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An electrostatic mass spectroscope
SESSION I I Methods of Measuring Residual Gases during the Manufacture and Life of Tubes
An Electrostatic Mass Spectroscope WERNER TRETNER Fernseh G,m.b.H, Darmstadt, Germany During the last few years, several methods have been known to analyse the residual gas o f high vacuum devices by means of small mass spectrometer tubes. In these tubes which have a resolving power o f about 20 or 30, the principle o f mass separation by different times o f flight is employed. Unfortunately, at pressure below 10-6 Torr, the signal currents generated by these tubes are so small that for the registration o f the whole mass spectrum a comparably long time is required. In order to avoid this drawback a new mass spectrometer tube has been developed. Due to the high signal-output o f this tube the whole mass spectrum, ranging from H + up to Hg +, can be dtsplayed on an oscilloscope at 5O Hz. Therefore, a practtcally contmuous control o f the residual gas can be achieved. The signal o f the mass spectrometer tube is generated by a cloud o f charged particles o f equal relative masses which oscillates along the axis o f an electrostatic fteM with axial symmetry. This ion cloud consists o f numerous superposed elementary clouds each o f which being created by an electron beam in a strong accelerating fieM near one end o f the tube. An electrode on the other end o f the tube is used to pick up the displacement-current induced by the ion cloud. The iomzmg electron beam is intensity-modulated with a variable frequency. Therefore, an wn cloud Is built up only when the variable modulation frequency o f the beam coincides with the oscillation frequency o f any sort o f ions. At frequencies other than those o f the ions present m the residual gas, the elementary ion clouds cannot form one big cloud because practically no phase relation between them exists. The space-charge action o f the elementary clouds explains that the functional dependence between mass intensity and partial pressure is not linear. A special tube having a resolvingpower m/Am .~ 20 at all masses, is described which can be used at pressures below 5 × 10-5 Torr. This upper limit is set by the mean free path o f the ions. The lower limit depends on the shot noise o f the first amplifier stage. Thus, at a registration speed o f 50 Hz, partial pressures m the order o f 10-8 Torr can be detected. A t still lower pressures a second operation method shouM be adopted whwh even permits the use of a n electron multiplier. DURING the last few years the measurement of partial pressures of the residual gas has been one of the aims of development in the technique of high vacuum. The scientific approach to this problem is based on the use of the well known mass spectrometer types. These devices have a high resolving power and a very good sensitivity. In the spectrogrammes of such devices the spectral lines of motopes may easily be separated and coordinated to mass numbers. Even the Fein-Struktur may be investigated. On the other hand, considering the technical applications of high vacuum, such as the manufacture of vacuum tubes, the problems are set quite otherwise. Here, high resolution is not absolutely necessary because in most cases it is known what sorts of vapours and gases can be expected in the residual gas. In the first place one wants to know approximately the quantity of each gas. These informattons should be given immediately. Furthermore, to meet the requirements of mass production, the spectrometer tube should be easy to outgas, and the control unit should be as compact as possible. Thus, resolving power and sensitivity may be neglected in favour of speed and simplicity of operation. Speed and
simphclty of operation, these are the predominant features of the tube to be discussed now, the features mostly desired for technical work. In order to give an introduction to the operation principle of the new tube, it seems favourable to proceed from the well known Omegatron. This tube is based on the principle of the Cyclotron and, therefore, requires a magnetic field. Unfortunately, even at the simplified Omegatron which is quite often used in the techmque of high vacuum, this magnetic field has to be generated by a comparatively ponderous permanent magnet. Thus, a large scale application of the simplified Omegatron in the production of vacuum tubes presents some difficulties. For technical applications there is still another drawback which the Omegatron has in common with some other mass spectrometers. This is the relatively long time required to record the whole mass spectrum. In view of these difficulties, when designing the new tube, we tried to avoid the use of a magnetic field by application of an electrostatic field. For this purpose, we replaced the almost circular motion of the ions in the Omegatron by 31
32
WERNER TRETNER
approximately linear oscillations in an electrostatic field. This principle shall be explained schematically m Fig. 1. The ion source is represented by an electron gun, the beam of which effects the ionization of the residual gas near one of the electrodes. For simplicity let us first consider only one sort of ions, for instance mercury ions. These ions, el.ektr~sches Fetd
--~
/ _I ]--J ~" Gun FIG. 1. The principle of the spectrometer tube. one of which is represented in Fig. 1 near the electrode on the left hand side, are forced by an inhomogeneous electrostatic field to oscillate between the two plain electrodes. It is this periodic motion which replaces the circular motion of the ions in the Omegatron. If now, an electrical field is superimposed which alternates at the same frequency as the mercury ions oscillate, these ions may gain momentum and, therefore, may reach one of the field confining electrodes. This kind of operation is analogous to the operation of the Omegatron. There is another mode of operation which is more practicable for technical applications than the Omegatron-like operation just described. Due to this mode of operation our tube will have an outstanding high output signal, naturally at the expense of resolution. It is this extremely high output signal which finally allows a high speed recording of the whole mass spectrum. Considering again F~g. 1, let us suppose that not only mercury vapour is present in the residual gas but also some other gases. Then, we are able to select one sort of ions by
modulating the ionizing electron beam with the same frequency as this sort of ions oscillates. During the first cycle, for instance, n ions may be generated. These move to and fro. During the second cycle n ions more are created, taking part on the discharge. Thus, at the beginning of each cycle, an elementary cloud of n Ions joins the famtly of ions. Finally a big cloud of ions is formed consisting of numerous elementary clouds which are separated by phase differences of 2n. This cloud of charged particles of equal relative masses generates an output signal by inducing displacement currents to the confining electrodes. All ions with oscillation frequencies other than that apphed to the control grid of the tube cannot form an ~on cloud because the phase d~fferences of their elementary clouds differ from 2n or a multiple of it. The high frequency signal can be amplified without the difficulties encountered at dc-amphficatlon. Furthermore, in order to measure all mass intensities practically at the same time, the modulation frequency of the ionizing electron beam may be sweeped periodically at mains frequency to cover the frequency band containing the oscillation frequencies of all masses to be investigated, for instance from mass 1 up to mass 300. The whole spectrum, therefore, may be recorded at 50 c.p.s. Before going into details, let us first have a look on the configuration of the electrodes of an experimental tube. In Fig. 2 we see a cage consisting of several electrodes. The Steuerelekt rode
Vertauf des A Potentlats - (~ tangs der Achse
FIG 2. The arrangement of the electrodes and the shape of the electric potentml q~along the axts
FIG. 3. An experimentalspectrometer tube,
An Electrostatic Mass Spectroscope voltages applied to them are chosen so that an ion created at the left hand electrode of the cage is accelerated towards the centre of the tube. It then enters the decelerating field at the other end of the tube. Here it is repelled, and the whole process begins again. The ionizing electron beam originates from a tungsten wire cathode. The beam is accelerated by the following electrode of the cage, penetrates a fine mesh screen and enters the electrode cage. By the decelerating fields at both sides of the mesh, the electrons are forced to oscillate around the mesh wires. Thus, an electron either creates an ion or will be captured by the mesh screen without having produced an ion. Just in front of the cathode, the control grid is arranged by which the modulation of the ionizing beam is effected. By the end electrode at the opposite side of the tube the displacement current is picked up and fed to the first stage of the preamplifier. The shape of the electrical field along the axis of the tube is shown below the electrodes. It is symmetric with respect to the centre of the cage. Each half of this ion-optical lens system is an immersion lens and, therefore, has a focussing action upon the ions. A n experimental tube designed according to the principles just described is shown in Fig. 3. The glass envelope has a length of 4 in. and a diameter of 1 in. The leads are strong enough to form a socket. The conductor of the signal electrode is arranged separately in order to avoid crosstalk from the modulated control grid. The various elements of the control unit by which the tube is operated are shown in the following block diagram, Fig. 4. The upper box on the right side symbolizes the voltage
r 0szJtt FIG
4 Block-dmgramof the control umt.
divider for the dc-voltages and the supply of the filament. Below this box we see the generator of the modulated, i.e. sweeped high frequency which is fed to the control grid. It is synchronized at mains frequency which also holds for the horizontal deflection of the oscilloscope. The output signal of the tube is fed to an ac-amphfier having a gain of about 10,000. The signal, after being rectified, passes a low pass filter in order to cut off the shot noise carried by the high frequencies, and then reaches the oscilloscope. Here, the mass spectrum may be observed in the co-ordinates of intensity and mass number. A photograph of a mass spectrum taken in the manner just described is shown in Fig. 5. As indicated by the mass numbers the different lines correspond to the ions H2 +, C +,
33
CH4 +, H20 +, CO +, CO2 +, Hg +, Hg +÷. The spectrum w a s taken at a total pressure of 2.10 -7 Torr on a mercury pumping system with liquid nitrogen on the mercury trap. The spectral lines at the mass numbers 7 = 28:4 and 112 = 28 × 4 are ghost lines of mass 28. The line at mass number 7 is generated by higher harmonics contained in the alternating current induced by the ions of relative mass 28. Ghost masses at mass numbers higher than that of the original mass, i.e. at lower frequencies, occur when the phase difference of two consecutive elementary ion clouds is a multiple of 2zt.
FIG 5. A mass spectrum of the residual gas of a mercury-d~ffus)on pumping system at a total pressure of 2 × 10--7Torr. Generally, only ghost lines of the first order occur, their intensities being less than 10 per cent than that of the main line. The resolving power wsually estimated according to the recorded mass spectrum on the oscilloscope is about ten for all masses. The limiting resolving power amounts to approximately 20 and may be obtained by selecting a small frequency interval and spreading it over the horizontal scale of the oscilloscope. The limiting sensitivity corresponds to partial pressures of somewhat less than 10-8 Torr. The limiting factor for the sensitivity is the shot noise of the first amplifier stage. Therefore, still lower pressures may be detected by using a slower registration speed. This can be done by varying by hand the modulation frequency applied to the control grid. In this case, optimum resolving power is about 20 too. Towards higher pressures a limitation is set by the mean free paths of the ions. Therefore, the upper pressure limit should be between l and 5 "< 10-5 Torr. It is important to know that the heights of the spectral lines do not exactly represent the partial pressures of the corresponding gases. This is due to the fact that space charge effects play a dominating role for the signal generating mechanism. Nevertheless, in a rough approximation the heights of the spectral lines give a fair impression of the contribution of the partial pressures to the total pressure. A n interesting mode of operation of the tube at low pressures is possible by applying an electron multiplier In connection with the Omegatron-hke operation principle. Fig. 6 shows how the tube construction then has to be modified. The former signal electrode should be perforated so that the ions which gamed momentum by the alternating
34
WERNER TRETNER
electrical field may pass the holes and enter the multiplier structure. Summarizing, it can be stated that this novel mass spectrometer tube, whose technical designation will be " Farvitron '" m a y be applied with success in the technique of high vacuum.
ii
The Farvitron as described in this paper, may be used at pressures ranging from 5 × 10-5 T o r r down to somewhat less than 10-8 Torr. Its o p t i m u m resolving power is about 20. Its main features are speed and simplicity of operation, which m a k e it most suitable for supervising work.
llr--II
I
cathode Lenssystem~ FIG. 6 Mass spectrometer tube w~th electron multlpher.
An Electrostatic Mass Spectroscope WERNER TRETNER Fernseh G,m.b.H, Darmstadt, Germany During the last few years, several methods have been known to analyse the residual gas o f high vacuum devices by means of small mass spectrometer tubes. In these tubes which have a resolving power o f about 20 or 30, the principle o f mass separation by different times o f flight is employed. Unfortunately, at pressure below 10-6 Torr, the signal currents generated by these tubes are so small that for the registration o f the whole mass spectrum a comparably long time is required. In order to avoid this drawback a new mass spectrometer tube has been developed. Due to the high signal-output o f this tube the whole mass spectrum, ranging from H + up to Hg +, can be dtsplayed on an oscilloscope at 5O Hz. Therefore, a practtcally contmuous control o f the residual gas can be achieved. The signal o f the mass spectrometer tube is generated by a cloud o f charged particles o f equal relative masses which oscillates along the axis o f an electrostatic fteM with axial symmetry. This ion cloud consists o f numerous superposed elementary clouds each o f which being created by an electron beam in a strong accelerating fieM near one end o f the tube. An electrode on the other end o f the tube is used to pick up the displacement-current induced by the ion cloud. The iomzmg electron beam is intensity-modulated with a variable frequency. Therefore, an wn cloud Is built up only when the variable modulation frequency o f the beam coincides with the oscillation frequency o f any sort o f ions. At frequencies other than those o f the ions present m the residual gas, the elementary ion clouds cannot form one big cloud because practically no phase relation between them exists. The space-charge action o f the elementary clouds explains that the functional dependence between mass intensity and partial pressure is not linear. A special tube having a resolvingpower m/Am .~ 20 at all masses, is described which can be used at pressures below 5 × 10-5 Torr. This upper limit is set by the mean free path o f the ions. The lower limit depends on the shot noise o f the first amplifier stage. Thus, at a registration speed o f 50 Hz, partial pressures m the order o f 10-8 Torr can be detected. A t still lower pressures a second operation method shouM be adopted whwh even permits the use of a n electron multiplier. DURING the last few years the measurement of partial pressures of the residual gas has been one of the aims of development in the technique of high vacuum. The scientific approach to this problem is based on the use of the well known mass spectrometer types. These devices have a high resolving power and a very good sensitivity. In the spectrogrammes of such devices the spectral lines of motopes may easily be separated and coordinated to mass numbers. Even the Fein-Struktur may be investigated. On the other hand, considering the technical applications of high vacuum, such as the manufacture of vacuum tubes, the problems are set quite otherwise. Here, high resolution is not absolutely necessary because in most cases it is known what sorts of vapours and gases can be expected in the residual gas. In the first place one wants to know approximately the quantity of each gas. These informattons should be given immediately. Furthermore, to meet the requirements of mass production, the spectrometer tube should be easy to outgas, and the control unit should be as compact as possible. Thus, resolving power and sensitivity may be neglected in favour of speed and simplicity of operation. Speed and
simphclty of operation, these are the predominant features of the tube to be discussed now, the features mostly desired for technical work. In order to give an introduction to the operation principle of the new tube, it seems favourable to proceed from the well known Omegatron. This tube is based on the principle of the Cyclotron and, therefore, requires a magnetic field. Unfortunately, even at the simplified Omegatron which is quite often used in the techmque of high vacuum, this magnetic field has to be generated by a comparatively ponderous permanent magnet. Thus, a large scale application of the simplified Omegatron in the production of vacuum tubes presents some difficulties. For technical applications there is still another drawback which the Omegatron has in common with some other mass spectrometers. This is the relatively long time required to record the whole mass spectrum. In view of these difficulties, when designing the new tube, we tried to avoid the use of a magnetic field by application of an electrostatic field. For this purpose, we replaced the almost circular motion of the ions in the Omegatron by 31
32
WERNER TRETNER
approximately linear oscillations in an electrostatic field. This principle shall be explained schematically m Fig. 1. The ion source is represented by an electron gun, the beam of which effects the ionization of the residual gas near one of the electrodes. For simplicity let us first consider only one sort of ions, for instance mercury ions. These ions, el.ektr~sches Fetd
--~
/ _I ]--J ~" Gun FIG. 1. The principle of the spectrometer tube. one of which is represented in Fig. 1 near the electrode on the left hand side, are forced by an inhomogeneous electrostatic field to oscillate between the two plain electrodes. It is this periodic motion which replaces the circular motion of the ions in the Omegatron. If now, an electrical field is superimposed which alternates at the same frequency as the mercury ions oscillate, these ions may gain momentum and, therefore, may reach one of the field confining electrodes. This kind of operation is analogous to the operation of the Omegatron. There is another mode of operation which is more practicable for technical applications than the Omegatron-like operation just described. Due to this mode of operation our tube will have an outstanding high output signal, naturally at the expense of resolution. It is this extremely high output signal which finally allows a high speed recording of the whole mass spectrum. Considering again F~g. 1, let us suppose that not only mercury vapour is present in the residual gas but also some other gases. Then, we are able to select one sort of ions by
modulating the ionizing electron beam with the same frequency as this sort of ions oscillates. During the first cycle, for instance, n ions may be generated. These move to and fro. During the second cycle n ions more are created, taking part on the discharge. Thus, at the beginning of each cycle, an elementary cloud of n Ions joins the famtly of ions. Finally a big cloud of ions is formed consisting of numerous elementary clouds which are separated by phase differences of 2n. This cloud of charged particles of equal relative masses generates an output signal by inducing displacement currents to the confining electrodes. All ions with oscillation frequencies other than that apphed to the control grid of the tube cannot form an ~on cloud because the phase d~fferences of their elementary clouds differ from 2n or a multiple of it. The high frequency signal can be amplified without the difficulties encountered at dc-amphficatlon. Furthermore, in order to measure all mass intensities practically at the same time, the modulation frequency of the ionizing electron beam may be sweeped periodically at mains frequency to cover the frequency band containing the oscillation frequencies of all masses to be investigated, for instance from mass 1 up to mass 300. The whole spectrum, therefore, may be recorded at 50 c.p.s. Before going into details, let us first have a look on the configuration of the electrodes of an experimental tube. In Fig. 2 we see a cage consisting of several electrodes. The Steuerelekt rode
Vertauf des A Potentlats - (~ tangs der Achse
FIG 2. The arrangement of the electrodes and the shape of the electric potentml q~along the axts
FIG. 3. An experimentalspectrometer tube,
An Electrostatic Mass Spectroscope voltages applied to them are chosen so that an ion created at the left hand electrode of the cage is accelerated towards the centre of the tube. It then enters the decelerating field at the other end of the tube. Here it is repelled, and the whole process begins again. The ionizing electron beam originates from a tungsten wire cathode. The beam is accelerated by the following electrode of the cage, penetrates a fine mesh screen and enters the electrode cage. By the decelerating fields at both sides of the mesh, the electrons are forced to oscillate around the mesh wires. Thus, an electron either creates an ion or will be captured by the mesh screen without having produced an ion. Just in front of the cathode, the control grid is arranged by which the modulation of the ionizing beam is effected. By the end electrode at the opposite side of the tube the displacement current is picked up and fed to the first stage of the preamplifier. The shape of the electrical field along the axis of the tube is shown below the electrodes. It is symmetric with respect to the centre of the cage. Each half of this ion-optical lens system is an immersion lens and, therefore, has a focussing action upon the ions. A n experimental tube designed according to the principles just described is shown in Fig. 3. The glass envelope has a length of 4 in. and a diameter of 1 in. The leads are strong enough to form a socket. The conductor of the signal electrode is arranged separately in order to avoid crosstalk from the modulated control grid. The various elements of the control unit by which the tube is operated are shown in the following block diagram, Fig. 4. The upper box on the right side symbolizes the voltage
r 0szJtt FIG
4 Block-dmgramof the control umt.
divider for the dc-voltages and the supply of the filament. Below this box we see the generator of the modulated, i.e. sweeped high frequency which is fed to the control grid. It is synchronized at mains frequency which also holds for the horizontal deflection of the oscilloscope. The output signal of the tube is fed to an ac-amphfier having a gain of about 10,000. The signal, after being rectified, passes a low pass filter in order to cut off the shot noise carried by the high frequencies, and then reaches the oscilloscope. Here, the mass spectrum may be observed in the co-ordinates of intensity and mass number. A photograph of a mass spectrum taken in the manner just described is shown in Fig. 5. As indicated by the mass numbers the different lines correspond to the ions H2 +, C +,
33
CH4 +, H20 +, CO +, CO2 +, Hg +, Hg +÷. The spectrum w a s taken at a total pressure of 2.10 -7 Torr on a mercury pumping system with liquid nitrogen on the mercury trap. The spectral lines at the mass numbers 7 = 28:4 and 112 = 28 × 4 are ghost lines of mass 28. The line at mass number 7 is generated by higher harmonics contained in the alternating current induced by the ions of relative mass 28. Ghost masses at mass numbers higher than that of the original mass, i.e. at lower frequencies, occur when the phase difference of two consecutive elementary ion clouds is a multiple of 2zt.
FIG 5. A mass spectrum of the residual gas of a mercury-d~ffus)on pumping system at a total pressure of 2 × 10--7Torr. Generally, only ghost lines of the first order occur, their intensities being less than 10 per cent than that of the main line. The resolving power wsually estimated according to the recorded mass spectrum on the oscilloscope is about ten for all masses. The limiting resolving power amounts to approximately 20 and may be obtained by selecting a small frequency interval and spreading it over the horizontal scale of the oscilloscope. The limiting sensitivity corresponds to partial pressures of somewhat less than 10-8 Torr. The limiting factor for the sensitivity is the shot noise of the first amplifier stage. Therefore, still lower pressures may be detected by using a slower registration speed. This can be done by varying by hand the modulation frequency applied to the control grid. In this case, optimum resolving power is about 20 too. Towards higher pressures a limitation is set by the mean free paths of the ions. Therefore, the upper pressure limit should be between l and 5 "< 10-5 Torr. It is important to know that the heights of the spectral lines do not exactly represent the partial pressures of the corresponding gases. This is due to the fact that space charge effects play a dominating role for the signal generating mechanism. Nevertheless, in a rough approximation the heights of the spectral lines give a fair impression of the contribution of the partial pressures to the total pressure. A n interesting mode of operation of the tube at low pressures is possible by applying an electron multiplier In connection with the Omegatron-hke operation principle. Fig. 6 shows how the tube construction then has to be modified. The former signal electrode should be perforated so that the ions which gamed momentum by the alternating
34
WERNER TRETNER
electrical field may pass the holes and enter the multiplier structure. Summarizing, it can be stated that this novel mass spectrometer tube, whose technical designation will be " Farvitron '" m a y be applied with success in the technique of high vacuum.
ii
The Farvitron as described in this paper, may be used at pressures ranging from 5 × 10-5 T o r r down to somewhat less than 10-8 Torr. Its o p t i m u m resolving power is about 20. Its main features are speed and simplicity of operation, which m a k e it most suitable for supervising work.
llr--II
I
cathode Lenssystem~ FIG. 6 Mass spectrometer tube w~th electron multlpher.