Raster Scope

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Raster scope monitor and controls

Where the raster scope is

The raster scope is located in the 2nd from left rack of electronics in the electronics room and is about 3 feet off the ground. It is an X Y scope and should have some pattern displayed on it whenever the raster is on. It is displaying the X and Y currents going through the raster magnet coils. The raster amplitude set values should be displayed in the Hall A General Tools GUI and also the Target GUI.

What a raster is

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).

Updating Raster Sync with Function Generator

The raster was set up on 5/30/2019 (halog) - Reference photos: Function Generator Locking on Scope, Raster Current Readbacks on Scope, Raster X vs Y on a 2D scope trace, What the Agilent Controls look like, The Raster Current set values controlled by knobs (experts/MCC only),

To sync the MPS signal to the raster we use an Agilent 33500 signal generator. It has a 10MHz input to sync the outputs with the signal coming in. The raster will utilize these 10MHz clock based output frequencies to determine its X and Y ramp frequencies. The output of the signal generator is synced to the MPS 120Hz signal from the injector manually by putting the MPS signal into a scope and tuning the Agilent function generator until both output frequencies (X and Y are different by an integer multiple of MPS) are locked w.r.t. each other and the 120Hz signal.

This setup is stored in the memory of the Agilent Function Generator and can be recalled by doing the following:

1. • Press the system button.
   • Under the screen press the save/recall button.
   • Press the recall state button.
   • Look down the list and select the TREX-30Hz.sta button and press select and the state will automatically return.

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).