Fast Raster

The beam is rastered on target with an amplitude of several millimeters to prevent overheating. The raster is a pair of horizontal (X) and vertical (Y) air-core dipoles located 23 m upstream of the target. The raster has been used in two different modes, sinusoidal and amplitude modulated. In the sinusoidal pattern both the X and Y magnet pairs are driven with pure sine waves with $90^{\circ}$ relative phase, and frequencies which do not produce a closed Lissajous pattern. In the amplitude modulated (or square root of time) mode both the X and Y magnets are driven at 18 kHz with a $90^{\circ}$ phase between X and Y producing a circular pattern. The radius of this pattern is changed by amplitude modulation at 1 kHz. The radius modulation is controlled by a function generator whose function creates a uniform distribution of the area swept out by the beam motion. It is not possible to switch on the fly between the two modes of operation as hardware changes are required. One can view the status of the raster in the EPICS overview screen called ``General Accelerator Parameters'' where the set-point for the radius amplitude and the readback of the peak-current in the raster are displayed. Control of the raster is done by first asking the MCC operators to set up the raster for a particular radius, typically 2.5 mm. The control software assumes a field-free region between the raster and the target, so it is only approximately correct because there are four quadrupoles in this region. It is important to check the raster spot size and make adjustments if necessary. The main adjustment is made by asking MCC to change the radius. Relatively small independent adjustments to the gains on the X and the Y raster coils are available in the middle room of the hall A counting room using the ``PGA Controller'' knobs. Near these knobs is also located an oscilloscope X-Y trace of the current in the raster. A fast shutdown (FSD) shuts the beam down within 0.1 msec if the raster fails, thus affording some protection of the target. NOTE: If you are unsure of the status of the raster, measure the spot size with very low current ($\le 2 \mu$A) or with the target out of the beam. An unfortunately common and potentially fatal error is to check the beam spot size with high current on target; by the time you check it, the target might already be destroyed. The rastered beam spot on target can be checked with plots in ESPACE or by using the stand alone code called ``spot''. Spot is probably already running somewhere; it runs on the Linux or Sun computers in the counting room; if it's not running, type ``spot''. When a new CODA run is started, spot automatically clears its histograms and displays in a window the X-Y beam position monitor coordinates from the first few thousand events from the start of the run. For more details on usage, type ``spot -h'' (help).