Summary form only given. The Naval Research Laboratory has developed a new type of plasma processing reactor called the Large-Area Plasma Processing System (LAPPS). This device uses a magnetically confined, sheet electron beam to produce planar plasmas with densities up to 5/spl times/10/sup 12/ cm/sup -3/, area /spl sim/1 m/sup 2/ and thickness /spl sim/1 cm. Other LAPPS attributes include: high uniformity; operation over a wide range of gas type and pressure; independent control of the ionization rate; efficient production of ions and free radicals; low metastable density; modest gas heating; a low and partially adjustable electron temperature; and independent bias control. Disadvantages of LAPPS include: the need for an energetic electron beam (a few keV and 10's mA/cm/sup 2/); the need for a longitudinal magnetic field (/spl sim/200 G) to confine the beam; cross-field restrictions on the plasma flow; beam-plasma instabilities; and energy losses to the beam dump. This talk will present the theory underlying LAPPS, while the experimental results and future plans will be presented elsewhere.