Raster Scope
Raster scope monitor and controls
What a raster
Raster is the German word for "screen", which was incorporated into English with the advent of scanning CRT TVs.
The electron beam in the CRT (CEBAF) is pushed by electrodes (magnets) so that it goes from side to side. For the parity quality experiments we want to prevent the beam from blasting a hole in one single spot in the target, and so we have the beam scan from side to side rapidly to distribute heat across it.
The Hall A Raster is two sets of two sets of coils, of which only one is in use for the low energy Prex/Crex experiments: one in X and the other in Y. Their frequencies are ~25kHz, meaning that they trace a continuous diagonal (nearly 45 degree) Lissajous figure.
How the raster should be set
Their frequencies should be mutually harmonic, meaning that the difference in frequencies between them is some multiple of the helicity window flip rate. The reason for this is so that the pattern traced in X and Y will repeat itself an integer number of times in each of the helicity windows of one Quartet (or other multiplet) pattern of windows used to calculate the asymmetries.
This is important to avoid having differences in the integrated electron beam position between helicity flip windows (that could either broaden our asymmetries or introduce false asymmetries from pick-up noise in the raster electronics itself).
Beam/60Hz Line Synch
The helicity multiplet start is synched with the 60Hz line power phase - this means that the last window in a multiplet will be shorter or longer than the first windows (randomly, on the order of 35us jumps between multiplets), and the Lissajous figure will start at a slightly different phase on each new multiplet (which for Prex is every 30Hz) which will make the raster pattern not stand still.
Also the "60Hz" timing can get generally shorter or longer over the course of a day, meaning that the "good" mutual harmonicity of the X and Y coils can "go bad" over the course of a few hours. This is potentially very bad, especially when the target densities are degraded by melting in the center (the fate rastering is intended to avoid).
