A Three Dimensional Bunch Shape Monitor (3D-BSM) has been developed for the CERN Proton Linac 2. A new area for beam studies at high intensities has been opened by this detector. Bunch density distributions in all three dimensions and their variations along the beam pulse can be obtained. Changing field gradients in linac quadrupoles, emittance variation along the bunch has been calculated. Measurements of beam halos become possible thanks to the large dynamic range of the device. Beam parameters at various linac settings have been measured and analysed. Introduction The new detector [1] allows the measurement of the three dimensional density distribution I(x,y,z) of a bunch and its evolution along the beam pulse. For example, using this distribution the first and second moments and the beam profiles in each direction (x,y,z) have been obtained. The CERN Linac 2 is a high intensity accelerator consisting of an RFQ and three Alvarez tanks producing 140 mA of protons at 50 MeV [2]. As proven during the detector commissioning, the 100 μm tungsten wire can operate safely with pulse lengths up to 145 μs. The insertion of the target in the beam does not disturb injection into the downstream booster synchrotron. Therefore the 3D-BSM can be used as a nondestructive beam diagnostic tool during linac operation. Bunch Shape Measurements of the 50 MeV Beam In Fig.1 the evolution of the longitudinal profile along the beam pulse is presented. The analysis of the figure as well as of the evolution of other beam parameters along the bunch shows that beam-loading is well compensated in Linac 2. There is no variation either of the bunch centre or the bunch shape along the entire pulse length. It has been demonstrated that the bunch shape changes along the pulse if beam-loading is not sufficiently compensated. This can be seen in Fig. 2 where the RF field in tank 3 has been increased by 4% while keeping the maximum power to the tank constant. 0.0 22.9 45.8 68.8 91.7 0 10 20 30 40 50 60 70