6.2.5 Extraction Lens

The ions from the conversion surface are accelerated in the extraction lens to higher energies allowing the use of conventional detection techniques for the particles. Due to this postacceleration the information about the primary energy of the particles would be lost. This is prevented by an extraction lens [4]. This lens is dispersive in energy space but focuses different angles and start locations at the conversion surface to one line at the exit aperture of the lens. Figure 6.6 depicts a SIMION simulation of the lens for particle energies right after reflection from the CS of 5eV to 75eV. As is clearly visible, particles scattered at different angles away from the conversion surface, but with the same energy, are approximately focused to the same location of the exit aperture of the lens. Particles with a lower energy are focused towards one edge of the exit aperture whereas particles with high energies are focused towards the other edge.

Figure: SIMION simulation of negative ion paths in the extraction lens. A situation with the conversion surface CS on ground potential is shown (for the actual neutral measurements in the NICE setup the voltages were reversed due to limitations of the TOF used). Neutral particles enter the lens from the left where they get negatively ionized at the CS. Ion paths for three different particle energies after reflection are shown (5eV, 25eV, and 75eV). Particles with different energies are mapped to different positions at the plane where the TOF would be attached. Slightly different angles between the CS and the particle velocity vector after ionization at the surface have only a minor influence on the position where the particles arrive at the TOF plane as shown in the inset. At angles closer to 90 \ensuremath{} the resulting position at the TOF plane is slightly shifted towards higher energies.

Results from earlier measurements with a position sensitive detector (PSD) mounted at the exit aperture instead of a TOF section are presented in Figure 6.7. As no mass resolution was available for the PSD measurements the different peaks had to be identified using the TOF measurements made for this work. The peak denoting the converted ions is shifted towards the low energy end of the exit aperture of the lens as the primary energy of the particles is lowered. The constant peak at the low energy end of the conversion surface is due to sputtered particles and secondary electrons from the conversion by TOF measurements. The peak at the high energy end is caused by secondary electrons emitted from one of the plates on the extraction lens. This peak is not visible in the TOF measurements because these electrons were suppressed by the TOF due to too short a time-of-flight.

Figure 6.7: Energy resolution of the extraction lens as measured some years ago using a CH \( ^{+}_{3}\) ion beam at 79 \ensuremath{} angle of incidence with respect to the conversion surface normal. Cuts across the exit aperture of the lens are shown. The arrow denotes the negative ions peak. The shaded area I is due to sputtered particles and secondary electrons from the conversion surface, the shades area II is due to secondary electrons emitted from parts of the lens close to the exit aperture. Peak II is not visible in the TOF measurements because electrons are suppressed. The intensity of the negative ions peak decreases towards lower energies due to the lower primary beam intensity at lower beam energies in the CASYMS ion source.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland