"A radio pulsar is formed when the iron core of a massive star collapses in a supernova explosion. Due to their extreme compactness and supernuclear density radio pulsars are extremely stable rotators. Our goal is 1) to use radio pulsars to improve our limited understanding of some of the most energetic events known in the universe and the source of the elements that make up planets and all forms of life: supernovae 2) to find individual radio-pulsar systems that are such stable rotators that they can be used for the first-ever direct detection of gravitational waves or in the first ever strong-field test of gravity. We can do this with LOFAR, a revolutionary new radio telescope that host institute ASTRON is currently completing. LOFAR has more collecting by itself than all other radio telescopes in the world combined. In the course of his research in the US, van Leeuwen has acquired the radio-telescope survey experience, the instrument-development skills and the pulsar-population knowledge that will be needed for this project to push the boundary of our understanding of the universe. As the host institute is close to completing the most sensitive radio telescope in the world, the researcher will have unsurpassed means to accomplish the objectives laid out above. The host institute offers the researcher a tenure position in its scientific staff where he can be at the global forefront of telescope possibilities for the next decade. We expect to foster several intense decade-long EU-US co-operations between the host and the researcher's current US collaborators and Berkeley group to 1) adapt algorithms, monitor new discoveries and improve supernova modelling; and 2) design and build several leading-edge signal processing machines. For the next decade this project can define the global scientific and technological state-of-the-art within its field."