Stability and End-Biasing in the PFRC-2

The RF-heated Princeton Field Reversed Configuration (PFRC-2) plasma exhibits a variety of macro-instabilities, including m=1, m=2, and chaotic azimuthal modes. The application of weak quadrupole magnetic fields effectively suppresses the instabilities for 10’s of milliseconds, but only in high-AMU gases such as argon. The large-amplitude m=1 and m=2 modes often present in hydrogen and helium plasmas are instead enhanced and even triggered from quiescent plasmas by the quadrupole field. Moreover, non-axisymmetric fields open field lines which degrade confinement, meaning that future reactor devices likely cannot rely on this method for stabilization. We find that fast gas puffing into unstable plasmas causes a tenfold reduction in instability amplitude, consistent with ion momentum loss through charge exchange. Moreover, gas puffing into plasma that decays in density during the RF pulse restores the electron density to its initial value, provided that the plasma is radially constrained by LN2-cooled flux conservers. Recently, a pair of concentric five-electrode end-bias rings were installed in the PFRC-2 to enable measurement and control of the radial electric field structure in the scrape-off layer. Herein we report the dependence of the electric field structure on a variety of machine parameters including gas species, neutral fill pressure, and axial magnetic field strength. We find that in a stable plasma the radial electric field is typically directed outward, which is inconsistent with the hypothesis that a balance between ion diamagnetic and E×B drifts prevents plasma spin-up, at least within the scrape-off layer. At the onset of an instability, however, the electric field reverses direction, indicating that the field plays a fundamental role in how instability develops. In addition, the stability properties of the plasma under active end-biasing and future work are discussed.