Designing RF Power Systems for Plasma Devices : Mark Carter (Ad Astra Rocket Co.)

RF power is used to create, heat, and control plasma in devices ranging from compact fluorescent light bulbs to semi-conductor chip reactors to plasma propulsion devices such as VASIMR, to thermonuclear fusion experiments such as ITER.  Successful operation requires that three basic conditions be satisfied simultaneously.  First, the required absorption conditions inside the plasma must exist to support the desired effect.  These effects vary widely depending on the application of the device and are constrained by the physics of RF-particle interactions in the plasma.  Second, RF power must efficiently couple from the RF launcher through the plasma to the desired absorption location. For frequencies below the microwave range, electromagnetic coupling across a vacuum layer inside which the desired wavelengths are cut off, can further complicate the launcher design, especially when static magnetic fields are present.  The plasma dielectric value for select frequencies and wavelengths can also become extremely high as the plasma density increases from almost vacuum conditions near the launching structure to those near the absorption location.  These transitional regions require careful, typically numerical analysis to make sure the power is delivered with the absorption requirements for the application. Third, the coupling circuit between the RF amplifier and the RF launcher must appropriately match the power between the RF amplifier and the load presented by the plasma.  Tuning this match can be complicated by transients in the plasma conditions that cause power to reflect back to the amplifier, which may be unable to tolerate the resulting standing waves for significant lengths of time. All three of these conditions must be kept in mind when designing and modeling the RF system.  For devices in which the plasma is both initiated and sustained by the RF alone, the design is further complicated by the change in dielectric properties as the system evolves from start-up conditions with no plasma present to its final state with plasma.  Self-consistent modeling of electromagnetic and plasma transport processes can be highly effective in assisting the design of the RF circuitry and the geometry of the launching structure.  VASIMR designs require additional special considerations, including very high efficiency, long lifetime, radiation hardening, and low weight   In this seminar, various general techniques for designing and modeling RF plasmas, including VASIMR, will be discussed.

Host: Prof. E. A. Bering (ext. 3-3543)

634 S&R 1 - 4PM