A system has been developed specifically for the calibration and development of thermal ion instrumentation. The system is optimized to provide an extended, approximately 80‐cm2beam, with usable current rates ∼1 pA/cm2at beam energies as low as 1 eV, with much higher values available with increasing energy. The beam energy spread is typically less than 2 eV/charge and the average angular divergence is approximately 2.5°. A tandem electrostatic and variable geometry magnetic mirror configuration within the ion source optimizes the use of the ionizing electrons, thus decreasing the gas and nonthermal electron throughput to the instrument chamber while improving the current density uniformity. The system is integrated under microcomputer control to allow automatic control and monitoring of the beam energy and composition and the mass‐ and angle‐dependent response of the instrument under test. The data can be transmitted in nearly real time to the interested investigators for comparison with expected results over existing computer networks. The system is pumped by a combination of carbon vane and cryogenic sorption roughing pumps and ion and liquid‐helium operating pumps. This allows testing and final calibration of flight instrumentation in an ultraclean environment.