- Email: [email protected]
Ion accelerators in space plasma mass spectroscopy
Nuclear Instruments and Methods in Physics Research B40/41(1989)
750
750-754
North-Holland, Amsterdam
ION ACCELERATORS D.T. YOUNG, Department
IN SPACE PLASMA
J.L. BURCH
MASS SPECTROSCOPY
and J.A. MARSHALL
of Space Sciences, Southwest Research Institute, San Antonio, TX 78284, USA
Plasmas found in the solar wind and the magnetospheres of the Earth and other planets represent formidable experimental challenges in the area of ion mass spectroscopy. Spectrometers must cope with low density ( < 1 ion/cm3) high temperature plasmas (kT > 10 kev) that are highly anisotropic and heterogeneous. Ions may include singly and multiply charged atomic species such as hydrogen, helium, nitrogen, and oxygen, as well as, in the case of comets and other planets, a variety of molecular species. Calibration of space plasma mass spectrometers offers a comparable challenge: it requires an ion source and acceleration stages that will produce a uniform beam of large area ( f 5% over - 20 X 20 cm), and moderate intensity (lo2 -lo9 ions/cm’ s) from a wide variety of gases. Furthermore, the beam must be well collimated ( 2 0.5 o FWHM) and resolved (M/AM > 50, E/A E > 200) and have a stability of + 1% over periods > 8 h. Instruments to be calibrated are oriented in the beam using a 4 degree-of-freedom gimbal and translation system. A system which meets these requirements is currently being designed at Southwest Research Institute.
1. Introduction Instrumentation designed for in situ space research often bears little resemblance to its ground-based counterpart. Because of the great expense of fabricating and placing instruments in orbit (~$10~ per kilogram of experimental payload is not unusual) there is an understandable emphasis on obtaining maximum performance from every gram flown. In the case of space plasma physics, instrument types include electric and magnetic field probes and a wide variety of particle detectors including electrostatic analyzers and mass spectrometers. This paper will review briefly the design of the latter and will then concentrate on calibration methods with emphasis on the characteristics of ion accelerators designed specifically for this application.
Knowledge of plasma composition is essential to the study of the origins, acceleration, transport and loss processes of these populations. The problem, then, is to span as much of this complex 4-dimensional measurement space as is possible with as few instruments as possible (ideally one!). In space plasma mass spectroscopy it is generally acknowledged that three instrument types, characterized primarily by their energy ranges, are required. These ranges, together with the general instrument characteristics, are listed in table 1. Because the subject of instrument design is vast and complex we will confine the remainder of our discussion to middle energy range spectrometers spanning - 1 eV to - 50 keV. Reviews of instrumentation appropriate to the lower range can be found in ref. [l] and to the upper range in ref. [2]. Other reviews of mid-range instruments are found in refs. [3,4].
2. Space plasma instrument design Experimental space plasma physics is concerned with in situ measurement of the diverse plasmas found in the solar system. Typical plasma temperatures range from - 0.1 eV in the upper ionospheres of planets to - 50 keV in the trapped plasma populations of planetary magnetospheres. Corresponding densities vary from lo6 down to 1 ion cme3 for cold plasmas found in ionospheres and high altitude polar regions, and from 10 to 10-l ions cme3 for hot plasmas in auroral zones and outer magnetospheres of the magnetized planets such as Earth, Jupiter and Saturn. Plasma composition depends on a number of factors but the most interesting species in the terrestrial magnetosphere include H+, He2+, He+, 06+, 02+, and O+ with traces of molecular species. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Table 1 Categories of instruments used in space plasma mass spectroscopy Energy range
Techniques
- 0.1-10
1) retarding potential with magnetic analysis 2) quadrupole 3) rf time-of-flight 1) differential electrostatic with magnetic analysis 2) differential electrostatic with timsof-flight (TOF) 1) differential electrostatic with TOF and solid state detectors
eV
- 1 eV-5 X lo4 eV
-1eVto -5keVto
>3OOkeV(H+) >3OOkeV(O+)
751
D. i7 Young et al. / Ion accelerators in space plasma mass spectroscopy
Table 2 Mass spectrometer design parameters, range and resolution Parameter
Range
Resolution (FVJ’HM)
Energy per charge Mass per charge Field of view (instantaneous) Solid angle coverage Aperture area Sensitivity Count rate Temporal resolution Single measurement Full distribution
1 eV-50 keV l-100 amu loo ~360~ 4~ sr -1-5 cm2 21 count per lo* ions cm-*
E/AE=5 M/AM = 5-20
Number of pixels 64 64
loo XIOO 3X10e2 sr
-1300 _
s-l sr-l eV-’ _
.
Table 2 lists typical requirements on current state-ofthe-art performance for medium-energy-range mass spectrometers. From table 2 a number of conclusions can be drawn about space plasma mass spectrometers and their calibration: a) Energy, angular, and areal acceptances are unusually broad compared to laboratory instruments. Therefore the range of energy and angle to be covered in calibration is large as is the beam area. The total number of measurement pixels can be as large as - 64 x 64 x 1300 = 5.3 x 106. In order for a large fraction of these pixels to be stimulated during calibration the beam must be reliable, reproducible, and stable. b) The mass range is relatively large although mass resolution is not very great. Hence a few selected ion species may be sufficient for calibration purposes. c) High sensitivity and wide count rate dynamic range are necessary. This implies a wide dynamic range in beam intensity.
few X10W3 s 2s
beam (fig. 1). It is essential that the incoming ion beam completely fills the instrument aperture (hence the need for large beam area) and that the beam be collimated to an angular divergence less than - 10% of the instrument angular passband. Angular calibration is then achieved by rotating the instrument in the beam and measuring the detector count rate as a function of the two angles. Usually a normalizing measurement and absolute intensity calibration is made with the instrument aperture looking directly into the beam. If the energy acceptance of the instrument is < 2000 eV (for the system described by table 3) then the energy of the beam is modulated at a relatively high frequency (10-100 Hz) compared to detector sampling times, so as to uniformly cover the energy passband. In this way energy response can be measured at the same time as angular response and the energy-geometric factor determined from a single set of measurements. Outside of the voltage modulation range of - f 1000 V it is necessary to measure energy response separately (usually by sweeping the instrument plate voltages rather than the beam energy) at a number of angles and then fold this
3. Principles of calibration The basic idea of space plasma mass spectrometer calibration is to measure its response function in 4-dimensional phase space (2 angles, energy, mass) by using an ion beam with well-defined properties similar to those that are summarized in table 3. We emphasize that table 3 is a “wish list” rather than the specification for any system currently operating, with the possible exception of the system at the University of Bern [5]. The system currently operating at Southwest Research Institute will meet some, but not all of the above requirements. However, a new system is being designed at SwRI that will in principle satisfy all of the above requirements. The method of calibration is to place the mass spectrometer on a 2-axis-of-rotation turntable with the axes oriented perpendicular to, and centered on, the ion
Table 3 Ion accelerator requirements Parameter
Operating values
Energy range Energy resolution
- 1 eV-50 keV - 1 eV (minimum) < 5X10e3 (AE/E, maximum) - * 1 eV to f 1000 eV (triangular) l-100 amu > 50 (M/AM) 5 0.5O FWHM 2OX2Ocm *540ver90%0f20x20cm lo*-10’ ions/cm* s f 1% (up to 8 h) < 10e4 contamination of main beam
Energy modulation Mass range Mass resolution Angular collimation Cross section Uniformity Intensity Stability Purity Vacuum
VI. MATERIALS ANALYSIS FACILITIES
152
D. T. Young et al. / Ion accelerators in space plasma mass spectroscopy
Fig. 1. Schematic diagram showing instrument mounted in vacuum chamber with drift tube. Ion accelerator is located to left, out of figure. Note the relative orientation of instrument aperture, ion beam, and the orthogonal axes of instrument rotation (8, g) and translation (Y, 2).
in. The mass spectrum and resolution are generally calibrated with a few characteristic species (e.g., H+, He+, N+, NC) to establish peak shape and differential detection efficiencies due to differences in ion species. A further important consideration is the establishment of the whereabouts of any “ghost” peaks in the mass spectrum, usually down to a level commensurate with the desired sensitivity to minor ion species, e.g., to 10e4 of the main peak. (Ghost peaks are caused mainly by ion scattering from instrument surfaces and stray fields.) This is most easily achieved by summing over all angle-energy calibration data and then carefully examining the resultant mass spectrum. In order for ghost peaks to be accurately characterized the integrated primary ion peak must have at least 106-10’ counts.
4. Ion accelerator systems Fig. 2 is a block diagram of the SwRI ion accelerator system designed to carry out the calibrations discussed above. Fig. 2 is drawn to reflect both the present and planned functions in the SwRI systems. The present ion source uses electron bombardment ionization with electrons of variable energy (70-150 eV> and emission current (SO-2000 PA). Gases are leaked with a Granville-Phillips leak controller, however there is no closed loop control of beam intensity. In the new system this will be remedied by feeding a beam monitor signal from the main chamber back into the source emission control circuit. The beam monitor will be located near the aperture of the instrument under calibration. Because the electron bombardment source does not use magnetic confinement of the ionizing electrons, it is relatively inefficient and is not intense enough for all calibration
requirements. The new source will be a duoplasmatron or a similar discharge source in order to achieve higher beam current densities than the - lo5 ions/cm’s presently available with the electron bombardment source. Because of its brightness of - 10 A/cm2 sr, the duoplasmatron must be defocused in many instances in order to avoid burning out sensitive microchannel plate detectors in the instruments to be calibrated. After passing through the alignment electrodes and focus/defocus optics, the beam will be mass analyzed with a resolution M/AM - 50-100 (at present M/AM - 10). The resulting monomass beam is 5 mm in diameter. It is then rastered transverse to the beam axis in order to begin the process of expansion to the desired 20 X 20 cm area. Up to this point the beam is transported at an energy that is most convenient for source extraction and mass analysis (1 keV at present, - 5 keV with the duoplasmatron). The beam is decelerated to nearly zero energy by concentric spherical grids whose radius of curvature is centered on the raster plates. This causes the beam to expand further, and ions with high transverse energies may be entirely lost from the system. The expanded low-speed beam is then accelerated through a graded potential to the final beam energy and, if required, energy modulation up to &-1000 V is applied to the entire system. In order to collimate the beam to 0.5O FWHM without significant intensity losses, the beam travels 5 m (3 m in the present system) down a drift tube before entering the instrument chamber. If ions are required with energies below the source potential, then they are decelerated with a series of grids immediately before entering the chamber. In this way beam energies below 10 eV (- 100 eV in the present system) can be achieved without severe intensity losses. One problem encountered with the long drift tubes used in our accelerators is charge exchange of the primary beam which produces a fast neutral beam component. The new system will remedy this by having a pump dedicated to the beam tube in order to maintain pressure below - 5 x 1O-8 Torr. Immediately in front of the instrument to be calibrated there will be a beam monitor and diagnostic package (absent in the present system). A monitor signal from a channel-electron multiplier (CEM) will be fed back to control the source intensity. Other diagnostics will include an imaging MCP with 40 mm diameter sensitive area that can be swept across the beam to provide a flux intensity map with - 1 mm2 resolution. A high resolution beam energy measurement for both E. and AE will be provided, as will absolute current (using a large area (10 cm2) Faraday cup) and angular collimation measurements (using a pinhole collimator and CEM). Our intention is that the new system will provide all beam diagnostic information as well as other source and system status data on a video monitor dedicated to this purpose. While this is a common
Ion accelerators
D. T. Young et al. /
in space plasma mass spectroscopy
153
1
YIWEHT
I-
HAIf UHVCHAm ------------I RUAt_AXIS ROTATION -I-----r
I QUA I I I I I I I I_--_-____-I
I_
-
1
I I I I
_---____----I
I I___._--_---_------
__--___-__-----
I
Fig. 2. Block diagram of system componentsin the SwRI ion accelerator facility. Arrows show the direction of beam flow and are roughly proportional in cross-section to the beam intensity. Each box in the diagram represents one or more optical elements describedin the text.
practice on large accelerator systems it has not, to our knowledge, been done on space calibration chambers. The instrument to be calibrated is mounted on dualaxis rotation and dual-axis translation systems that provide f 180 * azimuth and f45’ elevation of rotational motion as well as & 10 “ ( f 2” in the present system) of travel for each of 2 orthogonal axes of translation. The motion system as well as all other system and instrument functions are monitored and controlled via CAMAC interfaces to standard PC equipment and to a larger VAX minicomputer. 5. system parformance The present SwRI system has been used over the past 7 years to calibrate a number of satellite and rocket-borne plasma analyzers including instruments
flown on Dynamics Explorer, the AMPTE/Charge Com~sition Explorer, and the Giotto mission. It has also been used extensively for prototype development of magnetic and time-of-flight mass spectrometers that will be flown on the Comet Rendezvous Asteroid Flyby (CRAF) and International Solar Terrestrial Physics (ISTP)-Polar spacecraft. Our new system is planned for ~nst~ction during the next 3” years and will incorporate the improved features outlined above. These features reflect the evolution of satellite-borne mass spectrometers. In particular, the required large beam area and high intensity derive from trends toward very large aperture and 360° fieldof-view instruments [f&7], while the large mass range and resolution are needed for planetary missions in which the number of ion species of interest is greater and the species are more closely spaced in M/Q. VI. MATERIALS ANALYSIS FACILITIES
154
D. T. Young et al. / Ion accelerators in space plasma mass spectroscopy
This work was supported Institute.
by Southwest
Research
References [l] V.M. VasyIinnas, Meth. Exp. Phys. 9B, eds. R.H. Lovberg and H.R Griem (Academic, New York, 1971) p. 49. [2] B. WiIken, T.A. Fritz and W. Studemami, Nucl. Instr. and Meth. 196 (1982) 161.
[3] H. BaIsiger, Adv. Space Res. 2 (1983) 3. [4] D.T. Young, Amer. Geophys. Union Monogr. Ser. 46 (1989) in press. [5] A.G. Ghielmetti, H. Balsiger, R. Banninger, P. Eberhardt, J. Geiss and D.T. Young, Rev. Sci. Instr. 54 (1983) 425. [6] D.T. Young, S.J. Bame, M.F. Thomsen, R.H. Martin, J.L. Burch, J.A. Marshall and B. Reinhard, Rev. Sci. Instr. 59 (1988) 743. [7] D.T. Young, J.A. Marshall, J.L. Burch, S.J. Bame and R.H. Martin, Amer. Geophys. Union Monogr. Ser. 46 (1989) in press.
750
750-754
North-Holland, Amsterdam
ION ACCELERATORS D.T. YOUNG, Department
IN SPACE PLASMA
J.L. BURCH
MASS SPECTROSCOPY
and J.A. MARSHALL
of Space Sciences, Southwest Research Institute, San Antonio, TX 78284, USA
Plasmas found in the solar wind and the magnetospheres of the Earth and other planets represent formidable experimental challenges in the area of ion mass spectroscopy. Spectrometers must cope with low density ( < 1 ion/cm3) high temperature plasmas (kT > 10 kev) that are highly anisotropic and heterogeneous. Ions may include singly and multiply charged atomic species such as hydrogen, helium, nitrogen, and oxygen, as well as, in the case of comets and other planets, a variety of molecular species. Calibration of space plasma mass spectrometers offers a comparable challenge: it requires an ion source and acceleration stages that will produce a uniform beam of large area ( f 5% over - 20 X 20 cm), and moderate intensity (lo2 -lo9 ions/cm’ s) from a wide variety of gases. Furthermore, the beam must be well collimated ( 2 0.5 o FWHM) and resolved (M/AM > 50, E/A E > 200) and have a stability of + 1% over periods > 8 h. Instruments to be calibrated are oriented in the beam using a 4 degree-of-freedom gimbal and translation system. A system which meets these requirements is currently being designed at Southwest Research Institute.
1. Introduction Instrumentation designed for in situ space research often bears little resemblance to its ground-based counterpart. Because of the great expense of fabricating and placing instruments in orbit (~$10~ per kilogram of experimental payload is not unusual) there is an understandable emphasis on obtaining maximum performance from every gram flown. In the case of space plasma physics, instrument types include electric and magnetic field probes and a wide variety of particle detectors including electrostatic analyzers and mass spectrometers. This paper will review briefly the design of the latter and will then concentrate on calibration methods with emphasis on the characteristics of ion accelerators designed specifically for this application.
Knowledge of plasma composition is essential to the study of the origins, acceleration, transport and loss processes of these populations. The problem, then, is to span as much of this complex 4-dimensional measurement space as is possible with as few instruments as possible (ideally one!). In space plasma mass spectroscopy it is generally acknowledged that three instrument types, characterized primarily by their energy ranges, are required. These ranges, together with the general instrument characteristics, are listed in table 1. Because the subject of instrument design is vast and complex we will confine the remainder of our discussion to middle energy range spectrometers spanning - 1 eV to - 50 keV. Reviews of instrumentation appropriate to the lower range can be found in ref. [l] and to the upper range in ref. [2]. Other reviews of mid-range instruments are found in refs. [3,4].
2. Space plasma instrument design Experimental space plasma physics is concerned with in situ measurement of the diverse plasmas found in the solar system. Typical plasma temperatures range from - 0.1 eV in the upper ionospheres of planets to - 50 keV in the trapped plasma populations of planetary magnetospheres. Corresponding densities vary from lo6 down to 1 ion cme3 for cold plasmas found in ionospheres and high altitude polar regions, and from 10 to 10-l ions cme3 for hot plasmas in auroral zones and outer magnetospheres of the magnetized planets such as Earth, Jupiter and Saturn. Plasma composition depends on a number of factors but the most interesting species in the terrestrial magnetosphere include H+, He2+, He+, 06+, 02+, and O+ with traces of molecular species. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Table 1 Categories of instruments used in space plasma mass spectroscopy Energy range
Techniques
- 0.1-10
1) retarding potential with magnetic analysis 2) quadrupole 3) rf time-of-flight 1) differential electrostatic with magnetic analysis 2) differential electrostatic with timsof-flight (TOF) 1) differential electrostatic with TOF and solid state detectors
eV
- 1 eV-5 X lo4 eV
-1eVto -5keVto
>3OOkeV(H+) >3OOkeV(O+)
751
D. i7 Young et al. / Ion accelerators in space plasma mass spectroscopy
Table 2 Mass spectrometer design parameters, range and resolution Parameter
Range
Resolution (FVJ’HM)
Energy per charge Mass per charge Field of view (instantaneous) Solid angle coverage Aperture area Sensitivity Count rate Temporal resolution Single measurement Full distribution
1 eV-50 keV l-100 amu loo ~360~ 4~ sr -1-5 cm2 21 count per lo* ions cm-*
E/AE=5 M/AM = 5-20
Number of pixels 64 64
loo XIOO 3X10e2 sr
-1300 _
s-l sr-l eV-’ _
.
Table 2 lists typical requirements on current state-ofthe-art performance for medium-energy-range mass spectrometers. From table 2 a number of conclusions can be drawn about space plasma mass spectrometers and their calibration: a) Energy, angular, and areal acceptances are unusually broad compared to laboratory instruments. Therefore the range of energy and angle to be covered in calibration is large as is the beam area. The total number of measurement pixels can be as large as - 64 x 64 x 1300 = 5.3 x 106. In order for a large fraction of these pixels to be stimulated during calibration the beam must be reliable, reproducible, and stable. b) The mass range is relatively large although mass resolution is not very great. Hence a few selected ion species may be sufficient for calibration purposes. c) High sensitivity and wide count rate dynamic range are necessary. This implies a wide dynamic range in beam intensity.
few X10W3 s 2s
beam (fig. 1). It is essential that the incoming ion beam completely fills the instrument aperture (hence the need for large beam area) and that the beam be collimated to an angular divergence less than - 10% of the instrument angular passband. Angular calibration is then achieved by rotating the instrument in the beam and measuring the detector count rate as a function of the two angles. Usually a normalizing measurement and absolute intensity calibration is made with the instrument aperture looking directly into the beam. If the energy acceptance of the instrument is < 2000 eV (for the system described by table 3) then the energy of the beam is modulated at a relatively high frequency (10-100 Hz) compared to detector sampling times, so as to uniformly cover the energy passband. In this way energy response can be measured at the same time as angular response and the energy-geometric factor determined from a single set of measurements. Outside of the voltage modulation range of - f 1000 V it is necessary to measure energy response separately (usually by sweeping the instrument plate voltages rather than the beam energy) at a number of angles and then fold this
3. Principles of calibration The basic idea of space plasma mass spectrometer calibration is to measure its response function in 4-dimensional phase space (2 angles, energy, mass) by using an ion beam with well-defined properties similar to those that are summarized in table 3. We emphasize that table 3 is a “wish list” rather than the specification for any system currently operating, with the possible exception of the system at the University of Bern [5]. The system currently operating at Southwest Research Institute will meet some, but not all of the above requirements. However, a new system is being designed at SwRI that will in principle satisfy all of the above requirements. The method of calibration is to place the mass spectrometer on a 2-axis-of-rotation turntable with the axes oriented perpendicular to, and centered on, the ion
Table 3 Ion accelerator requirements Parameter
Operating values
Energy range Energy resolution
- 1 eV-50 keV - 1 eV (minimum) < 5X10e3 (AE/E, maximum) - * 1 eV to f 1000 eV (triangular) l-100 amu > 50 (M/AM) 5 0.5O FWHM 2OX2Ocm *540ver90%0f20x20cm lo*-10’ ions/cm* s f 1% (up to 8 h) < 10e4 contamination of main beam
Energy modulation Mass range Mass resolution Angular collimation Cross section Uniformity Intensity Stability Purity Vacuum
VI. MATERIALS ANALYSIS FACILITIES
152
D. T. Young et al. / Ion accelerators in space plasma mass spectroscopy
Fig. 1. Schematic diagram showing instrument mounted in vacuum chamber with drift tube. Ion accelerator is located to left, out of figure. Note the relative orientation of instrument aperture, ion beam, and the orthogonal axes of instrument rotation (8, g) and translation (Y, 2).
in. The mass spectrum and resolution are generally calibrated with a few characteristic species (e.g., H+, He+, N+, NC) to establish peak shape and differential detection efficiencies due to differences in ion species. A further important consideration is the establishment of the whereabouts of any “ghost” peaks in the mass spectrum, usually down to a level commensurate with the desired sensitivity to minor ion species, e.g., to 10e4 of the main peak. (Ghost peaks are caused mainly by ion scattering from instrument surfaces and stray fields.) This is most easily achieved by summing over all angle-energy calibration data and then carefully examining the resultant mass spectrum. In order for ghost peaks to be accurately characterized the integrated primary ion peak must have at least 106-10’ counts.
4. Ion accelerator systems Fig. 2 is a block diagram of the SwRI ion accelerator system designed to carry out the calibrations discussed above. Fig. 2 is drawn to reflect both the present and planned functions in the SwRI systems. The present ion source uses electron bombardment ionization with electrons of variable energy (70-150 eV> and emission current (SO-2000 PA). Gases are leaked with a Granville-Phillips leak controller, however there is no closed loop control of beam intensity. In the new system this will be remedied by feeding a beam monitor signal from the main chamber back into the source emission control circuit. The beam monitor will be located near the aperture of the instrument under calibration. Because the electron bombardment source does not use magnetic confinement of the ionizing electrons, it is relatively inefficient and is not intense enough for all calibration
requirements. The new source will be a duoplasmatron or a similar discharge source in order to achieve higher beam current densities than the - lo5 ions/cm’s presently available with the electron bombardment source. Because of its brightness of - 10 A/cm2 sr, the duoplasmatron must be defocused in many instances in order to avoid burning out sensitive microchannel plate detectors in the instruments to be calibrated. After passing through the alignment electrodes and focus/defocus optics, the beam will be mass analyzed with a resolution M/AM - 50-100 (at present M/AM - 10). The resulting monomass beam is 5 mm in diameter. It is then rastered transverse to the beam axis in order to begin the process of expansion to the desired 20 X 20 cm area. Up to this point the beam is transported at an energy that is most convenient for source extraction and mass analysis (1 keV at present, - 5 keV with the duoplasmatron). The beam is decelerated to nearly zero energy by concentric spherical grids whose radius of curvature is centered on the raster plates. This causes the beam to expand further, and ions with high transverse energies may be entirely lost from the system. The expanded low-speed beam is then accelerated through a graded potential to the final beam energy and, if required, energy modulation up to &-1000 V is applied to the entire system. In order to collimate the beam to 0.5O FWHM without significant intensity losses, the beam travels 5 m (3 m in the present system) down a drift tube before entering the instrument chamber. If ions are required with energies below the source potential, then they are decelerated with a series of grids immediately before entering the chamber. In this way beam energies below 10 eV (- 100 eV in the present system) can be achieved without severe intensity losses. One problem encountered with the long drift tubes used in our accelerators is charge exchange of the primary beam which produces a fast neutral beam component. The new system will remedy this by having a pump dedicated to the beam tube in order to maintain pressure below - 5 x 1O-8 Torr. Immediately in front of the instrument to be calibrated there will be a beam monitor and diagnostic package (absent in the present system). A monitor signal from a channel-electron multiplier (CEM) will be fed back to control the source intensity. Other diagnostics will include an imaging MCP with 40 mm diameter sensitive area that can be swept across the beam to provide a flux intensity map with - 1 mm2 resolution. A high resolution beam energy measurement for both E. and AE will be provided, as will absolute current (using a large area (10 cm2) Faraday cup) and angular collimation measurements (using a pinhole collimator and CEM). Our intention is that the new system will provide all beam diagnostic information as well as other source and system status data on a video monitor dedicated to this purpose. While this is a common
Ion accelerators
D. T. Young et al. /
in space plasma mass spectroscopy
153
1
YIWEHT
I-
HAIf UHVCHAm ------------I RUAt_AXIS ROTATION -I-----r
I QUA I I I I I I I I_--_-____-I
I_
-
1
I I I I
_---____----I
I I___._--_---_------
__--___-__-----
I
Fig. 2. Block diagram of system componentsin the SwRI ion accelerator facility. Arrows show the direction of beam flow and are roughly proportional in cross-section to the beam intensity. Each box in the diagram represents one or more optical elements describedin the text.
practice on large accelerator systems it has not, to our knowledge, been done on space calibration chambers. The instrument to be calibrated is mounted on dualaxis rotation and dual-axis translation systems that provide f 180 * azimuth and f45’ elevation of rotational motion as well as & 10 “ ( f 2” in the present system) of travel for each of 2 orthogonal axes of translation. The motion system as well as all other system and instrument functions are monitored and controlled via CAMAC interfaces to standard PC equipment and to a larger VAX minicomputer. 5. system parformance The present SwRI system has been used over the past 7 years to calibrate a number of satellite and rocket-borne plasma analyzers including instruments
flown on Dynamics Explorer, the AMPTE/Charge Com~sition Explorer, and the Giotto mission. It has also been used extensively for prototype development of magnetic and time-of-flight mass spectrometers that will be flown on the Comet Rendezvous Asteroid Flyby (CRAF) and International Solar Terrestrial Physics (ISTP)-Polar spacecraft. Our new system is planned for ~nst~ction during the next 3” years and will incorporate the improved features outlined above. These features reflect the evolution of satellite-borne mass spectrometers. In particular, the required large beam area and high intensity derive from trends toward very large aperture and 360° fieldof-view instruments [f&7], while the large mass range and resolution are needed for planetary missions in which the number of ion species of interest is greater and the species are more closely spaced in M/Q. VI. MATERIALS ANALYSIS FACILITIES
154
D. T. Young et al. / Ion accelerators in space plasma mass spectroscopy
This work was supported Institute.
by Southwest
Research
References [l] V.M. VasyIinnas, Meth. Exp. Phys. 9B, eds. R.H. Lovberg and H.R Griem (Academic, New York, 1971) p. 49. [2] B. WiIken, T.A. Fritz and W. Studemami, Nucl. Instr. and Meth. 196 (1982) 161.
[3] H. BaIsiger, Adv. Space Res. 2 (1983) 3. [4] D.T. Young, Amer. Geophys. Union Monogr. Ser. 46 (1989) in press. [5] A.G. Ghielmetti, H. Balsiger, R. Banninger, P. Eberhardt, J. Geiss and D.T. Young, Rev. Sci. Instr. 54 (1983) 425. [6] D.T. Young, S.J. Bame, M.F. Thomsen, R.H. Martin, J.L. Burch, J.A. Marshall and B. Reinhard, Rev. Sci. Instr. 59 (1988) 743. [7] D.T. Young, J.A. Marshall, J.L. Burch, S.J. Bame and R.H. Martin, Amer. Geophys. Union Monogr. Ser. 46 (1989) in press.