Low Energy Ion Source : 9783838105987 : 3838105982 : This PhD thesis describes the design, construction,and implementation of a Low Energy Ion Source (LEIS)for the ion mode calibration of the two ROSINA massspectrometers on board ESA's Rosetta spacecraft tocomet 67P/Churyumov-Gerasimenko.The low velocity of the ions in the vicinity of thecomet as well as the low relative speed between thespacecraft and the comet leads to ion energies oftypically less than 20 eV. In combination withspacecraft charging that occurs in such anenvironment the detection of cometary ions makes fora very challenging task to be performed by the twomass spectrometers. For the preparation of thesemeasurements LEIS is used in our lab to calibrateboth instruments in ion mode.We show the physics that are relevant for thedevelopment of a new design for this type of ionsources. As part of this thesis, a correspondingsource has been built and existing calibrationfacilities have been upgraded. We discuss thesensitivity,