  In this review paper on heavy ion inertial fusion (HIF), the state-of-the-art
scientific results are presented and discussed on the HIF physics, including
physics of the heavy ion beam (HIB) transport in a fusion reactor, the HIBs-ion
illumination on a direct-drive fuel target, the fuel target physics, the
uniformity of the HIF target implosion, the smoothing mechanisms of the target
implosion non- uniformity and the robust target implosion. The HIB has
remarkable preferable features to release the fusion energy in inertial fusion:
in particle accelerators HIBs are generated with a high driver efficiency of ~
30-40%, and the HIB ions deposit their energy inside of materials. Therefore, a
requirement for the fusion target energy gain is relatively low, that would be
~50-70 to operate a HIF fusion reactor with the standard energy output of 1GW
of electricity. The HIF reactor operation frequency would be ~10~15 Hz or so.
Several- MJ HIBs illuminate a fusion fuel target, and the fuel target is
imploded to about a thousand times of the solid density. Then the DT fuel is
ignited and burned. The HIB ion deposition range is defined by the HIB ions
stopping length, which would be ~1 mm or so depending on the material.
Therefore, a relatively large density-scale length appears in the fuel target
material. One of the critical issues in inertial fusion would be a spherically
uniform target compression, which would be degraded by a non-uniform implosion.
The implosion non-uniformity would be introduced by the Rayleigh-Taylor (R-T)
instability, and the large density-gradient-scale length helps to reduce the
R-T growth rate.
