  We present the design and science goals of SPT-3G+, a new camera for the
South Pole Telescope, which will consist of a dense array of 34100 kinetic
inductance detectors measuring the cosmic microwave background (CMB) at 220
GHz, 285 GHz, and 345 GHz. The SPT-3G+ dataset will enable new constraints on
the process of reionization, including measurements of the patchy kinematic
Sunyaev-Zeldovich effect and improved constraints on the optical depth due to
reionization. At the same time, it will serve as a pathfinder for the detection
of Rayleigh scattering, which could allow future CMB surveys to constrain
cosmological parameters better than from the primary CMB alone. In addition,
the combined, multi-band SPT-3G and SPT-3G+ survey data will have several
synergies that enhance the original SPT-3G survey, including: extending the
redshift-reach of SZ cluster surveys to $z > 2$; understanding the relationship
between magnetic fields and star formation in our Galaxy; improved
characterization of the impact of dust on inflationary B-mode searches; and
characterizing astrophysical transients at the boundary between mm and sub-mm
wavelengths. Finally, the modular design of the SPT-3G+ camera allows it to
serve as an on-sky demonstrator for new detector technologies employing
microwave readout, such as the on-chip spectrometers that we expect to deploy
during the SPT-3G+ survey. In this paper, we describe the science goals of the
project and the key technology developments that enable its powerful yet
compact design.
