2.2.3.1 ACIS Thermal Design

This section describes the thermal design of the ACIS Detector Assembly and the Thermal Control Subsystem (TCS). Both of these assemblies are installed onto the Science Instrument Module (SIM) which is attached directly to the optical bench of the AXAF telescope. Currently, it is planned to launch the AXAF-I into a highly elliptical 10,000 by 140,000 kilometer orbit. With this high orbit, the CCDs will have a higher radiation exposure than low earth orbit. It is desirable to improve the radiation tolerance of the CCDs with increased radiation shielding and lower CCD operating temperatures made possible in this higher orbit. The current design requirement is to operate the CCDs at -120 C. The cooling approach calls for a two stage passive radiator which radiates to deep space. The details of the design are discussed in the following sections.

Detector Assembly

The Detector Assembly has several design features which help maintain the focal plane at -120 C. The titanium collimator provides thermal isolation from the SIM which is at 0 C. Titanium provides the greatest thermal isolation of any commonly used metal and provides half of the total thermal isolation from the SIM. Therefore, the camera body temperature can be maintained at -60 C. The inside of the collimator has been gold plated to provide a low emittance surface to minimize the radiative heat absorbed from the telescope which is at 10 C. (This coating also reduces Ti K$\alpha$ and L fluorescence from the collimator.)

The camera body is constructed of aluminum to maximize the amount of radiation shielding close into the CCDs and to provide an isothermal environment for the focal plane. The inside of the camera is gold plated to minimize the radiated heat from the camera body which can be absorbed by the focal plane. The temperature of the camera body is maintained with heaters bonded to the -Z side of the housing. The controller for the heaters is located in the PSMC and maintains the camera body at -60$\pm$1 C except during bakeout when the housing is heated to +25 C. Thermistors mounted next to the heaters provide feedback to the PSMC to maintain temperature control. Since the camera body is constructed of 0.5" thick (min) aluminum, the temperature gradient is less than 1 C between the heaters on the -Z side of the camera and the warm straps on the +Z side. A 20 layer MLI blanket provides a thermal radiation shield between the Support Structure which is at 30 C and the detector housing which is a -60 C.

Thermal isolation is provided between the -60 C camera body and the -120 C focal plane using four Torlon standoffs. Torlon has a low conductivity and does not contribute significantly to the parasitic heat leaks between the focal plane and the camera body. External surfaces of the standoffs are gold coated (sputtered) to achieve low emittance. The top of the standoffs attach to the cold fingers and the cold stubs. Copper cold fingers provide high thermal conductivity and pass through the +Z standoffs to the outside of the camera. They provide a conductive path between the focal plane and cold straps which attach to the cold radiator.

The primary conduction path for heat from the camera body to the focal plane is through the interconnect flex cables. Copper cross sections have been minimized in these cables and the cables have been formed in a S-bend to maximize the length. They also have a low emittance surface to minimize radiation exchange between the camera body and focal plane.

The focal plane is constructed primarily of beryllium which has a very high conductivity providing an isothermal mounting surface for the CCDs. Four RTDs monitor the temperature of the focal plane. Two sets of heaters are used for controlling the temperature at -120$\pm$1 C during normal operation and +30$\pm$1 C during bakeout mode. The controller for the heaters is located in the DEA. Outside surfaces of the focal plane are gold plated to minimize the radiative heat exchange from the camera body.

Thermal Control Subsystem

 

Figure 2.9:   Thermal Analysis Results - Hot Case

The Thermal Control Subsystem (TCS) for the ACIS consists of two passive radiators, thermal standoffs, telescope shade, sun shade and the thermal control surfaces. Cut away views of the TCS can be seen in Figure 2.9. The two stage passive radiators cool the camera housing to -60 C with the first stage (warm) radiator and the focal plane to -120 C with the second stage (cold) radiator. The view to earth is small in this highly elliptical orbit. However, for the cold radiator, heating does occur at perigee when the radiators are facing the earth. The focal plane increases by 10-20 C with full earth IR and albedo, but is back to normal operating temperature within 8 hours. The radiators have no sun exposure and have a direct view of space at all other times enabling low operating temperatures.

The warm radiator is used to reject parasitic heat from the SIM (0 C) and the telescope (10 C) to deep space. The approximately 4 square feet radiator uses a high emittance Martin Black finish to reject 12 Watts to space at a temperature of -84 C for hot case conditions. Only about 2 square feet of the radiator is effective at radiating the heat due to cold radiator shadowing. A 20 layer Multi-Layer Insulation (MLI) blanket provides thermal radiation shielding from the SIM +Z panel.

The cold radiator rejects the conducted and radiated heat (parasitic) from the camera housing and the thermal dissipation of the CCDs. The total area of this radiator is 2.0 square feet and also has a Martin Black finish. This radiator rejects 3 Watts to space at a temperature of -135 C for hot case conditions. A 20 layer MLI blanket provides thermal radiation shielding from the warm radiator.

To minimize the delta T between the radiators and the detector assembly, the radiators are located as close as possible to the focal plane. When the ACIS focal plane is in the X-ray beam, a telescope shade is required to block views of the telescope and the SIM which are at around 10 C. A low emittance gold coating is required on the internal surface of the shade which views the radiators. Goldized Kapton is used for this surface and acts as a mirror to reflect the parasitic infrared radiation (heat) from the radiators to deep space.

Uncoated metallic surfaces will get very hot when exposed to the sun. Since the planned orientations of the AXAF will allow full sun on the aft end of the telescope, a sun shade is required to shield the radiators from the sun. The external surface of the sun shade uses a MLI blanket to minimize absorbed heat. As with the telescope shade, the internal surface of the sun shade will be goldized to provide a low emittance and highly reflective surface.