  Wavefront engineering is an important quantum technology. Here, we
demonstrate the design and production of a robust C-shaped and orbital angular
momentum (OAM) carrying beam in which the doughnut shaped structure contains an
adjustable gap. We find that the presence of the vortex line in the core of the
beam is crucial for the robustness of the C-shape against beam propagation. The
topological charge of the vortex core controls mainly the size of the C, while
its opening angle is controlled by the presence of vortex-anti-vortex loops. We
demonstrate the generation and characterisation of C-shaped electron vortex
beams, although the result is equally applicable to other quantum waves.
Applications of C-shaped vortex beams include lithography, dynamical atom
sorting and atomtronics.
