Allison scanner for high-current ion beam emittance measurements
A high-current ion beam emittance measurement unit Allison scanner was introduced.The scanner consists of two slits and a pair of electrical deflection plates driven by a ±1.2 kV bipolar power supply.When the scanner moves to a measurement position,the step-changed voltages are fed to the deflection plates.The divergence angle of the ions passing through the rear slit can be calculated from the voltages.The data of position,divergence angle and beam current are collected to the computer through an AD card.A program of measuring and data processing can handle the case of pulsed beam.In order to obtain correct emittance results,subtracting background and setting threshold value are important.The program can calculate the root-mean-square emittance,the beam size,the biggest positive and negative divergence angle and the boundary emittance.The root-mean-square emittance of a pulsed beam extracted from a 50 keV ECR proton source was measured,which took about 2.5 min.The average beam intensity was 4 mA with beam pulse of 1 ms at 166 Hz,and its normalized emittance was 0.27 πmm·mrad.