1. THEMIS Engineering Peer Review 1 UCB, October 15-16, 2003 Electric Field Instrument (EFI) Engineering Peer Review Overview Dr. John W. Bonnell and the THEMIS EFI Team Space Sciences Laboratory University of California - Berkeley

2. THEMIS Engineering Peer Review 2 UCB, October 15-16, 2003 Overview THEMIS EFI Personnel and Organization Requirements and Specifications Top-Level Design Design Drivers – DC Error Budget Design Drivers – AC Error Budget Active Design Trades

3. THEMIS Engineering Peer Review 3 UCB, October 15-16, 2003 Personnel and Organization Organizational Chart: Prof. F. Mozer (THEMIS EFI Co-I). Drs. J. Bonnell, G. Delory, A. Hull (Project Scientists) P. Turin (THEMIS Lead ME) Dr. D. Pankow (THEMIS Advising ME) B. Donakowski (THEMIS EFI Lead ME, SPB) R. Duck (AXB ME) D. Schickele (Preamp, SPB ME) S. Harris (THEMIS BEB Lead EE) H. Richard (BEB EE) G. Dalton (EFI GSE Mechanical) J. Lewis, F. Harvey (THEMIS GSE) UCBSSL Technical Staff (H. Bersch, H. Yuan, et al.)

4. THEMIS Engineering Peer Review 4 UCB, October 15-16, 2003 Requirements & Specs (1) Science Requirements EFI-1: The EFI shall determine the 2D spin plane electric field at the times of onset at 8-10 Re (4.1.1.7, 4.1.1.9). EFI-2: The EFI shall determine the dawn-dusk electric field at 18-30 Re (4.1.1.3). The EFI shall measure the 3D wave electric field from 1-600 Hz at the times of onset at 8-10 Re (4.1.1.11). The EFI shall measure the waves at frequencies up to the electron cyclotron frequency that may be responsible for electron acceleration in the radiation belt (radiation belt science).

5. THEMIS Engineering Peer Review 5 UCB, October 15-16, 2003 Requirements & Specs (2) Performance Requirements The EFI shall measure the 2D spin plane DC E-field with a time resolution of 10 s (EFI-1, EFI-2). The EFI shall measure the 3D AC E-field from 1 Hz to 4 kHz (EFI-3). The EFI shall measure the Spacecraft Potential with a time resolution better than the spin rate (3 s; from ESA to compute moments). The EFI FFT Spectra range shall be 16 Hz to 4 kHz, with df/f~25% (EFI- 3). The EFI shall measure DC-coupled signals of amplitude up to 300 mV/m with 16-bit resolution. The EFI shall measure AC-coupled signals of amplitude up to 50 mV/m with 16-bit resolution. The EFI noise level shall be below 10 -4 (mV/m)/Hz 1/2. The EFI HF RMS (log power) measurement shall cover 100-500 kHz with a minimum time resolution of the spin rate (on-board triggers). The EFI shall achieve an accuracy better than 10% or 1 mV/m in the SC XY E-field components during times of onset (EFI-1, EFI-2).

6. THEMIS Engineering Peer Review 6 UCB, October 15-16, 2003 EFI Block Diagram A High-Input Impedance Low-Noise Voltmeter in Space sensor preamp sheath Floating ground generation Bias channels BIAS USHER GUARD BRAID VBraid Vref VBraidCtrl

7. THEMIS Engineering Peer Review 7 UCB, October 15-16, 2003 Top-Level Design (1) Diagram of THEMIS EFI Elements

8. THEMIS Engineering Peer Review 8 UCB, October 15-16, 2003 Top-Level Design (2) Description of THEMIS EFI Elements Three-axis E-field measurement, drawing on 30 years of mechanical and electrical design heritage at UCBSSL. Closest living relatives are Cluster, Polar and FAST, with parts heritage from CRRES (mechanical systems, BEB designs, preamp designs).

9. THEMIS Engineering Peer Review 9 UCB, October 15-16, 2003 Top-Level Design (3) Description of THEMIS EFI Elements Radial booms: –20.8 to 27.8 m long. –8-cm dia., DAG-213 or Ti-N-coated spherical sensor. –3-m fine wire to preamp enclosure. –USHER and GUARD bias surfaces integral to preamp enclosure. –BRAID bias surface of 3 to 6-m length prior to preamp (common between all 4 radial booms). –Sensor is grounded through TBD Mohm resistance when stowed, providing ESD protection and allowing for internal DC and AC functional tests. –External test/safe plug (motor,door actuator,turns click, ACTEST) to allow for deploy testing/safing and external signal injection.

10. THEMIS Engineering Peer Review 10 UCB, October 15-16, 2003 Top-Level Design (4) Description of THEMIS EFI Elements Axial booms: –4-m stacer with ~1-m DAG-213-coated whip stacer sensor. –Preamp mounted in-line, between stacer and sensor. –USHER and GUARD bias surfaces integral to preamp enclosure. –No BRAID bias surface. –Sensor is grounded through TBD Mohm resistance when stowed, providing ESD protection and allowing for internal DC and AC functional tests. –External test/safe plug (deploy actuator, ACTEST) to allow for deploy testing/safing and external signal injection.

11. THEMIS Engineering Peer Review 11 UCB, October 15-16, 2003 Top-Level Design (5) Performance Specification EFI radial sensor baseline will be 41.6 m, tip-to-tip. EFI axial sensor baseline will be ~10 m, tip-to-tip. 16-bit resolution. Spacecraft potential: +/- 60 V, 1.8 mV resolution, better than 46 uV/m resolution (allows ground reconstruction of E from spacecraft potential to better than 0.1 mV/m resolution). DC-coupled E-field: +/- 300 mV/m, 9 uV/m resolution. AC-coupled E-field: +/- 50 mV/m, 1.5 uV/m resolution. AKR log(Power) channel: <= 70 uV/m amplitude, 100-500 kHz bandwidth.

12. THEMIS Engineering Peer Review 12 UCB, October 15-16, 2003 DC Error Budget (1) The estimated electric field along the direction between the two probes is E=(v1-v2)/2L. Errors arise from and are mitigated by: Errors in baseline (L). Errors in v1 and v2; eg. (v1-v2) or each individually.

13. THEMIS Engineering Peer Review 13 UCB, October 15-16, 2003 DC Error Budget (2) Errors in baseline (L). Fly as long of booms as possible, given resources (41.6-m baseline, ~55-m possible w/in mass resources). Control boom length to 1%, trim deploy length to 4-cm accuracy. Increase fine wire length to reduce boom shorting effect (observed up to 20% on Cluster; predicted 5% on THEMIS (better Lf/L)).

14. THEMIS Engineering Peer Review 14 UCB, October 15-16, 2003 DC Error Budget (3) Errors in v1 and v2; eg. (v1-v2) or each individually. Use TI-N coating on sensors (DAG-213 on AXB) for uniform photoemission. Keep all sensors clean pre-launch. Use high-impedance preamp (10 12 ohm) to reduce DC attenuation. Current-bias sensor to reduce sheath impedance and susceptibility to photoemission asymmetries (20-100 Mohm typ.). Mount sensor on fine wire and reduce emission area of preamp to reduce magnitude and effect of asymmetric photoemission (3-10 times smaller than Cluster). Use USHER and GUARD surfaces to control photocurrents to sensor (>= 20-V bias range, well above bulk of photoelectron energies). Use fine wire and BRAID bias surface to reduce cold plasma wake effects (scale with D/L or 1/L; roughly equivalent to Cluster). Enforce 1.0 to 0.1-V electrostatic cleanliness specification on THEMIS to reduce SC potential asymmetry effects to < 0.1 mV/m on all axes.

15. THEMIS Engineering Peer Review 15 UCB, October 15-16, 2003 AC Error Budget EFI Spectral Coverage and System Noise Estimates AKR band CDI BBF Preamp and R bias Current Noise Preamp Voltage Noise axial radial Maximum Spectra (DC-Coupled) 1-LSB Spectra (DC-Coupled) flat 1/f 1/f 3 Spin frequency 4-kHz Anti-aliasing roll-off 10-Hz Ac-coupled roll-in

16. THEMIS Engineering Peer Review 16 UCB, October 15-16, 2003 Active Design Trades (1) Active Trades—Mechanical 20.8-m versus up to 27.8-m SPB boom length. –Mass hit (134 g/SPB (80 g spool, 54 g wire)). –Must deploy through torsional resonance (22.1 to 25.6-m length). –Improves DC error budget by at least 30%. SPB cant angle in spin plane (85/95 versus 90/90). –Driven by fuel tank accommodation. –Mass hit to go back to 90/90 (10 g/SPB). –Small science hit for 85/95 (few percent increase in relative error between spin-plane E-field components). –Small flight software hit for 85/95 (angular offset between booms). Ti-N vs. DAG-213 coating for SPB spheres. –Ti-N is more scratch and abrasion resistant than DAG-213, and has a smoother photoemission variation (factor of four in lab tests), leading to easier handling and better photoemission uniformity. –DAG-213 predicted to run cooler in sunlight than TI-N (0 C versus 100 C), leading to easier materials selection for fine wire mechanism and less potential for thermal stress. –No mass or schedule hit.

17. THEMIS Engineering Peer Review 17 UCB, October 15-16, 2003 Active Design Trades (2) Active Trades—Electrical Inclusion of BRAID bias surface. –BRAID biasing adds mass to BEB (66 g). –BRAID biasing adds complexity to BEB design and SPB cable assembly. –Effects of cold plasma wake severe in lobe, but impact on THEMIS main science could be small due to unknown, but probably limited occurrence rate in CPS (10 % based on small Cluster study on nightside). Size of BRAID bias surface. –Balance between need for longer BRAID for ES wake mitigation and use of the remainder of braid for bias current collection (wire booms account for ~30% of collection area). +/- 20-V versus +/- 40-V bias offset range. –Science trade between full range of bias voltages and allowed range of sensor potential; larger range of bias voltages allows for greater control of photocurrents and wake effects; larger range of sensor potential allows for operation in lower plasma density environments. –No resource impact.