  A new 4-rod RFQ was commissioned at the UNILAC in 2009, it went into
operation in 2010. At high rf amplitudes strong modulations of the rf
reflection emerge. They are attributed to mechanical oscillations of the rods,
excited by the rf pulse. As these modulations could so far only be seen during
the rf pulse by means of rf measurements, a direct observation of the
mechanical vibrations was desirable. Such measurements have been conducted
using a commercial laser vibrometer, allowing the investigation of the
mechanical behavior of the RFQ in situ and independent of the presence of rf
power. Results from investigations under standard operation conditions confirm
the vibrations as the source of the modulations observed by rf as well as their
excitation by the rf pulse. Further measurements revealed more details about
the excitation process, leading to a better understanding and possibly new ways
of mitigation.
