1. Technical Field
The present invention relates to a charged-particle transport device in which a system of electrodes is installed in that is designed to transversally focus the charged particles. More specifically, the present invention relates to forming the required transverse fields by electrode strips on flat plates arranged in a substantially parallel configuration, and variations and applications thereof.
2. Related Art
In the related art, charged particles are transported in vacuum or in a buffer gas with transverse forces caused by electric multipole fields formed by rod-like electrodes that are arranged parallel to and around the particle beam. Such fields can be DC-field, i.e. constant direct current fields, or RF-fields, i.e. quickly varying high-frequency fields. Such fields are important for many applications in which the transport of charged particles is required over reasonably long distances without intensity losses. Generally these applications can be characterized by the pressure of the buffer gas in the transport region or the residual gas pressure in a vacuum.
Related art particle transport schemes include:                1. The transport of charged particles in a region in which the buffer gas has pressures from several mbar to several bar. As discussed in [G. Eiceman, “Ion Mobility Spectrometry”, CRC-Press, Boca Raton, 2006] one can house in such a region:                    1.1. an ion-mobility spectrometer (IMS) or            1.2. a differential mobility spectrometer (DMS) also known as FAIMS.                        2. The transport of charged particles in a region in which the buffer gas has pressures from below μbar to several mbar. In such a region one can house:                    2.1 a “beam-cooler” in which ions collide with atoms or molecules of the buffer gas and in this process distribute kinetic energy to them. Thus the ions will be “cooled” and the phase-space the ion beam occupies be reduced.            2.2 a “collision cell” in which molecular ions will be broken into fragments by collisions with atoms or molecules of the buffer gas.            2.3 a “transfer line” in which ions can be transported from a high pressure region to a low pressure region or vice versa.                        3. The transport of charged particles in a region in which the residual gas pressure is lower than about one μbar. In such a region one can house                    3.1 a beam transport channel as in particle accelerators or particle-beam guidance systems or            3.2 a mass spectrometer built from a sector field, an RF-quadrupole or an energy-isochronous time-of-flight system [P. H. Dawson, “Quadrupole Mass Spectrometry and its Application”, Elsevier, Amsterdam 1976]                        
In the foregoing examples, it is important that substantially all (or at least a large portion of) the initially existing charged particles arrive at the end of the transport line. In the related art, this can be achieved by using one or a number of lenses that repeatedly refocus the charged particle beam along the transport line with a curvilinear beam axis Z.
Assuming Cartesian X,Y coordinates perpendicular to this beam axis, rotationally symmetric electric or magnetic lenses can be used to simultaneously focus the charged particles towards the beam axis in X- and in Y-direction. However, magnetic or electric quadrupole lenses, i.e. 4-poles, can also be used advantageously to focus the charged particles in either the X- or the Y-direction and defocus them in the other direction, as discussed in [H. Wollnik, “Optics of Charged Particles”, Acad. Press, Orlando, 1987].
In some cases, magnetic or electric hexapoles or octupoles, i.e. 6-poles or 8-poles, can be used that exhibit nonlinear forces that drive the charged particles towards or away from the beam axis. In all 4-, 6-, or 8-pole devices—usually referred to as multipoles—the overall action on the charged particles is such that the charged particles are driven towards the beam axis both in X- and in Y-directions. This overall action occurs because the charged particles experience overall larger forces towards rather than away from the beam axis in each multipole. The reason is that these forces increase with the distance from the axis when they pass through a multipole, and since the beam diameters are always larger when the particles experience forces towards the beam axis, while the beam diameters are smaller when the particles experience forces away from the beam axis.
Although beams of charged particles can be transported efficiently by separated short 4-, 6-, and 8-pole devices, it is also possible to use a longer single device if one applies high-frequency RF-potentials to its electrodes [P. H. Dawson, “Quadrupole Mass Spectrometry and its Application”, Elsevier, Amsterdam 1976]. In this case, an ion will experience similar focusing and defocusing forces during its passage through the RF-multipole.
In the related art, electric multipoles are formed by 2N=4, 6, 8 . . . rod-like electrodes arranged parallel to and around the ion beam axis at equal azimuthal intervals Δθ=π/N in which case one applies to the even-numbered electrodes, the potential +VN0, and to the odd-numbered electrodes the potential −VN0. The most common multipoles are quadrupoles that have 2N=4 poles the geometry, as illustrated in FIG. 1, characterized by 4 electrodes arranged at azimuthal intervals of π/2 around an aperture of diameter 2G0.
In cylindrical R,θ,Z-coordinates, the potential VN(R,θ) in a 2N-pole is independent of Z, and can be described as:VN(R,θ)=VN0(R/G0)N*cos [N(θ−Φ)]  (1)Here, θ is the azimuthal angle around the Z-axis, and Φ is the azimuthal angle by which the arrangement of the 2N electrodes has been rotated relative to the θ=0 plane, i.e. the XZ-plane in FIG. 1. However, the exact potential distribution of Eq. (1) is usually only approximated when realistic electrodes are used that have finite shape tolerances.
In case of a classical electric 2N-pole with N=2, as is shown in FIG. 1, the potential distribution V2(R, θ−Φ) of Eq. (1) is approximated by applying potentials +V20, −V20, +V20, −V20 to electrodes with apices at θ−Φ=0, π/2, π, 3π/4, and with θ being the azimuthal angle. In order to represent V2(R, θ−Φ) of Eq. (1) exactly, the electrodes must be hyperbolically shaped in the XY-plane. Due to the symmetry of the electrodes, the potential distribution V(R, θ−Φ) has a 4-fold symmetry with Φ being the angle by which the electrode arrangement has been rotated relative to θ=0 as is illustrated in FIG. 1 for the cases Φ=0 and Φ=π/4. The potential vanishes along the dashed lines in FIG. 1 while the field E2=gradV2(G0) is constant along the so-called aperture circle of diameter 2G0. The resulting forces on charged particles thus have a constant magnitude along this circle but change direction as indicated by the small arrows in FIG. 1.