Finite element simulations of the laser-heated diamond-anvil cell

Abstract

Axial and radial temperature gradients in the laser-heated diamond-anvil cell are examined using finite element simulations. Calculations are carried out for an optically thin silicate or oxide sample separated from the diamonds by an insulation medium and heated by a TEM00 mode from an infrared laser. The peak temperature of the simulations was chosen to be a representative value (2200K) and sample dimensions are typical for experiments in the 20–50-GPa range. The distance between the anvils is 30μm. The total temperature drop across the sample in the axial direction is controlled by two parameters: the filling fraction (thickness of sample∕distance between anvils) and the ratio of thermal conductivity between the sample and insulator (kS∕kI). The results of the numerical calculations agree well with a one-dimensional numerical model. For a sample filling fraction of 0.5, the axial temperature drop will range from about 1000K (>45%) for a thermal conductivity ratio of 1 to about 200K (<10%) for a conductivity ratio of 10. If the conductivity ratio between sample and insulator is reduced to 1, then a sample filling fraction of less than 0.1 is required to keep the axial temperature decrease to be less than 10%. The effects of asymmetric samples and variations in absorption length are also examined. For a given gasket thickness and conductivity ratio, we find that radial gradients are minimal at a filling fraction of about 50% and then increase at higher and lower filling ratios. The anvil surface remains close to room temperature in all calculations. Our results demonstrate that reduction of axial temperature variations in optically thin laser-heated samples requires the use of thick, low thermal conductivity insulation media.