1. INTRODUCTION
The purpose of this document is to describe the EIS entrance filter vacuum chamber and to suggest desired features and performance.
2. PURPOSE
The EIS entrance aperture is covered by a thin aluminum filter supported on a nickel mesh. This filter is very fragile and would likely be destroyed by pressure differentials or acoustic vibrations during the launch of Solar-B. It is highly desirable to launch these filters in an evacuated chamber to protect them from these two effects. It is not possible to evacuate the entire EIS structure, so a filter compartment is required.
3. FILTER INFORMATION
The EIS entrance filters are 1500Å thick aluminum foils supported on a fine nickel mesh. They cover a circular entrance aperture of ~200 mm diameter. The filter is segmented into four pie shaped quadrants supported on separate aluminum frames. The surface properties should be those of evaporated Aluminum for thermal calculations. While the surface is specular on a fine scale, the filters are hung “loose” on the frames and ripples and waves have been observed to spread reflected light ~5° or so from the specular direction. The filters should be canted a few degrees so that the reflected sunlight goes out the center of the entrance aperture.
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To avoid damage to the filters, they should be launched in an environment with a pressure of approximately 1 Torr (TBC). The environment must be kept clean to avoid contamination of the filters, it must be free of particles and dust, and pressure differentials across the filters must be kept below 1 Torr (TBC). Any operation that generates pressure waves must be avoided, and venting must be done with clean dry filtered gas. Both venting and pumping must be done very slowly.
The vacuum vessel should be constructed with a minimum of outgassing materials, and organic lubricants and coatings must be avoided. High quality hermetic feedthroughs should be used wherever such are needed. The vessel should be evacuable to pressures below 10-3 Torr, and be capable of holding pressures of less than 1 Torr for up to 3 weeks without being pumped.
O-rings:
Captive virgin viton o-rings should form the main seals for the doors. The o-rings should be molded, not glued or vulcanized viton rope. No lubrication of the o-rings should be required. The front door o-ring should be located in the door itself or otherwise shaded from direct sunlight when the door is open. The o-rings should be cleaned and vacuum baked before use. Use of solvents on o-rings can cause unwanted adhesion to sealing surfaces and should be avoided. Aluminum surfaces in the vicinity of an o-ring joint should be given a protective coating such as hard anodize to protect them from scuffing and scratches.
Pressure Sensor:
An appropriate pressure sensor must be provided as an integral part of the vacuum vessel. It must be capable of measuring the absolute pressure to 20% accuracy (of actual reading) over the range 10-3 – 20 Torr. It must operate over the (TBD) temperature range, and perform equally well when the chamber is surrounded with ambient air at 1 atm or in a large test chamber at high vacuum. The pressure sensor must be able to survive all anticipated test levels with the vacuum chamber and maintain its calibration. It is TBD whether the pressure gauge will be read out through the EIS telemetry system or through a connection to EGSE. The Hastings Model 2000 series sensors have been suggested. They are rugged and have been flown on the HRTS and VAULT rockets by NRL. They combine a thin film Pirani gauge with a Piezoresistive sensor in a single gauge tube. Its range is from less than 10-4Torr to 1000Torr. The chamber might carry two sensor heads, one connected to GSE and one connected to the MHC box.
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The clamshell is nominally in the form of a vacuum tight tube with large doors at either end. Internally it has a cross-shaped support that holds the four filter segments. It should be very similar in form and function to the TRACE filter vacuum compartment.
Test Lamp & Photodiode:
Glenn Holland, who worked on the EIT filters for SOHO has suggested that the Clamshell be equipped with a LED and a photodiode on opposite sides of the filters. Nominally, these would be mounted in the center of each door. By turning on the diode and observing the photodiode signal with GSE, the filters can be checked for light leakage in place without venting and opening the doors. It would require low voltage electrical feedthroughs and a small GSE set and a connection for use in testing.
Light Baffles:
A highly reflective sunshade should cover as much of the filter frame as possible to prevent the filter frames from heating up. This could be made from thin sheet metal on standoffs.
Air Path:
There must be a light tight air path within the clamshell so that the pressure can equalize on either side of the filters. Normally this is done with a labyrinth air path.
Thermal Finish:
Since the front door will remain closed for some time after launch, it will be exposed to full sunlight. The thermal finish on the sunward side of this door should replicate that of the filters themselves as far as possible so that there is no thermal transient occurring when the door opens. A thin, highly polished aluminum sunshield may be the method of choice for this. This sunshield should be canted to reflect solar heat back out the front aperture.
6. CLAMSHELL ACTUATORS:
The baseline door release mechanisms include a wax actuator. For this application, we would probably use an 80° C actuator. Data from Starsys Corp:Model EH-3525 (or similar): According to the manufacturer, it is tested in vacuum at 80-82°C for one half hour with no extension. The actuator actually extends at around 86°C. There is also a 5 mil film Fasar (sp?) which the manufacturer uses on the housings for high temp applications. A thin metal sunshade should be provided to protect any exposed wax actuator from activation by solar heat. The wax actuators should be located outside the clamshell vacuum enclosure. The Actuator should include a device for overcoming any adhesion that the o-ring might develop. A tool should be devised for manually resetting the wax actuators.
Torsion springs provide the opening action and cushions or bumpers will damp the shock of the door opening. When thin aluminum filters are in place, the door should only be opened if both the clamshell and its surroundings are at vacuum since a rush of air would destroy the filters.
The doors must be reclosable after test operations on the ground. A tool must be devised to carryout this operation without entirely disassembling the EIS forward structure. Manual operation is envisaged. Alternatively, a red-tag vacuum compatible motor may be required, or door activation during S/C TV test may be waived.
7. CLAMSHELL MOUNTING
The mounting of the clamshell to the EIS structure is TBD.
8. FILTER MOUNTINGS
The thin aluminum filters are supplied by Luxel Corp. and comprise four pie shaped segments covering a 90° section of the entrance aperture. Each filter is mounted on an aluminum frame with three mounting feet. The frame is mounted to the filter support structure in the clamshell by three machine screws through the mounting feet. The mounting feet are staggered so that all the filters can be identical and mounted with minimal obstruction of the light path. See SAO drawing TRAC-1017 Rev2 for a filter frame concept.
Figure 1 A Trace Filter in its shipping container
9. VACUUM HARNESS:
A vacuum harness must be provided to evacuate and backfill the filter chamber. The harness should attach to the clamshell main valve by a VCR type fitting. Figure 3 is a schematic of the proposed vacuum harness. It should be made up into a portable system that can travel with the filter chamber. The VCR fittings allow it to be attached & removed as necessary during testing. A leak checker port will allow the connection of a helium leak tester whenever needed.
A safety feature is an electromagnetic valve above the pump. In the event of a power interruption, this valve should close and must be manually reset to open. A second safety feature is an overpressure relief valve in the backfill line.
The gauges marked 0 – 20Torr in the figure should be capable of measuring to 10-3 Torr or less. They might well be the Hastings 2002 series units.
To evacuate: Starting from one atmosphere, the chamber should be pumped very slowly through the needle valve only. With filters in place, the pumping rate should not exceed 2 Torr/sec until a pressure of <50 Torr is reached. Thereafter, the bypass valve can be slowly opened and the chamber pumped to its ultimate vacuum.
To Backfill: First evacuate the entire harness and verify that the pressure in the harness and the clamshell are equal or nearly so. If the clamshell has leaked up, it should be evacuated first, see above. Once everything is evacuated, the needle valve and bypass can be closed and the backfill started. Using the needle valve, the backfill should be done at 2 Torr/sec or less.
A complete written procedure will be provided for all evacuation and backfilling operations once the vacuum harness is assembled and calibrated.