  We have performed inelastic neutron scattering on the near ideal spin-ladder
compound La4Sr10Cu24O41, which will serve as a model system for the
investigation of doped ladders. A key feature in the analysis was the
separation of one-triplon and two-triplon scattering contributions due to an
antiphase rung modulation. This enabled extraction of the one-triplon
dispersion which has a spin gap of 26.4+/-0.3 meV, and for the first time
two-triplon scattering is clearly observed. The exchange constants are
determined by fitting the data to be Jleg=186 meV and Jrung=124 meV along the
leg and the rung respectively. In addition to the surprisingly large difference
between Jleg and Jrung a substantial ring exchange of Jcyc=31 meV was
confirmed.
