Title Instrument Simulator and L1B Processor Meeting Swarm CEFI LP Instrument Simulator and L1B Processor Meeting ESTEC, June 7-8, 2006

What we expect from this meeting Understanding of requirements and context Schedule for IS/L1B development Definitions of interfaces and who does what

Basic principles of Langmuir probes Bias voltage Vb applied on probe Resulting current measured Depends on ne, Te, Vsc, ram speed, and ni and Ti for all ion species Derive ne, Te, Vsc from probe response Many variants depending on how bias is varied

CEFI LP particulars Normal mode: one probe operated in each gain state Two Langmuir probes Two gain states in h/w to cover all parameter range 100 - 50,000,000 cm-3 Normal mode: one probe operated in each gain state Allows largest dynamic range

LP functionality A: Sweeps Most detailed information using probe bias sweeps Vary Vb, record current-voltage relation Drawbacks: large data volume time consuming e- retardation region e- “saturation” region ion “saturation” region

LP functionality B: Fix bias Constant Vb (usually positive) Current variations depend on ne and Te Advantage: very high time resolution Drawback: cannot disentangle variations in ne and Te Occasional sweeps can be used to calibrate large scale trends in ne/Te, but there is no chance to catch rapid variations

LP functionality C: Track Software can be used to track a constant current, i.e. to vary the bias so as to keep the probe current constant Allows tracking of Vsc variations at high time resolution Sheath variations also influence result Use occasional sweeps to calibrate result

LP functionality D: Ripple (1/3) Rippled Langmuir probes used to measure I and dI/dV simultaneously at a few points on a probe curve Quasi-sinusoidal dI Sinusoidal dV

LP functionality D: Ripple (2/3) 2nd point: positive side, e- saturation 3rd point: e- retardation region Vb 1st point: negative side, ion saturation

LP functionality D : Ripple (3/3) Ripple technique advantages: Fast operation Small data volume Low noise (averaging) compared to sweeps If points are judiciously chosen, variations in ne and Te can be disentangled CEFI LP: Digital implementation in FPGA Flexibility in design and operations Resource economy H/w stability

CEFI LP particulars Normal mode: one probe operated in each gain state Two Langmuir probes Two gain states in h/w to cover all parameter range 100 - 50,000,000 cm-3 Normal mode: one probe operated in each gain state Allows largest dynamic range

LP mode concept High-level modes: Normal Calibration Single-submode modes TBD (see below) Submodes composing the high-level modes: SM: Sweep mode ZM: (Zero-)tracker mode HM: Harmonic (ripple) mode (including Vsc track) FM: Fix bias mode Diagnostic modes

LP normal mode Normal mode is a combination of fundamental mode blocks Sweeps (SM) every 128 s Time between sweeps divided into 0.5 s blocks In each 0.5 s block: Track constant current (ZM) for ~0.2 s Measure I and dI/dV (HM) for ~0.3 s Resulting data High resolution ne, Te and Vs from HM Independent dVs from ZM Independent dn from probe current (HM)

LP HM submode Uses three points One in each (±) saturation region One in retarded electron region (gives unique ne/Te separation) 3rd point: e- retardation region Vb 1st point: negative side, ion saturation

LP simulator/L1B processor status Used for design purposes (h/w & s/w) Probe size Gain resistors FPGA filter evaluation Operational submode sequencing Algorithms for operational modes Data analysis algorithms Error analysis

Present simulator s/w (1/4) Works on one 0.5 s segment at a time Input for each time segment: ne, Te, ion species, Ti, ram speed Instrument setup S/c potential calculated from input plus a 0.5 V rms random value added Bias voltage Vb time series generated using specified instrument setup

Present simulator s/w (2/4) Probe current time series calculated from Vb and plasma using Langmuir probe theory (Mott-Smith & Langmuir, 1926) Combined ram+thermal expression for each ion species (Medicus, 1962) Sheath model for electrons by Walker (1961) Photoelectrons by Pedersen (1995)

Present simulator s/w (3/4) Analog electronics op amp noise (the dominating noise source) added ADC quantization noise added

Present simulator s/w (4/4) FPGA operations Full simulation of FPGA code at internal resolution Mixing and filtering (including correct CIC filters) NCO (numerically controlled oscillator) as sine table with truncated values Flight s/w functionality included except timing and ISP production

Plan

Present L1B s/w No ISP unpacking Analyzes normal mode data Nominal analysis of HM Does not yet use ZM SM implementation needs update Uses simplified probe equations Leads to systematic errors... ... which are collected in tables Output parameters corrected using the tables (TBI)

IS/L1B example result: Random errors Contours indicate IRI parameter range 8 orbits at 450 km Conditions for 1999 (i.e. 2010)

Systematic errors Systematic errors are less important: correction to be included in analysis

Errors in s/c potential Small errors in important parameter range

Plan

Plan

SM L1B algorithm (1/3) Find Vsc from max(d2I/dV2) Noise reduction in differentiation Model of ion and photoemission current: Use the 75% of the probe curve lowest in potential Linear least square fit Positive values replaced by zeros Subtract modelled current from data

SM L1B algorithm (2/3) Get Te from electron retardation part Non-linear logarithmic least squares fit Do linear fit above Vsc Ie(Vb) = e + f Vb Calculate ne = c sqrt(Te) f Do a full non-linear fit Values above used as input First leave only Vsc free (with necessary coupling to other params) Finally adjust all parameters

SM L1B algorithm (3/3) Correct for systematic errors Errors from using simplified model, imperfect algorithms and noise Calibration table determined from simulations Calibration table updated post- commissioning for identified on-orbit error sources

HM L1B algorithm (1/4) Estimate Vsc: Get line from I and dI/dV for point on positive side Get a second line from I and dI/dV in electron retardation region Use intersection of the two lines as Vs

HM L1B algorithm (2/4) I Vb Vsc estimate

HM L1B algorithm (3/4) Model of ion and photoemission current: Use I and dI/dV from point at negative side to derive linear expression Electron temperature: Derived from I/(dI/dV) at electron retardation region (intermediate point) Corrected using linear expression from ion side

HM L1B algorithm (4/4) Get ne and Te from di/dV on electron side ne = c sqrt(Te) dI/dV Correct for systematic errors (table lookup) Summary of algorithm: “Lite version” of sweep analysis Good Te, ne and Vsc at (better than) 0.5 s resolution Can fail in extreme situations (highest and lowest ne, highest Te). Fallback solutions: ZM data is fallback for dVsc I data from HM is fallback for dne Both are calibrated using the succesfully analyzed data points just before and after the failed point (HM or SM) Conflict with ISP-to-L1B data mapping requirement