Precise control over the potential of an electrically isolated proof mass is critical for many devices, such as the MGRS. During flight, direct charging and secondary electron emission can lead to a charge imbalance between the proof mass and housing, leading to electrostatic forcing. The UV LED project aims to develop a full non-contact charge control system using AlGaN UV LEDs operating at 255 nm.
Charge management basics:
Charge control is achieved through controlled photoemission. By modulating the UV LED, charge control can be performed outside the science band. The rate of charging depends on UV optical power, coating properties such as workfunction, quantum efficiency, and reflectivity, and the surface roughness of the proof mass and bias plate.
The solid lines describe the electron path while the dashed lines describe the UV path. When the bias plate voltage relative to the housing (Vbias) is positive, (1) photoelectrons generated from the proof mass and (2) photoelectrons generated from the bias plate travel to the bias plate leading to an increase in the proof mass potential (VPM). When Vbias is negative, (3) photoelectrons generated from the proof mass and (4) photoelectrons generated from the bias plate travel to the proof mass leading to a decrease in VPM.
Space qualification testing of UV LEDs
We have successfully tested AlGaN UV LEDs to MIL-1540E levels of thermal, thermal-vacuum, and vibration; these include 27 thermal and thermal-vacuum cycles over the range -34 to +71C, and 9 minutes of 14.07 g RMS vibration. There is less than a 3% change in current draw, less than a 15% change in optical power, and no change in spectral peak or FWHM (full width at half maximum power). The V-I (voltage-current) curve for a diode is the fundamental measure of its PN junction characteristics and a family of well behaved V-I curves is a good indication of diode chipset quality.
Characteristic performance plots of an uncollimated UV LED taken during MIL-1540 level laboratory testing. Figures show data taking before testing, after thermal vacuum cycling, after shake, and after thermal cycling (post test). From left: Voltage (V) vs. Current (mA), Current (mA) vs. Optical Power (uW), and Spectrum
Proof mass coatings:
Selection of a proper proof mass coating is critical for both charge management and general performance of the GRS. The photon energy Ephoton from a 255 nm UV LED source is 4.86 eV, and this sets the upper bound on the candidate coating workfunctions. The selected coating should also act as a protective layer, allowing the proof mass to retain surface optical and geometric properties during caging, uncaging, thermal cycling, and during any possible collision with GRS housing walls. The coating must also strongly adhere to the proof mass during launch vibrations of up to 50 g and during thermal cycling; there must also be minimal adhesion between the surface coating and the GRS housing. In addition, the coating must be highly reflective in the IR (1064 and 1550 nm) for optical proof mass readout and be electrically conductive to help minimize patches.
Coated aluminum samples. Top row (from left): Au, Nb, Ir, SiC. Bottom row (from left): TiC, Mo2C, ZrC, TaC.
Last modified Mon, 6 May, 2013 at 19:34