Professor Felix Parra Diaz

Abstract:

Simulating electrostatic turbulence in tokamaks on transport time scales requires retaining and evolving a complete turbulence modified neoclassical transport description, including all the axisymmetric neoclassical and zonal flow radial electric field effects, as well as the turbulent transport normally associated with drift instabilities. Neoclassical electric field effects are particularly difficult to retain since they require evaluating the ion distribution function to higher order in gyroradius over background scale length than standard gyrokinetic treatments. To avoid extending gyrokinetics an alternate hybrid gyrokinetic-fluid treatment is formulated that employs moments of the full Fokker-Planck kinetic equation to remove the need for a higher order gyrokinetic distribution function. The resulting hybrid description is able to model all electrostatic turbulence effects with wavelengths much longer than an electron Larmor radius such as the ion temperature gradient (ITG) and trapped electron modes (TEM). © 2008 IOP Publishing Ltd.

Abstract:

We present a new recursive procedure to find a full f electrostatic gyrokinetic equation correct to first order in an expansion of gyroradius over magnetic field characteristic length. The procedure provides new insights into the limitations of the gyrokinetic quasineutrality equation. We find that the ion distribution function must be known at least to second order in gyroradius over characteristic length to calculate the long wavelength components of the electrostatic potential self-consistently. Moreover, using the example of a steady-state -pinch, we prove that the quasineutrality equation fails to provide the axisymmetric piece of the potential even with a distribution function correct to second order. We also show that second order accuracy is enough if a more convenient moment equation is used instead of the quasineutrality equation. These results indicate that the gyrokinetic quasineutrality equation is not the most effective procedure to find the electrostatic potential if the long wavelength components are to be retained in the analysis. © 2008 IOP Publishing Ltd.

Abstract:

Time-centering algorithms to increment the temporal accuracy of the state of macroparticles in the hybrid code HPHall-2 are presented. As a result, one orderof-magnitude increment in temporal precision is achieved. Errors in the conservation of mass flow along the chamber are reduced by one order-of-magnitude too. In a second part, the algorithm controlling the population of particles per cell is improved with the aim of reducing both the statistical oscillations and the computational cost.

Abstract:

Particle-in-cell methods are used for ions and neutrals. Probabilistic methods are implemented for ionization, charge-exchange collisions, gas injection, and particle-wall interaction. A diffusive macroscopic model is proposed for the strongly magnetized electron population. Cross-field electron transport includes wall collisionality and Bohm-type diffusion, the last one dominating in most of the discharge. Plasma quasineutrality applies except for space-charge sheaths, which are modeled taking into consideration secondary-electron-emission and space-charge saturation. Specific weighting algorithms are developed in order to fulfil the Bohm condition on the ion flow at the boundaries of the quasineutral domain. The consequence is the full development of the radial plasma structure and correct values for ion losses at lateral walls. The model gains in insight and physical consistency over a previous version, but thrust efficiency is lower than in experiments, indicating that further model refinement of some phenomena is necessary. © 2006 American Institute of Physics.

Abstract:

The effect of a third, active electrode placed inside the ceramic chamber of a Hall thruster is analyzed. Both electron-collecting and electron-emitting modes are considered. Significant efficiency enhancement with respect to the single-stage operation can be obtained for a good electron-emitting electrode, placed in an intermediate location of the acceleration region, and for an anode-to-electrode (inner-stage) potential significantly larger than the ionization potential. Optimum values of the electrode location and voltage are determined. The performance improvement is due to a reduction of the energy losses to the chamber walls. This is the consequence of lower Joule heating and thus lower electron temperature in the outer stage. When the ionization process is efficient already in the single-stage operation, (i) two-stage operation does not affect practically the propellant and voltage utilizations and (ii) thrust efficiency decreases when the intermediate electrode works as an electron collector. © 2005 American Institute of Physics.