Upload
tekla
View
31
Download
0
Embed Size (px)
DESCRIPTION
EPHIN Status and Alternatives. Michael Juda. Outline. EPHIN description Thermal issues +27V rail anomaly and impacts Operations constraints Contingencies Future plans. EPHIN Description. - PowerPoint PPT Presentation
Citation preview
QuickTime™ and aNone decompressor
are needed to see this picture. CXC
EPHIN Status and Alternatives
Michael Juda
EPHIN Status Page 2
QuickTime™ and aNone decompressor
are needed to see this picture. CXCOutline
1. EPHIN description2. Thermal issues3. +27V rail anomaly and
impacts4. Operations constraints5. Contingencies6. Future plans
EPHIN Status Page 3
QuickTime™ and aNone decompressor
are needed to see this picture. CXCEPHIN Description
• EPHIN (Electron, Proton, and Helium Instrument) provides on-board particle radiation sensor for safing function– Flight-spare of EPHIN unit in COSTEP on SOHO
– Contains 7 detectors• Passivated ion-implanted Si (detectors A, B, and F)
• Lithium-drifted Si (detectors C, D, and E)• Scintillator with PMT readout (detector G)
– Signals combined to provide 13 particle “coincidence” channels• 4 electron channels covering 0.25-10.4 MeV• 4 proton channels covering 5-53 MeV• 4 alpha-particle (He) channels covering 5-53 MeV/nucleon
• 1 “Integral” channel for particles with energies higher than the above ranges
EPHIN Status Page 4
QuickTime™ and aNone decompressor
are needed to see this picture. CXCEPHIN Description
EPHIN Status Page 5
QuickTime™ and aNone decompressor
are needed to see this picture. CXCEPHIN Location
EPHIN Status Page 6
QuickTime™ and aNone decompressor
are needed to see this picture. CXCEPHIN in RADMON
• EPHIN data is provided to the on-board computer for potential use in radiation monitoring (RADMON)– Rate data from the 13 coincidence channels– Rate data from the individual detectors (not in RADMON now)
– “Aliveness” data
• The RADMON process currently monitors three of the coincidence channels to identify a high-radiation environment– In high-radiation an on-board sequence is run to safe the science instruments and stop the observing program
Channel Particle Energy Range (MeV) Coincidence ConditionP4GM Protons 5.0 - 8.3 A1 A4 B0 C0 D0 E0 F0 G0P41GM Protons 41.0 - 53.0 A1 A2 B0 C0 D0 E0 F0 G0E1300 Electrons 2.64 - 6.18 A0 A1 B0 C0 D0 E0 F0 G0Xn indicates a detector threshold levelStrike-thru indicates a "NOT" condition
EPHIN Status Page 7
QuickTime™ and aNone decompressor
are needed to see this picture. CXCThermal Issues
• EPHIN is mounted on the sun-ward side of the spacecraft
• Degradation of passive thermal control surfaces (e.g. MLI) has led to temperatures increasing faster than pre-launch expectations
• High temperatures have caused anomalous EPHIN performance– High detector leakage currents at high temperature exceed design capability of +27V supply leading to a current-limit condition
– Drop in +27V supply output that leads to a drop in detector HV
– Hysteresis in temperature to recover from anomaly
• High temperatures could lead to permanent degradation or failure of EPHIN– Drop in HV may lead to loss of compensation in Si(Li) detectors
– Component/workmanship-related failure
EPHIN Status Page 8
QuickTime™ and aNone decompressor
are needed to see this picture. CXC+27V Rail Anomaly
EPHIN Status Page 9
QuickTime™ and aNone decompressor
are needed to see this picture. CXCImpact of HV reduction
• Reduced HV on detector G reduces its anticoincidence efficiency– Higher E1300 rate observed which could lead to unnecessary radiation safing and lost science time• No evidence in past events of lowered sensitivity to radiation
• Reduced HV on detectors C, D, and E could lead to permanent degradation in their performance– Si(Li) detectors require sufficient HV bias to maintain compensation• HV level unknown (not available in telemetry)
– Increased noise in detectors is expected to lower the sensitivity in the EPHIN coincidence channels
– No degradation observed to-date that can be attributed to the anomaly events (16 episodes)
EPHIN Status Page 10
QuickTime™ and aNone decompressor
are needed to see this picture. CXCImpact of Reduced HV on E1300
EPHIN Status Page 11
QuickTime™ and aNone decompressor
are needed to see this picture. CXCOperations Constraints
• Avoid episodes of +27V rail anomaly as much as possible– Plan observations such that the attitude profile keeps the predicted EPHIN temperature below the onset temperature with a margin• Margin selected to limit episodes to ~5/year• Limit on duration of observations in the 60-130 deg pitch range– Pitch range of concern grows with time as the degradation of thermal control surfaces continues
• Requires extensive (re)work in the long-term schedule
• Constrained science targets are occasionally expected to trigger the anomaly– Schedule a long-duration, cold attitude to follow the science target to speed recovery from the anomalous condition
– Adjust safing time before radiation zone entry to minimize possibility of safing trigger from higher E1300 level
EPHIN Status Page 12
QuickTime™ and aNone decompressor
are needed to see this picture. CXCContingencies
• Change thresholds of monitored EPHIN channels or which EPHIN channels are monitored in response to degraded EPHIN performance
• RADMON process has been modified to read HRC anticoincidence and MCP total rate data– HRC antico shield and MCP trigger rates replaced He coincidence channel rates
– HRC rates only reflect the high-energy end of the EPHIN measurements• OK match to P41GM but dynamic range is more limited
• Less good match to E1300 and none to P4GM
EPHIN Status Page 13
QuickTime™ and aNone decompressor
are needed to see this picture. CXCHRC vs EPHIN
• Ceiling on HRC rate is lower than where we would normally safe for high-radiation
• Using HRC for safing could lead to unnecessary safing and lost science time
• Use it when EPHIN cannot deliver high-energy monitoring capability
EPHIN Status Page 14
QuickTime™ and aNone decompressor
are needed to see this picture. CXCFuture Plans
• Raise E1300 threshold to minimize possibility of unnecessary safing during +27V rail anomaly episodes
• Investigate the gains from turning off the detector G HV– Less current draw should raise the temperature of the onset of the +27V rail anomaly
– Thresholds in the RADMON process may require modification
• Investigate modifications to the temperature margin used in scheduling observations– Budget to allow for more anomaly occurrences
EPHIN Status Page 15
QuickTime™ and aNone decompressor
are needed to see this picture. CXCReference Links
http://hea-www.harvard.edu/~juda/memos/ephin/index.html
General EPHIN Information
http://hea-www.harvard.edu/~juda/memos/ephin/leakage_current/index.html
EPHIN Leakage Currents
EPHIN +27V-rail Supply Current-Limit Episodeshttp://hea-www.harvard.edu/~juda/memos/ephin/current_limit/index.html
HRC Use in RADMON Processhttp://hea-www.harvard.edu/~juda/memos/FN443_HRC_in_RADMON.pdf