Stabilization of Rotational Modes in the PFRC-2 Using Quadrupole Magnetic Fields

The m=1 and m=2 rotational modes are the most prevalent macro-instabilities present in the PFRC-2 device, often distorting or moving the plasma column by several core radii. Here we report on the stabilizing effects of a weak quadrupole magnetic field and its dependence on the fill gas pressure, gas species, and axial magnetic field strength. We find that a quadrupole field effectively suppresses — for 10's of milliseconds — chaotic and periodic rotating plasmas of high-AMU gases with a threshold that increases with the axial field strength. The instability fundamental, typically ~5-15 kHz, shifts higher in frequency and shrinks in amplitude with an increasing quadrupole field. Light gases, most notably hydrogen, in otherwise-stable discharges often grow large rotational modes on millisecond timescales upon application of the quadrupole field. Plasma column stability is enhanced by a high fill pressure — an effect attributed to drag — thus lowering the threshold quadrupole field. In addition, interferometer measurements indicate that the quadrupole field increases the mean electron line density by up to ~10%, implying that ionization is enhanced or loss is reduced.