Phase Space Density Studies on Cyclotron Ion Sources

Using the MSU source testing facility, the phase space density of a cyclotron-type ion source has been computed from axial and radial emittance-area measurements at a variety of arc conditions and for a number of source geometries. Studies to date have all employed a 1.5 × 9 mm source output slit. Arc currents have been varied from 0.5 to 5 amps. Geometrical variations have thus far been principally concerned with explorations of the focusing effects of various source face shapes. Both axial and radial emittance areas from a normal flat source are found to have a considerable admixture of coherent motion which results in inefficient use of aperture and also, in combination with non-linear fields, can lead to an effective dilution of the phase space density. Moderate recessing of the source contributes a focusing force in both r and z and makes the phase space volume much more compact. The effects of plasma boundary and space charge are evidenced in the axial measurements. The axial plasma boundary is found to be concave, with the results that ions emitted from the top and bottom of the ion source slit are focused toward the median plane. Space charge effects are clearly discernable; the axial width of the beam linearly increases as a function of total beam current.