A device for separating high-mass ions (having cyclotron frequency .OMEGA..sub.h) from low-mass ions (having cyclotron frequency .OMEGA..sub.l) in a plasma includes a chamber. Coils are provided to generate a substantially uniform magnetic field in the chamber. An antenna is provided to launch a left-hand elliptically polarized electromagnetic wave into the chamber along the stationary magnetic field that is evanescent in the multi-species plasma. Importantly, the E vector of the elliptically polarized electromagnetic wave rotates at a frequency, .omega., where .OMEGA..sub.h <.omega.<.OMEGA..sub.l. Ponderomotive forces are generated by the electromagnetic wave that cause the low-mass ions to move toward the antenna while causing the high-mass ions to move away from the antenna.
There is disclosed a method of selectively extracting ions comprising the steps of:providing a supply of ions in a body of gas;generating a ponderomotive ion trapping potential generally along an axis;generating further potentials to provide an effective potential which prevents ions from being extracted from an extraction region;trapping ions in said effective potential; andselectively extracting ions of a predetermined m/z ratio or ion mobility from the extraction region;in which the characteristics of the effective potential which prevent ions from being extracted from the extraction region are caused, at least in part, by the generation of the ponderomotive ion trapping potential.
A plasma potential measuring method is conducted by: providing a measurement space surrounded by a radio-frequency electric field in plasma atmosphere; varying a floating potential at an electrode located in the measurement space by the ponderomotive effect acted only on electrons; and determining as a plasma potential a value of the floating potential at the time when an ion current flown into the electrode begins to lower.
