Ion energies from a permanent-magnet helicon thruster

Summary form only given. A small helicon discharge¹ 5 cm in diam and 5 cm high with a DC magnetic field from a neodymium (NdFeB) magnet has been studied for use as an ion thruster. The antenna is a single loop of 1/8-inch copper tubing. Normal operating conditions are with 400W of 27.12 MHz RF power and 15 mTorr of argon, but powers of up to 1.8 kW and pressures down to 0.5 mTorr have been tested. The discharge ejects plasma downwards into a large chamber, and the retarding-field ion energy distribution function RFID is measured at various distances downstream. As expected, the RFID peaks at the normal sheath drop of ~5KTe, or about 10V, increasing slightly with power. No double-layer is seen because the RFID sensor is a large disk that interferes with it. Applying a positive bias to the discharge increases the ion energy, especially at low pressure, when it reaches 25V. A commercially available magnet 2 × 4 × 0.5 inches can be used in a compact configuration.