  Plastic tuning of multicell superconducting accelerating cavities is crucial
in the development cycle of cavities for particle accelerators. Cavities must
meet stringent requirements regarding the operating mode frequency, field
flatness, and eccentricity before lining them up in a cryomodule string. After
dressing bare cavities with helium vessels, the welded vessel prevents access
to individual cavity cells disallowing any further localized tuning. Currently,
there is no straightforward way to tune dressed cavities other than cutting the
vessel and then tuning the bare cavity and dressing it back, which would
significantly impact cost and schedule. In this paper, we present a novel
tuning technique for already jacketed cavities that is non-invasive and
cost-effective. The proposed scheme employs pressurized balloons to be
temporarily deployed inside the cavity as a means to localize mechanical
deformation in specific cells. The proposed tuning technique was successfully
utilized to recover a 9-cell 1.3 GHz tesla-style cavity.
