In 2010 our group custom designed and built a computer-controlled stepper-motor driven X-Y stage which utilized zero-backlash acme threads to sweep a magnetic field probe through the median plane. An F. W. Bell 7010 hall probe based gauss meter was used for the AVF measurements; the gauss meter was outfitted with an RS232 data port. The computer program which controlled the X-Y stepper motors also recorded the gauss meter data, fully automating the measurement process. An overall view of the measurement apparatus is shown in Figure 1.
A Programmable Logic Controller (PLC) based machine-protection system was implemented to allow safe, un-attended operation of the 12-Inch magnet. In the event of high-temperature condition or a coil cooling-water flow loss for more than 10 seconds, the PLC will slowly ramp the magnet down and latch it off, requiring an operator to reset. The PLC safety system was necessary as the scans could take in excess of 24 hours: a standard measurement grid of 129 x 129 points equals 16,641 measurement points, each measurement required about 5 seconds each, totaling an excess of 23 hours scan time.
To calibrate the hall probe.s position against the magnet.s mechanical center, magnetized iron needles were precisely placed around the pole tip to create field bumps, one such needle is displayed in Figure 2. The field-bump calibration was performed with the 12-Inch magnet de-energized. A full 2-D scan was completed; peaks found by fitting to the measured field bump were taken to be the center location of the needles and calibrated to the X-Y stage position. A result of a of field-bump calibration scan is shown in Figure 3, it also reveals the residual magnetization of the 12-Inch magnet. Four needles were needed to scale both dimensions; the fifth needle was used to break the symmetry, removing orientation ambiguities.
The variation of the peak amplitudes indicate the probe was traveling in a plane slightly tilted with respect to the median plane. However, this effect seems to be insignificant in the measurement of actual AVF field. A vertical sensitivity study will be done.
The first set of AVF pole tips measured were of the simplest design, and are shown installed with the cyclotron chamber removed in figure 4. With a periodicity of four, the hills and valley are each 45 degrees wide. They maintain a constant thickness out to the pole edge, except for a quarter-inch chamfer to break the sharp corners. The data from the first scan is plotted in Figure 5. The four hills are prominent, h\ wever a small central bump is observed from the slug that ties the four vanes together. This is required to promote a centrally localized weak focusing field since the flutter will be too small to be effective at the central convergence.