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Energy-mass spectrometer for low energy weak ion flux analysis
Acta Astronautica Vol. 17, No. 5, pp. 539-544, 1988
0094-5765/88 $3.00 + 0.00 Pergamon Press pie
Printed in Great Britain
E N E R G Y - M A S S SPECTROMETER FOR LOW ENERGY WEAK ION FLUX ANALYSISt Y. SEMKOVAl, R. KOLEVAl, P. BAYNOVl, E. SAVOV1, N. KANCHEVl, O. VAYSBERG2, V. SMIRNOV2, A. FEDOROV2 and A. LEYBOV2 qnstitute for Space Research at the Bulgarian Academy of Sciences, 6 Moskovska Street, Sofia-1000, Bulgaria and 2Space Research Institute, U.S.S.R. Academy of Sciences, Profsojusnaj Street 84/32, 117810, GSP-7, U.S.S.R. (Received 19 August 1986; revised version received 17 September 1987) Abstract--An instrument for energy- and mass-spectrum measurements of ion fluxes in the Earth magnetotail is described. The ion selection by energy and mass is provided by sequential analysis of the incident ion beam in the electrostatic field of a cylindrical detector and the field of a permanent magnet. The measurement of ion energy distribution is carried out through scanning the voltage supplied to the electrostatic deflector. At a fixed energy the intensities of the different ion fluxes are measured simultaneously. An electrostatic angular scanner allows for the variation of the direction of the polar view-angle (in a frame of reference with a polar axis coincident with the spin axis of the spacecraft). The scanning of the azimuth angle is obtained while the satellite is spinning. The operational principle and the design of the instrument are described and the results from the laboratory testing are presented.
l. INTRODUCTION The multiparametric spectrometry of magnetotail plasma is a challenging experimental problem. The magnetotail consists of three distinct plasma regimes: the plasma sheet, the tail lobes and the boundary layer or mantle. Plasma density, temperature and composition in the three configurations are quite different. The temperature can vary from few electronvolts to several kiloelectronvolts. The ion populations can be of different origin which specifies their different composition. Besides the isotropic plasma background, ion streams are continuously observed. The can flow both Earthwards or antisunwards and in dawn-dusk direction, though the latter case being more seldom. The velocity of these plasma fluxes can vary from 20km/s in quiet conditions, up to 500-700 km/s during substorms, while in the boundary layer and in the mantle very high speed flows of about 1000 km/s are observed sometimes. The flux density is very low--from 1 to 105 (cm 2 s sr eV) -l. The substorm processes, being in the core of modem investigations, have characteristic times of about 10s. On the other hand, magnetotail investigations are conducted by high apogee satellites at strongly elliptic orbits. For that reason the plasma populations--the plasma sheet, boundary layer--are sometimes crossed very quickly within a few minutes or sometimes this crossing lasts for hours. These complicated conditions impose very strong requirements upon an energy-mass spectrometer, detBased on paper IAF-86-292 presented at the 37th Congress of the International Astronautical Federation, Innsbruck, Austria, 4-11 October 1986. 539
signed for magnetotail measurements: (1) large integrated geometric factor, providing the necessary rapid functioning of the instrument; (2) the view angle of the device should enable the angular analysis of scanning through the field of view; (3) large energy range; (4) ability for an adaptive experiment control, onboard data processing and flexible information capacities. A simplifying condition is the fact that a high mass resolution is not needed for the magnetotail--it is enough to identify ions H +, He 2+ and O +. In this paper, the Analyzer by Mass and Energy of the Ions (AMEI) instrument is described. The device will be flown onboard the high apogee satellite Magnetotail Probe of the Interball Project. A M E I is designed to measure H +, He 2+, He + and O + ions in the energy range 0-10 keV and it is also able to perform angular scanning. The instrument consists of two identical, mounted, sensors, one viewing in the direction of the sun, the other viewing in an antisunward direction, and an electronics assembly.
2. DESCRIPTION OF THE SENSOR
2.1. Main ion optics The ion optics is schematically shown in Fig. 1. The main energy-mass separating part is similar to that described by Moor[l] based on the geometry optimized by Hinterberger and K6nig[2]. By means of a 53 ° sector cylindrical analyzer, the incident ion beam is filtrated by energy through the analyzer and only particles with an energy-to-charge ratio E/q in the energy bandpass, defined by the voltage UCA supplied to the deflector's electrodes, will pass.
Y. SEMKOVAet al.
540
ANGUt SCANI~ AS
TRAJECTORIES
Fig. 1. A schematic view of AMEI ion optics. By scanning UCA in steps, the necessary energy range is covered. Further, the ion beam passes through the magnet assembly, consisting of a permanent 90 ° sector magnet and a yoke; the magnet being with a field intensity of 4500 G in the 4 mm gap. As the gyro path of the particles with equal E/q depends on the ratio M/q (where M is the mass of the particle), a space distribution of the particles dependent on their M/q is obtained at the magnet exit. The particle space distribution is recorded by means of a coordinate-sensitive detector (CSD). The ratio M/q of the particle can be identified on the basis of its E/q ratio, determined by UCA, and its position on a CSD. The disposition of the entrance aperture, the cylindrical analyzer, its sector angle, the magnet, the form of its entrance (to the ion beam) surface and the position of the focal (detector) plane, proposed by Hinterberger and K6nig[2], provides for secondorder focusing.
2.2. Peculiarities of the sensor In the instrument suggested by Moor[l] the characteristic (scale) size determining the linear dimensions of the sensor, r0, is 50 mm. A sensor with a similar scale factor analyzing particles with ME in the range 0-160 (M in a.m.u., E in keV) would be too large and heavy for operating aboard a high apogee satellite. In our sensor r0 = 25 mm. As for the cylindrical analyzer with equal sector angles the geometric factor is proportional to their linear size[3], it is natural to
expect decreasing in the geometric factor of our instrument. An alternative has been found by maximum opening of the entrance aperture and mounting a slit in front of the magnet entrance. Thus, the high mass resolution of the system is decreased to the lowest possible limit--resolving only particles with M/q = 1, 2, 4 and 16, but increasing the geometric factor to the possible upper limit. Furthermore, in order to minimize the size of the magnet which is the main weight in the instrument, two modifications of Moor's[l] ion optics are carried out. First, the part of the magnet, just opposite its entrance, is cut in an appropriate way, so that particles with suitable ME could get out of the analyzer through this second exit; we will refer to it as the "cut of the magnet". The particles going out through the cut of the magnet are recorded by the purpose-constructed microchannel detector. No focusing properties are expected at this exit. Second, the whole assembly cylindrical analyzer-magnet~letectors, is mounted in an electrostatic shield. Accelerating or retarding voltage is supplied to the latter. In this way the range of ME values for the particles passing through the magnet is limited, without diminishing this range for the particles incident to the device. Besides, the measurement scheme is chosen so that up to E/q = 2.7 keV for the particles passing through the magnet, the four masses with M/q = 1, 2, 4 and 16 are positioned on the CSD. When the energy of the particles in the magnet is greater than 2.7 keV, then particles with M/q = 1, 2 and 4 hit the CSD, and only O ÷ ions hit the microchannel detector at the cut of the magnet. So the CSD is not needed at this exit of the magnet and one has only to count the leaving particles.
2.3. Angular scanner At the entrance of the energy-mass analyzing part, an angular scanner, proposed by Vaysberg et al.[4] is mounted. It represents a 180 ° cylindrical analyzer, whose outer electrode is a wire grid of 95% transparency and is kept at zero potential. A negative voltage UAS is supplied to the inner electrode. At a given ratio between the design kinetic energy E0 of the cylindrical analyzer (i.e. Uc^) and the voltage UAS, the view angle + Act of the cylindrical analyzer (CA) is tilted to a polar angle fl, i.e. the field of view is shifted in the range (fl - Act~, fl + Act~). In this way, by varying UAS we can sequentially scan the polar angle (in a frame of reference with a polar axis coincident with the spin axis of the satellite, parallel to the central axis of the sensor view angle at UAS = 0). The azimuth angle is scanned while the satellite is spinning. 3. T H E ELECTRONICS ASSEMBLY
3.1. Block-diagram The electronics system of the instrument is spatially separated into three structural units; two identical units (high voltage supplies and preamplifiers) moun-
Energy-mass spectrometer
541
ted in the two sensor boxes and one unit (two identical analog-digital parts and one central data processing unit), installed in the electronics box. In this way, the device comprises two identical measurement channels, connected with the two sensors, and a common data processing unit (DPU). Only one of the two channels operates at any time. The choice is made by the DPU. The block-diagram of one of the measurement channels and the DPU is shown in Fig. 2. At the beginning of each measurement cycle the DPU sets the entire electronics circuitry of the device at the initial state and feeds the control signals for fixing the initial voltages to: (1) The HVAS-the power supply of the negative voltage step; /-/As, for the inner electrode of the angular scanner. The outer electrode of the scanner is grounded. (2) The HVCA--the power supply of symmetric bipolar high voltage step, UCA, for the electrodes of the cylindrical analyzer. (3) The HVES---the power supply of bipolar high voltage step, UEs, for the electrostatic shield and the magnet. (4) The HVMCP--the high voltage power supply for the bias voltage of the microchannel plates of the coordinate-sensitive detector and the microchannel detector. Scanning the voltage UCA supplied by HVCA at a fixed tilt of the field of view, determined by the voltage UAS supplied by HVAS, the energy spectrum of the four ion species is measured.
3.2. Particle registration Leaving the mass separator, the ions are trapped either by the CSD or by the microchannel detector (MCD). The MCD is composed of two cascadeconnected microchannel plates and a charge collecting anode at their output. The MCD has a rectangular form with the relevant dimensions. The CSD consists of an input charge multiplying element---cascade-connected microchannel plates and an anode for charge collecting and coding the coordinate. We use the two-electrode anode in the form of strips and wedges, proposed by Martin et aL[3] with analog coding of the charge position. A characteristic feature of the coordinate X of the input for the CSD charge is the ratio X = 2b/(a + b), where a and b are the amplitudes of the relevant signals of the two-charge-sensitive amplifiers AMPa and AMPb, connected with the two electrodes of the anode. When the ion is trapped by the CSD the signals a and b are supplied through AMP a and AMPb to the respective peak detectors, fixing their maximum amplitude values, and, further on, to a summator. The signals from one of the peak detectors and the summator, necessary to obtain the ratio b/(a + b) are supplied to the signal and reference inputs, respectively, of a fast analog-to-digital convertor
I DATA PROCESSING UNIT (DPU} ] MICROPROCESSORADDRESS/ I DATA/ CONTROL BUS I EXPERIMENT CONTROL
I
STORAGE
DATA FORMATING
C.A E
L L°_ NTROL I
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Fig. 2. A block-diagram of AMEI sensor and electronics assemblies. (ADC) with para!lel convertion. When the maximum amplitude values are fixed, the peak detectors feed signal "START" to the ADC. At the end of the conversion the output information of the ADC represents the digital code of the ratio b/(a + b). This code and the address signal through the ADC are then supplied to the buffer memory of the DPU for intermediate memorizing. Thus in the DPU's memory information is accumulated about the number and coordinate distribution of the registered by the CSD ions at fixed values of the analyzed energy and tilt of the view angle. At the values thus fixed, the boundary coordinates of the positions of the different ion species of the CSD, preliminary computed and specified during the calibration of the instrument, are recorded in the prom of the DPU. The latter identifies the mass of the registered particles comparing their position on the CSD with these boundary coordinates. In addition, in the DPU's memory the number of the registered MCD O + ions is accumulated. The
Y. S~IKOVAet al.
542 B=o UAS : 0
(a)
(b)
1.0
o lt I I I I I I /) : 28o30 ~ UAS: 0 . 2 9 EO
1.0 E Z Z
0.5
I
I
= 50" UAS: 0 . 4 3 Eo 1.0
0.5
-10"
I I I I I I I I I 0 ° 10" 2 0 ° 30 ° 4 0 " 5 0 e 6 0 ° 70 ° 8 0 e
a
I
IL
IL~I
I
l
0.85 0.90 0.95 1.00 1.05 1.10 1.20 1.25 1.30
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Fig. 3. Measured characteristics of the system angular-scanner-cylindrical-analyzer. (a) Angular characteristics; (b) energy characteristics. DPU provides the necessary data compression and formating and the link with the telemetric system of the satellite. 4. P R O T O T Y P E L A B O R A T O R Y R E S U L T S
4.1. Testing of the angular scanner For laboratory testing purposes of the sensor, two different laboratory sensor units were constructed: an energy-mass analyzing unit and a sensor box comprising the system angular-scanner-cylindrical analyzer. The angular and energy characteristics of the latter system are shown in Fig. 3 (a) and (b), respectively. They are measured at different voltages of the scanner, corresponding to different tilt angles/~ of the field of view. The characteristics are measured in the following way: the sensor box was exposed to an ion beam well collimated, monoenergetic, with energy of 1 keV, wider than the surface of the scanner, with an angle of incidence/~. The most appropriate pair of voltages (U^s, Uc^) were determined by the maximum of particles leaving the cylindrical analyzer, detected by a channel electron multiplier. Then, the
voltages were fixed and the energy and the angle of the incident particles were varied sequentially, thus measuring the matrix ~(fl; E, a). The angular characteristics in Fig. 3(a) are obtained through integrating this matrix by energy, and the energy characteristics in Fig. 3(b) are obtained by integrating the matrix by angle. As the nominal field of view of the system (that at/~ = 0) is too wide--12 ° at half width, when tilted it becomes much more wider and it is more difficult to shift. This is evident from the second and the third panels in Fig. 3(a). There E0 is the design kinetic energy of the CA, in this case it is 1 keV. Therefore, the scanner is not going to be used for the angular analysis of the ion fluxes but for suitable tilting the field of view, thus enabling the device to trap the ion fluxes, incident to the instrument at different angles. The energy curves display a 16% resolution, with a typical high-energy wing.
4.2. Energy-mass analyzing properties The testing of the energy-mass analyzing unit was performed as follows: at an angle/3 = 0 and a design
Energy-mass spectrometer
20
30
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io
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.............
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Fig. 4. Pattern of the ion beams leaving the energy-mass analyzing system. kinetic energy of the CA, E0 = 2 keV, the sensor box was exposed sequentially to mass-calibrated ion beams, namely H + (M/q = 2), He + (M/q = 4) and 0 + (M/q = 16). The incident ion beam was well colimated and strictly monoenergetic. The sensor box could be rotated around the axis fl = 0. Particle detection at the magnets' exits was done by a channel electron multiplier with an entrance slit of 1 mm width, mounted so that it could be remotely moved along axes parallel to the exit and the cut of the magnet, and spaced at different distances from them. Thus, the pattern of the analyzed ion beams with different masses, different initial energies and angles of incidence, was obtained. The results are shown in Fig. 4. The beams with energies E0 = 2 keV are given in thick solid lines; those with E = 0.9 E 0 by thin solid lines; E = 1.09 E0 by dashed lines, and E = 1.2 E 0 by dotted lines. At each energy value measurements with a step A~ = I ° are performed for every leaving beam with an angle of incidence in the entire angular bandpass (specified by the energy). Attention should be drawn to the peculiarities of the disposition of the leaving He + ion beams, i.e. MEo = 8. There exists such a distance from the magnet, at which the four different energies of the He + ions are well separated, i.e. the system can resolve M E = 7.2, M E = 8.4 and M E = 9.6. This allows for the use of the system in experiments where greater mass resolution is needed, preserving the small size and the good sensitivity of the sensor. At the cut of the magnet only the size of the microchannel detector, positioned at 11 mm from the magnet, was determined. At the near end of the
cut (Fig. 4, lower) the position of particles with M E = 43.2(O +, E = 2.7 keV) is drawn by a thin sold line, and that of particles with M E = 4 8 (O +, E = 3 keV) by a thick solid line. At the upper end of the MCD the far-most position of the O + ions is shown. Naturally the geometric factor of the system depends on the lengths of the gyropass of the particles as the magnet gap limits the azimuth angle of the passing particles. The geometric factor measured varies from 0.9.10 -4 x E0 up to 3.6.10 -4 x E0, for the ion trajectories permitted by the modes of operation.
5. CONCLUSION The Analyzer by Mass and Energy of the Ions proposed here, possess the capabilities necessary for providing energy-mass analysis of the weak low energy ion fluxes in the magnetotail--good sensitivity, capacity for directing the field of view towards the ion flux, capability for onboard control of the experiment and data processing. The M E bandpass ( M - - t h e mass number, E - - t h e energy of the ions) could be enlarged significantly by means of the electrostatic shield.
REFERENCES
1. T. E. Moor, EOS Trans. 60, 326 (1979). 2. H. Hinterberger and L. A. K6nig, Advances in Mass Spectrometry, p. 16. Pergamon Press, London (1959).
544
Y. SEMKOVAet al.
3. E. Savoy, Some general relations between the geomet~ and the parameters of cylindrical electrostatic spectrometers. C.r. Acad. bulg. Sci. 38, 1645-1648 (1985). 4. O. L. Vaysberg, B. I. Hazanov and P. L. Shifrin, Author's Certificate 7776, 395. Bulletin of Inventions,
Discoveries, Symbols and Blanks, No. 13 (in Russian) (1983). 5. C. Martin, P. Jelinsky, M. Lampton, R. E. Malina and H. O. Anger, Wedge-and-strip anodes for centroidfinding position sensitive photon and particle detectors. Rev. scient. Instrum. 52, 1067-1074 (1981).
0094-5765/88 $3.00 + 0.00 Pergamon Press pie
Printed in Great Britain
E N E R G Y - M A S S SPECTROMETER FOR LOW ENERGY WEAK ION FLUX ANALYSISt Y. SEMKOVAl, R. KOLEVAl, P. BAYNOVl, E. SAVOV1, N. KANCHEVl, O. VAYSBERG2, V. SMIRNOV2, A. FEDOROV2 and A. LEYBOV2 qnstitute for Space Research at the Bulgarian Academy of Sciences, 6 Moskovska Street, Sofia-1000, Bulgaria and 2Space Research Institute, U.S.S.R. Academy of Sciences, Profsojusnaj Street 84/32, 117810, GSP-7, U.S.S.R. (Received 19 August 1986; revised version received 17 September 1987) Abstract--An instrument for energy- and mass-spectrum measurements of ion fluxes in the Earth magnetotail is described. The ion selection by energy and mass is provided by sequential analysis of the incident ion beam in the electrostatic field of a cylindrical detector and the field of a permanent magnet. The measurement of ion energy distribution is carried out through scanning the voltage supplied to the electrostatic deflector. At a fixed energy the intensities of the different ion fluxes are measured simultaneously. An electrostatic angular scanner allows for the variation of the direction of the polar view-angle (in a frame of reference with a polar axis coincident with the spin axis of the spacecraft). The scanning of the azimuth angle is obtained while the satellite is spinning. The operational principle and the design of the instrument are described and the results from the laboratory testing are presented.
l. INTRODUCTION The multiparametric spectrometry of magnetotail plasma is a challenging experimental problem. The magnetotail consists of three distinct plasma regimes: the plasma sheet, the tail lobes and the boundary layer or mantle. Plasma density, temperature and composition in the three configurations are quite different. The temperature can vary from few electronvolts to several kiloelectronvolts. The ion populations can be of different origin which specifies their different composition. Besides the isotropic plasma background, ion streams are continuously observed. The can flow both Earthwards or antisunwards and in dawn-dusk direction, though the latter case being more seldom. The velocity of these plasma fluxes can vary from 20km/s in quiet conditions, up to 500-700 km/s during substorms, while in the boundary layer and in the mantle very high speed flows of about 1000 km/s are observed sometimes. The flux density is very low--from 1 to 105 (cm 2 s sr eV) -l. The substorm processes, being in the core of modem investigations, have characteristic times of about 10s. On the other hand, magnetotail investigations are conducted by high apogee satellites at strongly elliptic orbits. For that reason the plasma populations--the plasma sheet, boundary layer--are sometimes crossed very quickly within a few minutes or sometimes this crossing lasts for hours. These complicated conditions impose very strong requirements upon an energy-mass spectrometer, detBased on paper IAF-86-292 presented at the 37th Congress of the International Astronautical Federation, Innsbruck, Austria, 4-11 October 1986. 539
signed for magnetotail measurements: (1) large integrated geometric factor, providing the necessary rapid functioning of the instrument; (2) the view angle of the device should enable the angular analysis of scanning through the field of view; (3) large energy range; (4) ability for an adaptive experiment control, onboard data processing and flexible information capacities. A simplifying condition is the fact that a high mass resolution is not needed for the magnetotail--it is enough to identify ions H +, He 2+ and O +. In this paper, the Analyzer by Mass and Energy of the Ions (AMEI) instrument is described. The device will be flown onboard the high apogee satellite Magnetotail Probe of the Interball Project. A M E I is designed to measure H +, He 2+, He + and O + ions in the energy range 0-10 keV and it is also able to perform angular scanning. The instrument consists of two identical, mounted, sensors, one viewing in the direction of the sun, the other viewing in an antisunward direction, and an electronics assembly.
2. DESCRIPTION OF THE SENSOR
2.1. Main ion optics The ion optics is schematically shown in Fig. 1. The main energy-mass separating part is similar to that described by Moor[l] based on the geometry optimized by Hinterberger and K6nig[2]. By means of a 53 ° sector cylindrical analyzer, the incident ion beam is filtrated by energy through the analyzer and only particles with an energy-to-charge ratio E/q in the energy bandpass, defined by the voltage UCA supplied to the deflector's electrodes, will pass.
Y. SEMKOVAet al.
540
ANGUt SCANI~ AS
TRAJECTORIES
Fig. 1. A schematic view of AMEI ion optics. By scanning UCA in steps, the necessary energy range is covered. Further, the ion beam passes through the magnet assembly, consisting of a permanent 90 ° sector magnet and a yoke; the magnet being with a field intensity of 4500 G in the 4 mm gap. As the gyro path of the particles with equal E/q depends on the ratio M/q (where M is the mass of the particle), a space distribution of the particles dependent on their M/q is obtained at the magnet exit. The particle space distribution is recorded by means of a coordinate-sensitive detector (CSD). The ratio M/q of the particle can be identified on the basis of its E/q ratio, determined by UCA, and its position on a CSD. The disposition of the entrance aperture, the cylindrical analyzer, its sector angle, the magnet, the form of its entrance (to the ion beam) surface and the position of the focal (detector) plane, proposed by Hinterberger and K6nig[2], provides for secondorder focusing.
2.2. Peculiarities of the sensor In the instrument suggested by Moor[l] the characteristic (scale) size determining the linear dimensions of the sensor, r0, is 50 mm. A sensor with a similar scale factor analyzing particles with ME in the range 0-160 (M in a.m.u., E in keV) would be too large and heavy for operating aboard a high apogee satellite. In our sensor r0 = 25 mm. As for the cylindrical analyzer with equal sector angles the geometric factor is proportional to their linear size[3], it is natural to
expect decreasing in the geometric factor of our instrument. An alternative has been found by maximum opening of the entrance aperture and mounting a slit in front of the magnet entrance. Thus, the high mass resolution of the system is decreased to the lowest possible limit--resolving only particles with M/q = 1, 2, 4 and 16, but increasing the geometric factor to the possible upper limit. Furthermore, in order to minimize the size of the magnet which is the main weight in the instrument, two modifications of Moor's[l] ion optics are carried out. First, the part of the magnet, just opposite its entrance, is cut in an appropriate way, so that particles with suitable ME could get out of the analyzer through this second exit; we will refer to it as the "cut of the magnet". The particles going out through the cut of the magnet are recorded by the purpose-constructed microchannel detector. No focusing properties are expected at this exit. Second, the whole assembly cylindrical analyzer-magnet~letectors, is mounted in an electrostatic shield. Accelerating or retarding voltage is supplied to the latter. In this way the range of ME values for the particles passing through the magnet is limited, without diminishing this range for the particles incident to the device. Besides, the measurement scheme is chosen so that up to E/q = 2.7 keV for the particles passing through the magnet, the four masses with M/q = 1, 2, 4 and 16 are positioned on the CSD. When the energy of the particles in the magnet is greater than 2.7 keV, then particles with M/q = 1, 2 and 4 hit the CSD, and only O ÷ ions hit the microchannel detector at the cut of the magnet. So the CSD is not needed at this exit of the magnet and one has only to count the leaving particles.
2.3. Angular scanner At the entrance of the energy-mass analyzing part, an angular scanner, proposed by Vaysberg et al.[4] is mounted. It represents a 180 ° cylindrical analyzer, whose outer electrode is a wire grid of 95% transparency and is kept at zero potential. A negative voltage UAS is supplied to the inner electrode. At a given ratio between the design kinetic energy E0 of the cylindrical analyzer (i.e. Uc^) and the voltage UAS, the view angle + Act of the cylindrical analyzer (CA) is tilted to a polar angle fl, i.e. the field of view is shifted in the range (fl - Act~, fl + Act~). In this way, by varying UAS we can sequentially scan the polar angle (in a frame of reference with a polar axis coincident with the spin axis of the satellite, parallel to the central axis of the sensor view angle at UAS = 0). The azimuth angle is scanned while the satellite is spinning. 3. T H E ELECTRONICS ASSEMBLY
3.1. Block-diagram The electronics system of the instrument is spatially separated into three structural units; two identical units (high voltage supplies and preamplifiers) moun-
Energy-mass spectrometer
541
ted in the two sensor boxes and one unit (two identical analog-digital parts and one central data processing unit), installed in the electronics box. In this way, the device comprises two identical measurement channels, connected with the two sensors, and a common data processing unit (DPU). Only one of the two channels operates at any time. The choice is made by the DPU. The block-diagram of one of the measurement channels and the DPU is shown in Fig. 2. At the beginning of each measurement cycle the DPU sets the entire electronics circuitry of the device at the initial state and feeds the control signals for fixing the initial voltages to: (1) The HVAS-the power supply of the negative voltage step; /-/As, for the inner electrode of the angular scanner. The outer electrode of the scanner is grounded. (2) The HVCA--the power supply of symmetric bipolar high voltage step, UCA, for the electrodes of the cylindrical analyzer. (3) The HVES---the power supply of bipolar high voltage step, UEs, for the electrostatic shield and the magnet. (4) The HVMCP--the high voltage power supply for the bias voltage of the microchannel plates of the coordinate-sensitive detector and the microchannel detector. Scanning the voltage UCA supplied by HVCA at a fixed tilt of the field of view, determined by the voltage UAS supplied by HVAS, the energy spectrum of the four ion species is measured.
3.2. Particle registration Leaving the mass separator, the ions are trapped either by the CSD or by the microchannel detector (MCD). The MCD is composed of two cascadeconnected microchannel plates and a charge collecting anode at their output. The MCD has a rectangular form with the relevant dimensions. The CSD consists of an input charge multiplying element---cascade-connected microchannel plates and an anode for charge collecting and coding the coordinate. We use the two-electrode anode in the form of strips and wedges, proposed by Martin et aL[3] with analog coding of the charge position. A characteristic feature of the coordinate X of the input for the CSD charge is the ratio X = 2b/(a + b), where a and b are the amplitudes of the relevant signals of the two-charge-sensitive amplifiers AMPa and AMPb, connected with the two electrodes of the anode. When the ion is trapped by the CSD the signals a and b are supplied through AMP a and AMPb to the respective peak detectors, fixing their maximum amplitude values, and, further on, to a summator. The signals from one of the peak detectors and the summator, necessary to obtain the ratio b/(a + b) are supplied to the signal and reference inputs, respectively, of a fast analog-to-digital convertor
I DATA PROCESSING UNIT (DPU} ] MICROPROCESSORADDRESS/ I DATA/ CONTROL BUS I EXPERIMENT CONTROL
I
STORAGE
DATA FORMATING
C.A E
L L°_ NTROL I
l
TM
Fig. 2. A block-diagram of AMEI sensor and electronics assemblies. (ADC) with para!lel convertion. When the maximum amplitude values are fixed, the peak detectors feed signal "START" to the ADC. At the end of the conversion the output information of the ADC represents the digital code of the ratio b/(a + b). This code and the address signal through the ADC are then supplied to the buffer memory of the DPU for intermediate memorizing. Thus in the DPU's memory information is accumulated about the number and coordinate distribution of the registered by the CSD ions at fixed values of the analyzed energy and tilt of the view angle. At the values thus fixed, the boundary coordinates of the positions of the different ion species of the CSD, preliminary computed and specified during the calibration of the instrument, are recorded in the prom of the DPU. The latter identifies the mass of the registered particles comparing their position on the CSD with these boundary coordinates. In addition, in the DPU's memory the number of the registered MCD O + ions is accumulated. The
Y. S~IKOVAet al.
542 B=o UAS : 0
(a)
(b)
1.0
o lt I I I I I I /) : 28o30 ~ UAS: 0 . 2 9 EO
1.0 E Z Z
0.5
I
I
= 50" UAS: 0 . 4 3 Eo 1.0
0.5
-10"
I I I I I I I I I 0 ° 10" 2 0 ° 30 ° 4 0 " 5 0 e 6 0 ° 70 ° 8 0 e
a
I
IL
IL~I
I
l
0.85 0.90 0.95 1.00 1.05 1.10 1.20 1.25 1.30
E~ E o
Fig. 3. Measured characteristics of the system angular-scanner-cylindrical-analyzer. (a) Angular characteristics; (b) energy characteristics. DPU provides the necessary data compression and formating and the link with the telemetric system of the satellite. 4. P R O T O T Y P E L A B O R A T O R Y R E S U L T S
4.1. Testing of the angular scanner For laboratory testing purposes of the sensor, two different laboratory sensor units were constructed: an energy-mass analyzing unit and a sensor box comprising the system angular-scanner-cylindrical analyzer. The angular and energy characteristics of the latter system are shown in Fig. 3 (a) and (b), respectively. They are measured at different voltages of the scanner, corresponding to different tilt angles/~ of the field of view. The characteristics are measured in the following way: the sensor box was exposed to an ion beam well collimated, monoenergetic, with energy of 1 keV, wider than the surface of the scanner, with an angle of incidence/~. The most appropriate pair of voltages (U^s, Uc^) were determined by the maximum of particles leaving the cylindrical analyzer, detected by a channel electron multiplier. Then, the
voltages were fixed and the energy and the angle of the incident particles were varied sequentially, thus measuring the matrix ~(fl; E, a). The angular characteristics in Fig. 3(a) are obtained through integrating this matrix by energy, and the energy characteristics in Fig. 3(b) are obtained by integrating the matrix by angle. As the nominal field of view of the system (that at/~ = 0) is too wide--12 ° at half width, when tilted it becomes much more wider and it is more difficult to shift. This is evident from the second and the third panels in Fig. 3(a). There E0 is the design kinetic energy of the CA, in this case it is 1 keV. Therefore, the scanner is not going to be used for the angular analysis of the ion fluxes but for suitable tilting the field of view, thus enabling the device to trap the ion fluxes, incident to the instrument at different angles. The energy curves display a 16% resolution, with a typical high-energy wing.
4.2. Energy-mass analyzing properties The testing of the energy-mass analyzing unit was performed as follows: at an angle/3 = 0 and a design
Energy-mass spectrometer
20
30
4'0
io
50
io
H'+
----
~
.............
1.09
8o
543
9o mm !O00 /
o*
Eo
1.2Eo
Fig. 4. Pattern of the ion beams leaving the energy-mass analyzing system. kinetic energy of the CA, E0 = 2 keV, the sensor box was exposed sequentially to mass-calibrated ion beams, namely H + (M/q = 2), He + (M/q = 4) and 0 + (M/q = 16). The incident ion beam was well colimated and strictly monoenergetic. The sensor box could be rotated around the axis fl = 0. Particle detection at the magnets' exits was done by a channel electron multiplier with an entrance slit of 1 mm width, mounted so that it could be remotely moved along axes parallel to the exit and the cut of the magnet, and spaced at different distances from them. Thus, the pattern of the analyzed ion beams with different masses, different initial energies and angles of incidence, was obtained. The results are shown in Fig. 4. The beams with energies E0 = 2 keV are given in thick solid lines; those with E = 0.9 E 0 by thin solid lines; E = 1.09 E0 by dashed lines, and E = 1.2 E 0 by dotted lines. At each energy value measurements with a step A~ = I ° are performed for every leaving beam with an angle of incidence in the entire angular bandpass (specified by the energy). Attention should be drawn to the peculiarities of the disposition of the leaving He + ion beams, i.e. MEo = 8. There exists such a distance from the magnet, at which the four different energies of the He + ions are well separated, i.e. the system can resolve M E = 7.2, M E = 8.4 and M E = 9.6. This allows for the use of the system in experiments where greater mass resolution is needed, preserving the small size and the good sensitivity of the sensor. At the cut of the magnet only the size of the microchannel detector, positioned at 11 mm from the magnet, was determined. At the near end of the
cut (Fig. 4, lower) the position of particles with M E = 43.2(O +, E = 2.7 keV) is drawn by a thin sold line, and that of particles with M E = 4 8 (O +, E = 3 keV) by a thick solid line. At the upper end of the MCD the far-most position of the O + ions is shown. Naturally the geometric factor of the system depends on the lengths of the gyropass of the particles as the magnet gap limits the azimuth angle of the passing particles. The geometric factor measured varies from 0.9.10 -4 x E0 up to 3.6.10 -4 x E0, for the ion trajectories permitted by the modes of operation.
5. CONCLUSION The Analyzer by Mass and Energy of the Ions proposed here, possess the capabilities necessary for providing energy-mass analysis of the weak low energy ion fluxes in the magnetotail--good sensitivity, capacity for directing the field of view towards the ion flux, capability for onboard control of the experiment and data processing. The M E bandpass ( M - - t h e mass number, E - - t h e energy of the ions) could be enlarged significantly by means of the electrostatic shield.
REFERENCES
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544
Y. SEMKOVAet al.
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