Appendix: Report on NASA Advanced Functional Test

Figure A.01: Complete flight prototype Spacecraft Charge Monitor

Spacecraft collect enough electrostatic charge to have a potential of between a few volts to tens of thousands of volts relative to the surrounding space plasma. The magnitude of the charge depends on orbit, solar activity, and other factors. In low earth orbit, most of the time, spacecraft have a floating potential that ranges from few volts positive to a negative charge approaching the voltage of the power system (e.g.; the ISS charges to somewhere in the region of -100 volts in daylight under normal conditions). Spacecraft charging is known to cause problems. At the very least, charging biases space plasma measurements and has led to great uncertainty in the interpretation of some data. In extreme cases, spacecraft charging and subsequent arcing has caused the loss of spacecraft systems. Additionally, it is thought that spacecraft charging could cause loss of life during extravehicular activity (thus, the requirement that the ISS charge must be reduced to a safe level during EVA's). Spacecraft charge monitors are needed to evaluate charge reduction schemes, study the charging phenomenon, correct biases to plasma measurements, and so on.

There are currently very few ways to measure spacecraft charge. In 2001 Goembel Instruments won SBIR funds to develop a laboratory prototype, then a space flight prototype of the Spacecraft Charge Monitor (SCM). The 650-gram, 2 Watt SCM has a 300-fold better sensitivity-per-unit-weight than its predecessor due to its electron-optical design. The SCM will enable one spacecraft charge determination per second with 0.1-volt accuracy in low earth orbit. The SCM promises to be 10 times more accurate than other devices now used to measure charge in low earth orbit, will not suffer from loss of calibration over time, is compact, low power, and returns easily interpreted data. The SCM may also prove useful for measuring charge in interplanetary space, on the moon, and in other orbits. (See www.goembel.biz for more information.)

It was desired to perform several experiments at KSC, including

1. collecting an electron spectrum to independently verify the SCM operation,
2. collecting several spectra with the SCM floating at various voltages to demonstrated the ability of the SCM to measure space craft potential, and
3. collecting electron energy spectra at various incident energies, and various incidence angles.

With the exception of the incident angle testing, all experiments were performed.

II. Experimental Setup

Figure A.02: SCM and basic test apparatus for this project

Figure 2 shows the test apparatus. The test apparatus was set up in a high vacuum (~10-6 torr) chamber. Figures 3 and 4 provide electrical schematics for the KSC experiments. SCM serial number 006 was used for these experiments. It is important to note that the SCM power connections (pins 5 & 8) are different than the document provided to KSC by Goembel Instruments ("Requirements for The Kennedy Space Center Advanced Functional Test of the Goembel Instruments Spacecraft Charge Monitor" dated March 15, 2005). Figure 3 gives the correct pin assignments. Although not required, this experiment used separate feed-thrus for SCM power and communications.

Figure A.04: Diagram of the Electron Source Electronics

The DB9 vacuum feed-thru does NOT reverse the pin assignments. As a result, the external pins must be the vertical mirror image of the internal (and true) pins. This is given in Table 1.

Figure A.06: The Entire Experimentation Area

The photograph in Figure 5 shows the control station for the KSC SCM experiments. The control station provided convenient control of the SCM via computer, easy control of the various power supplies, and direct access to the power feed-thrus for measurement with a digital voltmeter. Figure 6 provides a photo of the entire experimental work area.

Communication between the SCM and the computer can be established at room pressure.

III. Experiments - Verify Proper Setup

On Monday 5 December 2005 the vacuum chamber was pumped down and power was applied to the SCM. This verification test demonstrated that the vacuum chamber was leak free - getting down to ~2 x 10-5 torr after a few hours. Unfortunately, when the SCM was powered up, it was noticed that it was drawing excess current. (It drew so much current that the power supply was unable to regulate.) As a result, the power was removed from the SCM. On Tuesday 6 December 2005, Luke Goembel suggested that the +28 V power pin assignments for the SCM may be inverted. The power connections were reversed and the SCM was shown to power up properly. (The SCM was powered up at atmospheric condition. This is acceptable with the default start-up configuration.) After identifying the power pin assignment issue, it was determined that the DB-9 vacuum feed-thru was not a "cross-over" as is common for many gender changers. As a result, the pin assignments needed to be reversed for the communications wires. A review of the correct pin assignments is given in Table 1. (The ability of the SCM to withstand the pin connector mix-up is an indication of its ruggedness.)

After correcting the pin assignment problems, the SCM was set up in the vacuum chamber and the chamber was evacuated. After a few hours the pressure reached ~4 x 10-5 torr. At this time the filament operation was tested with 6 Amps (~2V). This was sufficient power to visually observe the filament through the 6" CF site glass. The SCM was then turned on and shown to draw 0.07A at 28V, corresponding to a power dissipation of 1.96W.

IV. Experiments - Demonstrate Operation with Floating Potential

On Wednesday 7 December 2005, the SCM was powered up to test its functionality with a floating chassis potential (to simulate spacecraft charging). The SCM was configured using the parameters set forth by Luke Goembel. The pressure in the vacuum chamber was measured at 3 x 10-6 torr. The electron gun was set at 24 eV and a current of 5.5 Amp. This current was determined to be optimum using the SCM to collect spectra. With a filament current of 5.6 Amp, the electron spectrum was bimodal. Under these conditions, spectra were collected with a floating potential varying from 0 to -45 Volts. Electron spectra were only observed to a floating potential of -41V at which point the intensity approached zero. After operating the SCM for about 45 minutes, the temperature was approximately 50°C. The maximum operating temperature is about 60°C. The unit was powered down at that time. For the testing performed at KSC, the SCM was thermally isolated. When deployed for actual operation, the unit will be thermally connected to the vehicle, thus providing the appropriate thermal sink.

On Thursday 8 December 2005, experiments similar to the previous day were performed. With a filament current of 5.2 Amps and base pressure of 1.4 x 10-6 torr, the chassis potential was varied between 0 and -40V, using both 30eV and 20eV electrons. Figure 7 shows the data collected for 7 December and 8 December. Similar to the data obtained on the previous day with 24 eV electrons, the 30 eV and 20 eV electron spectra had points where the peak intensity went to zero. For the 30 eV electrons, that chassis potential was -36V and for the 20 eV electrons the chassis potential was -31V. Note that on the previous day, and with a different filament current, 5.5 Amps, the maximum float voltage for 24 eV electrons was -41V. Goembel concluded that the negative potential of the SCM chassis at the protruding electronics cover directly above the entrance aperture was deviating the beam of electrons from the aperture. Subsequently, a beam shield, as described in Section V) was used to allow the electrons to travel in a straight path. In space, the electrons detected by the SCM will originate from a large area rather than a small diameter beam so that the deflection of the electrons by the electronics cover will not reduce signal strength as it did in this experiment. Figure 8 shows the various peak shapes and intensities for various float voltages. The awkward looking peak shapes are due to the inability of the simple electron production apparatus used to produce a high intensity, single energy beam of electrons under certain conditions. After the beam shield was added (see section V) to the apparatus to exclude the asymmetrical electric field from the SCM chassis the sort of single energy peak one would expect the SCM to detect in space was produced (see Fig. 10). It is important to note that the variation in peak shape and intensity shown in Figure 8 is due to the actual energy distribution of incident electrons energy analyzed by the SCM, it does not indicate a change in SCM performance with floating potential.

Figure A.08: Peak shapes and intensities as function of SCM floating potential

V. Experiments - Beam Shield

Figure A.09: Photo of Beam Shield

In order to better simulate the space environment, a beam shield was constructed by Luke Goembel. The beam shield prevents the electron beam from defocusing as a result of the various conductors at varied potentials in the vacuum chamber. The beam shield was installed on Thursday 15 December 2005 (see Figure 9). After installation, the vacuum chamber was evacuated and allowed to pump down overnight. On Friday 16 December, the chamber had reached a pressure of 7 x 10-7 torr.

The SCM was powered on and was able to collect electron spectra with float voltages up to -145V. The limit of the float voltage was due to the DC/DC converter of the electronics within the SCM. Figure 10 shows several spectra with float voltage up to -145V.

Figure A.10: 24 eV, demonstrates that beam shield allows operation up to -110 eV (the limit of the DC/DC converter)

VI. Experiments - Float Positive and Negative

On Monday 19 December 2005, another experiment was performed to demonstrate that the SCM would operate with floating potentials in both the negative and positive directions. Using 24 eV electrons, spectra were observed with float potentials between -139 and +42V. Using 20 eV electrons, spectra were observed with float potentials between -135 and +45V. These ranges correspond to a range from "-110eV" to "+67eV" apparent pass energy. This scan range is the result of the electronics and is not a fundamental physical limitation. Figure 11 shows the linearity and chassis floating potential dynamic range of the SCM. Table 2 summarizes all linearity data collected with the SCM at KSC.

VII. Conclusion

A detailed description of the experiments performed on the spacecraft charge monitor (SCM) at KSC during December 2005 was given. The goal of the testing was to independently demonstrate that when the chassis potential of the SCM is varied the SCM is capable of measuring a corresponding change in the measured kinetic energy of an electron beam. The tests to meet that goal were successful. The SCM chassis potential was varied from -145 to +45 Volts. A linear relationship was demonstrated over that potential range using electron beams of 20 eV, 24 eV and 30 eV. It was desired to perform an additional test by varying the electron beam incident angle. Such a test is very time intensive, since a 2-day vacuum cycle must occur to change the incident angle. Also, there is no expectation that the system will not operate at various incident angle, just that the signal intensity will drop off rapidly outside of the approximately ~60° x ~20° fan-shaped field-of-view of the SCM. Since the test was so labor intensive and the expected return data was not considered critical, the incident angle test was not performed. Other than that, the testing at KSC is considered successful.

Figure A.11: Successful testing

This graph demonstrates the successful testing performed on the SCM at KSC. The goal of the project was to demonstrate the ability of the SCM to vary in potential and measure a corresponding peak shift for the electron energy. Shown here is the results for 24 eV electron beam (solid line and circles) and a 20 eV beam (dashed line & diamonds).

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