Orbit Stability in a Helically Perturbed Magnetic Field

The stability of particle orbits in resonant helices (``corkscrews'') is investigated for axial acceleration and deceleration of the injected particles. It is shown that deviations from the unperturbed orbit lead to growing oscillations about that orbit for a decelerating corkscrew. The growth rate is small in devices suitable for injection into mirror fields. The numerical results are in agreement with experimental observations of injection parameters and output properties. Deviations from the equilibrium orbit in an accelerating corkscrew lead to damped oscillations.