Permanent magnet structures for XUV image sensors

In space astronomy there has been considerable recent interest in the spectral region of wavelengths below 1200 Å. The investigation of such radiation is often carried out by means of a windowless detector placed on a Rowland-circle together with a collimating slit and grating as in figure 1. After the to-be-investigated beam passes through the rectangular opening in the detector it impinges upon a photocathode from which it ejects electrons. The electrons are subsequently accelerated toward a collecting anode by an electric field between the electrodes. The rectangular, slab-like electron beam thus formed requires a magnetic field of several hundred Oersteds to keep it focused on the anode. This paper discusses several suitable permanent magnet structures designed to optimize the balance among field uniformity, light weight, small bulk, simple, rugged design and minimum magnetic flux leakage to the detector exterior. The latter is important to allow tight packing of field-sensitive instruments in the space vehicle.