Rapid Non-scanning Ion Distribution Measurements Using Electrostatic Mirror and Multichannel Collimator for the INTERBALL and MARS-96 Missions

Rapid Non-scanning Ion Distribution Measurements Using Electrostatic Mirror and Multichannel Collimator for the INTERBALL and MARS-96 Missions A.O. Fedorov, O.L. Vaisberg Space Research Institute, Russian Academy of Science Moscow, Ru33ia A.D. Johnstone, A.M. James, R.D. Woodliffe Mullard Space Science Laboratory, University College London, UK Combination of electrostatic mirror cylindrical or hyperbolic shape with multichannel collimator and particle analyser, which forms the image on position sensitive detector, allows one to measure 3-D mass-resolved spectra without any scanning in relatively short time. The mirror and collimator form ад angular diagram of detector. Each collimator channel has an individual direction. Particles, coming through collimator move along focused trajectories. In the case of INTERBALL's instrument TRIPLET the energy analysis of these particles is made by wide cylindrical electrostatic analyser, and for MARS-96's instrument FONEMA the Thomson parabola mass-energy analyzer is used. The TRIPLET instrument gives energy spectra in range of 300eV 4000eV for 6 separate narrow-angle channels. The maximum temporal resolution of this instrument is 1 sec. The FONEMA instrument has a 3-D field of view, divided by 36 separated angular windows. The particles, within one angular subrange are analysed by mass and energy. The maximum temporal resolution is 0.13 sec. The paper describes design of both instruments. Choice of specific shape of mirrors, collimators and analysers is discussed. 1. INTRODUCTION One of the most important problems in observing of space plasma, is to measure the ion distribution function near the thin plasma boundaries. The difficulty is, Measurement Techniques in Space Plasmas: Particles Geophysical Monograph 102 Copyright 1998 by the American Geophysical Union that the characteristic time for crossing these boundaries, such as the bow shock, is very small. The time scale of many processes at thin boundaries lies near the ion gyro period ( about 1.5s for solar wind ), and bow shock ramp has a length about c/w p i = 80km, so the spacecraft crosses the bow shock in about 8 s. Where с is the speed of light and w p i is the ion plasma frequency. Common ion analysers accumulate a distribution by using energy and angular sweeping techniques so the resulting distributions are not adequately resolved. For example, if temporary variations of ion distribution func-

222 RAPID NON-SCANNING ION DISTRIBUTION MEASUREMENTS Detector Steps 10 ms each Scanning versus 320 ms spectrum Parallel accumulation 1000 >00 10 3 1/s ma* count fcik u 400 200 10 15 SO 15 30 Energy etep number Ч LU LU LU ы В В в с г F : В с в в u и и U CSD 10 3 1/s max count Parallel accumulation 20 ms per spectrum Figure 1. Explanation of advantages of parallel way of energy spectra measurement. See text for details. tion are comparable with the time of accumulation of complete energy spectrum, the scanning instrument obtains spectrum with low and high energy parts corresponding to two very different distribution functions. This can lead to misinterpretation of results. The same is applicable to angular sweeping usually perfomed by rotating spacecraft. The fastest sweeping instruments aimed for the investigation of thin boundaries such as the ISEE Fast Plasma Analyser [Вате et al., 1978], the AMPTE UKS Three Dimensional Ion Instrument [Coates et al., 1985] and the AMPTE IRM Plasma Analyser [Pashman et al, 1985] have a fast spectral sweeping time, about 0.14 sec but all of them have the important disadvantage that they depend on the spacecraft rotation (и 3sec) to obtain the angular distribution. Russian spacecraft provide a good opportunity to resolve this problem because they do not rotate or only rotate very slowly. But the problem of sweeping through different parts of the distribution function at different times remains. The good solution is to design an imaging instrument producing an image of the energy/twoangular distribution of incoming particles at a Coordinate Sensitive Detector (CSD). Even if the times of complete spectrum accumulation of the non-scanning and scanning instruments are the same, the first one produces time-averaged spectra. In the case of a high variability distribution function such spectra are more accurate than spectra obtained by scanning instrument. Apart from this obvious advantage of non-scanning measurements there are other not so obvious advantages to this type of instrument. Figure 1 is an example of typical solar wind spectrum, displayed on a linear scale. Suppose you have a sweeping instrument, where the detector has a maximum count rate of 10 s s -1, and the Geometrical Factor of the instrument is specified for this count rate. Then let us assume that 1000 counts at the maximum of the spectrum provides good enough statistics for the measurement. It means that the accumulation time must be about 10ms and then a spectrum of 32 steps takes 320ms. But if you have a 32-energy-channel ion analyser, which accumulates counts simultaneously ( in a "parallel" way), using single detector with the same maxi-

FEDOROVETAL. 223 mum countrate for all channels, then because the solar wind spectrum actually only occupies 3 channels you can make exposure time 20ms. Two right panels of Figure 1 explain the previous statement. Two upper panels show the diagram of sweeping analyzer with energy range about 100eV 4-10 kev (left) and pattern of the energy sweeps. Shaded rectangulars show energy steps with counts. One can see, that the detector registrates particles during only 10% of the full accumulation time. Two bottom panels present the non-sweeping "parallel" 32-channel analyser. All particles enter the single Coordinate Sensitive Detector. Dark bars on the right panel show relative countrate of the channels. Actualy only 3 channels are working. It allows accumulation of enough counts at maximum, in a short time, in spite of the restricted detector countrate. Note, that the situation does not change after, for example, crossing the bow shock and the thermalisation of distribution function. The spectral sensitivity will be the same. These considerations have lead us to design an imaging energy-angular spectrograph for investigating short time scale discontinuities in the solar wind and a three dimensional energy-mass-angular spectrograph for studying Mars/solar wind interactions. Actually, the main goal of continuation of its design was to check and adjust methods of "parallel" measurements of angular-energy spectra of charged particles. Figure 2 shows the basic scheme of the instrument. The main idea of the instrument is as follows: particles come through a pinhole entrance aperture and axe reflected by the cylindrically-shaped electrostatic mirror. Then they pass through the multichannel collimator, which is transparent only for particles moving along the X axis of the system. The combination of the collimator and mirror defines the correspondence between the angle of the incoming particle and the position along Y coordinate of the system. Now parallel beams of particles of all energies, reflected by the mirror, enter into a cylindrical energy analyser with a wide range. This analyser disperses the particles according to their energy, while particles, having different coordinates along Y on the collimator and the same energy, will be focussed at the same point on the detector. Each channel of the collimator produces an energy strip on the sensitive surface of the detec- 2. TRIPLET INSTRUMENT The first attempt to make a parallel image device was the TRIPLET instrument for INTERBALL project. It was proposed that new principles help us to design an instrument for taking a momentary "snapshot" of the solar wind ion distribution function. To observe the whole sphere or hemisphere was not the purpose of the instrument. It was supposed to make measurements in angular range ±20 for both angles and in energy range from 100 ev up to 3000 ev. It was impossible to receive one image in a time less than 1 s due to telemetry rate problems. It is difficult to compare this ideology with principles of AMPTE and ISEE plasma analysers. The angular range of TRIPLET was supposed to be relatively small, but with high angular resolution (1.5 ). The full spectra accumulation times of instruments are similar (Is for TRIPLET, and spin period for ISEE and AMPTE). So the main advantage of new ideology is the "parallel" accumulation of the whole distribution function. During the instrument design phase, due to technical problems, it was decided to remove the second angular dimension. Thus TRIPLET became a 2-dimensional instrument, losing a major part of its competitive ability. Collimator Layout Location of Energy Strips on Detector Surface Figure 2. Basic schematic of the TRIPLET instrument. Points in the Collimator Layout correspond to pinhole channels of collimator. Each pinhole apperture gives one energy strip on the Detector Surface. See text for detailed explanation.

224 RAPID NON-SCANNING ION DISTRIBUTION MEASUREMENTS tor. We use a 2-D position sensitive detector made by a chevron assembly of two MCPs(microchannel plates) and a wedge and strip anode [Martin et al., 1981]. The second dimension of the collimator and CSD is used to separate the angular and energy spectra of ions. The channels of the collimator located at different Y coordinates have a different positions along Z axis as well. So each angular beam produces an energy spectra along separate strips on the CSD. The following sections describe behaviour of each part of TRIPLET instrument in more detail. The first problem of the elecrostatic mirror is the scattering of particles as a function of energy. The scattering appears due to nonuniformities in the electric field around the wires of the mirror grid. More energetic particles penetrate into the mirror deeper than low energy particles. As a result low energy ions are scattered much stronger than high energy ions, which are approximately specularly reflected. Computer simulation combined with vacuum chamber experiment gives the behaviour of scattering versus energy, displayed in Figure 3. The vertical axis is the full width of the scattering and horizontal axis is the normalised energy, i.e. the ratio of the energy to the mirror voltage multiplied by the ratio of mirror's depth and the grid space. It gives us the empirical formula: FWHM = ^ + 2.0[<feflr] Z75 (1) Here E is ions energy per charge [ev], U is internal electrode voltage [ev], L is the distance between grid and internal electrode and S is grid space. The physical basis of this equation is: Each grid cell sets up an electrostatic lens, with the focus distance dependent on field strength ( U/L) and particle energy. But the scattering is a ratio of grid space S and focus distance. So, actually, the scattering depends on the value of E/U L/s. If you change all dimensions with the same scale - nothing changes, also increasing E and U by a factor of two leads to nothing changing. The Electrostatic mirror has an obvious limitation for high energy particles. Ions with energy per charge ratio more than mirror voltage will stall on inner electrode, in the case of a trajectory perpendicular to the mirror. Low energy particles will be scattered in the grid and will lose initial angle. So they are not detected. For technical reasons we chose the depth of mirror to be 5mm and a grid space of 0.1mm. This mirror geometry provides particle reflection within 5 scattering angle for particles with Emax/Emin 20. This is the first limitation of the TRIPLET instrument. We chose the cn <D "O X 40 30 20 10 0 1 я 0 2 4 E/U L/S -Ж -ж: Figure 3. Electrostatic mirror scattering FWHM as function of normalized energy. E, U, L, S are particles energy, mirror voltage, depth of mirror and grid step respectively. energy as 150eV -=- ЗОООеК. The energy deviation of the angle of the channel is 1 per 500eV. The analyser has a small inner electrode with a 30mm radius and a voltage of minus 3850 V (Figure 2). The outer electrode has a 80mm radius and 1180V voltage. The 5cm gap and non-symmetric voltage allows particles in a wide energy range and with widely varying starting points to pass through. The sides of analyser are closed by ceramic walls with co-axial electrodes. The voltage on each electrode corresponds to the local potential of electric field inside analyser. This removes nonuniformity of field of analyser along Z axis. To correct for field disturbances at the entrance and exit of analyser, each electrode is connected to a similar electrode on the opposite wall by thin wire. These wires form entrance and exit grids of analyser with a distributed voltage, not shown in figure). The entrance grid supplies deceleration of particles at top part of analyser and accelertion of particles at the bottom part. This acceleration and decceleration is the reason for the wide energy range of this analyser. The wide energy range and focusing characteristics are obtained by the Y-coordinate dependent acceleration of the particles. Figure 4 shows vacuum chamber measurements of the analyser behaviour. Each peak is the CSD response for the particle flux at one energy, passing through the collimator with all possible angles. The lowest energy is 470 ev and the highest energy is 2750eV. Figure 5 shows the CSD coordinate versus energy of the particles for 3 rays with different entrance positions. The gap between the two lines is the FWHM of the detector response.

FEDOROVETAL. 225 The disadvantage of the analyser is the movement of energy range with changing collimator channel location. The collimator ( see Figure 2 ) is made from three thin plates with different gaps. The pinhole layout is shown in the bottom left panel of Figure 2. The gaps between plates and the allocation of holes is chosen to prevent rays passing through holes in different collimator channels. The number and size of the holes is another problem for this device. To produce a large number of angular channels we have to make the holes very small, but this reduces the geometrical factor of the instrument. The compromise was 10 holes of 0.6mm diameter each. As mentioned above, ions passing one collimator pin-hole channel are spread along Y axis of CSD (see Figure 2). Thus 10 pin-holes project onto 10 strips on the CSD surface. The coordinate-sensitive detector itself has a 256 pixels resolution along Y axis, but particles accumulate in 32 cells, uniformly filling Y space. Each cell is adjusted to specific angle and energy, i.e. represents the detector for a fixed energy with specific apperture and energy passband. Figure 6 shows the TRIPLET response characteristics. The top panel of the figure illustrates the total field of view of the instrument. It occupies the area from 6 up to 24 with respect to direction of spacecraft spin axis (the sunward direction). The shape of the response area of instrument is not rectangular and low energy ions are not registered by instrument in the 10 24 range. The bottom panel of the figure shows the response of one cell of the CSD in Energy/Angular space. It is the typical resolution. Because the particles are accelerated before entering the analyser the i\e/e ratio is not constant and varies from 50% down to 6%. The geometric factor of one bin is 7 10~ 7 sradcm 2, but it is necessary to note that there are 32 energy bins of the same collimator channel (defining the GF) measuring simultaneously, which increases efficiency of the CSD cooraimie Figure 4. CSD response of one collimator hole for various ion energies. Left peak is response for 470 ev ions, right peak is response for 2750 ev ions. >00 J50 t CSD coordinate 1500 1000 Figure 5. CSD coordinate as function of energy of particles for 3 different collimator channels. The space between 2 nearest lines shows the width of distribution on the plane of CSD. instrument as it was shown of Figure 1. The viewing angle along the direction normal to the fan-shape plane of view of instrument is 6. The geometric factor allows spectra of solar wind and magnetosheath ions to be measured with a time resolution of about lsec. 3. FONEMA INSTRUMENT Another instrument using a similar idea for parallel measurements of the ion distribution function is FONEMA for the MARS-96 mission [James et al., 1996]. We have tried to create an instrument which would be suitable for the Martian plasma environment. We needed to measure the 3-dimensional distribution function of ions with mass discrimination and with fast time resolution [Vaisberg et al., 1990\. The entrance aperture was formed by a hyperboloidal electrostatic mirror (Figure 7, top panel). A paraboloidal shape of mirror would have been better, but it was very difficult to find a method for manufacturing such a mirror. The actual shape was made by winding straight wires between rings of different radii. This mirror has the same

226 RAPID NON-SCANNING ION DISTRIBUTION MEASUREMENTS therefore, m / q can be derived. To avoid influence of ultraviolet solar photons which can reflect from the mirror, pass the collimator and enter the focus point of detector, the focus point is protected with blackened screen. Top panel of Figure 8 shows result of Thompson Analyser simulation. H +, He ++, 0 + and particles with masses greater than 20, uniformly distributed over all allowed energies and all directions passing collimator were tracked through realistic magnetic and electric fields of analyser. The picture shows the particle impact points on the detector surface. Different curves correspond to 10 IS 20 25 Alpna, I. FONEMA Sensor Design > 2500 2000 WL t 1500 с Ш Particles trajectories Hyperboloidal electrostatic mirror 1000 500 0 ' 1 1 1 1 1 1 10-5 0 5 10 15 20 25 Alpha, dag Figure 6. TRIPLET response area. Top panel shows the total response area of instrument. Bottom panel shows response of one channel of the detector. Thomson analyser Thomson analyser limitations as the previous mirror for TRIPLET. The ratio Emax/Emin is still only 20. Because we would like to get a wider energy range from looev up to 8keV, we split the range into 4 overlapping subranges each with a dynamic range of 20. The maximum voltage of the mirror is 8 kv. Reflected particles enter the collimator, designed in a similar manner to that of TRIPLET, but with one important difference. The collimator forms a set of converging trajectories. The focus point is exactly on the surface of the detector. There are 18 separate collimator sections and detectors. The Thomson Analyser [Thomson, 1911] (bottom panel of Figure 7) is used as the energy-mass resolving element. It consists of a magnetic field region followed by an electrostatic field region. The magnetic and electrostatic fields are parallel. Magnetic deflection is proportional to the ion momentum per charge p / q, whereas electrostatic deflection is proportional to E / q. As illustarted at Fig.8, the measured ion position enables unique identification of both E/q and p/q and, Particles trajectories 1 ; 11 Thomson Analyser Collimator Magnetic poles Electrode MCPs of CSD Chart Figure 7. Top panel presents cross section of sensor of FONEMA instrument. Bottom panel displays cross section of Thomson Analyser.

FEDOROVETAL. 227 ся I О <л 60 CL ~ 50 CL О о 40 О) Ь 30 Tj В 20 О ч-» ю А ' О Ч> О 30 25 20 I' 10 5 0 10 20 30 40 50 60 Magnetic deflection (pixels) 0 0 10 20 30 40 50 60 CSD channel model of FONEMA. The model connects each allowed point in ion velocity space with a point on the detector surface. Figure shows energy spectra (the distribution of counts along vertical (see top panel of the figure) axis of detector accumulated for 0.8 s in magnetosheath and magnetotail regions. One problem of the analyser is that it has a small energy range of a factor of 5 under normal circumstances. The range of the analyser can be increased to match the range of the mirror by accelerating the particles in the collimator. As shown in the top panel of Figure 7 the set of analysers and mirror has 3 angular apertures: 0 -f- 40, 40 4-70 and 70 4-90. The assembly of three analysers is repeated 6 times around the azimuthal direction. The FONEMA instrument has 2 sensors with mirror axes pointed in the Solar and anti-solar directions. Although each collimator has many channels, the Geometric Factor of the instrument is relatively small. Nevertheless the sensitivity is high because all energies angles and masses are accumulated continuously. The angular field of view of each sector is large in order to cover the full range of angles with a reasonable number of analysers. In the case of low-temperature distributions this could lead to a misinterpretation of results if the distribution is confined within the field of view of one sector. The instrument can be described as a small sensitive sphere with 2-10 -5 cm 2 cross section. The bottom panel of Figure 8 shows a simulation of instrument spectra in the magnetosheath of Mars and of oxygen in the tail region. The count scale of each graph is linear and the accumulation time is 0.8 sec. Figure 8. Top panel shows the simulation of the response of Thomson analyser to a distribution of particles of H +, He ++, 0 + and particles with mass greater, then 20.The energy range of test particles was 150 ev 3000 ev. The lowest energy corresponds to the top of vertical axes of detector. Bottom panel represents response of analyser for magnetosheath protons ( V = 300 km aec~ 1, T = 20 ev, n = 4 cm~ 3 ) - solid line and magnetotail 0 + ions (V = 31fcm sec -1, T 40eV, n = 4cm -3 ) - dashed line. Accumulation time is 0.8sec in both cases. different sorts of particles, as is labeled on the figure. The energy range of test particles was 150 ev 3000 ev. The lowest energy corresponds to the top of vertical axes of detector. The bottom panel of Figure 8 represents simulation of response of instrument to a supposed near Mars plasma environment. To make this simulation both results of ray-tracking analysis of mirror and Thomson analyser were combined in mathematical 4. CONCLUSIONS Our experience with the design of these 3D ion analysers shows that the method of combining mirrors, multichannel collimators, analysers and position sensitive detectors makes it possible to get relatively fast, parallel measurements of the energy/angular/mass distributions of ions. The geometric factor of TRIPLET'S one channel is 7 10" 7 srad cm*. This corresponds to the geometric factor of about 2 10 -o sradcm 2 for a sweeping device. This value is enough only to register solar wind and magnetosheath ions. Another (already mentioned) disadvantage of this instrument is that it is only capable of 2-dimensional measurements. Insufficient geometric factor is a disadvantage of the FONEMA instrument as well. But in this case the latter is compensated by 3-dimensional measurements and mass analysis.

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