  We study the ground state phase diagram of a frustrated spin tube in a strong
external magnetic field. This model can be viewed as two coupled zigzag spin
chains, or as a two-leg spin ladder with frustrating next-nearest-neighbor
couplings along the legs, and its study is motivated by the physics of such
materials as Sulfolane-Cu_{2}Cl_{4} and BiCu_{2}PO_{6}. In magnetic fields
right below the saturation, the system can be effectively represented as a
dilute gas of two species of bosonic quasiparticles that correspond to magnons
with inequivalent incommensurate momenta at two degenerate minima of the magnon
dispersion. Using the method previously proposed and tested for frustrated spin
chains, we calculate effective interactions in this two-component Bose gas. On
this basis, we establish the phase diagram of nearly-saturated frustrated spin
tube, which is shown to include the two-component Luttinger liquid, two types
of vector chiral phases, and phases whose physics is determined by the presence
of bound magnons. We study the phase diagram of the model numerically by means
of the density matrix renormalization group technique, and find a good
agreement with our analytical predictions.
