDB (text or NetCDF), PlasmaState, PEQDSK (Elite PFILE), CORSICA and UFILE data, and produce a standard input.profiles file for use in transport-timescale simulation. In this way the conversion from a base format (like ITERDB) to the input.profiles standard is transparent and not buried inside the application (e.g. transport or gyrokinetic) code.
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These highlights are reports of research work in progress and are accordingly subject to change or modification