Patent Number: 
Section: claims

1. A sensor for the measurement of a potential of an ion beam including:a faceplate having a front wall and back wall, the faceplate defining an aperture; andan electrode mounted to the back wall of the faceplate, concentric to the aperture defined by the faceplate, wherein the electrode is configured to capacitively couple to fringe fields associated with the ion beam. 2. The sensor of claim 1 wherein the sensor includes a box, and wherein the faceplate comprises a side of the box. 3. The sensor of claim 1 wherein the faceplate is configured to be grounded. 4. The sensor of claim 1 wherein the aperture of the sensor is in the form of a rectangle. 5. The sensor of claim 4 wherein the width-to-length aspect ratio of the rectangle is about 2/1 to 5/1. 6. The sensor of claim 4 wherein the electrode is in the form of a rectangle, and defines a central opening at least as large as the aperture defined by the faceplate. 7. The sensor of claim 6 wherein the electrode is is positioned a distance from the back wall of the faceplate. 8. The sensor of claim 1 wherein the electrode is mounted to an insulating layer, and wherein the insulating layer is mounted to the back wall of the faceplate. 9. The sensor of claim 8 wherein the insulating layer is an insulating plastic. 10. The sensor of claim 1 wherein the electrode is configured to be maintained at a negative potential. 11. A method for determining a potential of an ion beam, including:generating an ion beam;positioning a sensor in the vicinity of the ion beam, the sensor including:a faceplate having a front wall and back wall, the faceplate defining an aperture;an electrode mounted to the back wall of the faceplate concentric to the aperture defined by the faceplate, wherein the electrode is configured to capacitively couple to fringe fields associated with the ion beam; andmeasuring the charge induced in the electrode. 12. The method of claim 11 wherein the sensor is positioned within a halo portion of the ion beam. 13. The method of claim 11 wherein the electrode of the sensor is maintained at a negative potential relative to ground. 14. The method of claim 11 wherein the faceplate of the sensor is maintained at ground potential and the electrode of the sensor is maintained at a negative potential. 15. The method of claim 11 wherein the sensor is positioned relative to the ion beam such that the ion beam crosses the aperture defined by the faceplate in a direction parallel to its shortest dimension. 16. The method of claim 11 wherein the sensor is positioned along a drift section of a beam line apparatus used to transport the ion beam. 17. The method of claim 16 wherein the beam line apparatus includes a beam pipe through which the beam is transported, and wherein the faceplate of the sensor is radially positioned from the center of an axis of the beam pipe at a distance equal to a beam pipe radius of the beam pipe.