Reference: Moore, T.E., C. R. Chappell, M. O. Chandler, S. A. Fields, C. J. Pollock, D. L. Reasoner, D. T. Young, J. L. Burch, N. Eaker, J. H. Waite, Jr., D. J. McComas, J. E. Nordholt, M. F. Thomsen, J. J. Berthelier, and R. Robson, The Thermal Ion Dynamics Experiment and Plasma Source Instrument, Space Sci. Rev., 1995.
Rejection/Deflection System:
EUV-induced electron contamination of the TOF system is a very serious concern for TIDE, for in contrast to the situation with parallel curved plate electrostatic analyzers, TIDE's relatively open M/RPA system provides little rejection of EUV photons which are incident upon the mirror grid wires. The grids have a reflectivity for such photons which is the complement of the grid transmission, approximately 10% for each of the three grids, multiplied by the reflectivity of the surfaces. This reflectivity is diffuse rather than specular due to the curvature and roughness of the electroformed surface. Nevertheless, measures have been taken in TIDE to introduce EUV rejection requiring at least two additional bounces between the grids and the TOF START foils. This is the purpose of the "S" shaped trajectories of ions passing from the M/RPA through the rejection/deflection system to the TOF section, as may be seen in the raytracing of Figure 4.
Click the image for a larger version.
Figure 4. Section view of the TIDE sensor showing the optics path through the mirror, RPA, immersion lens, UV rejection deflector, START foils, and STOP detectors for a single channel, with rays plotted for parameters which completely fill the instrument aperture in space, angle, and energy.
Displacement of the ion rays sufficient to cut off all direct paths from the mirror grids to the START foils is accomplished by means of electrodes biased so as to accelerate the ions and accomplish the two opposite deflections. The net translation is radially inward with respect to the symmetry axis of the instrument. The detailed configuration of this Rejection/Deflection section was developed by means of iterative potential solution and raytracing with the goal of accomplishing a deflection and straightening of the ion rays with as little dependence on starting energy as possible, over the range 0-300 eV. The result is effective in rejecting EUV photons and in preserving the bundle of rays.
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