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High speed flows with ion transport

Lupe
Lupe

Background

Gas flows transporting charged particles occur in different fields. Our focus are vacuum devices, where molecules are ionized to be manipulated and analyzed. An important application is mass spectrometry, which, in all its variations and extensions, is one of the work horses in chemical and biochemical research.
Ionization under atmospheric conditions, as in electrosprays, allows ionization of molecules which would be destroyed by other, more direct methods.


In such devices, particles, such as complex biochemical molecules, are typically ionized under atmospheric conditions and transferred to low pressure conditions, whereby the gas is removed and intact ions of theses molecules in vacuum are obtained. This is the prerequisite for many accurate and detailed investigation methods.

The vacuum transfer inevitably yields high speed flows over high-pressure range, i.e. high speed sonic and supersonic flows. These take place in complex geometries dictated by the need to create special, appropriate electric fields to guide the ions, i.e. ion optics.

Research

We investigate vacuum interfaces mainly by numerical simulations and comparison with our experimental partners. Due to the high losses in this first stage, an improvement is pivotal for increasing the sensitivity of the full facility.

This is achieved by simulating the flows with sonic up to highly supersonic speeds and, in part, strong heating. We also incorporate the complex geometries of the ion optics. On this base we simulate particle transport of a large number of charged and interacting ions in the complex and time dynamical electric field. We can thus predict essential properties of such devices and compare them with existing experiments. This research aims at improving these devices by a better understanding of the set-up, which has, despite its importance, only received little attention. Two main parts are studied:

1.) Transfer capillary

The atmospheric part is often connected to a first pumping stage or low pressure chamber by a capillary, a straight tube of typically less than a millimetre diameter, with a length of a few centimeters or more. Often, high losses of ions are observed within the capillary.

The heating yields a thermal choking, which strongly influences the overall flow field [1]. A simple, one-dimensional investigation already reproduces very well experimental findings.
In a more detailed investigation, these findings where mainly confirmed. The flow is accelerated to sonic and even slightly supersonic conditions. A strong heating, as applied in the experiment, creates a thick thermal boundary layer and nontrivial flow profile [2]. A simple estimate based on space charge expansion of the ion cloud and the transport by the gas flow along the axis was found to describe fairly well many of the aspects. While the gas flow is mainly independent of the capillary length, the ion transfer strongly drops with the capillary length. The simulation is in line with the available experimental measurements of ion transfer.

2.) Ion funnel

The capillary ends in the first pumping stage. Here, due to a much lower pressure of a few millibar, a highly supersonic jet (highly underexpanded) forms, often terminated by a Mach disk. Complex ion optics like the ion funnel, a stack of disks with progressively reducing inner diameter, is used to guide the ions, while the gas can pass between the disks. The gas flow is influenced not only by the pressure difference and the capillary diameter, but also by the geometry of the ion funnel. Different flow patterns can be observed. These are visible in the videos as intermediate states until the near steady flow establishes. For different set-ups a mach disk, i.e. straight shock, or reflected, oblique shocks can occure. To simulate the ion transport, the complex electric fields of the ion optics need to be incorporated. A detailed publication is in preparation.

Video of 2D ion funnel simulation

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