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ALUMNOS CON DISCAPACIDADES
The method of measuring test mass charge for the single-mass pendulum is analogous to that which will be applied for Pathfinder, (Weber et al., 2006). A low frequency ‘dither’ voltage is applied to the x electrodes which produces a response in the test mass which is related to the test mass charge. For Pathfinder the polarity of the applied voltages are such that a translation in produced, but given that in the torsion pendulum case the test mass is rotationally free, the polarities are chosen so as to produce a rotation. By measuring the rotation φ with the autocollimator, the frequency dependant torque N (f ) can be obtained via the pendulum transfer function: N (f ) = I (2πf )2 1 − (fT0)2− i Q φ (f ) (3.2)
where I is the moment of inertia, T0 is the pendulum’s free period and Q is the quality factor of the torsion fibre. I was calculated to be 4.31 × 10−5
kg m2, T0 was measured and varied from 564 to 600 s and Q was measured to be 2880. To obtain a coherent torque NM OD, a sinusoidally varying voltage with an amplitude of VM OD and a frequency of fM OD is applied to the four x sense electrodes. This torque is then proportional to the test mass potential VT M, and for a centred test mass is
given by: NM OD= −4 ∂C ∂φ VT M − P j(x)δVj 4 ! VM ODsin (2πfM ODt) (3.3) where ∂C ∂φ
is the partial derivative of the x electrodes capacitance with respect to the rotation and δVj is any stray DC voltages on the x electrodes. The charge on the test mass is finally related to VT M by:
VT M = q CT + X i CiδVi CT (3.4)
where q is the free charge on the test mass, Ci is the capacitance between the housing surface i and the test mass, CT is the total capacitance of all i housing surfaces and Vi is the potential of the ith housing surface. For the measurements discussed here VM OD = 1 V and fM OD = 3 mHz, which led to a measurement error of approximately 5000 elementary charges per 333 s measurement cycle. In addition to the dither voltages a 4.47 V, 100 kHz AC injection bias, VInj, was applied to the y and z injection electrodes. In some of the tests, DC biases were also applied to the z sense electrodes, VZBIAS, in order to either enhance or suppress certain photoelectric currents. The polarity of the applied dither and injection voltages as well as the DC biases are shown in Figure 3.7.
Figure 3.7: A diagram showing the AC and DC voltages that were applied while measuring test mass charge. Also shown in purple is the UV illumination of both the test mass and upper z electrodes. Adapted from Weber et al. (2006).
3.3.3
Results
Using the setup described, the engineering model Charge Management System was tested with the engineering model inertial sensor at the single-mass torsion pendulum facility, during 2006. The results of the testing are discussed in detail in Wass et al. (2006b) and Weber et al. (2006). Here, a summary of their findings are presented.
After measuring the output power from the two lamps being used for the test- ing, the level of attenuation by the optical fibres were measured. The throughput for the fibre system delivering UV to the test mass was measured as 0.005 and for the electrode housing as 0.012. The cause of such a large proportion of the ini- tial light being lost was later found to be poor coupling between fibre connections and has been significantly improved since to values ∼ 0.5. The test mass was then charged alternately positive/negative by illuminating either the test mass or elec- trode housing, allowing the discharging rate around VT M = 0 to be calculated. This was repeated for several lamp powers with VZBIAS = 0 and the results are shown in Figure 3.8.
Figure 3.8: A plot showing the measured discharge rates at VT M = 0 for
both test mass illumination (Red) and electrode housing illumination (Blue). No VZBIASwas applied and as expected both illuminations show a linear relationship. Figure taken from Weber et al. (2006).
As expected, a linear relationship between the UV power and the discharging rate was found. There was also a clear asymmetry between the discharging rates
for test mass illumination and electrode housing illumination which was quantified by fitting s straight line to each set of data. A value of 6830 ± 15 e−1
s−1
nW−1 was calculated for test mass illumination and −27460 ± 140 e−1 s−1 nW−1 for the electrode housing illumination. This asymmetry is thought to be due to the differ- ent geometries, surface reflectivities and photoelectric properties of the illuminated surfaces. Nevertheless, the charge management device was observed to be capable of bipolar discharge.
The next test involved discharging in one direction until a test mass equilibrium potential, VEQ, was reached. The test mass is only at an equilibrium potential if one integrates over an entire injection bias cycle. An equilibrium is reached because as the test mass potential increases the photoelectric flow in the dominant direction is gradually suppressed. Eventually no net photocurrent flows and an equilibrium potential is therefore reached. This typically took 1 to 2 hours and was repeated for various VZBIAS. The results for both illuminations are shown in Figure 3.9.
Figure 3.9: A plot showing the test mass equilibrium potential reached for different applied VZBIAS for both test mass illumination (Red) and electrode
housing illumination (Blue). Note the negative equilibrium reached upon test mass illumination with VZBIAS = −2. Figure taken from Weber et al. (2006).
The results demonstrate the effect that the applied VZBIAS has in either enhanc- ing or suppressing the photoelectric current and show that at least some absorption took place in the z sense electrode regions. The measurements again show an asym- metry between both illuminations. Of particular interest is the reading made with
VZBIAS = −2 for the test mass illumination which shows a negative VT M equilibrium was reached. This means that with this setup bipolar discharge is possible using a single lamp and an appropriate applied bias.
A continuous discharge experiment was also carried out. This involved having both lamps turned on simultaneously and adjusting the lamp powers so that the photoelectric currents match in both directions. This allowed the test mass potential to be held near zero while simulating the situation expected in flight. One current acted to ‘charge’ the test mass, in analogy with the radiation environment in space, while the other acted to ‘discharge’. Four separate tests were performed with the test mass potential held near zero, to within 10 mV or ∼ 106 individual charges, for between one and two days. The pendulum torque noise was then analysed during this time and no detectable excess noise was seen in the Pathfinder bandwidth. This was despite the fact that the magnitudes of the photoelectric currents were 100 times larger than are expected in flight.
3.3.4
Conclusions
The testing of the engineering model Charge Management System with the single- mass pendulum was deemed a complete success. The system showed it was capable of discharging in either direction and at the rates required for Pathfinder. A simple demonstration of the systems capability of continuous discharging was also shown. However, as stated in Sections 3.3.0.1 and 3.3.1, the setup studied had many, possibly significant, departures from the flight system. Later measurements with a four-mass pendulum experiment had a setup more representative of flight.