Pitman, S. Apsimon, R. Burt, G. Catalán Lasheras, N. Grudiev, A. Owen, H.L. Wuensch, W. JACoW Publishing Geneva, Switzerland 2226-0366 978-3-95450-194-6 10.18429/JACoW-LINAC2018-THPO108 English 924-927 THPO108 Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-LINAC2018-THPO108 http://jacow.org/linac2018/papers/thpo108.pdf The PROBE project aims to develop a high gradient proton accelerator for protons with energy around 250-350 MeV for proton radiography. Detailed studies have shown that the optimum design is a side coupled cavity at S-band. With an aperture of 8 mm a gradient of 54 MV/m can be obtained with 13 MW of RF power in a 30 cm structure. A prototype cavity has been machined by VDL and diffusion bonded by Bodycote. We present initial measurements of the prototype.