Rutgers 12-Inch Cyclotron Ion Source Studies: Part II (Ion Source Simulations and Measurements)

Since the publication of the first Ion Source Studies document there have been improvements in both the understanding and implementation of the ion source system.[1] The understanding has been greatly assisted by the 3-dimensional E&M particle tracking code SIMION. Several experiments in the adjustment of the ions’ initial condition have benchmarked our 3D model as well as identified the source of the large initial ion launch angle. From these results an electrically symmetric chimney has been machined and installed. Additionally, to enhance the peak field near the ion source aperture, pullers have been mounted to the face of the DEE. These improvements have resulted in an hundred fold increase in proton beam intensity at the target and further improvements are anticipated VERTICAL MOTION. An earlier document “Observation of Betatron Motion in the 12-Inch Cyclotron” concluded by inquiring as to what the source of the large observed betatron amplitude was – the answer lies in the ions initial conditions.[2] Since the ion source aperture is located in the median plane, an initial vertical displacement can not be the culprit, thus an initial launch angle must be to blame. Evidence to back up the accusation presented itself when, after a particularly long beam run, a spiraled discoloration appeared on the copper chimney. The discoloration began at the aperture and wrapped in the direction of the beam rotation and with downward pitch as shown in figure 1. The discoloration is taken to be tracks of ions launched during the early portion of the RF phase that were not energetic enough to clear the chimney. It was suspected that the slight vertical asymmetrical geometry of the ion source chimney was the cause of the vertical electric field. In obtaining the order of magnitude of the vertical field a slope of 4.3 degrees was calculated from the spiral track. A right-handed coordinate system is used, the ion source aperture points in the direction of the x-axis, the DEE-Dummy DEE gap is parallel to the y-axis, and the vertical axis is parallel to the vertical magnetic field. The vertical electric field was estimated from the measured slope. If one calculates that in one half of an RF cycle, the ion vertically declines 0.032 inches in height the effective integrated electric field is simply calculated from: