Characteristics of Low-q(a) Disruptions in the Compact Toroidal Hybrid

\newcommand{\g}{\raisebox{-3pt}{$\mathchar'26\mkern-14mu$} $\iota$} Tokamak disruptions are dramatic events that lead to a sudden loss of plasma confinement. Disruptions that occur at low edge safety-factor, $q(a)$, limit the operation of tokamaks to $q(a)\ge2$. The Compact Toroidal Hybrid (CTH) is a torsatron-tokamak hybrid with a helical field coil and vertical field coils to establish a stellartor equilibrium, while an ohmic coil induces plasma current. A feature of the CTH device is the ability to adjust the vacuum rotational transform, \g$_{vac}$ (\g$=\frac{1}{q}$), by varying the ratio of current in the helical and toroidal field coils. The value of edge \g$_{vac}$ can be varied from about 0.02 to 0.3 ($q_{vac}(a)\ \sim$ 50 to 3.3). Plasma discharges in CTH are routinely observed to operate with $q(a)<2$, and in some cases as low as $q(a)\sim 1.1$. In CTH, low-q(a) disruptions are observed with a dominant m/n=3/2 precursor. The disruptivity of plasma discharges is over 80\% when \g$_{vac}(a)<0.04$ ($q_{vac}(a)< 25$) and as \g$_{vac}(a)$ is increased further, the disruptivity of the plasma discharges decreases. The disruptions are completely suppressed for \g$_{vac}(a)>0.07$ ($q_{vac}(a) \sim$14).