Outer electron zone. Inner proton zone. Circuit concepts for flight experiments. Experiment and interesting issues. Idealized view of the Earth’s radiation belts in 3 dimensions.
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Inner proton zone
Circuit concepts for flight experiments.
Experiment and interesting issues
Idealized view of the Earth’s radiation belts in 3 dimensions
As the need for integrating emerging technologies into today’s space missions continues to exist, so too the need for qualifying new devices persists. Test beds of experiments designed to study radiation effects on microelectronics and photonics have been developed and flown. These test beds can serve to get flight heritage for new devices such as advanced DRAMs, microprocessors, and analog to digital converters. They can be a proving ground for new technologies pushing the envelope on feature size or introducing revolutionary materials and they can be used to further the understanding of effects such as Enhanced Low Dose Rate (ELDRS) degradation and the understanding of the space environment through in-situ measurement.
The Microelectronics and Photonics Test Bed (MPTB) was launched in November of 1997. The data presented here was collected by two of the experiments in the MPTB suite. The first experiment board tested field programmable gate arrays for single event upsets (SEU) in the different cell types and the second board measured the occurrence and amplitude of transients at the outputs of an operational amplifier and a comparator.
The fact that there is a surprising effect from the spot shielding of the FPGAs is a strong argument for the necessity of test beds for the validation of ground studies and the understanding of space environment effects.
The field programmable gate array board tests Actel’s A1460A devices. The parts are programmed with long strings of flip-flops configured as shift registers. Each string is made up of particular libraries which utilize only one each of the 3 basic building blocks: the s-module, the c-module, and the io-module. A total of 5 shift registers are tested: a 100-element chain of c-modules, 2 separate 100-element chains of s-modules, a 50-element chain of io-modules, and a 50-element chain of TMR modules. The TMR module is simply made up of 3 s-modules in a voting scheme. All chains are clocked by the global clock except for one of the s-module chains which is clocked by the high-speed clock.
Ground-Based Testing Results
The Analog SEU board tested an operational amplifier (the LM124) and a comparator (the LM139) for transients at the output. An event would be validated and binned by amplitude. The circuits for the 2 device types are basically the same except that the output level on the op-amp would be expected to lie somewhere between the two rail voltages but the output of the comparator would be at one rail. Transients cannot exceed the rail level, so while an SET at the output of the op-amp can be either negative or positive, the transients at the output of the comparator can only go in one direction.
The orbit in which MPTB is flying is a highly elliptical orbit, passing through the proton belts twice a day. It has a perigee of 600km, apogee of 46000km and an angle of inclination of about 63º.
The data has been correlated to the environment using data from dosimeters on the same spacecraft as well as data from SAMPEX .
In order to easily correlate the flight results, data was collected using a duplicate of the flight board in an accelerator beam at Lawrence Berkeley National Laboratories (LBNL). The test devices are exposed to a particle beam tuned from a "cocktail" of heavy ions. A range of energies can be tuned such that data is collected for linear energy transfer (LET) of 0.5 MeV/(mg/cm2) to 63 MeV/(mg/cm2). The upsets are counted (N) along with the fluence of particles (F). From this and the angle of incidence (Q), the cross-section (s) is calculated .
A set of cross-section versus LET curves is generated for the different device types. From these curves, Weibull parameters are derived using Space Radiation™ software. These are in turn used in CREME96 calculations of the expected upset rate for the given orbit and period.
The calculated upset rates shown in the table were derived from cross-section curves like the ones below. Data was taken for several parts and Weibull parameters were derived from the combined curves. These parameters were combined with the orbital parameters and entered into CREME96.
Results from the ASEU Flight Experiment
The Earth’s radiation belts as measured in total dose behind 82.5 mils of aluminum on CRRES
There have been 4 events in the LM139 since July of 1999 when the input differential voltage was lowered from 1V to 0.1V for most of the orbit. These few transients are not correlated with any storms but are most likely attributable to very high-energy cosmic galactic rays.
The Earth’s magnetic field lines trap the particles to form the radiation belts. An “L-shell” measures the height of a field line at the equator (in Earth radii) and is a convenient measure of the belt locations (inner proton belt at L~1.5 and the outer electron belt at L~3-4.5).
Solar proton events on 20 April 1998 & 14-15 July 2000 caused the highest weekly rates of MPTB single event transients above L=3. The proton event of 8 November 2000 also should have caused a response, but the device was powered off.
The LM124 on the Analog SEU board on MPTB has been experiencing upsets since launch. All the events have been plotted in terms of where the spacecraft is relative to the magnetic field lines. The figure shows several populations of events: a nearly continuous rate near L~2, sporadic events at higher L that appear to occur at random times, and a cluster of events in coincidence with the intense solar particle event of 14-15 July 2000.
A typical data set for the LM124.
A typical data set for the LM139.
The schematic shows the locations of trapped and more transient energetic particles in the magnetosphere. Solar energetic particles can penetrate deep within the magnetosphere during geomagnetic storms. Each of these populations varies in intensity and location on different time scales. During intense storms there can be injections of enough protons to form new temporary or shorter lived belts.
The monthly rate of MPTB single event transients in the inner radiation belt fell by a factor of ~2 from late 1997 to early 2001. Galactic cosmic rays are the source of the inner zone; their decreasing intensity, as shown by the minimum rate of protons on SAMPEX, may have led to the lower SET rate.
By binning the events, it is shown that most transients occurred near L~2, the proton belt. Of those at higher L, nearly half occurred on 14-15 July 2000, which was an intense proton event.
Data for the A1460A powered at 3.3V is shown to the left and 5V shown to the right. The trends to note are that the TMR modules and the c-modules are harder to SEU than the others. There is no difference in being clocked with the global clock or the high-speed clock. Lowering the supply voltage increases the cross-section.