Research in fundamental physics is different from research in astronomy/astrophysics or solar system science. In the area of fundamental physics, much of the research is carried out during the study, design and development phases of a project. Error analysis, comprising many different elements, is perhaps the most important aspect and has to begin right at the start. In the past couple of years, research in SSD (Y. Jafry) has concerned the development of fundamental physics experiments intended for flight on STEP, MiniSTEP and LISA.
On missions in fundamental physics, drag-free control is usually considered as part of the payload as the drag-free control system uses the signals from the accelerometers and, in turn, directly determines the scientific sensitivity of the experiment. In collaboration with Stanford University, SSD is planning to develop the drag-free control system for MiniSTEP. This research primarily involves the development of optimised control laws and algorithms for the drag-free system, culminating in the flight software. The main aims of the work are to achieve the best possible performance of the MiniSTEP system by optimally matching the drag-free controller with the MiniSTEP accelerometer design under development at Stanford University and with the spacecraft hardware. In support of this work, a Mechatronics Laboratory has been initiated, focusing on the blend of electronics, mechanics and digital control. This enables a sophisticated emulation of the spacecraft dynamics, and the drag- free control sensors, actuators and control laws in a realtime digital control environment.
Research is continuing in the important area of electrostatic charging. Sophisticated software tools (such as GEANT from CERN) is being used to evaluate the consequences of energetic particle bombardment on fundamental physics payloads. In particular, the STEP and LISA test masses have been extensively modelled in 3-D.
The GEANT software has been used to assess the adverse effects from cosmic rays, geomagnetically trapped particles and solar flare protons. The results have been used extensively by the respective science teams in the design of the instruments. Some of this work is being carried out in collaboration with Imperial College (UK), where a prototype discharge system is under development.
Extensive analysis has recently been carried out in the context of evaluating a proposal for an orbiting Equivalence Principle (EP) experiment featuring elastically-connected bodies in high speed co-rotation. The research involved a blend of analytical methods based on linearised dynamical modelling and control system design, and computational methods using sophisticated symbolic dynamics software. The results of the investigations reveal that the few advantages of high speed rotation are offset by the many disadvantages associated with the dynamical interactions. Imperfections in the sensing and actuation systems are shown to limit the achievable performance in EP measurement sensitivity severely.
An analytical approach is being developed to investigate the effects of various orbital effects (eccentricity, Earth oblateness, etc) on the EP measurement sensitivity. The approach is to extend the well known Hill's equations to account for the non-circular effects, and to introduce the constraints imposed by the test mass suspensions. Preliminary results reveal that a modified Hill's formalism (constrained with additional non- circular terms) is useful for assessing the spectral characteristics of gravity-gradient disturbances. This work also addresses the gravity-gradient parametric excitation of the EP test masses. Preliminary results suggest important design constraints on the instrument.
SP1211