Plasma Production Using Radiofrequency Fields Near Or Below The Ion-Cyclotron Range Of Frequencies

The use of radiofrequency (RF) fields in the megahertz range to produce the initial plasmas for nuclear fusion experiments can provide a broader operating regime for many stellarator devices and can result in a saving of volt-seconds in tokamaks. This process has been used for many years in various devices at the Institute of Physics and Technology at Khar'kov. The initial breakdown of neutral gas by non-resonant RF near-fields can be understood in terms of RF electric fields parallel to the static magnetic field and a ponderomotive trap with a long scale length parallel to the static magnetic field. If the parallel fields are not too strong for a given antenna geometry, the concept of a ponderomotive trap is valid and the trapped electrons can oscillate in the near-field region for many RF periods. Efficient breakdown can begin if the oscillation energy of these trapped electrons is sufficient to ionize the gas. Good agreement between the experimental observations and an analytic model for the early phase of the breakdown process is presented.