Ion beam formation in a very low magnetic field expanding helicon discharge

An ion beam has been measured emerging from a low pressure (0.04 Pa) helicon plasma reactor over a narrow range of magnetic field values (1 mT< B₀ <3 mT). The presence of the ion beam occurs simultaneously with a large increase in the plasma density for the same applied magnetic field, produced using a single solenoid half the length of the m=1 rf antenna. The peak central plasma density of 1.5× 10¹⁷ m^-3 is measured to be almost 15 times larger than that occurring before or after the increase, and is associated with a steep axial density gradient which follows the gradient of the magnetic field. During this low magnetic field transition the antenna power transfer efficiency is measured to increase from less than 10% to 50%, suggesting some form of localized bulk electron heating in the source region associated with the helicon wave.