  A prototype plastic scintillating fibre based beam profile monitor was tested
at the Heidelberg Ion Therapy Centre / \textit{Heidelberg Ionenstrahl
Therapiezentrum} (HIT) in 2016 to determine its beam property reconstruction
performance and the feasibility of further developing an expanded system. At
HIT protons, helium, carbon, and oxygen ions are available for therapy and
experiments. The beam can be scanned in two dimensions using fast deflection
magnets. A tracking system is used to monitor beam position and to adjust
scanner currents online.
  A new detector system with a finer granularity and without the drift time
delay of the current MWPC system with a similar amount of material along the
beamline would prove valuable in patient treatment. The sensitive detector
components in the tested prototype detector are double-clad Kuraray SCSF-78MJ
scintillating fibres with a diameter of 0.250~mm wound as a thin multi-layer
ribbon. The scintillation light is detected at the end of the ribbon with
Hamamatsu S11865-64 photodiode arrays with a pitch of 0.8~mm.
  The results shown in this paper include the linearity with respect to beam
intensity, the RMS of the beam intensity as measured by two planes, along with
the RMS of the mean position, and the measured beam width RMS. The
Signal-to-Noise ratio of the current system is also measured as an indicator of
potential performance. Additionally, the non-linear light yield of the
scintillating fibres as measured by the photodiode arrays is compared to two
models which describe the light yield as a function of the ion stopping power
and Lorentz $\beta$.
