Experimental investigation of a conical helicon double layer thruster arrangement

A conical pyrex plasma source with a conical helicon antenna surrounded by two cylindrically wound solenoids has been used in a helicon double layer thruster (HDLT) arrangement with the aim of investigating a new plasma cavity geometry and imposing a misalignment between the geometric and magnetic axis. An ion beam is measured with an energy analyser placed 6 cm downstream of the source exit for a low pressure argon plasma (similar to 0.4 mTorr) for various currents in the two solenoids. Operation with current in the exhaust solenoid only also shows the presence of the ion beam. For a given radiofrequency power and similar total solenoidal current, the ion beam current measured downstream of the conical HDLT is about seven times greater than that previously measured downstream of the cylindrical HDLT. This may result from a larger plasma density in the source and from a source peak density shift by about 10 cm towards the tube exit measured in the conical arrangement compared with previous observations in the cylindrical arrangement. When an angle between the geometric and magnetic axis of up to 9 degrees is mechanically imposed, the ion beam is detected for angles less than 5 degrees.