The deflector-pickup system: A novel approach to measure online the energy spread of DC ion beams

Abstract

A new quite flexible system is proposed for measuring the energy spread of low velocity dc ion beams. The online detection can be done for different charge/mass ratios and variable particle energies. The ion current can be as low as 500 nA. All hardware components pertaining to the proposed measurement system do not interfere with the normal beam line operations.

The major components of the system are a rf deflecting cavity and two rf pickups. By deflecting most of the beam to a limiting aperture a periodic pulse signal is created. With the pickups the voltage signal is measured at two different positions. The difference of the voltages which corresponds to a change of the pulse form is caused by the energy spread. The rms energy spread can be determined by reconstruction techniques from amplitude measurements of the Fourier components. The analysis is independent of the actual pulse shape and of pickup details.

For a beam with β = 3% or E = 420 keV/N a detailed description of the complete system is presented. A prototype 20 MHz deflection cavity was built, where 25 W average power gave rise to 5 kV peak deflecting voltage sufficient for He⁺. With commercially available pickups, positioned 2 m apart, a rms energy spread of ±3×10⁻³ can be detected. Two more ambitious designs are finally presented. One detects a rms energy spread of ±1×10⁻³. The other one is able to measure an energy spread of ±3×10⁻³ for particle velocities varying from 3% to 6%.