Observation of bifurcation properties of radial electric fields using a heavy ion beam probe

The bifurcated nature of the potential profile of a toroidal helical plasma is investigated in CHS using a heavy ion beam probe. The measurements reveal that three main branches of potential profile exist in ECR heated plasmas with a low density of n̄_e ≈ 0.5 × 10¹⁹ m^-3. The branches with higher central potential exhibit a rather strong radial electric field shear that should result in fluctuation reduction and formation of a transport barrier. An analysis of the pulsating behaviour in the potential suggests the existence of a fine structure in the potential profiles between the main branches. Lissajous and histogram expressions are useful in extracting the bifurcation characteristics of the potential structure.