Critical Design Elements

Two requirements on the CODIF, TEAMS and ESIC sensors had a large impact on the final design: 1) In order to measure the 3D distribution function with reasonable angular resolution the electrostatic analyzer must image the angular distribution to the carbon foil and the cylindrical TOF section must be divided into at least 16 pixels. 2) In order to provide mass resolution for ions with masses up to 16 (or even 32 for molecular ions) at plasma energies a post-acceleration voltage of at least 15 kV is needed. A schematic radial cross-section of the TOF system of the sensor with the most important elements is shown in Fig. 3.

The focal plane of quadrispherical analyzers coincides with their exit. Because of the post-acceleration gap the incoming ions would already be divergent in the carbon foil plane where the position is determined. Therefore, a toroidal ESA was chosen. The optimization for this application was reached through careful computer simulations before the final design. A detailed description may be found in a paper by McFadden and Carlson (1996) in this volume.

In order to achieve position resolution with 16 pixels, while minimizing the TOF electronics, the electron signals were split within the sensor. The Start and Stop signals for the time-to-amplitude converter electronics are taken from metallic grids at 10 mm above the position pixels on a printed circuit board. In this way an almost complete separation of both electronic systems can be achieved except for stray capacitances between the different electrodes. It turned out that unipolar signals without overshoot for both timing and position could be achieved with the grid and pixels on exactly the same, i.e. ground, potential. It was derived experimentally that an even split of the signal between both terminals is reached with an 80-90% transmission grid. This can most probably be attributed to fringing fields near the grid, but this has not been modeled during the design of the instrument. To reduce capacitive cross-talk between adjacent pixels, ground traces and CuBe separators have been implemented. In this way a suppression of cross-talk to as low as 1% of the original signal height has been achieved.

To provide the necessary post-acceleration the complete interior part of the TOF section, including the MCPs has be at negative potential ( 20 - 25 kV). In order to avoid elaborate high voltage interfaces for the MCP signals the signal electron cloud that leaves the MCP is accelerated by the post-acceleration voltage to electrodes at ground potential. Thus all signal connections can be referenced to ground. The only high voltage interfaces are the power connections to the deflection electrodes and the MCPs in the TOF section. The configuration still leaves us with several challenges.

Any conducting surfaces at negative high potential tend to emit electrons. Depending on where these electrons strike a solid surface in the sensor they can contribute substantially to instrument background. For example, secondary electrons emitted by the carbon foil of the TOF section into the direction of the ESA may produce secondary ions of gases that are attached to the surface of the ESA or the mounting plate (a). If created at the exit of the ESA they can be accelerated into the TOF section and thus are identified as ions which cannot be distinguished from natural ions entering the analyzer with very low energies. Therefore, great care has been taken to focus secondary electrons of this kind deep into the ESA. Such focusing becomes almost impossible to achieve for secondary electrons from the edge of the carbon foil where it meets the mounting frame. Unfortunately, it cannot be guaranteed that all 16 foils are perfect throughout all the handling. A tiny rupture next to the edge can therefore provide a strong source of secondary electrons in the post-acceleration electric field. Background increases which could be related to foil damage after the test were observed during sensor operation in the laboratory. Therefore, a second grid (denoted "Protection Grid" in Fig. 3) was added in front of the carbon foil to significantly reduce the electric field at the foil surface. This protection has removed the secondary electron background.

The mounting of the TOF structure with an insulator to the housing, the MCP assembly, and the high voltage connection in the center of the sensor are areas where triple points (junctures of metallic and insulator surfaces exposed to vacuum) cannot be avoided (b) in Fig. 3). These triple points are known as potentially dangerous starting points for electron migration in high voltage environments. Therefore, all unavoidable triple points are either placed in regions with dramatically reduced field strength by field shaping electrodes or buried under a shield.

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