CLOSED SUPERSTRINGS IN MAGNETIC-FLUX TUBE BACKGROUND

We consider a solvable closed superstring model corresponding to the K aluza-Klein Melvin magnetic flux tube background. The model describes superstring moving in a flat but non-trivial 10-dimensional space-time containing a 3-dimensional factor which is a 'twisted' product of a 2 -plane and an internal circle. The spectrum of string states has simil arities with the Landau spectrum for a charged particle in a uniform m agnetic field. The presence of spin-dependent 'gyromagnetic' interacti on implies breaking of supersymmetry and possible existence of tachyon ic instabilities. We also discuss the compact analogue of the Melvin m odel based on 'twisted' SU(2) x U(1) WZW model and its possible relati on to models of spontaneous supersymmetry breaking.