Adverse consequences of a moving vacuum-plasma boundary on axisymmetric ac helicity injection

The recent prediction of Liewer, Gould, and Bellan that a moving plasma-vacuum boundary significantly lowers the effectiveness of ac helicity injection is generalized by resolution of the apparent discrepancy between the helicity-conservation equations of Jensen and Chu and of Moffatt. It is shown that, if there are axisymmetric circular flux surfaces and a moving vacuum-plasma boundary, the helicity injected by oscillating fields (if net injection occurs) is simply consumed by an increase in helicity dissipation due to the same oscillating fields