Langmuir probe measurements in the intense RF field of a helicon discharge

Helicon discharges have extensively been studied for over 25 years both because of their intriguing physics and because of their utility in producing high plasma densities for industrial applications. Almost all measurements so far have been made away from the antenna region in the plasma ejected into a chamber where there may be a strong magnetic field (B-field) but where the radiofrequency (RF) field is much weaker than under the antenna. Inside the source region, the RF field distorts the current–voltage (I–V ) characteristic of the probe unless it is specially designed with strong RF compensation. For this purpose, a thin probe was designed and used to show the effect of inadequate compensation on electron temperature (Te) measurements. The subtraction of ion current from the I–V curve is essential; and, surprisingly, Langmuir’s orbital motion limited theory for ion current can be used well beyond its intended regime. (Some figures may appear in colour only in the online journal)