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Part 3.4: Thermal-Vacuum Tests

I took the advice given by staff at the Air Force Research Laboratory to add a pair of thermoelectric coolers to the Goembel Instruments SCM test apparatus so that I could perform the thermal-vacuum tests. The addition of thermal control turned out to be simple and inexpensive. Goembel Instruments can now save time and money by performing thermal-vacuum tests in-house rather than subcontracting the work. The thermal control also allows the calibration of the SCM with an electron beam at various temperatures so that it can be determined if there has been any change in performance during thermal-vacuum cycling. The Goembel Instruments vacuum chamber is equipped with a water-cooled cold-plate that is in turn attached to a pair of thermoelectric coolers. The thermoelectric coolers are in contact with an aluminum plate that is bolted to the SCM chassis.

Figures 3.4.01 and 3.4.02: Goembel Instruments thermal-vacuum apparatus and Thermal blanket over SCM

The ability to thermally cycle the SCM while collecting electron energy spectra turned out to be important. The test revealed a limitation to the 'calibrated operating range' of the flight prototype SCM, as will be described below. Initial thermal vacuum testing of the flight prototype SCM took place in June 2005. At that time the test was performed to check our thermal vacuum apparatus and see if the SCM had any temperature-related performance problems.

During thermal cycling from -24 to +61°C I found that the SCM is not able to energy analyze electrons when turned on at temperatures above 40°C. The quirk is due to poor high temperature performance by a 600 volt DC-DC converter. When the SCM is turned on at temperatures about 40°C it is unable to deliver the voltage needed to supply a the correct electrostatic potential to the inner hemisphere of electron energy analyzer. Interestingly, if the SCM is turned on at temperatures below 40°C the SCM can collect spectra without a problem at up to 50°C. All other function of the SCM remains stable when operated between -24 to 61°C. Furthermore, all units tested exhibit this quirk and all recover when allowed to cool. Therefore, I think the behavior is innocuous and report an operating temperature range of -24 to 61°C and a 'calibrated operating range' of -24 to 40°C. I would like to either replace the responsible component with one rated for higher temperatures or increase thermal conductivity between the component and the chassis on future versions of the SCM, but in the meantime 40°C is the upper limit for 'calibrated operating range turn on' for the SCM flight prototype delivered to NASA.

The extensive thermal vacuum test of the unit delivered to NASA (SN001) took place from 2/6/06 to 2/8/06 in the Goembel Instruments Electron Spectroscopy Laboratory. The SCM was taken through six thermal cycles with a total duration of 41 hours. The SCM was turned on and off at various times and electron energy spectra of a bare filament electron source were collected by the SCM during the test.

Figure 3.4.03: SN001 Thermal-Vacuum test duration: six cycles between -24°C and +61°C

All indications are that thermal cycling has not changed the SCM. Therefore it has passed the thermal-vacuum test.

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