Ion Cyclotron Resonance Spectrometry
Ion Cyclotron Resonance Spectrometry (Lecture Notes in Chemistry, 7)
[54] METHOD AND APPARATUS FOR THE
ACCUMULATION OF IONS IN A TRAP OF AN ION CYCLOTRON RESONANCE SPECTROMETER, BY TRANSFERRING THE KINETIC ENERGY OF THE MOTION PARALLEL TO THE MAGNETIC FIELD INTO DIRECTIONS PERPENDICULAR TO THE MAGNETIC FIELD [75] Inventor:
Pablo Caravatti, Winterthur, Switzerland
[73] Assignee: Spectrospin AG, Switzerland
[21] Appl. No.: 251,192
[22] Filed: Sep. 29,1988
[30] Foreign Application Priority Data
Fast Wave Resonance Near the Ion Cyclotron Frequency A Dissertation in Electrical Engineering Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
Oct. 7, 1987 [DE] Fed. Rep. of Germany 3733853
[51] Int. CI.5 H01J 49/36
[52] U.S. Q 250/290; 250/291
[58] Field of Search 250/290, 291, 293, 281,
250/282, 288
Ion Cyclotron Resonance Spectrometry II (Lecture Notes in Chemistry)
A semiannual status report on the study of the multi-ion, multi-event test of ion cyclotron resonance heating, reporting period, May 8, 1993 - August 30, 1993 (SuDoc NAS 1.26:194134)
U.S. PATENT DOCUMENTS
3,502,867 3/1970 Beauchamp 250/290
Ion cyclotron resonance spectrometry II
3,922,543 11/1975 Beauchamp 250/290
The effect of finite ion and electron temperatures on the ion cyclotron resonance instability (A.E.R.E. reports;no.CLM;R32)
3,984,681 10/1976 Fletcher et al 250/290
Multi-species test of ion cyclotron resonance heating at high altitudes (SuDoc NAS 1.26:206145)
4,563,579 1/1986 Kellerhals et al 250/291
Electron Cyclotron Resonance Ion Sources: The Basis, Developments and the State of the Art
4,581,533 4/1986 Littlejohn et al 250/282
Electron Cyclotron Resonance Ion Sources and ECR Plasmas
4,686,365 8/1987 Meek et al 250/281
4,739,165 4/1988 Ghaderi et al 250/290
4,746,802 5/1988 Kellerhals 250/291
4,761,545 8/1988 Marshall et al 250/291
4,818,864 4/1989 Alleman 250/291
Ion cyclotron wave growth calculated from satellite observations of the proton ring current during storm recovery: By J.A. Joselyn and L.R. Lyons, Space ... Research Laboratories (SEL preprint)
Method and apparatus for introducing ions into an ion trap of an ion cyclotron resonance spectrometer, the ion trap being arranged in a constant homogeneous magnetic field and comprising walls which are designed as electrodes and extend in parallel and/or perpendicularly to a symmetry axis having the direction of the magnetic field and which are supplied with electric trapping potentials retaining the ions in the ion trap, one of the walls which extend perpendicularly to the magnetic field being provided with a hole, the method including the steps of generating the ions outside the ion trap, forming the ions into an ion beam, directing the ion beam upon the hole in the one wall of the ion trap, in the direction of the magnetic field, and reducing thereafter the velocity component, in the direction of the magnetic field, of the ions which have passed the hole and entered the ion trap, below the value determined by the trapping potentials which is needed by the ions for leaving the ion trap, characterized in that the ions that have entered the ion trap are imparted a second motion component in a direction perpendicular to the magnetic field, such that the magnitude of the vector sum of the two ion velocity components remains the same.
11 Claims, 1 Drawing Sheet
130
109
METHOD AND APPARATUS FOR THE ACCUMULATION OF IONS IN A TRAP OF AN
ION CYCLOTRON RESONANCE
SPECTROMETER, BY TRANSFERRING THE 5
KINETIC ENERGY OF THE MOTION PARALLEL
TO THE MAGNETIC FIELD INTO DIRECTIONS
PERPENDICULAR TO THE MAGNETIC FIELD
The present invention relates to a method for intro- 10 ducing ions into the ion trap of an ion cyclotron resonance spectrometer, the ion trap being arranged in a constant homogenous magnetic field and comprising walls which are designed as electrodes and extend in parallel and/or perpendicularly to the direction of the 15 magnetic field and which are supplied with electric trapping potentials retaining the ions in the ion trap, one of the walls which extends perpendicularly to the magnetic field being provided with a hole, the method including the steps of generating the ions outside the ion 20 trap, forming the ions into an ion beam, directing the ion beam upon the hole in the one wall of the ion trap, in the direction of the magnetic field, and reducing thereafter the velocity at which the ions which have passed the hole and entered the ion trap move in the direction of 25 the magnetic field, below the value determined by the trapping potentials which is needed by the ions for leaving the ion trap.
A method of this kind has been known before from DE-OS 35 15 766. The known method has two variants. 30 One of these variants consists in increasing temporarily the gas pressure in the ion trap for reducing the velocity of the ions that have entered the ion trap, in order to slow down the ions. This variant requires that gas has to be pumped off after the ions have been shot in, which 35 not only extends the process time but may lead also to ion losses and fragmentation of the ions.
According to other variants, the velocity of the ions is reduced by a decelerating electrode arranged upstream of the ion trap, and at the same time the trapping 40 potentials are switched off to enable the ions to enter the ion trap in spite of their reduced velocity. Thereafter, the trapping potentials are switched on again so as to trap the ions present in the ion trap. However, this variant is also not capable of achieving in the ion trap 45 the ion concentration maximally possible and desirable to obtain the best possible sensitivity for recording the ion cyclotron resonance spectrum.
Now, it is the object of the present invention to provide a method for slowing down the speed, in the direc- 50 tion of the magnetic field, of those ions which have entered the ion trap, which method be carried out easily and results in increased density of the trapped ions.
This object is achieved according to the invention by the fact that the ions that have entered the ion trap are 55 imparted a motion component in a direction perpendicular to the magnetic field.
Consequently, the velocity of the ions in the direction of the magnetic field, which enables the ions to leave the ion trap, is slowed down in the case of the method 60 according to the invention not by increasing the gas pressure, or by providing a decelerating electrode, but rather by causing the ions to drift off their original path of movement in the direction of the magnetic field so that once the ions have entered the ion trap they will 65 move along a path which results in an extension of their average dwelling time in the ion trap. This increases considerably the period of time during which the ions
are permitted to accumulate, and the ion flow can be maintained until the maximum ion density, which is limited by the average dwelling time, has been reached in the ion trap. It is a particular advantage in this connection that no critical operating parameters have to be adhered to, as regards the value or the duration of the potentials to be applied.
According to a particularly simple embodiment of the method according to the invention, the ions are introduced into the ion trap along an axis set off laterally from the axis of symmetry of the ion trap extending in parallel to the magnetic field. This can be achieved simply by arranging the ion beam and the ion trap with a certain lateral offset relative to each other. Due to this lateral offset, the ions entering the ion trap get into an area where the electric field prevailing due to the potentials applied to the walls of the ion trap exhibits a transversal component which leads to a certain lateral deflection of the ions. The ions are thereby forced to perform a cyclotron movement along paths which result hi the desired extension of the dwelling time of the ions in the ion trap.
A Brief History of the Harvard University Cyclotrons (Department of Physics)
Cyclotrons and Their Applications: Proceedings of the 14th International Conference, Cape Town, South Africa 8 - 13 October 1995
The cyclotron (Methuen's Monographs on physical subjects series)
According to another variant of the method according to the invention, an electric field directed transversely to the direction of the magnetic field is generated during the time of application of the ion beam, and preferably in the direct neighborhood of that wall of the ion trap which is provided with the hole. Such a field may be generated in a simple manner by means of additional electrodes arranged in the ion trap. Neither the value nor the duration of application of the field are critical. However, the field has to be switched off before the spectra-recording process proper is commenced.
It may be convenient for both variants of the method to reduce the potential of that wall of the ion trap, which is provided with the hole, below the trapping potential during application of the ion beam, so that the ions can be shot into the ion trap at reduced axial velocity, which has a favorable influence on the trapping process.
Axial velocity measurements in reacting and non-reacting flow in a centerbody combustor
The effect of change in axial velocity on the potential flow in cascades (Aeronautical Research Council. Reports & memoranda, no.3547)
Axial velocity streaks in the jet stream: Ageostrophic "inertial" oscillations (Technical report)
Particle Impact on the Wall of Gas/Solids Transfer Lines: A Non-Intrusive Probe of Axial Flow Velocity
The effect of change in axial velocity on the potential flow in cascades, (Aeronautical Research Council. Reports and memoranda, no. 3547)
Analysis of geometry and design-point performance of axial-flow turbines using specified meridional velocity gradients (NASA contractor report)
The present invention further relates to an ion cyclotron resonance spectrometer adapted for carrying out the method according to the invention. It comprises in the conventional manner an ion trap which is arranged in a constant homogeneous magnetic field and comprises walls which are designed as electrodes and extend in parallel or perpendicularly to the direction of the magnetic field and which are supplied with electric trapping potentials retaining the ions in the ion trap, one of the walls extending perpendicularly to the magnetic field being provided with a hole. The spectrometer further comprises means for introducing ions into the ion trap comprising an ion source, means for generating an ion beam which is emitted by the ion source in the direction of the magnetic field and directed upon the hole in the one wall of the ion trap, and means for reducing the velocity at which the ions which have passed the hole and entered the ion trap move in the direction of the magnetic field, below the value determined by the trapping potentials which is needed by the ions for leaving the ion trap.
According to the invention, the means for reducing the velocity of the ions in the direction of the magnetic field are adapted for imparting to the ions that have entered the ion trap a motion component perpendicular to the direction of the magnetic field.
4,924,089
According to one embodiment of the spectrometer according to the invention, the hole arranged in one wall of the ion trap is laterally offset relative to that axis of symmetry of the ion trap which extends parallel to the magnetic field. 5
According to another embodiment of such a spectrometer, electrodes which are insulated from the wall and which are connected to a voltage source that can be switched on in pulse-like manner are arranged on both sides of the hole provided in the one wall of the ion trap. 10 It will be appreciated that such electrodes can be used also when the hole provided in the one wall of the ion trap is set off from the center of the wall.
Further, the potential of the wall opposite the wall provided with the hole may differ from the potential of 15 the wall provided with the hole, with respect to the ionic charge.
It will be appreciated that the method according to the invention does not require any complicated measures regarding the design of the spectrometer, but can 20 be effected with relatively small modifications which do not oppose the application of the method according to the invention.
The invention will now be described and explained in more detail with reference to the embodiments illus- 25 trated in the drawing. The features that can be derived from the description and the drawing may be used in other embodiments of the invention either individually or in any desired combination. In the drawing:
FIG. 1 shows a diagrammatic representation of a first 30 embodiment of an ion cyclotron resonance spectrometer according to the invention;
FIG. 2 shows a diagrammatic representation of a second embodiment of an ion cyclotron resonance spectrometer according to the invention; and 35
FIG. 3 shows a time diagram illustrating the different process steps to be carried out when operating the ion cyclotron resonance spectrometer according to FIG. 2.
The ion cyclotron resonance spectrometer illustrated diagrammatically in FIG. 1 comprises an ion source 1 in 40 the form of a cell coacting with an electron gun 2 by which an electron beam 3 indicated by a broken line can be shot into the chamber 1 for ionizing the gas contained in the said cell. A wall 4 of the ion source 1 is provided with a small hole 5 through which the ions 45 can leave the ion source 1. The ion source 1 is followed by a flight channel 6 which extends coaxially to the hole 5 in the wall 4 of the ion source 1 and which, when the system is operated with positive ions, is maintained in operation at a relatively high potential of — 1 kV to — 3 50 kV. The end of the flight channel 6 opposite the ion source 1 is equipped with a mask 7 provided with a hole 8 through which the ion beam 9 formed by means of the flight channel 6 and indicated in the figure by a broken line is permitted to escape from the flight channel 6. 55 The flight channel 6 is followed by an ion trap 10 comprising two walls 11, 12 extending perpendicularly to the direction of the ion beam 9, and four walls extending in parallel to this direction. Of the last-mentioned four walls, only the two walls 13, 14 extending perpendicu- 60 larly to the drawing plane can be seen, while the other two walls extend in parallel to the drawing plane. The wall 11 of the ion trap neighboring the flight channel 6 is provided with a hole 15 upon which the ion beam 9 is directed. The ion beam 9 extends in parallel to the axis 65 16 of the ion trap, but is laterally offset relative to this axis. Between the end of the flight channel 6 and the ion trap 10 a decelerating electrode 17 is arranged for slow
ing down the ions to a suitable potential before they enter the ion trap. Typical operating potentials for the walls of the ion trap are 0 V for the wall 11 neighboring the flight channel 6, +0.5 V for the wall 12 extending in parallel thereto, 1 V for the walls which extend in parallel to the ion beam and of which only the walls 13, 14 are shown, and —0.5 V for the decelerating electrode. It is noted here once more that all these values apply to the examination of positive ions. If negative ions are to be examined, potentials with inverse signs are employed. In operation, the ion trap is located in a constant homogeneous magnetic field B extending in parallel to the direction of the ion beam 9 and to the axis 16 of the ion beam 10. The magnetic field is indicated in the drawing by arrows.
Although in operation of the ion cyclotron resonance spectrometer illustrated in FIG. 1 the impulse of the ions introduced into the ion trap 10 in the form of the ion beam 9 is largely reduced, it must still be sufficiently great to enable the ions to overcome the potential of the wall 11 of the ion trap adjacent the flight channel 6. This impulse is generally sufficient also to enable the ions to reach the other wall 12 extending perpendicularly to the direction of the ion beam and to the magnetic field B, and to get lost—either by impinging upon this wall or by escaping from the ion trap through a hole 18 arranged in the wall 12 concentrically to the axis 16 of the ion trap 10—in case the ion beam would enter the ion trap along the axis 16. However, in the case of the embodiment illustrated in FIG. 1, the ion beam 9 is offset relative to the axis 16 of the ion trap 10 so that it enters an area of the ion trap 10 where the electrostatic field existing inside the ion trap 10 and obtained inside the ion trap as a result of the potentials applied to the walls, comprises certain component directed transversely to the axis 18 with the result that when the ions enter the ion trap 10 they are deflected from their straight path, due to the prevailing magnetic field and the electrostatic field, whereby their impulse component in the direction of the cell axis 16 is reduced below the value needed to cause the ions to leave the cell immediately. It is ensured in this manner that the dwelling time of the ions that have entered the ion trap 10 is increased quite considerably and that, accordingly, a very high ion density can be obtained by accumulation of the ions during their dwelling time. The duration of the ion beam required for achieving a high ion density in the ion trap corresponds to the maximally achievable dwelling time of the ions; it is in the range of between 10 and 500 ms and determined, amount other things, by the intensity of the ion flow.
The embodiment of an ion cyclotron resonance spectrometer illustrated in FIG. 2 comprises again an ion source 101 in the form of a cell filled with gas into which an ionizing beam 103 an be shot by means of an electron gun 102 or a laser. The ions leaving the ion source 101 are again formed into an ion beam 109, by means of a flight channel 106, and the ion beam 109 is permitted to leave the flight channel through the hole 108 in a mask 107 provided at the end of the flight channel opposite the ion source 101. The ion beam 109 is directed upon an ion trap 110 which, just as in the case of the embodiment of FIG. 1, comprises walls 111 and 112 extending perpendicularly to the ion beam 109 and walls 113 and 114 extending in parallel to the beam. The wall 111 facing the flight channel 106 is again provided with an opening 115, but here the opening 115 is arranged concentrically to the axis 116 of the ion trap.
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