The dynamic range extension system for the LHAASO-WCDA experiment

The Large High Altitude Air Shower Observatory (LHAASO) will be constructed at Mountain Haizi, in Sichuan Province, China (4410m a.s.l.). The Water Cherenkov Detector Array (WCDA), one of the major component of the LHAASO, will focus on surveying the northern sky for gamma ray sources from 100 GeV to 30 TeV. The whole WCDA, owing an area of 78,000 m^{2}, is subdividing into 3120 detector cells (5×5 m^{2} each, with a water depth of 4m). Every cell has an 8 or 9-in PMT residing at the bottom to collect Cherenkov lights produced by the charged secondary particles of air showers. To observe high-energy shower events, especially to measure precisely the shower cores of events at the energy range from 100 TeV to 10 PeV for an analysis of the cosmic ray spectrum, as a dynamic range extension system, another sets of small PMTs (1-2 inch) will be added to 900 cells of the WCDA array. In this talk, the design and status of this dynamic range extension system is presented.