Biological process

positive regulation of thymic T cell differentiation, positive regulation of thymocyte cell differentiation, positive regulation of thymocyte differentiation

Any process that activates or increases the frequency, rate or extent of T cell differentiation in the thymus.

positive regulation of T cell development in thymus

Note that immunologists typically use the word 'development' to refer to cells of B or T cell lineages undergoing the process that GO describes as 'cell differentiation'.