Beam Profile Wire-scanner/halo-scraper Sensor Analog Interface Electronics

The halo experiment presently being conducted at the Low Energy Demonstration Accelerator (LEDA) at Los Alamos National Laboratory utilizes a generally traditional wire scanner for measurement of the beam core profile and a graphite scraper for measurement of the tails of the beam distribution. A lossy integrator is used to detect the replacement charge flowing to the wire and scraper. Independent programmable dc-bias voltages are applied to the wire and the scraper through the analog electronic interface to optimize charge capture from the two sensors. A programmable guard voltage is applied to isolate the scraper from the resistivity of the cooling system. Programmable gain provides a total dynamic range in the analog electronics of greater than about one part in 10. The analog signal is digitized to 14 bits plus sign, and the equivalent input noise is nominally 30fC. 1 WIRE-SCANNER/HALO-SCRAPER SIGNALS The signal from the wire-scanner/halo-scraper (WS/HS) sensors is the charge required to be delivered to the sensor to replace the charge imbalance caused by the interaction of the sensor with the accelerator beam [1]. Secondary electrons are radiated from the wire and protons are collected in the scraper in the proton beam of the LEDA. Therefore, the charge that must be delivered to both sensors to maintain charge balance in normal operation is an electron current to the sensor. It is reasonable to expect that under various conditions of operation, for example different Z positions and various accelerator tunes, the WS/HS sensors will collect other particles. Although it is not intended that these particles be collected, it is important to know that they are collected. The charge of these particles may be either positive or negative. Therefore, the displacement current may be an electron current either to or from the sensor. To provide maximum versatility in the analog-front-end electronics (AFE) collecting the sensor signals, the AFE must be capable of processing bipolar input signals. The displacement charge collected is a function of both the intensity of the illumination and the duration of the illumination. The secondary-emission current from the wire sensor is a small percentage of the beam current. Microampere wire currents result from milliampere beam currents. The nominal macropulse width in the LEDA is 30μs. The collected wire charge for the beam currents of nominally 100mA is on the order of 10ηC at the beam core. The maximum scraper signal is substantially higher than the wire signal, but the scraper is intended to collect charge information in the outer tails of the distribution to the limits of the beam pipe. Therefore, the scraper actually presents with both the highest and lowest signals. Therefore, the scraper sensor has the more demanding dynamic-range requirements. 2 ELECTRONIC INTERFACE A block diagram of two AFE channels is shown in Figure 1. The input stage is configured as a lossy integrator. The integrating capacitance provides the integration constant, and the shunt resistance provides integrator reset. The response pole is set at nominally 160Hz (1ms reset time constant) which can reasonably accommodate an accelerator pulse-repetition frequency as high as 20Hz. The AFE is provided with three integration constants roughly at decade intervals and remotely selectable under program control. The response poles for all three integration constants are made equal so that the frequency response is consistent among the three integration values. The macropulses comprise a substantial RF component from the beam micropulses. The RF component is of no particular interest in the WS/HS measurements, but it must be accommodated to assure that the desired signals are not compromised. The RF component is terminated at the AFE input by means of a passive surge termination with a comparatively high response pole. The response pole of this surge termination is accommodated in the lossy integrator by adding a corresponding lead (response zero) in the integrator function. The combined result of the surge termination and the lossy integrator is a continuous integration function over the length of the macropulse. The lossy integrator is followed by a programmable-gain stage which is also under program control. The combination of the integration and gain control provides a total full-scale charge range of about 500:1. The gain stage is followed by an absolute-value stage. This allows a unipolar 14-bit analog-to-digital converter to be utilized. A sign bit is provided output from the absolute value stage so that the total digitized resolution is 14 bits plus sign. * Supported by US DOE, Office of Defense Programs, and the Office of Nuclear Energy, Science and Technology. 0-7803-7191-7/01/$10.00 ©2001 IEEE. 2314 Proceedings of the 2001 Particle Accelerator Conference, Chicago