  In this white paper, we present the MegaMapper concept. The MegaMapper is a
proposed ground-based experiment to measure Inflation parameters and Dark
Energy from galaxy redshifts at $2<z<5$. In order to achieve path-breaking
results with a mid-scale investment, the MegaMapper combines existing
technologies for critical path elements and pushes innovative development in
other design areas. To this aim, we envision a 6.5-m Magellan-like telescope,
with a newly designed wide field, coupled with DESI spectrographs, and
small-pitch robots to achieve multiplexing of at least 26,000. This will match
the expected achievable target density in the redshift range of interest and
provide a 10x capability over the existing state-of the art, without a 10x
increase in project budget.
