Applications of Free Fields in 2D Current Algebra
  In this thesis steps are taken in the direction of formulating non-critical strings in the framework of the $G/G$ approach. A major part of the thesis is concerned with conformal field theory based on affine $SL(2)$ current algebra, in particular for admissible representations which are relevant in the $SL(2)/SL(2)$ description of coupling conformal minimal matter to 2D gravity. By virtue of fractional calculus, free field realizations are made applicable of producing integral representations of chiral blocks, even in the case of admissible representations where ghost fields raised to fractional powers are inherent. The famous work on minimal models by Dotsenko and Fateev is then generalised, working out monodromy invariant 4-point Greens functions and the operator algebra coefficients. Furthermore, the fusion rules are re-derived within the context of free field realizations by introducing the notion of over-screening. For higher groups an explicit Wakimoto free field realization is presented of affine current algebras based on simple Lie groups. Furthermore, a study is undertaken of primary fields in the framework of extra $x$ variables, and screening currents of both kinds are discussed. Several new and definite results are presented. A quantum group structure relying on both kinds of screening currents is discussed along the lines of Gomez and Sierra. 
J. Rasmussen