SOLPS-ITER simulation of MAST neutral penetration*

In this research, we will study the decay length of neutrals penetrating into the separatrix as a function of the aspect ratio and core electron density using SOLPS-ITER.  The aspect-ratio is varied from 1.5 (MAST-like) to 3.16 (DIII-D like) and the core electron density range from  m³ to  m³. In regular tokamaks, to compare the neutral penetration depth inside the separatrix, an analogy with optics is used and the concept of opaqueness is introduced. To simplify comparison across devices with fusion relevant temperature, opaqueness is approximated by , where  is the average of pedestal and separatrix electron densities, and a is the minor radius of the tokamak _[1]. However, the relationship between machine-size and minor radius does not hold for low aspect ratio tokamaks. The dimensionless version of opaqueness is adopted. It is defined as the ratio of the width of the pedestal, , to the neutral penetration decay length  _[2] .We run SOLPS-ITER_[3] simulations of MAST and match experimental H-mode conditions to extract transport coefficients and set boundary conditions. Keeping all other input parameters constant, we change the major radius and the core electron density separately and plot dimensionless opaqueness with respect to them. Opaqueness is calculated at different poloidal angles.