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.
2. Under the screen press the save/recall button.
3. Press the recall state button.
4. 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).

Possible tuning to an independent HelBoard's 120Hz

It is possible to set up a circuit with a pair of 794 gate generators to hand-produce a stable and ~120Hz or ~60Hz fake "beam synch" signal and convert it into the optical signal we need. Thankfully we commissioned a send/recieve fiber/TTL converter box and can get more if needed. This hand-made "beam synch" can be plugged into a spare Helicity Control Board that we have in the counting house VME crate and use it to generate a much more stable MPS signal with no back and forth jitter, and nearly perfectly synched to 120 Hz, within a few microseconds truncation/extension on the Nth multiplet window length.

This significantly more stable and hopefully accurate copy (assuming all helicity boards have the same 120Hz definition too) of the intra-Multiplet helicity window timing can now be used to actually tune the Agilent Raster X-Y frequency generator to a safe helicity window-harmonic Lissajous configuration. Though it will still be the case that the phase of this Lissajous pattern w.r.t. subsequent multiplets of the injector HelBoard's actual beam synch will definitely precess around as its Nth multiplet helicity window changes its time.

HelBoard software update

See also - Helicity Control

The HelBoard software has been updated (with advice from Gary Croke) - The Counting House VXWorks Bootscript's HelBoard.o executable is now in a new ~/devices/helbrd-sync/ copy folder. This update allows for VXWorks command line user editing of the "Clock Mode" register - now we have a 5th register to control the beam synch mode (beam synch vs. free clock) and its fixed output frequency (30, 120, 240):

• WriteHelBoard(4,0) -- 30 Hz
• WriteHelBoard(4,1) -- 120 Hz
• WriteHelBoard(4,2) -- 240 Hz
• WriteHelBoard(4,3) -- Free Clock

All require some clock signal to be plugged into the front, but these are the exact frequencies that it will generate in the first N-1 multiplet windows before resetting the QRT trigger (I think... maybe it resets QRT/relative MPS trigger on each external "beam synch"?)

Note that in Free Clock mode the user has control over Tsettle and Tstable, while in Beam Synch mode the user only has control over the relative amount of the helicity window that is occupied by Tsettle. In both modes the user can control delay and multiplet pattern type.