  A compact helicon plasma thruster that features a cusp in its internal
magnetic field is analyzed with experiments and simulations. A compensated
Langmuir probe and a Faraday cup are used in the former, while a hybrid
PIC/fluid transport model combined with a frequency-domain electromagnetic
field model are used in the latter. Measurements serve to tune the anomalous
transport parameters of the model and overall show the same trends as the
numerical results, including a secondary peak of electron temperature
downstream in the magnetic nozzle, where electron cyclotron resonance
conditions for the 13.56 MHz excitation frequency are met. The cusp plays a
central role in determining the plasma losses to the walls and the profile of
electron temperature, which in turn defines the excitation and ionization
losses. While losses to the rear wall are reduced, losses to the lateral wall
are increased, which, together with the low production efficiency, limit the
performance of the device. Shorter chamber lengths and optimization of antenna
and cusp location are suggested as potential ways to improve performance.
