SPP-FIELDS Team Mtg - 13 Dec SPP-FIELDS Slow Sweeps, Auto Bias, and All That J. W. Bonnell, UCB SSL Past experience - back to ISEE - has shown that the correct current and voltage biasing of E-field antenna elements is crucial for making accurate quasi-DC and LF E-field estimates. Challenges to a successful biasing strategy on SPP FIELDS: –The finite number of perihelion encounters on SPP – few dozen at most. –The limited opportunities for data download and commanding – less than once per encounter, and only pre- and post-encounter. –The large dynamic range in environmental drivers of antenna properties (e.g. factor of 32 in solar flux!). –The significant uncertainties in plasma environment variability (thermal, suprathermal and energetic) during encounters.

SPP-FIELDS Team Mtg - 13 Dec 2014 Sensor Biasing on SPP FIELDS Why Bias? FIELDS biasing system capabilities – PAs, AEBs, FSW. Finding Our Way – Slow Sweeps. Keeping On Track – AutoBias0. Responding to Surprises – AutoBias1. 2

SPP-FIELDS Team Mtg - 13 Dec LF Section Details: Biasing 20 Vpp IBIAS sets operating point on non-linear sheath I-V curve to reduce offset voltages due to stray currents. VSHIELD and VSTUB set up potential barriers between environmental current sources and WHIP (sensor) to reduce stray currents and improve DC sensor isolation from SC.

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB Requirements (stable since Dec 2012) Preamp signal characteristics –DC voltage level: ± 60Vdc w.r.t. AGND (± 60 Vdc at full bias offsets; up to +/- 100Vdc at reduced bias current and voltage levels). –AC voltage level: ± 10V w.r.t. floating ground up to 70 kHz (± 13V capability up to several 100 kHz) Floating Ground Driver –Input: LF Preamp signal –Input filter roll off: 500 Hz (~450 Hz actual, soft requirement due to limited AC dynamic range). –Output voltage level: ± 60Vdc w.r.t. AGND –Floating supply rails: ± 15Vdc. Bias, Stub, Shield Drivers (Bias and Box on V5) –Reference Input: LF Preamp signal –Reference input filter roll off: 500 Hz (~450 Hz actual, match FGND) –Output voltage level: Vref ± 40Vdc (max, programmable) w.r.t. AGND –DAC resolution: 12-bit (~.025%). Noise voltages at Bias, Stub, and Shield outputs consistent with noise floor requirements, downstream filtering and processing, and predicted coupling to antenna (TBD). 4

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB Requirements, con’t. Bias, Stub, Shield Drivers (Bias and Box on V5) – Output Currents –Bias ranges (AEB+PA, V5 is lowest range only; 12-bit resolution): +/- 802 nA (360 nA 1 AU; ~4 bits of next range) +/- 14 uA (0.25 AU; ~5 bits of next range). +/- 414 uA (184 uA 9 Rs). –PA1..4 Stub and Shield Currents (max is photoelectron-dominated): Stub:60 nA (1 AU), 30 uA (9.5 Rs, nominally shadowed!). Shield:200 nA (1 AU), 100 uA (9.5 Rs). –V5 Box Currents (nominally shadowed; max is photelectron-dominated): Box:~40 nA (1 AU). –Sunlit surfaces dominated by photoelectron and possibly thermionic electron emission, and so currents tend to be sinking of current from exposed surfaces (sourcing e- to surfaces consistent with sheath I-V curves). 5

SPP-FIELDS Team Mtg - 13 Dec 2014 Slow Sweeps (Sensor Diagnostic Tests, SDT) A Slow Sweep, aka. Sensor Diagnostic Test (SDT), is an essential part of the in- flight calibration of the FIELDS LF E-field system. Such a tool, usually implemented in FSW, has long heritage on E-field instruments (Polar, Cluster, THEMIS, RBSP, etc.) Conceptually straightforward (example shown on following page, changes from heritage marked in rust): –On a cadence of 2^D*(100 s) (TBR), loop over each antenna or pair of antennas: –Loop over Bias Current (DAC and Range Relay Setting), Stub Offset Voltage and Shield Offset Voltage on a single or opposed pair of antenna elements (e.g. V1 or V1 and V2), while keeping current and voltage biasing of other antennas constant. Ramps in parameters are linear, with the initial setting, step size, and number steps programmable. –Hold at each step for TBD seconds – e.g. 2 N s - (rather than for M spins), collecting single-ended (Vn) and differential sensor potentials (Enm = Vn-Vm), instrument HSK (esp. Bias readbacks) and other diagnostic data (e.g. survey plasma data). –Science Team then evaluates V and E data from swept and un-swept sensors to establish nominal current and voltage bias settings for the given plasma environment, and Ops modifies the on-board biasing tables to reflect the new default settings. Proposal: develop and implement this algorithm in DCB FSW for SPP FIELDS. 6

SPP-FIELDS Team Mtg - 13 Dec 2014 Slow Sweeps, con’t. 7

SPP-FIELDS Team Mtg - 13 Dec 2014 AutoBiasing-0 (For Predicted Solar Flux) Varying the zero-order current (and possibly voltage) biasing as a function of heliocentric distance (R H ) will be crucial to maintain DC and LF data quality: –To avoid saturation due to too high of current bias. –To maintain sheath biasing to minimize impacts of stray currents to the sensor. Implemented in the past with finite number of fixed on-board bias tables (2-4) controlled by time-tagged commands uploaded in batches. Not very feasible for SPP – –Large dynamic range in solar forcing – lots of tables needed, ~20 to keep within sqrt(2) of nominal “half the photemission” bias current setting over the ~32-fold change in solar flux from 0.25 AU to 9.5 Rs (closest perihelia). –Significant (?) uncertainty in ephemerides at time of planning (ToP) and time of command uplink (ToCU) – uncertain (?) when to execute the table switching commands. However Time and Status data distributed to instruments includes CBE ephemerides: –Expected to be more accurate than those available at ToP or ToCU. –Simple FSW algorithm and process can thus determine zero-order bias current (Range Relay and DAC settings) from R H (see diagram, next slide). PROPOSAL: Develop and implement such an algorithm for DCB FSW. 8

SPP-FIELDS Team Mtg - 13 Dec 2014 AutoBiasing-0, con’t. 9 Zero-Order Bias Update Waypoints: Sqrt(2) spacing. few hours to few tens of hours between updates. Finer gradiations possible if required or desired. Ibias ~ 0.5*Ipe ~ const*(Scaled Solar Flux) Scaled Solar Flux = (R 0 /R H ) 2

SPP-FIELDS Team Mtg - 13 Dec 2014 AutoBiasing-1 (For Environmental Variations) Sudden environmental changes (e.g. enhanced energetic electron fluxes) change the major contributors to SC and sensor current balance. Fixed current bias can then lead to sensor saturation and loss of E- field measurements (DC on up). Mozer, Harvey, and Bonnell developed and implemented an on-board AutoBias algorithm that determines the need to change the default bias settings using on-board SC_POT estimates, and adjusts the biases on a selectable cadence (32-s nominal for RBSP-EFW). Core of algorithm is a piecewise linear I-V curve that relates I_BIAS to SC_POT: Likely additions: –Independent I-V curves for each sensor (V1..V5), possibly with different weights for the different sensor potentials. –V-V curves for each sensor’s stub and shield offset potentials. 10

SPP-FIELDS Team Mtg - 13 Dec 2014 AutoBias-1 (example, RBSP-EFW) Plasmasphere (high density) Energetic e- IBIAS (V1+V2)/2

SPP-FIELDS Team Mtg - 13 Dec 2014 BACKUP SLIDES 12

SPP-FIELDS Team Mtg - 13 Dec 2014 Scope of AEB Design Effort Antenna Electronics Board (AEB, see following block diagram slide) –Full AEB functionality split into two boards: AEB1 (V1, V2, V5) – primarily controlled by DCB. AEB2 (V3, v4) – primarily controlled by TDS. –FGND Driver: 1 channel each for 4 forward whips (V1..4). 1 channel for aft antenna (V5). –Sensor bias current, Stub and Shield bias voltage drivers: 3 channels of each for V1..4. Sensor bias current and “Box” bias voltage driver for subset for aft whip or dipole. –Sensor preamp bias range relay control: 1 channel each for 4 forward whips (3 ranges). one for V5. –Floating Power Supplies: 2 floaters to support opposing pairs of forward whips. 1 floater to support aft whip or dipole (AEB1 only). –V5 Preamp Power and Heater Control: FSW-controlled floating power supply and pre-heater resistor. –HF Output Stage Power Regulation: +/- 6V from LVPS1/2 regulated down to +/- 5V for wide (4x) current consumption (signal amplitude). –Passthrough of LF signal to DFB. –Serial Command and Data (HSK) I/F to DCB/TDS. 13

SPP-FIELDS Team Mtg - 13 Dec 2014 SPP FIELDS PA 14

SPP-FIELDS Team Mtg - 13 Dec 2014 HF Section Details: Input Coupling 15

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB – FGND Bandwidth and LF Dyn Range AEB-16 The FGND bandwidth needs to be higher than one expects to allow for low-distortion measurement of large-amplitude LF fields. The top panel shows a 120-Vpp 100-Hz signal (solid black line) and the response of the Floating Ground Driver (FGND, green dashed) and positive and negative Preamp Floating Supply Rails (red and blue dash-dot). This example is consistent with the initial RBSP-EFW-BEB FGND design and the OP-15 used in the EFW PRE circuit (~2-V headroom on pos and neg supplies to give harmonic amplitudes < -40dB relative to fundamental). The FGND is a one-pole RC low-pass filtered version of the input signal, with corner (3-dB) frequency of 300 Hz (three times the input signal!). The region between the red and blue lines at any give time indicates the range of signal input voltages that will be faithfully reproduced. When the voltage margin on the pos or neg rail goes negative, the output will be distorted by clipping.

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB – LF->HF Dynamic Range AEB-17 The dynamic range (maximum amplitude signal measured meeting maximum distortion requirements) varies with input frequency: At very low frequencies, the dynamic range is set by the AEB HV output stage rails. At frequencies well above the FGND rolloff, the dynamic range is set by the PRE floating supply rails and the headroom the preamp requires on those rails. At intermediate frequencies, the dynamic range falls off remarkably fast as the rolloff frequency of the FGND is approached. 450 V pp (± 225 V, ~100 V/m), BUT… 26 V pp (± 13 V,~7 V/m) At FGND 3-dB Frequency 500 Hz: 36 V pp (± 18 V, ~9 V/m) Frequency Maximium Amplitude

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB – LF Response At Different Frequencies AEB-18 Examples using Flight RBSP-EFW AEB and PRE floating supply designs (500-Hz FGND bandwidth). Left plot is threshold case for 100-Hz input frequency (65 V amplitude [130 Vpp]). Right plot is threshold case for 500-Hz input frequency (18 V amplitude [36 Vpp]).

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB – LF->HF Dynamic Range AEB-19

SPP-FIELDS Team Mtg - 13 Dec 2014 AEB – Biasing Reduces LF Dynamic Range AEB-20 V_BIAS, V_GUARD, V_STUB, etc. (up to ± 40 V; typ. ≤ ± 20 V). The full HV rail-to-rail voltage is not available to the signal at low frequencies if current and voltage biasing is active and one wants stable DC current and voltage biasing. For typical conditions and designs (current biasing to half possible range, +/- 40 V offset system), this eats up another 20 V or so of the dynamic range in a conservative (worst-case) design. Similarly, any stable SC potential offset (SC floating potential, V SC ) eats up part of the LF dynamic range (few to tens of volts) in a conservative (worst-case) design. Large values of V SC are typically driven by biasing, but will also be driven on SPP by the space charge in the SC wake.