WO2017211627A1 - Magnet-free generation of ion pulses - Google Patents

Abstract

The invention relates to a device and to a method for generating, storing, and liberating ions from a residual gas atmosphere, comprising an electron source for liberating electrons, an anode that is permeable to the electrons liberated by the electron source and has a negative spatial charge distribution, formed by the electrons, within an ion storage chamber at least partly surrounded by the anode, and a pulse electrode, isolated electrically from the anode, for extracting the ions from the storage chamber, wherein there are no further electrodes within the ion storage chamber and the ion storage chamber has a spatial potential distribution that is attractive to the ions generated by ionization of the residual gas atmosphere exclusively on the basis of the negative spatial charge distribution produced by the electrons, and stores ions.

Description

 Magnet-free generation of ion pulses

description

The invention relates to an apparatus for generating, storing and releasing ions from a residual gas atmosphere having the features of claim 1 and a method therefor having the features of claim 11. The prior art already provides apparatus and methods for

Generating ions from a residual gas atmosphere known. Physically, the effect of impact ionization is usually used, in which electrons strike the particles of the residual gas atmosphere and thereby ionize them. At a first, known as EBIT (electron beam ion trap) and z. For example, in US Pat. No. 6,717,155, an electron beam is used to generate ions. In this case, high-energy electrons are used, with acceleration voltages in the range of more than 15 kV and electron currents of more than 20 mA is used. to

Focusing the electron beam on the smallest possible and sharply focused beam cross section requires elaborate magnetic fields with the corresponding coil devices, which must ensure a magnetic field strength in the range of more than 200 mT. The generation of the ions from the neutral gas particles takes place in the electron beam through the known process of impact ionization. The storage of the ions thus generated takes place in a cylindrically symmetrical structure, the inclusion of the ions in the radial direction via space charge forces and in the axial direction via additional electrodes. The EBIT process has a number of disadvantages. A first disadvantage is the considerable expenditure on apparatus for generating and bundling the electron beam, which in particular in the magnetic field device for

Expression comes. Therefore, the use of a large enough magnetic field strength in combination with devices close to the equipment that are sensitive to ambient magnetic fields is not easy for an EBIT device. There are thus corresponding Shielding or a minimum physical distance required, causing the

Space requirement of the device continues to increase accordingly.

In addition, in the EBIT process due to the high-energy

Produces electrons predominantly multiply ionized ions, wherein in the

Residual gas atmosphere existing molecules are also decomposed to a considerable extent in smaller fragments. However, such multiply charged ions and molecular fragments are a not inconsiderable problem for downstream analytical methods, in particular for the purpose of residual gas analysis. Because the ionized molecular fragments are in an excited, multi-ionized state. Their properties can not therefore easily be compared with those in relevant databases

stored data to be compared. In addition, when decomposing original compounds in the residual gas, the original molecular aggregates present there are destroyed so that they can no longer be detected directly, but only indirectly. This requires a certain analytical

Proof uncertainty. Another disadvantage is the high required

Emission current of> 20mA. The high heating power required to produce this emission current results in a high power and heat input into the entire arrangement and the adjacent vacuum chamber.

Another method known from US Pat. No. 4,904,872 also uses an electron beam for ionization. The focusing of the electron beam takes place there via an arrangement of repeller electrodes. For storing the ions, an additional electrode arrangement is used there, which generates an attractive potential for ions by means of an additional storage electrode within the ionization volume and in particular within the ion storage space. The method disclosed there has the disadvantage that the ions produced can have a short life, since they are attracted to the storage electrode and neutralized. Thus, the producible ion currents, i. H. Ultimately, the number of stored ions depends proportionally on the pressure of the residual gas atmosphere. Although the arrangement disclosed therein is free of magnetic fields, however, the arrangement shown there is also characterized by a complicated storage electrode structure, the acceleration voltages necessary there being present at the electron source in the

Range from 500 to 1000V. In both known methods, it is also the case that the ions produced an energy distribution is impressed, which is significantly higher than the thermal distribution in the residual gas atmosphere. This is especially with regard to a downstream residual gas analysis over z. B. a time-of-flight measurement of the ion packet disadvantageous because the individual mass-separated ion packets diverge spatially strong during the flight time, causing the

Resolution of the apparatus generally decreases or longer routes are needed.

It is therefore the task to remedy the disadvantages mentioned.

In particular, a method and an apparatus for generating, storing and releasing ions from a residual gas atmosphere are to be specified, in which the number of ions generated by the pressure of the residual gas atmosphere in

 Is essentially independent. The process should still be executable even at a low pressure in the residual gas and thereby deliver the largest possible ion quantities for a secured ion detection. The arrangement for carrying out the method should have a simple structure and can be carried out without the additional influence of magnetic fields.

In addition, the method should ensure the best possible thermal energy distribution within the generated quantity of ions. The energy distribution of the ions should thus by the process of ion generation itself no

distorting and additional signature to be imprinted. Finally, the desired process for ion generation and the device used in the process should leave the molecules within the residual gas atmosphere as intact as possible during the ionization, it should as possible only be easily charged ions are generated.

The above objects are achieved with a device for generating, storing and releasing ions from a residual gas atmosphere having the features of claim 1 and with a corresponding method for generating, storing and releasing ions from a residual gas atmosphere having the features of claim 10. The dependent claims contain appropriate or

advantageous embodiments of the device and the method. The apparatus for generating, storing and releasing ions from a residual gas atmosphere comprises an electron source for releasing electrons and an anode permeable to the electrons released by the electron source having a negative space charge formed by the electrons within an ion storage space at least partially surrounded by the anode ,

Here, the ion storage space for by ionization of

Residual gas atmosphere generated an attractive spatial potential distribution due to the negative space charge. Furthermore, an in

 Emission direction the ion storage space final, switchable to an electric potential and perforated pulse electrode to release a

Ion packet provided from the ion storage space. The device according to the invention is based on the idea of ionizing and storing the particles of the residual gas in the attractive potential of a negative space charge cloud. In contrast to the known EBIT method, in which a strongly collimated by a magnetic field electron beam is used, the negative space charge cloud or electron density is without further

Bundling or collimation formed and without the increase of additional electrodes in the axial direction. To form the negative space charge cloud, the anode is permeable to the emitted electrons. The emitted electrons collect in the region of the permeable anode and form the space charge cloud, in particular in its inner region.

The negative space charge cloud fulfills a dual function: first, it ionizes the particles of the residual gas and second, it forms an attractive potential for the positive ions formed therein. The positive ions accumulate in this attractive potential, so that the negative space charge cloud a

Storage area for the generated ions forms.

In one embodiment, the anode and the pulse electrode are at the same potential during the storage process. In one embodiment of the device, the electron source is in the form of a hot cathode surrounding the electron-permeable anode Ring filament formed. As a result, the electron-permeable anode is exposed from all sides with the electrons emitted from the ring filament. Also conceivable are other embodiments of the electron source in which the electrons are emitted from one or more sources and act on the anode at appropriate locations. The decisive factor is the generated negative space charge distribution within the anode at the end.

 In comparison with the known prior art methods, the acceleration voltage for the electrons can be significantly reduced. In one embodiment, the acceleration voltage applied to the hot cathode for the emitted electrons is at most 200 volts.

Compared to the known EBIT methods, the electron current can be significantly reduced. In one embodiment, the electron current exiting the hot cathode is not more than 10mA, in particular, 2mA is sufficient for typical applications. This significantly reduces the power consumption and the heat input in the sensor.

In one embodiment, a surrounding the electron source

electrostatic arrangement of focusing electrodes and / or a repeller for additional alignment and shaping of the electron emission provided. The focusing electrodes and / or the repeller are used, in particular, to direct the electrons not emitted in the direction of the electron-permeable anode in the direction of the electron-permeable anode, thus assisting the formation of the negative space charge.

The negative space charge distribution forms a potential well with respect to the anode and pulse electrode potentials, which, when the negative space charge distribution is not ion-compensated, is attractive to ions in the ionization volume and forms an omnidirectional electrostatic exit barrier for ions, thus enabling one

 Storage of ions up to the compensation of the negative Raumlaudung.

For ion extraction, the pulse electrode is switchable to a negative potential compared to the anode, whereby the collected ions are extractable in the direction of the pulse electrode. The frequency for switching the pulse electrode is expediently at least 0.1 Hz and at most 1 M Hz, in particular at least 1 Hz, and at most 100 kHz.

In particular, the electron current generated by the electron source is a minimum of ΙμΑ and a maximum of 15 mA, in particular a minimum of 5μΑ and a maximum of 2mA.

The electrons generated at the electron source (1) due to the potential difference between electron source (1) and anode (2) are effective

Acceleration voltage is a minimum of 30V and a maximum of 400V, in particular a minimum of 70V and a maximum of 150V.

In another embodiment, the anode which is permeable to the emitted electrons has a cylindrically symmetrical structure. In this case, the lateral surface of the cylinder parallel to the circumferential annular filament of the

Be formed electron source, while the pulse electrode forms one of the two top surfaces of the cylindrically symmetric anode. The cylindrically symmetric structure promotes the multiple passing of the electrons through the

Ionization space, whereby the electron yield is increased compared to directed beam paths and the storable charge at the same

Emission current is increased due to the increased space charge in the storage space.

In addition, a detector arranged in the direction of flight of the ions may be used for

Measurement of the ion current can be provided.

The method for generating, storing and pulsed release of ions from a residual gas atmosphere is carried out with the following method steps: There is an emitting of electrons from an electron source and a

Accelerating the electrons towards the transmissive anode assembly. If they do not die at the anode grid, the electrons pass the ionization space (possibly several times) and form a negatively charged one

Space charge cloud out. Among other things, within the negative space charge cloud occurs simultaneously an impact ionization of gas molecules and / or gas atoms. The generated positively charged ions are collected in the attractive potential of the negatively charged space charge cloud and form a positively charged ion reservoir stored there.

This is followed by switching an ion-permeable pulse electrode to a negative potential and accelerating the in the potential of

Space charge cloud ion storage in the direction of the pulse electrode. In conjunction with this, a release of at least one in the field of

 Pulse electrode accelerated from the attractive potential part of the Ionenvorrates in the form of an ion packet through the pulse grid.

The basic idea of the method is to use a negative space charge cloud formed from released electrons firstly for the impact ionization of neutral gas particles and secondly as the negative potential of the

Space charge cloud to use simultaneously for collecting and storing the ions generated. This negative potential is thus gradually filled with the generated positive ions until this potential is substantially balanced. Because the depth of the negative potential is not substantially dependent on the pressure of the surrounding gas atmosphere and this potential is always filled in the course of ionization of the gas particles, the number of ionized gas particles in the negative potential is largely independent of the pressure of the gas atmosphere, so that one of the pressure independent ion packet can be released. The energy distribution of the ions within the

Storage potential is essentially thermal, the ions themselves are usually only positively charged due to the weak low kinetic electron energy, with larger molecules are essentially not split into smaller fragments. The method does not require complicated magnetic focusing of an electron beam and can be operated at comparatively low electron energies compared to the prior art. Likewise, expensive storage electrodes for collecting the generated ions are eliminated. The emitting of the electrons from the electron source takes place in an advantageous embodiment of the method from an annular transmissive Anode arrangement surrounding hot cathode, wherein the electrons are accelerated in the field of the transmissive anode assembly and the negative

Train space cloud. The transmissive anode arrangement thus pursues only the purpose of concentrating the emitted electrons in a certain area of space and thereby the ionizing and simultaneously storing

Create space charge cloud.

In one embodiment of the method, the strength of the released ion pulse over a time interval between successive

Switching the pulse electrode set so that the strength of the

Ion pulse is proportional to the length of the time interval.

In a modification of the method, in addition to the release of the

Ion packets also a determination of a total pressure to be performed. In this case, a strength of the liberated ion current is measured at a fixed predetermined time interval between successive switching operations of the pulse electrode, wherein the strength of the ionic current is used as a measure of the total pressure to be measured. In a modification of the method, in addition to the release of the

 Ion packets also a determination of a total pressure to be performed. In this case, the time interval between successive switching operations of the pulse electrode is controlled to a fixed predetermined strength of the measured released ion packet, wherein the time interval between successive switching operations of the pulse electrode is a measure of the total pressure to be measured. Due to the independent of the pressure strength of the ion packet, the

Ion source can be operated in a very wide pressure range of le-12 mbar to 2e-2 mbar. The device and the method will be described below with reference to

Embodiments will be explained in more detail. For clarity, Figures 1 to 14 are used. The same reference numerals are used for identical or equivalent parts. It shows: Fig. 1 is an overall view of the device for ion generation, storage and release in section,

Fig. FIG. 2 shows a representation of the device from FIG. 1 in a perspective view, FIG.

Fig. 3 shows a representation of the process of ion generation, storage and release in the device according to the invention, FIGS. 4a and 4b show different representations of an exemplary pulse electrode with a pulse grid,

Fig. 5 shows an exemplary illustration of the device for generating ions in connection with a sensor for a time-of-flight spectroscopy,

Fig. 6 is an exemplary representation of various potential distributions in

Ion storage space due to the negative space charge caused by the electrons, Fig. 7 is an illustration of the dependence of the number of storable ions on the emission current at the electron source for two different acceleration voltages acting on the electrons,

Fig. 8 is an illustration of the dependency of the storable charge on

 Emission current at the electron source for two different on the

Electron-acting acceleration voltages,

Fig. 9 is an illustration of the time filling process for the attractive potential as a function of two different pressures,

Fig. 10 a representation of the simulated filling time as a function of the pressure,

Fig. 11 is a representation of a simulated collecting effect and a consequent filling time for a pressure of 10 ^"5 mbar, 12 shows a representation of the experimentally determined filling time as a function of the pressure,

13 shows exemplary determinations of the collected charge as a function of the collection time and the total pressure via an evaluation of a

Integrals via a measurement signal in a Faraday cup,

14 is a representation of the flight times of ions of different masses. Fig. 1 shows an overall view of the apparatus for ion generation, storage and release in section. The device contains a

Electron source 1, which is formed in the present example as a hot cathode in the form of a ring filament. The ring filament surrounds an anode 2 which is permeable to electrons. Within the anode and in the spatial region of the ring filament, a negative ionizing space charge 3 is generated by the emitted electrons, which is indicated in the present figure by a dashed line. The negative space charge extends in particular into an ion storage space 4, which is located in the interior of the anode 2. The anode 2 is electrically isolated from the pulse electrode 5.

In the example given here, the arrangement is additionally of

Focusing electrodes 6 surrounded and shielded to the outside. The

Focusing electrodes effect in particular an alignment of the

Electron source emitted electrons in the direction of the electron-permeable anode. 2

Fig. 2 shows the arrangement shown in Fig. 1 in a perspective

Presentation. The electron-permeable anode 2 is constructed here cylindrically symmetrical. It contains a lateral surface 7, which is designed as a sufficiently fine-meshed grid, sieve or conductive fabric. The front of the

 Lateral surface 7, d. H. one of the bases of the cylindrical anode is formed by the pulse electrode 8.

The entire anode arrangement, including the lateral surface 7 and the pulse electrode 8, is gas-permeable and at the same time partially transparent to electrons. The electron source used in the present example is a ring filament 9 in the form of a ring-shaped hot cathode which extends at a certain distance parallel to the lateral surface 7 and emits electrons at high temperature by means of thermal emission.

The electrons emitted from the hot cathode are mutually u .a. emitted in the direction of the lateral surface 7 of the anode and accelerated and penetrate the lateral surface 7, wherein these penetrate into the interior of the anode and ionize the gas particles present there via the process of impact ionization. At the same time, the electrons form a negative space charge cloud, which represents an attractive potential for the positively charged ions. The ions are thus collected in this attractive potential within the ion storage space 4 of the anode. By switching the pulse electrode 8 to a negative potential, the ions can now be expelled from the ion storage space to the outside

Pulse grid accelerated through. It is an ion packet extracted. The extraction direction of the ion packet is illustrated by an arrow.

In an alternative embodiment, the pulse electrode may be switchable to a positive potential compared to the anode in order to accelerate the collected ions against the direction of the pulse electrode. In this case, in addition, if the part of the anode lying in the direction of flight of the ions is permeable to the ions, the collected ions can be extracted from the ion storage space counter to the direction of the pulse electrode by a repulsive electrostatic interaction. The pulse electrode may in such a case be impermeable to the ions and drive the positive ions out through the body of the anode.

Fig. 3 shows the process of ion generation and storage in a series of process steps A to D. In state A, the electron source 1 is inactive. The anode 2 is at a positive potential V _AN , which is above the

Cross-section of the anode in the ion storage space 4 located therein is substantially location-independent and constant. In state A, the anode and the pulse electrode are at the same potential V _AN , so that a constant potential V _{AN is} established in the ionization volume. In state B, the electron source 1 is activated. The emitted electrons penetrate the anode and form, in particular in the ion storage space 4, a negative space charge cloud 3. As a result, the potential within the anode assumes a location-dependent value. In the central area of

Space charge cloud and thus also the ion storage space forms a potential well with a local potential _minimum V _min .

State C takes into account the impact ionization of the gas particles present in the ion storage space 4 by the influence of the emitted electrons. The positively charged ions are moved to the local minimum of the potential in the ion storage space and accumulate within the negative

Space charge cloud on. Due to the attractive potential well, the ions are held within the ionization volume, can not with the

Ionization space surrounding electrodes collide and neutralized. The potential is compensated with continuous time, the depth of the local potential minimum within the ion storage space decreases and the potential minimum becomes increasingly flatter. In state D, the potential minimum is filled up by the positive ions. The negative space charge of the electrons is completely through the positive

accumulated ions compensated. Now, with regard to the potential inside the anode, a ratio arises, as it was in state A: a substantially location-independent constant electrostatic potential inside the anode. The ions stored there can now be accelerated towards the pulse electrode under the attractive influence of the now activated pulse electrode 8 and released to the outside as an ion packet. For this purpose, the pulse electrode is switched from the anode potential V _AN to a more negative potential, whereby the ions generated from the ion storage space

subtracted from.

As a result, the state B is again assumed, in which there is an empty unfilled potential minima within the ion storage space and which can be refilled when the states C and D are re-run and emptied by the ions stored there. This cyclic operation thus allows ion packets or ion pulses, ie spatially limited Ion accumulations, generate. The time from state B to complete space charge compensation in state D will be referred to hereafter as the fill duration. From the positive ion charge, which is stored in the state D, results in the storage capacity of the ion source.

FIGS. 4a and 4b show a representation of the pulse electrode 8 with a pulse grid 5 incorporated therein. The basis for the pulse electrode 8 is a sheet-metal disk. In the inner surface of the sheet-metal disk, in the present example, a hexagonal honeycomb structure 10 is incorporated, in particular cut by means of laser radiation, which has a diameter D _PG . This increases the permeability of the pulse grid and ensures adequate shielding against external fields.

Via metallic feeds, the pulse electrode can be electrically contacted. The pulse electrode can thus be activated via an external wiring. It is necessary on the one hand, the pulse electrode at a constant potential V _PG = V _AN as shown in FIG. 3 to ensure the formation of the potential minimum in the anode compartment. On the other hand, the attractive pulses V _PU | _{S of} a pulse generator, not shown here, which are required for the extraction of the accumulated ions, transmitted to the pulse electrode.

4b shows a detailed view of an exemplary lattice structure of the pulse grid 5. In the example given here, the lattice structure 10 consists of honeycomb-like hexagonal openings 11, which are separated from one another by webs 12. The geometric dimensions of the grid structure of the pulse grid are chosen so that a loss-free as possible extraction of the ions from the anode chamber, with optimal shielding against external fields is guaranteed. Corresponding transmission losses, which are due to the fact that ions neutralize at the lattice webs, can be minimized by an appropriate dimensioning of the web width S and the mesh size G of the hexagonal openings 11.

Below, the operation of the device and the method will be explained in more detail. The following explanation of the ion source takes place in connection with the explanation of a time-of-flight spectroscopic device. The illustrated in Fig. 5 schematic structure of the cylindrically symmetric

Arrangement of the ion source and the time of flight spectroscopic device consists essentially of three modules. These assemblies are the ion source, consisting of the electron source 1, the anode 2 and the pulse electrode 8. Also provided is a repeller 13 as well as a second assembly of a time-of-flight (TOF) mass separator adjoining the ion source 14 with a detector unit as the third module, which is designed here as a Farady cup 15. The entire arrangement has the compact design of conventional ionization vacuum gauge.

From the example. Ring-shaped filament of the electron source electrons are released by thermal emission, due to the attractive

Potentials of the anode are directed into the anode compartment inside. Thereupon, by collisions of the electrons with the neutral residual gas particles, positive ions are formed which, as mentioned, can be accumulated to a certain extent in the anode.

By switching the pulse electrode to an attractive potential Vpuis for the ions, the accumulated ions are drawn out of the anode. The result is a potential attractive to the positive ions, which extracts and accelerates the collected ions into the time-of-flight mass separator. In this ion according to their mass according to the principle of TOF

Mass spectroscopy separated.

If the ions are separated according to their mass, time-separated signals can be detected at the detector, which is designed here in the form of a Faraday cup 15. The advantage of the time-of-flight mass separation lies in the fact that it is thus possible to record an entire mass spectrum with one single ion pulse with one and the same detector. In this case, the time t _T0F required for the ions is measured by a given distance S _T OF between the

Releasing the ion pulse and the impact on the detector cover. Since all ions have the same charge and the accelerating potential is the same for all ions, it can be assumed that all ions have the same kinetic energy. Massier ions will have a lower velocity than lower mass ions and thus later, ie after the lower mass ions, arrive at the detector. After a time-resolved detection at the Faraday Cup, a mass is now assigned to the respective flight times and thus a conclusion drawn on the prevailing in the vacuum chamber residual gas composition.

The separated on the flight time distance according to their mass ions can be detected electrically with other types of detectors. A Faraday Cup offers the advantage of a simple structure. This consists of a metallic cup 15c, which is held at a constant potential V _FC = 0V. Inside the cup is the metal plate 15b. A positive ion impinging on the metal plate now creates an additional one

Net charge. This positive charge is then passed through to the

Metal platelets are neutralized by flowing electrons. The current I _FC flowing during discharge of the metal plate is thus directly proportional to the number of impinging ions.

As described above, the time-of-flight mass separator separates light ions from the heavier ones. As light ions accelerate faster, they reach the Faraday Cup at an earlier point in time. In this case, according to the temporal detection at the Faraday Cup individual peaks visible. If individual masses differ sufficiently, the

corresponding signals are shown separated in time in the spectrum, otherwise they will partly overlap.

The sum over the entire temporal signal of an ion pulse at the detector provides a statement about the total pressure within the chamber, while the analysis of the individual signal peaks a conclusion on the respective

Partial pressure allowed.

The following is a theoretical characterization of the ion source. For this purpose, in the next sections, a computational estimation of selected characteristic variables, such as, for example, the field distribution within the anode, the storage capacity or the ion current and the resulting pressure-dependent filling duration are carried out. Since the anode is kept at a constant positive potential V _AN , the electrons, as already explained, are directed into the anode because of their attractive potential. Here they cause negative

Space charge effects a potential minimum that distorts the initially constant potential.

The negative space charge is subsequently assumed to be simplified as an electron density with a given radius r ₀ . The field distribution in the presence of the electron density decays here into two areas. The first region is formed by the space within the electron density with the radius r <r ₀ , the second region is formed by the outside of the electron density environment with r ₀ <r <r _AN , where as boundary condition V (r = r _AN ) = V _AN holds. The potential distribution V (r) for these two ranges can be calculated as a function of r. For a given distance r _AN , the potential in FIG. 6 graphically.

The potential dependent on the radius r can be approximated by the following course:

Within the electron density with the radius r ₀ , the potential for r <r ₀ assumes the following course:

Outside the electron density, for r> r ₀ , the potential can be set as follows:

It can clearly be seen that the depth of the potential minimum is strongly dependent on the radius of the electron density. For example, with an electron density of radius r ₀ = ₆ mm, a potential minimum of V _AN of approx. 2V results, whereas an electron density focused on the radius r ₀ = 1 mm already results Potential minimum of V _AN of about 8V generated. It is thus an advantageous embodiment to focus the electrons emitted by the filament to a sufficient extent in order to ensure effective storage of the ions within the potential minimum. In Fig. 1, the focusing electrodes 6 are indicated schematically. If the focusing electrode 6 is set to a potential smaller and equal to the potential of the electron source 1, the electrons emerging from the electron source 1 are focused in the direction of the anode.

For the mathematical estimation of the maximum storable number of ions, d. H. the storage capacity N _{+ of} the storage ion source, it can be assumed in good approximation that an electron density of the current density j _e = p _e 'v _{e contains} a negative charge of Q _e = p _e ' V within a volume V = A ^■ AL. Here A is the lattice surface through which the electrons enter the

Anode space, and AL corresponding to the length traveled by the electrons within the anode.

If one pulls the kinetic energy of the electrons after leaving the

Filamentes, the charge density of the electrons can be written to:

Where V _e = V _AN - V _FN which acts on the electrons

Acceleration voltage due to the potential difference between anode and filament. Furthermore, for the negative charge density:

If both equations are equated and resolved according to the number of electrons N _e , we obtain:

(A)

Die maximale Speicherkapazität ergibt sich dann, wenn die Anzahl der erzeugten Ionen N₊ gleich der Anzahl der Elektronen N_e ist und somit N₊ = N_e gilt. Fig. 7 zeigt hier die Abhängigkeit der Anzahl der speicherbaren Ionen Q₊ = N₊e vom Emissionsstrom I_e für zwei verschiedene auf die Elektronen wirkende

The maximum storage capacity results when the number of ions generated is N ₊ equal to the number of electrons N _e and thus N ₊ = N _e . 7 shows here the dependence of the number of storable ions Q ₊ = N ₊ e on the emission current I _e for two different electrons acting on the electrons

Acceleration voltages. The upper graph shows the progression for one

Voltage of V _e = 70 V, the lower graph for a voltage of V _e = 120 V.

As can be seen from equation (A), the storage capacity N _{+ is} the

Storage ion sources of the emission current I _e dependent. Increasing the emission current increases the space charge density, which allows more ions to be stored in the negative space charge potential. This

The relationship is shown graphically for the resulting storable charge Q ₊ = N ₊ e as a function of two different acceleration voltages of the electrons V _e in FIG.

With an emission current of I _e = 1 mA, an acceleration voltage of the electrons of V _e = 70 V and a length of the storage volume of L = 1 cm, the number of storable ions thus becomes N ₊ 1.3 ¹ 10 ⁷ . This corresponds to a charge of Q ₊ = N ₊ e "2.0 ^{1 10" 12} C, which can be maximally stored within an electron density at a current of I _e = 1mA. In general, the electron current generated by the electron source 1 is minimal ΙμΑ and a maximum of 15 mA, in particular minimal 5μΑ and a maximum of 2 mA. the heating power needed to generate this emission currents is sufficiently small to cause only a small power and heat input into the entire assembly and the adjacent vacuum chamber. the amounts of charge generated by Q ₊ "2.0 ^{1 10" 12} C are sufficiently high to be detected by simple detectors (for example, in the manner of a Faraday Cup) with sufficient signal-to-noise ratio. Further, Fig. 8 shows that as the accelerating voltage is increased from 70V to 130V, the storage capacity is decreased because the faster electrons generate a smaller negative space charge. In addition, a charge of approx. 10 ^"14 C to 10 ^{" 11} C can be stored at the selected acceleration voltages with an emission current in the range 10μΑ to 5mA. These amounts of charge can without additional expensive equipment, such. B. a photomultiplier can be detected with a simple Faraday Cup with a good signal-to-noise ratio.

The filling time, i. the time in which the negative electron space charge is completely compensated by stored ions depends on this

prevailing total pressure in the vacuum chamber. If one varies the collection time sammei, i. the time in which ion collecting process takes place unhindered, so will depend on this collecting time dependent signal that with

progressing t _Sa mmei will increase accordingly until the filling time t _{FÜM is} reached, as shown in Fig. 10.

From a certain point in time t _FÜM the potential _minimum is completely compensated with positive charges and the measuring signal V _FC decreases

constant value.

Furthermore, the filling time t _{FÜM is} also dependent on the prevailing pressure p, since at higher pressures correspondingly more neutral gas particles are present, which can fill up the potential minimum faster after ionization. With an increasing pressure Pi> p ₂ , a decreasing filling time t _{FÜM /!} <t _{FÜM / 2} and the qualitative course of these curves will be similar.

At this point it should be emphasized that the maximum number of ions released during a pulse depends very little on the pressure. Because the

The storage capacity of the ion source is determined solely by the depth of the potential formed by the negative space charge. As in the

13, the maximum storable charge changes by about half, while the pressure is varied by about 3 decades from about 5E-6 mbar to about 5E-9 mbar. Furthermore, the change in the storable charge decreases with decreasing pressure. This allows the use of the ion source over a very wide pressure range without significant loss of measurement sensitivity.

After the potential well has been filled up, further ions are formed which displace the already stored ions. In this case, ions of higher mass number are generally harder to displace than ions of small mass number. Therefore, an accumulation of heavy ions and a displacement of light ions occurs, whereby the sum of the ions and the stored charge are obtained stay. On the one hand, this effect can be exploited to the

 Detection capability of the arrangement for ions of higher mass number to increase.

 On the other hand, this effect can be minimized by ion extraction before the potential well is completely filled.

For the theoretical estimation of the above-mentioned filling time t _FÜM , one can once again make use of fundamental relationships. The starting point is again the electrical current density j, which describes the ratio of the current intensity I to a cross-sectional area A available to it. Furthermore, the current density can be about the space charge density p = n e ^■ and the average

also schreiben : Express drift velocity v of the respective charge carriers. Based on the electrons emitted by the filament with V _FM towards the anode, one can use the relationship

so write:

Je = - = Pe - v = n _e ev _e

 A

If the above equation is converted to n _e and multiplied by the volume V, one obtains the number of free electrons N _e within the considered volume. These generate, as described above, a negative charge Q _e = N _e e. In order to compensate for this charge with a correspondingly positive charge Q ₊ , ie to fill up the potential minimum, the positive ion current 1 _{+ must} flow over a corresponding time t. It can thus be summarized:

Q _e = Ne _e = Q ₊ = I ₊ t

The time t _FIVE required to fill up the negative charge of the electrons with correspondingly positive ions is thus:

t ^p ii

Somit wird die Abhängigkeit der Fülldauer von Totaldruck deutlich. Eine graphische Darstellung dieser Abhängigkeit ist in Fig. 11 dargestellt. Für die Druck-abhängige Fülldauer wird ersichtlich, dass bei Druckbereichen von p = 10^"7 . . . 10^"3 mbar Ionenströme von etwa 1₊ = 10^"10 . . . 10^"6 A zu erwarten sind . Bei einem Emissionsstrom von I_e = 1 mA wird also nach etwa t_FÜN = 10^"3 . . . 10^"7 s das Potentialminimum im Anodenraum aufgefüllt sein. Durch das Schalten der Pulselektrode vom Anodenpotential auf ein bspw.

Thus, the dependence of the filling time of total pressure becomes clear. A graphical representation of this dependence is shown in FIG. 11. For the pressure-dependent fill time that at pressure ranges of p = 10. ^{"7.. 10"} is visible, ³ mbar ion currents of about 1 ₊ 10 = ^{"10... 10" 6} A can be expected. In an emission current of _Ie = 1 mA is thus after about _Fuen t = 10 ^{"3... 10"} s have filled the minimum potential in the anode chamber ^7. By switching the pulse electrode from the anode potential to a bspw.

Negative extraction potential manipulates the electric field inside the anode so that the collected ions are accelerated out of the ionization volume and detected on the Faraday cup. When the pulse electrode is switched back to the anode potential, the original state sets in: electrons generate a potential minimum in which ions are generated and collected. The period of time ions are collected until they are extracted by the switching of the pulse electrode is the collection time.

If you wear, as in Fig. 11, the stored ion charge over the associated collection time, the assumption is confirmed that at a defined pressure, a corresponding number of generated ions slowly fill up the negative charge Q _e «- Γ 10 ^{" 12} C of the potential minimum as the collection time increases Therefore, the positive ion charge Q ₊ increases steadily and _changes to a constant value as of a corresponding time t _FÜM In the example selected here, the graph of FIG

Fill time to t _FIVE determine 50 ps.

With the storage ion source developed here, it is possible to different ion pulses as a function of the collection time in the anode

to generate. The storage was realized by having a

Low energy electron density due to space charge effects a sufficiently low potential minimum within the, otherwise on the

constant potential V _AN lying, anode causes. In continuing experiments, the clear dependency of, until

complete compensation of the potential minimum with positive ions, continuous time, the so-called filling time, are shown by the prevailing pressure, as shown in FIG. 12.

This gives the possibility of the storage ion source as a

To use the total pressure sensor and to use as measurement of the pressure the metrologically easily accessible required filling time. As can be seen in FIG. 13, there is a one-to-one correspondence between the time between two extraction pulses during steady-state operating parameters

collected ion charge and the prevailing total pressure. By means of a corresponding evaluation electronics, a conclusion can be drawn about the prevailing total pressure from the total extracted ion charge and the set time between the extraction pulses.

Likewise, it can be seen from FIG. 13 that the required collection time is, for example,.

25 nVs signal at the detector to generate less than ms for pressures p> 1 E-7 mbar. So this device is for use as "faster"

Total pressure sensor suitable for detecting fast pressure changes with reaction times <1 ms. As shown in FIG. 14, it is possible to temporally separate the helium signal from the other residual gas components. Therefore it is possible the

Total pressure sensor at the same time to use as a helium detector, whereby a helium leak test is possible. In this case, the flight path underlying the measured values in FIG. 14 is only 2 cm, so that the sensor has a high altitude

Has compactness.

The device treated here has a high compactness, a reliable total pressure determination and a helium mass separation. The dimension of the sensor corresponds to that of a conventional ionization vacuum gauge.

By the possibility of a subsequent data processing can be further calculated back by the determination of the integral over the entire measurement curve directly to the stored ion charge per pulse, whereby the comparison and the evaluation of different spectra at divergent experimental parameters is greatly facilitated. The device according to the invention and the method according to the invention were explained by means of exemplary embodiments. In the context of expert actions, further refinements are possible. These are also apparent from the dependent claims.

LIST OF REFERENCE NUMBERS

1 electron source

 2 Electron-permeable anode

 3 Negative ionizing space charge

 4 ion storage space

 5 pulse grid

 6 focusing electrode

 7 lateral surface

 8 pulse electrode

 9 ring filament

 10 honeycomb structure

 11 hexagonal breakthroughs

 12 bars

 13 repellers

 14 time-of-flight mass separator

 15 Faraday cups

 15a shielding grid

 15b metal plate

 15c metallic mug

Claims

Patent claims 1. Device for generating, storing and releasing ions from a residual gas atmosphere, full an electron source (1) for releasing electrons, an anode (2) permeable to the electrons released by the electron source (1) with a negative space charge distribution (3) formed by the electrons within an at least partially surrounded by the anode (2). Ion storage space (4) and a pulse electrode (8) electrically insulated from the anode (2) for extracting the ions from the storage space, whereby there are no further electrodes within the ion storage space (4) and the ion storage space (4) is exclusively due to the negative ones generated by the electrons Space charge distribution for the ionization of the Residual gas atmosphere generated ions attractive spatial Has potential distribution and stores ions. 2. Device according to claim 1, characterized in that the anode (2) and pulse electrode (8) are at the same potential during the storage process. 3. Device according to claim 1 or 2, characterized in that the negative space charge distribution during the storage process forms a potential well with respect to the anode and pulse electrode potential, which, if the negative space charge distribution is not compensated for with ions, has an attractive effect on ions in the ionization volume and forms an electrostatic exit barrier for ions in all directions and thus enables storage of Ions until the negative charge in the room can be compensated. 4. Device according to one of the preceding claims, characterized in that For ion extraction, the pulse electrode (8) can be switched to a negative potential compared to the anode, whereby the collected ions can be at least partially extracted in the direction of the pulse electrode. Device according to one of the preceding claims, characterized in that the frequency for switching the pulse electrode (8) is a minimum of 0.1 Hz and a maximum of 1 MHz, in particular a minimum of 1Hz and a maximum of 100 kHz. 5. Device according to one of the preceding claims, characterized in that the electron current generated by the electron source (1) is a minimum of ΙμΑ and a maximum of 15 mA, in particular a minimum of 5μΑ and a maximum of 2mA. 6. Device according to one of the preceding claims, characterized in that the effective acceleration voltage for the electrons generated at the electron source (1) due to the potential difference between the electron source (1) and the anode (2) is a minimum of 30V and a maximum of 400V, in particular a minimum of 70V and a maximum of 150V. 7. Device according to one of the preceding claims, characterized in that the electron source (1) is designed as a hot cathode surrounding the electron-permeable anode (2) in the form of a ring filament (9). 8. Device according to claim 1, characterized in that an electrostatic arrangement of focusing electrodes (6) and/or a repeller (13) surrounding the electron source (1) is provided for additional alignment and shaping of the electron emission. 9. Device according to one of the preceding claims, characterized in that the anode (2) which is permeable to the emitted electrons Has a cylindrically symmetrical structure. 10. Device according to one of the preceding claims, characterized in that a detector arranged in the direction of flight of the ions is provided for measuring the ion current. 11. Process for generating, storing and pulsed release of ions from a residual gas atmosphere with the process steps: - Emitting electrons from an electron source (1) and Acceleration towards the ionization space (4) through the permeable anode arrangement (2), - Creation of a negative space charge cloud within the Ionization space due to the electrons moving through the ionization space (4), - Impact ionization of gas molecules and/or gas atoms within the ionization space (4) and storage of the positively charged ions generated in the attractive potential of the negatively charged ones Space charge cloud as a positively charged ion supply, - Switching a pulse electrode (8) to a potential relative to the Anode potential and acceleration of the potential of the Space charge cloud out of the ion supply ionization room, - Extraction of at least part of the stored ion supply in the form of an ion packet. 12. Method according to claim 11, characterized in that The electrons are emitted from the electron source from a hot cathode surrounding the transmissive anode arrangement in a ring, with the electrons being accelerated in the field of the transmissive anode arrangement, passing through the anode several times and forming the negative space charge cloud in the ionization space (4). 13. Method according to one of claims 11 or 12, characterized in that the electrical charge of the released ion packet Time interval between successive switching operations Pulse electrode is set, whereby the electrical charge of the Ion packet is proportional to the length of the time interval. 14. Method according to one of claims 11 to 13, characterized in that a determination of a total pressure is carried out, with a fixed time interval between successive Switching processes of the pulse electrode a strength of the released Ion packet is measured, the strength of the ion packet being a measure of the total pressure to be measured. 15. Method according to one of claims 11 to 13, characterized in that a determination of a total pressure is carried out, whereby the Time interval between successive switching operations Pulse electrode is regulated to a fixed predetermined strength of the measured released ion packet, the time interval between successive switching operations of the pulse electrode being a measure of the total pressure to be measured.