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3. Instrument Description

3.1 Ion Optics

The TIMAS is a double focusing (in bend angle alpha and energy) spectrograph that simultaneously images in both momentum per charge (equivalent to mass per charge given the knowledge of the energy per charge) and azimuthal angle of incidence (beta). The principle ion optical elements are shown schematically in Figure 1 and the key dimensional parameters are listed in Table 2. All elements are axially symmetric about the vertical center line of Figure 1. The ions enter the TIMAS instrument on a 73 degree half-angle cone and are collimated to +/- 5 degrees in alpha. The ions next pass through a +10 volt repeller grid at C1 before being accelerated by the potential PA1 (-0.2 to -3 kV, see below for details) between C1 and the object slit (S1). The first toroidal electrostatic analyzer (EA1) is angle focusing in alpha at slit S2.

Following S2 the ions are further accelerated by a potential PA2 (-0.4 to -3 kV) applied between S2 and C2, to a minimum E/Q of approximately 1200eV/e over the remainder of their path. The second toroidal electrostatic analyzer (EA2), refocuses the ions in alpha at the MCP detector surface. In addition the combination of geometric parameters for EA1 and EA2 are chosen to produce energy focusing at the MCP. A general discussion of the theory and laboratory verification of the double focusing properties of toroidal ion optics systems is given in Ghielmetti and Young [1987] and Young et al. [1987]. The theory is extended to the poloidal geometry (i.e. the geometry of the TIMAS) in Ghielmetti and Shelley [1990]. Laboratory verification of TIMAS-like optics is found in Bratschi et al. [1993].

The combination of PA1 and EA1 voltages determines the mean ion energy per charge (E/Q) and together with the S2 slit width determine the energy per charge passband (delta E/Q). The internal energy passband (delta Ei</Ei), that is the passband relative to the energy after the first preacceleration (Ei), is approximately 8% FWHM. It is this internal passband (internal to EA1) that contributes to mass line broadening. However, from an energy resolution standpoint it is the energy passband relative to the initial or external energy (Ee) before preacceleration that is of interest. This passband scales inversely as the energy so that (delta Ee/Ee) = (Ei/Ee ) (delta Ei/Ei). The ratios of internal to external energies at the center of the channel range from approximately 9 at the lowest energy to 1 at the highest energy, resulting in passbands ranging from approximately 70% to 8%.

Ions exiting from slit S3 following EA2 are momentum dispersed radially outward in a toroidal magnetic field that is produced by 32 wedge shaped magnets arranged in a spoke-like pattern. Each magnet subtends an angle of approximately 3 degrees, thus obstructing somewhat more than 25% of each of the 11.25 degree sectors. The samarium-cobalt magnet material (RECOMA with BHmax >= 29 MGOe) was selected for high inductance at the working point. Magnets are precisely matched in total flux (<< 1% variation) to provide nearly equal magnetic induction of ~ 0.26 Tesla in all 32 gaps. In addition the magnets are arranged to minimize any residual dipole and higher order moments of the stray field.

Ions that strike the polefaces of the magnets can be scattered into the detector as either degraded ions or neutrals. This undesirable background is suppressed by providing each magnet face with an anti-scattering surface consisting of a set of parallel vertical slats. The slats, separated by 1 mm, are 240 microns high and are less than 10 microns wide at their peaks. They are produced by wire electron discharge machining a thin (~ 280 micron) layer of copper which was chemically deposited directly onto the magnets. The slatted surface produced in this manner has proven highly effective in reducing scattering background to less than 5 x 10^-4 of the ion flux impinging on the magnet faces [Bratschi et al., 1993].

The image on the MCP detector for a given EA setting ideally consists of a series of nearly concentric rings, each ring representing a different M/Q ion. As the EA settings are changed to transmit higher or lower E/Q ions, the radii of the family of rings decreases or increases accordingly. Acceleration to a minimum E/Q of approximately 1200eV/e within EA2 ensures that the radius of the lightest ion (H+) never exceeds the active radius of the MCP. The azimuthal position on the detector is a direct map of the incident azimuthal angle and is divided into 32 sectors, each 11.25 degree wide corresponding to the spacing of magnets. However, only 28 of these sectors are actually used, as will be explained below.

Before leaving the discussion of the ion optics, a few specific details are worth pointing out. Firstly, as discussed by Ghielmetti and Shelley [1990], the electrostatic field in a poloidal toroid varies with position along the principal path, resulting in non-circular paths and significant aberrations that degrade the image. Furthermore, the ion accelerations at C1 and C2, if not compensated for, result in beam refraction that alters the effective focal lengths of the respective analyzers EA1 and EA2. These effects are mitigated by making small alterations to the toroidal deflection plates and to the entrance and exit slit regions. Some of these features, such as the off-set between the starting angle of the inner and outer deflection plates of EA1 near S1 and the serrations on the EA2 deflection plates, can be seen in the details of Figure 1. These modifications minimize imaging errors while maintaining first order angle-energy focusing at the MCP. The final calculated full line width (including 3-D aberrations) is approximately 3.3 mm at the detector surface.

A plan view of the grid and detector geometry is shown in Figure 5. Note the front side of the MCP detector faces downward in Figure 1. The 90% transmission grid assembly, placed approximately 1.5 mm in front of the MCP (see Figure 1), not only terminates the PA2 equipotential but also acts as an important collimator in the azimuthal direction. This collimation is achieved by leaving unetched spoke regions approximately 3.3 degrees wide centered over each magnet and aligned with a second set of similar spokes (without grids) placed approximately 4.5 mm from the MCP. These spokes, combined with other stops at S3 and S2/C2, are designed to prevent ions from crossing over from one 11.25 degree sector to adjacent sectors and in effect limit the azimuth (beta) acceptance of ions from any given position at S1 to ~ +/ 2.5 degrees in the absence of preacceleration. The PA1 and PA2 preaccelerations lead to refraction at S1 and S2 respectively and expand the external acceptance in beta to as much as +/- 10 degrees at the very lowest energy. As a result there is some overlap in beta between sectors below about 1 keV/e.

The input side of the MCP is maintained at a fixed -3.5 kV to ensure a high quantum efficiency for all ions and to repell any secondary electrons produced on surfaces other than the MCP. This results in a potential difference of between 0.5 and 3.5 kV across the 1.5 mm gap between the grid and the MCP. While accelerating the ions into the MCP, this potential accelerates electrons produced on the interstitial surfaces between micropores away from the MCP. This eliminates image spreading that would otherwise result if these electrons were repelled back to the MCP and were detected at a position other than where the ion struck the surface. However it has the undesirable effect of reducing the overall MCP efficiency by approximately 50%.

3.2 Detector System

The active area of the MCP detector (see Figure 5) extends from approximately 26 mm inner radius to 71 mm outer radius and is divided into two independent arc sectors of 157.5 degrees each. Each sector is made up of two quadrants which have different micropore bias angle orientations so that the incident ions never enter parallel (within ~ 7 degrees) to the micropore axis. This minimizes the variation of sensitivity over the surface. The two inactive 22.5 degrees sectors of the optics are used to transfer high voltage to the inner electrodes and to provide mechanical support across the various apertures. This gap also facilitates the independent operation of the two MCP halves. The symmetry line between the two MCP halves is rotated by one and a quarter sectors (i.e. 1.25 x 11.25 degrees = 14.06 degrees) with respect to the spacecraft spin axis. This offset ensures that all solid angles in the field of view, including those near the spin axis, are covered at least once per spin. It furthermore provides for a more uniform coverage of view angles near the spin plane because the look directions shadowed by the magnets of one detector half fall into the field of view centers of the other detector half approximately one half spin later. The above configuration results in the fraction of the solid angle within 17 degrees +/- 5 degrees of either spin axis direction (~5% of 4 pi) being sampled only once per spin, while the major part (~93%) is sampled twice per spin. The remaining 2% of the 4 pi solid angle, within 12 degrees of either spin axis direction, is never sampled due to the conical shape of the field of view.

A two dimensional wedge and strip anode [Anger, 1966; Martin et al., 1981; Schwarz and Lapington, 1985] is placed approximately 3 mm behind the MCP. Its bias voltage relative to the MCP can be adjusted for optimum performance. The wedge pattern consists of 28 discrete steps that are oriented along concentric circles to uniquely identify each of the 28 azimuthal sectors, independent of radial distance. The strip pattern, interleaved with the wedge pattern, decreases nonlinearly with radius so that the relative signal S(R) = S(Ro) + log(Ro/R), where Ro is the inner radius; thus providing approximately constant delta R/R resolution. A higher positional resolution is required at smaller radii because both the mass line width and dispersion decrease with decreasing radius. The anode area not occupied by either the strip (A) or the wedge (B) pattern is filled in by a third electrode (C). Following preamplification of signals from each electrode, three signals are generated; A, B, and A+B+C (see Figure 6). These signals are shaped using a six pole critically damped LCR, RC, LCR, RC network which produces a pulse that peaks at 650 ns from the start of the pulse and recovers to 0.5% of this peak in 3.5 us. They are fed to two ratioing analog to digital converters (ADC) which generate an 8-bit A/(A+B+C) radial position output code and a 6-bit B/(A+B+C) sector, or azimuth, output code. In order to accommodate higher event rates, the positive signal from the MCP contact anodes (i.e. the positive terminals of the MCPs) is shaped using a 4 pole RC network with an RC time constant of 32 ns. The pulse peaks at 40 ns from the start of the pulse and crosses zero at 100 ns from the start. The default discriminator level for this pulse is set at 25% of nominal amplitude, thus it will respond to another pulse about 125 ns after the start of the first pulse. The precise response time depends on the amplitude of the first pulse and the actual setting of the programmable discriminator, but will be within 100 to 150 ns for the expected range of outputs of the MCP. Each fast pulse blocks the imaging system for 3.5 us to avoid pile-up. In addition, a singles counter, driven by a one shot whose pulse width is set to 140 ns is used to normalize the imaged events at high rates. In an alternate (calibration) mode the sum signal (A+B+C) is fed to an ADC with a 4-bit output code proportional to the MCP gain. Gain measurements will be performed periodically to determine whether the threshold discriminators or the MCP bias need to be adjusted.

Ideally, all ions with the same M/Q and E/Q are detected at the same radial position on the MCP. However, noncircularities in the deflection plates, misalignments of the analyzers or the detector relative to the optics axis, or differences in magnetic induction among magnets could result in a slight variation of radial position with azimuth. Furthermore, some electronic distortion results from cross talk between electrodes A, B, and C. The combined optical and electronic distortions are removed by transforming the 6×8 bit code to a 5-bit (actually 0-27) detector sector identification and a 6-bit psudo-radial code that is dependent only on mass and energy. This is accomplished using ROM look up tables (LUT) as shown in Figure 7, which is a block diagram of the instrument control and data processing system showing the flow of data from the detector system through the various stages of processing. These LUTs were defined on the basis of the detailed calibration of the complete optical system and will be installed into the instrument during the refurbishment period shortly before launch.

A graphic representation of the ROM LUTs themselves is shown in Figure 8, combining the radial (vertical bands) and azimuthal (horizontal bands) into a single two-dimensional grid pattern. Each radial mass step typically alternates in width between two and three radial detector bins, whereas the sector number spans either one or two azimuthal bins. Only 60 of the 64 available mass steps are shown here, running from MS = 2 on the far left to MS = 61 on the far right. Mass steps 0, 1, 62, and 63 have been assigned to the MCP edges and beyond, and are not expected to have any signal. It should be noted that the rightmost mass steps in Figure 8 are near the inner edge of the MCP, where the spatial density of detector radial bins is the highest. This accounts in large part for the greater undulation of the constant-rigidity loci on the right. As far as the field-of-view sectors (numbered 0 through 27) are concerned, it is worth noting that sectors 13 and 14 (midway up) are physically separated by a 22.5 degrees blind sector. The flight data products actually use a reversed sector numbering, running from top to bottom in Fiugre 8, in order to provide right-handed external coordinates. Data accumulation beyond this point will be discussed in a later section.

To facilitate ground testing at atmospheric pressure, when the MCP is non-functional, and to serve as a standard end-to-end data systems test, there are two independent stimulus points incorporated into the detector anode. When enabled, a fixed pattern of pulses are injected into these stimulus points, thus producing a unique image pattern. This stimulus calibration allows for testing of the gains of the analog components as well as the digital processing and telemetry formatting.

3.3 Analyzer High Voltage System

As described in the ion optics section, the TIMAS analyzer system consists of tandem toroidal electrostatic analyzers, each floating at a controllable acceleration voltage of up to -3 kV (see Figure 6). The two analyzers, EA1 and EA2, must be matched in energy transmission to a few tenths of one percent in order to achieve the desired instrument response characteristics. Thus the two preacceleration voltages, (PA1 and PA2), and the four analyzer plate voltages ( EA1 and EA2) must accurately track each other to ~0.1%. Furthermore, these analyzers represent relatively large capacitive loads (several hundred pf). Since the analyzer voltages are stepped rather than swept, they must settle to within a few tenths of a percent of their prescribed values within a few ms before data acquisition starts. In order to meet these requirements the TIMAS high voltage power system uses optically controlled linear regulators (HV601, manufactured by AMPTEK, Inc.) to supply regulated voltages to multiple loads from a common high voltage converter.

An optical regulator can be described as a light sensitive high voltage diode, an LED, and a light transparent medium between the two. The principle of operation is control of the leakage current through the high voltage diode by the amount of LED light incident on it. The device essentially behaves as a current controlled current source. As an added advantage, the LED and high voltage diode have high voltage isolation, making a four-leaded device which can control high voltages from an isolated potential. Arranging the regulators in a series-shunt configuration results in equivalent positive and negative going slew rates. This is particularly important for the two pre-acceleration potentials, PA1 and PA2, which do not follow a monotonic stepping pattern. In addition, this arrangement, which requires no passive pull-down to achieve the negative going slew rates, permits the use of very high impedance high voltage sensing resistors.

Running multiple regulators from a single DC high voltage supply allows for independent regulated outputs with the overhead of only one high voltage power supply. This method is particularly efficient in the case of capacitive loads since the optical regulators are using significant power only while slewing. Furthermore, since the loss of any one of the analyzer voltages would represent an instrument failure, the use of fewer supplies actually increases reliability.

The series-shunt configuration is achieved by placing the regulator’s LEDs inside of the feedback loop and closing the loop by sensing the forward current through these devices. This arrangement inherently protects the series and shunt regulators from both ever being “on” simultaneously. In addition, the current sensing scheme strongly reduces any crossover distortion when switching between the series and shunt devices. Since the error amplifier is contained within the compensated feedback loop, it is able to swing through this region in an open loop manner.

3.4 Calibration

Initial calibration of the TIMAS instrument, using the University of Bern ion beam test and calibration facility [Ghielmetti et al., 1983], verified most of the design characteristics. However, system noise coupled through the MCP reference high voltage supply resulted in significantly reduced imaging resolution and precluded the verification of the detailed mass peak shapes and thus the mass resolution. The noise coupling has subsequently been eliminated; however, a recalibration will not be possible until the instrument recalibration and refurbishment which is scheduled to occur after spacecraft integration and test.

The initial calibration consisted of measuring a combination of single parameter cuts through the energy-angle-mass response functions over a wide range of parameter space and more detailed multiple parameter matrix measurements over more limited ranges of parameter space. Figure 9 shows a typical energy passband for the TIMAS instrument. It was actually measured using a large area, uniform, monoenergetic ion beam incident normal to S1 and varying the EA1 plate potentials while holding the EA2 plate potentials constant of their nominal settings. Since the EA2 energy passband is much wider than that of EA1, it does not contribute to the total system passband limits for normal incidence ions. The mean value of the bandwidths in the 28 sectors was 8.06% with a maximum variation of +/- 0.35%. This is completely consistent with the optics design parameters. However; the RMS deviation of the passband centers was 0.75%. This is interpreted as variations in the plate spacings resulting from eccentricities and small distortions in the optics. The small error in defining the energy of the ion <<10% of the instantaneous bandwidth) is negligible in terms of defining the velocity distributions. However, these deviations, taken together with uncorrelated deviations of comparable magnitude in the EA2 passband, result in some fluctuations in the overall transmission as a function of sector. These fluctuations, combined with other instrumental effects, are corrected for in the detailed, end-to-end sensitivity measurements as a function of sector (cf. Figure 8).

Figure 10 displays characteristic elevation angle response curves for a single sector. The solid symbols were measured with a monoenergetic ion beam and display an almost ideal flat response with sharp cut-offs at both the upper and lower limits as expected for angle focusing optics (See Figure 11). The mean value of the cone angle, in this case measured from the horizontal, is approximately 16.9 degrees, within measurement error of the theoretical 17 degrees. Likewise, the measured 9.9 degrees FWHM is consistent with the design passband. The open symbols in Figure 10 were measured with a beam having an energy spread (~13%) significantly broader than the instrument energy passband (~8%, see Figure 9). As seen from Figure 11, the ray tracing predictions show both a low energy, low angle and a high energy, high angle cut off. These result from extreme rays striking respectively the inner and outer of EA2 plates. This should lead to a softening of both the upper and lower edges of the elevation angle response. By contrast the low angle cut-off is nearly identical (within measurement error) to that for a monoenergetic beam and the high angle cut-off is somewhat softer than predicted. This effect is a direct result of the small misalignments of the analyzer plates discussed above. In this particular sector the deflection in EA2 is insufficiently strong relative to EA1, so that too many rays are cut off by the outer EA2 plate. The primary effect is a small variation in total instrument sensitivity as a function of sector. The effective changes in look direction are negligible compared to the overall angular resolution.

Finally, a short sample of the azimuthal response of the TIMAS is shown in Figure 12. The look direction angle is defined in terms of the incident ion velocity vector relative to the spacecraft spin axis. Thus sector 13, which overlaps the 180 degrees look direction, views along the spin axis. Note that the response drops to zero at angles greater than about 181 degrees. This area corresponds to one of the “dead zones” that is masked off by the grid frame. By contract, the transmission does not go to background levels between sectors as it should if the internal collimation elements at S2/C2 plus the grid assembly between the magnets and the MCP performed exactly as designed. The details of the finite transmission between sectors is best seen near 160 degrees. As the beam direction approached a magnet, the transmission first drops nearly to background as expected, but then increases to about 15%-20% over a region of a few tenths of a degree. The transmission again drops to near background directly over the magnet and is symmetrical about the magnet meridian. The net result is that approximately one percent of the ions reaching any given sector are leaked from the adjacent sectors in this calibration set-up.

The azimuthal transmission band at each sector is approximately 6.5 degrees FWHM resulting in an azimuthal transmission of about 60%. This is somewhat less than would be expected based only on the widths of the magnets and grid spokes (~70%); however, a parallel beam incident on S1 does not remain parallel, but crosses over approximately mid-way through EA2. Thus the ~3.5 degrees wide stops at S2 will first cut out rays that are closer to the meridian of the collimating slits than the local meridional ray while the ~3.5 degrees stops at S3 and the 3.3 degrees stops at the MCP grid spokes will first cut out rays that are toward the center of the sector from the local meridional ray. The net result is that when the local meridional ray is coincident with the meridian of the collimating stops at S2 and S3, parallel rays have been cut off. Thus the full width, not the full width at half maximum, is about 7.8 degrees as observed in Figure 12.

It should be noted that the calibration data presented above did not involve any preacceleration at PA1 or PA2. Preacceleration has several effects. First, acceleration preceding S1 results in an increased azimuthal acceptance per sector due to refraction at S1. It simultaneously increases the area factor for a parallel beam for the same reason (i.e. the incident rays are bent toward the local meridian). Finally, the energy bandpass relative to the external ion energy increases in direct proportion to the increased energy. Preacceleration at S2 has much less effect on the geometric factor, but since it results in a smaller spread in angles within EA2, fewer rays are cut off by the analyzer plates and by the stops at S3. In particular it suppresses the cross-over between sectors seen in Figure 12.

Last modified February 1996 by Bill Peterson

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