Characterization of a neutral calcium plasma

Neutral calcium plasma is particularly interesting for studies of ion wave dynamics. Apart from the Barium Q-machine, laser induced fluorescence (LIF) measurements in most contexts rely on excitation from metastable states.  However, under many circumstances involving inhomogeneous plasma or large perturbations, the metastable distribution function may be very different from the total ion distribution and may even be closer to the neutral atom velocity distribution.  Achieving ground-state LIF in most plasmas (e.g.  He, Ar) requires pump laser frequencies on the order of petahertz, which is currently unfeasible.  In calcium, this can be achieved with a tunable diode laser operating at several hundred terahertz by pumping the 3p⁶4s ²S_1/2 to 3p⁶4p ²P⁰_3/2 transition (397nm) and observing emission as the ion deexcites to the 3p⁶3d ²D_5/2 state (854 nm). Ionized calcium is produced by passing a neutral calcium beam through a cylindrical cavity where the TE₁₁₁ mode is excited.  Ionization efficiency is increased by external permanent magnets tuned for electron cyclotron resonance (ECR) at the cavity mode frequency. The plasma passes into the Iowa Multi-dipole Experiment (IME) where it is trapped by rails of dipole magnets on the walls, but left unmagnetized in the region of measurements (~2 Gauss). This novel calcium plasma source is described and characterized using ground-state LIF measurements for varying neutral beam flux, ECR magnetic field, and RF power.