We have builtavacuumdoublecrystalspectrometer,whichcoupledtoanelectron-cyclotronresonance
ion source,allowsabsolutemeasurementsoflow-energyX-raytransitionsinhighlychargedionswith
accuracies oftheorderofafewpartspermillion.Wedescribeindetailtheinstrumentand
its performance.Furthermore,wepresentafewspectraof n ¼ 2-n ¼ 1 transitionsinAr14þ, Ar15þ
and Ar16þ.Wehavedevelopedan ab initio simulation codethatallowsustoobtainaccuratelineprofiles.
It canreproduceexperimentalspectrawithunprecedentedaccuracy.Thequalityoftheprofiles allows
the directdeterminationoflinewidth.