A high energy pulsed terahertz (THz) laser is studied experimentally. The laser cavity simply consists of a quartz glass waveguide, a coated GaAs input window, and a crystal quartz output window. NH₃ is filled in the cavity as gain medium, and pumped by an 8-J line-tunable transversely excited atmospheric (TEA) CO₂ laser. When 9R(16) transition acts as the pump line, 55.6-mJ THz radiation (90 \mu m) is obtained at 730-Pa NH₃ pressure. The corresponding conversion efficiency is 13.54%. Energy and optimal pressure of amplified spontaneous emission and laser oscillation are compared.