Characterizing the Performance of a Cylindrical Electrostatic Analyzer for Measurements of Ion Beam Energy

The direct measurement of electric potential in the plasma interior remains a need on many magnetic confinement devices. Such measurements can advance understanding of the electric field and its role in particle transport. Traditional beam probes can fulfill the need by measuring the energy difference between injected and detected beams, yet the diagnostic can be large and expensive. We are developing a new energy analyzer that uses cylindrical annulus sector electrodes, rather than flat electrodes, which lowers the maximum electrode voltage required, thereby reducing the overall cost of the diagnostic. It combines two subsystems, one that measures the angle of the beam ions entering the analyzer, and one that measures their energy via deflection through an electric field. Determination of the detected beam energy using this measurement combination achieves the energy resolution, 0.01% of the injected beam energy, of traditional analyzers. We have designed and built a prototype of the analyzer, installed it on a beam teststand, and assessed its performance by injecting a singly charged ion beam with precisely known energy while varying the cylindrical electrode voltages and beam angle. The data is then analyzed by performing a multiple linear regression of the beam energy in terms of the deflection and angle measurements. The resulting empirical analyzer response will be used to infer the beam energy and plasma potential when the system is operated on a plasma device. Measured signals and results of our characterization will be presented.