T5, T6 thrusters) radio frequency(RF) oscillation of an electric field induced by an alternating electromagnet, which results in a self-sustaining discharge and omits any cathode (RIT 10, RIT 22, µN-RIT thrusters) microwave heating (µ10, µ20) Related to the electrostatic ion production method is the need for a cathode and power supply requirements. Electron bombardment thrusters require at the least, power supplies to the cathode, anode and chamber. RF and microwave types require an additional power supply to the rf generator, but no anode or cathode power supplies. The positively charged ions diffuse towards the chamber's extraction system (2 or 3 multi-aperture grids). After ions enter the plasma sheath at a grid hole, they are accelerated by the potential difference between the first and second grids (called the screen and accelerator grids, respectively). The ions are guided through the extraction holes by the powerful electric field. The final ion energy is determined by the potential of the plasma, which generally is slightly greater than the screen grids' voltage. The negative voltage of the accelerator grid prevents electrons of the beam plasma outside the thruster from streaming back to the discharge plasma. This can fail due to insufficient negative potential in the grid, which is a common ending for ion thrusters' operational life. The expelled ions propel the spacecraft in the opposite direction, according to Newton's 3rd law. Lower-energy electrons are emitted from a separate cathode, called the neutralizer, into the ion beam to ensure that equal amounts of positive and negative charge are ejected. Neutralizing is needed to prevent the spacecraft from gaining a net negative charge, which would attract ions back toward the spacecraft and cancel the thrust. Performance Longevity The ion optics are constantly bombarded by a small amount of secondary ions and erode or wear away, thus reducing engine efficiency and life. Several techniques were used to reduce erosion; most notable was switching to a different propellant. Mercury or caesium atoms were used as propellants during tests in the 1960s and 1970s, but these propellants adhered to, and eroded the grids. Xenon atoms, on the other hand, are far less corrosive, and became the propellant of choice for virtually all ion thruster types. NASA has demonstrated continuous operation of NSTAR thruster for over 16,000 hours (1.8 years) and NEXT thruster for over 48,000 hours (5.5 years). In the extraction grid systems, minor differences occur in the grid geometry