Readout electronics of a prototype spectrometer for measuring low-energy ions in solar wind plasma

Readout electronics of a prototype spectrometer for measuring low-energy ions in solar wind plasma 1 2 first-author 1 2 2 3 4 1 2 1 2 corresp Corresponding author, fengcq@ustc.edu.cn Corresponding author, fengcq@ustc.edu.cn fengcq@ustc.edu.cn 1 2 State Key Laboratory of Particle Detection and Electronics (IHEP-USTC), Hefei 230026, China State Key Laboratory of Particle Detection and Electronics (IHEP-USTC), Hefei 230026, China Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China Division of Space Physics, University of Science and Technology of China, Hefei 230026, China Division of Space Physics, University of Science and Technology of China, Hefei 230026, China CAS Key Laboratory of Geospace Environment, Hefei 230026, China CAS Key Laboratory of Geospace Environment, Hefei 230026, China Readout electronics is developed for a prototype spectrometer for in situ measurement of low energy ions of 30 eV/e–20 keV/e in the solar wind plasma. A low-noise preamplifier/discriminator (A111F) is employed for each channel to process the signal from micro-channel plate (MCP) detectors. A high voltage (HV) supply solution based on a HV module and a HV optocoupler is adopted to generate a fast sweeping HV and a fixed HV. Due to limitation of telemetry bandwidth in space communication, an algorithm is implemented in an FPGA (field programmable gate array) to compress the raw data. Test results show that the electronics achieves a 1MHz event rate and a large input dynamic range of 95 pC. A slew rate of 0.8 V/μs and an integral nonlinearity of 0.7-LSB for the sweeping HV, and a precision of less than 0.8% for the fixed HV are obtained. A vacuum beam test shows an energy resolution of 12% ±0.7% full-width at half-maximum (FWHM) is achieved and noise counts are less than 10/sec, indicating that the performance meets the physical requirement. Solar wind plasma Energy spectrometer Readout electronics A111F Data compression 1 Introduction 1 Solar wind, a plasma stream from the upper atmosphere of the Sun, mainly consists of protons, electrons, helium ion, and other ions in a whole spectrum of ionization states [ 1 1 ]. The energy spectra of the ions is indispensable for observing solar wind activity, and important for exploring the source of the particles accelerated to produce the shock associated and storm ions and so on[ 2 2 , 3 3 ]. A low-energy ion prototype spectrometer is being developed to measure solar wind ions in the interplanetary space. It will cover the energy-per-charge range of 30 eV/e ~ 20keV/e in an energy resolution (Δ E / E ) of better than 15% (FWHM). As shown schematically in Fig. 1 Fig. 1 , it contains three structural elements: an entrance unit, a particle detection unit, and its readout electronics. The entrance unit is implemented by a symmetrical electrostatic analyzer (ESA) with a top hat, which is used to select incident ions at 32 energy steps controlled by the sweeping HV between its inner hemisphere and outer hemisphere. The detection unit, with a gain factor of about 10 6 , mainly comprises of two MCP multipliers in a chevron pair configuration and 12 position encoding discrete anodes which present the azimuth information, leading to 12 readout channels of the electronics. Low energy ions contribute most of the particle flux of solar wind, thus a large number of event signals are generated by the MCP detector, imposing a challenge on the readout electronics [ 3 3 , 4 4 ]. Meanwhile, to get the actual events, background noise must be rejected efficiently. A fast sweeping HV ranging from −2300 V to −3.5 V and a fixed HV at −2500 V~ −2300 V are required to apply to the ESA and MCP, respectively. Event signals of each channel at every energy step are counted by the electronics, which brings a data rate close to 15 kbps after count accumulation in one spin period of the spacecraft. Nevertheless, the transmission width allocated to the spectrometer is only 3.2 kbps, as the telemetry bandwidth between satellite and ground is confined in space mission. So an efficient algorithm is needed to compress the raw data with low distortion. Besides, the reliability of electronics has to be considered because of the space radiation environment. In this paper, we report the design of readout electronics to fulfill the requirements of spectrometer, and the performance test. 2 Electronics design 1 According to principle of the spectrometer, the main function of the readout electronics is to count MCP signals of each channel at different energy steps controlled by a HV supply circuit. In addition to high-event rate, a low noise level of the readout electronics is required to satisfy the demand of the energy resolution that is also devoted to by the ESA and MCPs. And the low noise level is critical, too, for its key role in threshold setting of discrimination. Some space programs used the front-end electronics to read detector signals, such as the Double Star mission [ 5 5 ], Spektr-R [ 6 6 ], CAPS [ 7 7 ], and CLUSTER [ 8 8 ]. In the Double Star and Spektr-R, the detector signals are amplified by A225 with a low-noise charge sensitive amplifier and a fast shaper with a high-count rate, and then discriminated by a comparator that outputs a digital pulse to a counter. Obviously, this readout scheme is not suitable for this application of low power consumption and tightly restricted space. Efforts have been made in improving the integration of readout electronics for space plasma instruments, such as developing a multi-channel, mixed-signal integrated circuit (IC) AIDA [ 9 9 , 10 10 ]. However, the radiation effects of semiconductor devices, especially the single events effect (SEE) and the total ionizing dose (TID), have to be taken into account [ 11 11 ]. Thus few of the multi-channel application special integration circuits (ASICs) are practical in the harsh space environment. In CAPS, an A111F was used as a charge sensitive preamplifier/discriminator, while in CLUSTER an A121 was used. Although they are single channel circuits, their reliability and counting rate satisfy the physical demand. Considering the power limit, the noise level and the telemetry rate constraint, a readout scheme based on A111F chips and RS422 interface, together with a HV supply circuit, is adopted for the Low-Energy Ion Spectrometer. As shown in Fig.2 Fig.2 , the top-level diagram of the readout electronics is mainly composed of the preamplifier & discriminator circuits, HV supply module, self-test circuitry, and control and data processing unit. 2.1. Preamplifier & discriminator circuits 2 The preamplifier and discriminator are implemented by the A111F hybrid chip. Due to the number of discrete anodes, 12 A111F chips are employed. Besides its high counting rate (~2.5 MHz) and low noise (0.44 fC for nominal threshold) performance, the A111F features a high sensitivity (8 fC), a 25 ns rise-time and a small size (occupying less space on the printed circuit board) [ 12 12 ]. And the discrimination threshold in the A111F is adjustable to filter non-interesting events. Although anode strips are connected to ground level via large resistors, a 1-nF high-voltage capacitor and a pair of clamping diodes are utilized at the input pin of A111F, to avoid possible high voltage discharge. 2.2. HV supply module 2 The HV supply module is designed to generate a sweeping HV and a fixed HV. Prior to event signals produced by the MCPs, a sweeping HV of −2300 V to −3.5 V is applied to the ESA, and a fixed HV at −2500 V~ −2300 V for biasing the MCPs. It includes a digital-to-analog converter (DAC), two amplifiers, a high voltage optocoupler, an HV module, an analog-to-digital converter (ADC) and some voltage-proof resistors, as shown in Fig. 3 Fig. 3 [ 13 13 ]. It generates a −3000 V output, which is divided by a resistor divider to get the fixed HV, meanwhile, is led to a fast HV optocoupler to generate the sweeping HV controlled by a DAC. 2.3. Self-test circuitry 2 To check operating status of signal chains of the readout electronics in a complex and harsh space environment, a built-in self-test circuitry is utilized ( Fig. 4 Fig. 4 ). The DAC is controlled by an FPGA to output an adequate analog voltage. A narrow signal enables the analog switch to turn on, producing a test pulse to trigger A111Fs by capacitive coupling. Apart from checking the operating status, the data compress algorithm in FPGA can also be verified by accommodating the frequency of the test pulse. 2.4. Control and data processing unit 2 The control and data processing unit is implemented in FPGA (ACTEL Corp.). Its block diagram is shown in Fig. 5 Fig. 5 . In order to make the instrument into operative mode, related commands, sent by an upper computer and arriving at the subcell manage block after UART receiving and command parsing, are used to control or configure DACs, ADCs and other components orderly. When entering into counting mode, 12 counters (24-bit) record the event signals which are from A111Fs at the period of corresponding energy step for each channel. Then the 12 counting values at each energy step are accumulated cycle by cycle, respectively. After 64 cycles per spin, the last accumulated values of 24-bit raw data are compressed into 8-bit data by a log compression algorithm [ 14 14 ]. The decompression is given by Eq.(1), and the maximum loss rate is less than 8%. There is about 15K bit/s transmission rate without data compression, which surpasses considerably the 3.2K telemetry width. While counters are counting, the status of HV supply and low power operating current, and the ambient temperature, are collected. Under different working modes, the compressed event data, or the monitoring data, or both, is sent to the upper computer after data packing and framing. <math display="block" id="M1"><mtable columnalign="left"><mtr><mtd><mi>M</mi><mo>=</mo><mi>N</mi><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mtext>when </mtext><mi>N</mi><mo><</mo><mn>32</mn></mtd></mtr><mtr><mtd><msub><mi>N</mi><mn>1</mn></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mi>M</mi><mrow><mo>[</mo><mrow><mn>3</mn><mo>:</mo><mn>0</mn></mrow><mo>]</mo></mrow><mo>+</mo><mn>16</mn></mrow><mo>)</mo></mrow><mo>×</mo><msup><mn>2</mn><mi>M</mi></msup><mrow><mo>[</mo><mrow><mn>7</mn><mo>:</mo><mn>4</mn></mrow><mo>]</mo></mrow><mo> </mo><mtext> when </mtext><mi>N</mi><mo>></mo><mn>32</mn></mtd></mtr></mtable></math> where, N is a raw 24 bit count datum, M is a 8 bit output code after compression, and N 1 is a datum after decompression. Considering the dead time τ ( < 0.6×10 −6 s) of the signal chain, the true counting rate is then: <math display="block" id="M2"><mrow><mi>R</mi><mo>=</mo><msub><mi>N</mi><mn>1</mn></msub><mo>/</mo><mo stretchy="false">(</mo><mn>1</mn><mo>−</mo><msub><mi>N</mi><mn>1</mn></msub><mi>τ</mi><mo stretchy="false">)</mo><mtext> </mtext><mo>.</mo></mrow></math> 3 Results 1 The prototype detector and readout electronics are shown in Figs.6(a) and 6(b) Figs.6(a) and 6(b) , respectively. An electronics test system and an ion beam test system are established for the performance test ( Fig. 6c Fig. 6c ), and the electronics test is shown schematically in Fig.6(d) Fig.6(d) . 3.1. Electronics test 2 As shown in Fig.6(d) Fig.6(d) , a signal generator (TEK AFG3252), together with a passive attenuator, outputs an appropriate negative pulse to preamplifiers in the electronics. The counting data are transferred to a remote PC for further analysis and display executed by a LabView program. An oscilloscope is used to measure the output signal for comparison. Against the background noise and crosstalk, an 184 fC threshold is configured for A111Fs. Under the condition of a 100 kHz narrow test signal, the input dynamic range expressed as the normalized threshold, is achieved by regulating the amplitude of input signal and the attenuation ratio. Fig.7 Fig.7 shows the counting rate verse the input charge, measured by both the readout electronics and the oscilloscope. There is a stable and flat count rates, about 98.3 kHz by the electronics and 100 kHz by the oscilloscope, in a wide input range from a 184 fC threshold to 95 pC (over 500 times of the threshold). For both ways of measurement, the counting rate drops sharply below the threshold of input, while it increases noticeably at input charges beyond 95 pC, which will be explained in Section 4. With a suitable width-fixed test pulse generated by the built-in test circuit to trigger A111Fs, detection efficiency of the readout electronics at different event rates can be measured by adjusting the frequency of test signal which is accomplished by a self-test control unit in the FPGA via sending different control commands. The detection efficiencies at expected event rates are listed in Table 1 Table 1 , which verifies the log compression algorithm. a Generated by built-in circuit 1 1 Event rate a (Hz) 1 1 By oscilloscope(Hz) 1 1 By Eqs (1,2)(counts/sec) 1 1 Detection efficiency (%) 1 1 1024,000 1 1 1023,980 1 1 973,677.7 1 1 95.09 1 1 512,000 1 1 511,992 1 1 486,838.9 1 1 95.09 1 1 256,000 1 1 255,996 1 1 243,419.4 1 1 95.09 1 1 128,000 1 1 127,998 1 1 121,709.7 1 1 95.09 1 1 65,536 1 1 65,535 1 1 63,195.4 1 1 96.43 1 1 32,768 1 1 32,768 1 1 31,597.7 1 1 96.43 1 1 16,384 1 1 16,384 1 1 15,798.9 1 1 96.43 1 1 4,096 1 1 4,096 1 1 3,949.7 1 1 96.43 1 1 1,024 1 1 1,024 1 1 994.3 1 1 97.10 1 1 512 1 1 512 1 1 497.1 1 1 97.10 1 1 64 1 1 64 1 1 62.9 1 1 98.21 A linear sweeping test and a fixed voltage deviation test are conducted for the HV supply module, with a relative precision of < 0.8% for fixed HV, a 0.7% integral nonlinearity ( Fig. 8 Fig. 8 ), and a fast enough slew rate (0.8 V/μs) for the sweeping HV. More details can be found in Ref.[ 13 13 ]. 3.2. Beam tests 2 A beam test system under ground-based vacuum facility is constructed to simulate the vacuum environment in space, as shown in Fig.9 Fig.9 . The ion source injects a 2.5 keV Ar + beam into the vacuum chamber, which covers the whole effective area of the sensor aperture. With the MCPs biased at −2500 V, the ESA is controlled to sweep in −150 V ~ −400 V to select the incident Ar + beams. Fig.10(a) Fig.10(a) shows the energy response represented by the counting rates of relevant channels. The energy resolution (Δ E / E ) is 12% ±0.7% (FWHM) for Channel 8 by a Gaussian fitting (dashed curve). Fig.10(b) Fig.10(b) shows the baseline noise measurement with the ESA sweeping from −100 V to −800 V for Ch7 to Ch9. The noise counts is less than 10/sec. 4 Discussion 1 4.1. Input dynamic range and double pulsing effect 2 Test results confirm that the readout electronics reaches an input dynamic range from 184 fC to 95 pC, as the counting rate is flat and accords with the compression algorithm which can be verified by a built-in circuit generating test pulses up to 1 MHz. The increase in counting rate with input charges beyond 95 pC is due to the double pulsing, which was also observed by lliev Metodi et al . [ 15 15 ].The double pulsing is yielded by the shape variation which creates the possibility that under the large input signals, the discriminator threshold may be crossed more than once during the duration of the pulse. 4.2. Energy resolution 2 In the Ar + beam test, sizeable flux of Ar + beams is injected into Ch8 and the counting rate of each channel is compared. The noise counting under different sweeping HV can manifest that the spectrometer is of low noise, since the count rate fits comfortably with the dark counts of MCP [ 16 16 ]. Due to the optimized ion optics design of the ESA and a low noise level, an energy resolution Δ E / E of 12% ±0.7% (FWHM) is achieved, which is better than the design specification of 15%. 4.3. Consideration for radiation mitigation 2 Because of the radiation environment in space, hazards can be imposed to the readout electronics. For example, TID can affect input bias current, offset voltage, transistor parameters, timing margins and so on for microcircuits, while SEEs can upset the logic state, cause malfunctions or even latch-up [ 17 17 ]. Apart from shielding, components with high TID tolerance have a higher priority during this electronics design. For example, A111F and LM6142 (operational amplifier) have a TID resistance over 100kRad (Si). There are also other measures to mitigate SEEs, such as adding current-limiting resistors to the input and power-supply pins of a component to reduce latch-up currents, using triple module redundancy (TMR) and parity check techniques for FPGA logic [ 18 18 ]. Some anti-fuse and Flash FPGA are much more suitable for aerospace applications because their logic elements are immune to SEE. For example, Actel ProASIC PLUS series Flash-based FPGAs are tolerant to single event upset (SEU), and their linear energy transfer (LET) threshold for single event latch-up (SEL) is over 104 (MeV-cm2/mg) [ 19 19 ]. However, in this prototype design, considering the cost and practical requirements, some commercial components including an FPGA are used instead of high-reliable ones. 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(Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0136-0 http://dx.doi.org/10.1007/s41365-016-0136-0 . 2016 Nuclear Science and Techniques 06 27