  A concept of SUb-atmospheric Radio-frequency Engine (SURE) designed for near
space environment is reported. The antenna wrapping quartz tube consists of two
solenoid coils with variable separation distance, and is driven by
radio-frequency power supply (13.56~MHz-1~kW). The discharge involves inductive
coupling under each solenoid coil and capacitive coupling between them. This
novel scheme can ionize the filling air efficiently for the entire pressure
range of 32\sim 5332 Pa in near space. The formed plasma density and
temperature are up to 2.23\times 10^{18}~m^{-3} and 2.79 eV, respectively. The
influences of separation distance, input power, filling pressure and the number
of solenoid turns on discharge are presented in detail. This air-breathing
electric propulsion system has no plasma-facing electrode and does not require
external magnetic field, and is thereby durable and structurally compact and
light.
