  Very Small Angle Neutron Scattering (VSANS) is an upgrade of the traditional
Small Angle Neutron Scattering (SANS) technique which can cover three orders of
magnitude of length scale from one nanometer to one micrometer. It is a
powerful tool for structure calibration in polymer science, biology, material
science and condensed matter physics. Since the first VSANS instrument, D11 in
Grenoble, was built in 1972, new collimation techniques, focusing optics
(multi-beam converging apertures, material or magnetic lenses, and focusing
mirrors) and higher resolution detectors combined with the long flight paths
and long incident neutron wavelengths have been developed. In this paper, a
detailed review is given of the development, principles and application
conditions of various VSANS techniques. Then, beam current gain factors are
calculated to evaluate those techniques. A VSANS design for the China
Spallation Neutron Source (CSNS) is thereby presented.
