Ball pen probe in strongly magnetised RF plasmas

Abstract

A study of ball pen probes (BPPs) in a rf strongly magnetised plasma is reported for the first time. These probes have been successfully used in fusion plasmas, with magnetic fields up to 2.5 T, to measure the plasma potential. In this paper experimental results of various ball pen designs (2 and 4 mm diameter with flat and conical collectors) are presented up to 0.5 T in a low pressure capacitively coupled rf plasma. A theory of the BPP is developed, showing that the increase of the collector potential and plateau region, with collector retraction, requires the electron current to decrease faster than the ion current. Experimentally, it is found that to develop effective electron screening the electron Larmor radius should be smaller than the tunnel internal diameter. Smaller tunnels improve screening due to the tunnel entrance wall sheaths. Inside the tunnel a plateau region forms at 81 mT reducing to a broad peak at higher field strengths. Ion shielding and surface losses (for small tunnel diameters) reduce the collector peak width and maximum potential with increasing magnetic field. Conical collectors were found to increase the length of the plateau region and broaden the peak. Particle in cell simulations were in good agreement with the experimental results. The electron shielding and plateau regions were reproduced but not the broad peak at higher field strengths. Good agreement between both 2 mm BPPs and an emissive probe was found only at 81 mT to within 3 V or 1.3 electron temperatures (Te). For all BPPs at higher field strengths (≥ 250 mT) the maximum collector potential underestimated the emissive probe by more than 2.7 Te (7 V). At these field strengths all BPPs agree with each other to within 1.5 Te (4.1 V). Possible reasons for these disagreements are discussed.