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University of Southern California


Optics Free Spectrometer II

This Space Sciences Cente (SCC) has proposed to developed a new, substantially improved and highly efficient, Extreme Ultraviolet (EUV) Optics Free Spectrometer (OFS) for measuring EUV solar radiation. The SSC has successfully fabricated, flown and recieved high quality data from an OFS through its sounding rocket program. The OFS II will be improved by optimizing the focusing system model to reach the highest spectral resolution and efficiency for the required dimensions, gas concentration, and potentials on electrodes in combination with optimal photoelectron acceleration to meet the detector requirements.

The earlier OFS photoelectron focusing system contained two concentric cylinders with a series of slits for selecting appropriate trajectories and directing the electron beam to a channeltron detector. The applied voltage between the cylinders created the necessary electric field to form, together with the slits, a beam for the selected range of electron energies. If the voltage was constant, the OFS worked as a monochromator. A scan through the spectra of electronenergies was realized by changing the voltage between the cylinder. To improve the spectral resolution, the beam intersected the axis of the cylinders (the optical axis) twice, forming a sine function trajectory in any azimuthal plane. The starting point on this trajectory was a gas ionization aperture, and the end point was the detector.

Figure 1. A model of the earlier OFS design shows photoelectron trajectories for electrons ionized by He II 30.4 nm radiation. The red beam shows ideal (stable) conditions with a gas-photon area of 6 mm in diameter and an elevation angle of 54.7 deg. The blue color beam shows trajectories for a larger interaction area (12 mm) and deviation of the elevation angle from - 54.0 to - 55.4 deg. Inner and outer cylindrical electrode potentials were set to about 5 and -11 V, respectively. Limiting diaphragms are not shown.

Figure 6. A 3-D view of the proposed focusing system. Red and blue photoelectron beams represent a vertical cut through an azimuthally symmetric distribution of electrons inside the system. Limiting diaphragms are not shown.

In contrast, the proposed design uses the photoelectron beams orthogonal to the incident EUV radiation, originate from a thin ionization area on the optical axis of the instrument. This solution has three major advantages compared to the prior prototypes. The first is the maximum luminosity angle Q = 90 deg, which supplies much more photoelectrons to the detector than designs with 50 – 60 deg angles. The second advantage is a weak dependence of the focusing characteristics, i.e. the size of the beam on a detector, on the size (diameter) of the ionization area. The third is the on-axis design, which allows the azimuthally symmetric distribution of photoelectrons in the spectrometer. This feature improves the efficiency compared to the orthogonal beam design with an off-axis photoelectron distribution.



    website last updated 5/31/2007