Instruments - SHRIMP RG

SHRIMP RG (reverse geometry) uses a different ion optical design to the other SHRIMPs. Matsuda (1990) produced a set of mass spectrometer designs to minimize third-order aberrations through the use of reverse-geometry double-focussing mass spectrometers. In reverse geometry, the magnet precedes the electrostatic analyser in the ion optical pathway. In the SHRIMP RG design, the ion optics is further complicated by the use of four quadrupole lenses. Q1 and Q2 are housed in a chamber immediately before the magnet. This QQH chamber also includes a hexapole lens to allow shaping of the beam particularly as a response to imperfect shaping of the magnetic field. The Q3 lens operates in a similar fashion to the quadrupole lens in the forward geometry design of SHRIMP I and allows matching between the magnetic and electrostatic sectors. The Q4 lens acts mainly as a projection lens to throw the image to an appropriate point for the collector.

The original Matsuda designs were modified so that the “momentum” crossover after the magnet occurred before the ESA. This allows “energy” filtering on SHRIMP RG. The design of the SHRIMP RG yields approximately four times the mass resolution for the same slit sizes compared to SHRIMP II.

SHRIMP RG was completed in 1997 however it was apparent that the instrument was not performing as well as it should. The voltages on the electrostatic quadrupole lenses were very different to the theoretical voltages and good peak shapes could only be obtained by severely limiting the beam divergence. After lengthy examination of the input parameters it was found that the RG electrostatic lenses were clamping the beam too hard and were not consistent with the expectation of Matsuda’s design. Following the re-design of the lenses, the voltages for the quadupole lenses were close to theoretical and high mass resolution could be readily obtained.

SHRIMP RG at RSES