  Fusion of two deuterons of room temperature energy is studied. The nuclei are
in vacuum with no connection to any external source (electric or magnetic
field, illumination, surrounding matter, traps, etc.) which may accelerate
them. The energy of the two nuclei is conserved and remains small during the
motion through the Coulomb barrier. The penetration through this barrier, which
is the main obstacle for low-energy fusion, strongly depends on a form of the
incident flux on the Coulomb center at large distances from it. In contrast to
the usual scattering, the incident wave is not a single plane wave but the
certain superposition of plane waves of the same energy and various directions,
for example, a convergent conical wave. As a result of interference, the wave
function close to the Coulomb center is determined by a cusp caustic which is
probed by de Broglie waves. The particle flux gets away from the cusp and moves
to the Coulomb center providing a not small probability of fusion (cusp driven
tunneling). Getting away from a caustic cusp also occurs in optics and
acoustics.
