Calibration system for satellite and rocket-borne ion mass spectrometers in the energy range from 5 eV/charge to 100 keV/charge.
The system has been developed for calibration of space plasma analyzers, in particular ion mass spectrometers. The system provides a large‐area (∼250 cm2), highly parallel (±0.5°), and spatially uniform (±5%) beam of ions over the energy per charge range from 5 eV/charge to 100 keV/charge. Other special features include variable energy spread from ΔE∼1 eV to ∼3 keV/charge and multiple charge state ions such as He2+ or Xe9+. Among several key ion optical elements are a high‐efficiency electron bombardment ion source capable of delivering ∼10^−8 A, a 90° crossed electric and magnetic field mass spectrometer designed to produce either a mixed or a mass‐selected beam, and a unique beam expansion system which produces the uniform large‐area beam. The system also includes automatic beam monitoring and control via a feedback loop, as well as provisions for semiautomatic control of angle and energy analysis. Use of the calibration system during its development phases has made possible the rapid calibration of five complex satellite ion mass spectrometers already flown. Data from one of these calibrations are discussed here.
The Figure shows a block diagram of the calibration sys- tem. To describe the system brieñy, ions are produced by an electron bombardment ion source and preaccelerated to an energy of 3 keV/charge. After passing through a mass/ charge analyzer which ñlters out undesired ion species, the beam cross section is expanded by a factor of 103. The ions are then accelerated or decelerated to their final energy (5- 105 eV/charge) before entering a drift tube that results in reduction of their angular divergence. The beam is then de- flected by high-frequency rastering to uniformly illuminate the target (i.e., the instrument sensor opening) which can be rotated about two perpendicular axes with respect to the beam. Beam current is monitored and automatically kept stabilized during the measurement period by means of a feedback circuit. Several diagnostic probes placed along the beam determine its essential characteristics such as ionic species, intensity profile, and absolute current. The clean UHV chamber operates at pressures 10^-8 Torr and the entire UHV system is placed in a laminar clean room that provides the proper environmental conditions for in- strument assembly and testing.
Test setup for LEIS (Low Energy Ion Source) with a CNT electron emitter. SSL Detector on y-stage, alpha-stage at +90°. This picture has been taken through the view port of the chamber.
In front left COPS, DFMS in the back right and RTOF is located in the vacuum chamber.