  The Ingot wavefront sensor is a novel pupil-plane wavefront sensor,
specifically designed to cope with the elongation typical of the extended
nature of the Laser Guide Star (LGS). In the framework of the ELT, we propose
an optical solution suitable for a Laser launch telescope, located outside the
telescope pupil. In this paper, we present the current optical design, based on
a reflective roof-shaped prism, which, at the level of the focal plane, splits
the light from an LGS producing three beams. The three images of the telescope
pupils can be then used for the retrieval of the first derivative of the
wavefront. The 3D nature of such a device requires new alignment techniques to
be determined theoretically and verified in the real world. A possible fully
automated procedure, relying solely on the illumination observed at the three
pupils, to align the prism to the image of the LGS is discussed. Careful
attention needs to be put both on the telecentricity of the system and on the
reference systems of the Ingot adjustments in the 3D space. This is crucial in
order to disentangle all the possible misalignment effects. In this context, we
devised a test-bench able to reproduce, in a scaled manner, the 3D illumination
that the Ingot will face at the ELT, in order to validate the design and to
perform preliminary tests of phase retrieval.
