Air Force Research Laboratory ยท Kirtland Air Force Base
Matching a broad ion beam to a narrow inlet
When a large quadrupole feeds a small downstream aperture, geometric matching can be as important as the performance of either component alone.
The challenge
The output of a 19 mm quadrupole had to be transferred into the narrower inlet aperture and skimmers of a flow-tube instrument.
The engineering
Ardara developed a sequence of four tapered rectilinear quadrupole ion guides. Flat-faced rods and the inscribed aperture were larger upstream and narrowed downstream, while the ratio of rod size to inscribed radius was maintained along each guide.
With the same RF frequency and amplitude applied along the guide, the narrowing geometry increased the local confinement parameter toward the exit. The purpose was to reshape the beam to better match the receiving aperture.
The tradeoff
Geometric beam compression should not be confused with lossless phase-space compression or cooling. The design involved a wider axial-velocity distribution. That tradeoff was acceptable for this application because the downstream, higher-pressure flow tube was intended to thermalize ions and allow them to react.
The reported result
The engineering team reports substantially improved transmission into the flow tube. No numerical gain is claimed here.