Enhancements have been made to the log-ratio analog front-end electronics based on the Analog Devices 8307 logarithmic amplifier as used on the LEDA accelerator. The dynamic range of greater than 85 dB, has been extended to nearly the full capability of the AD8307 from the previous design of approximately 65 dB through the addition of a 350 MHz band-pass filter, careful use of ground and power plane placement, signal routing, and power supply bypassing. Additionally, selection of highisolation RF switches (55dB) has been an integral part of a new calibration technique, which is fully described in another paper submitted to this conference [1]. Provision has also been made for insertion of a first-stage low-noise amplifier for using the circuit under low-signal conditions. BACKGROUNDA BPM signal processor based on the AD8307 was previously used by us on the Low Energy Demonstration Accelerator (LEDA), part of the Accelerator Production of Tritium (APT) project at Los Alamos. Experience with that system was reported at the 2000 Beam Instrumentation Workshop [2]. Several of the problems encountered with that system have been corrected, as well as new capabilities added, with the resultant system being presented here. Currently the LEDA accelerator is configured to conduct a beam halo experiment.Beam diagnostics include fifteen beam position monitors in the 52-quadrupole magnet focusingdefocusing (FODO) lattice, along with wire scanners, beam-loss monitors and current monitors [7]. The nature of the experiment places a requirement for additional dynamic range on the BPM electronics. Beam debunching, which occurs as the beam is transported through the lattice, reduces the signal power by about 40 dB from the last BPM with respect to the first. CIRCUIT DESCRIPTIONReferring to the block diagram in Figure 1, signals from the four electrodes of the beam position monitors are band-pass filtered and input to the AD8307 log amplifier through a transformer balun, which is required because of the differential inputs with an impedance of 1 kΩ shunted by 1.4 pf to common. The circuit layout can accommodate a Minicircuits Lab fixed attenuator (PAT-X) and/or a Cougar Components AC566 pre-amplifier [8] installed between the filter and the transformer. This attenuatoramplifier combination allows us to adjust the signal highend for the log amp's maximum input of approximately +17 dBm, thereby obtaining the greatest signal to noise ratio and dynamic range (approximately 92 dB is specified for the AD8307.) The log amp output is then amplified, scaled and offsetadjusted by an AD8041 amplifier stage for a 0 to approximately 4.6 V range. The signal is then low-pass ___________________________________________ * Supported by US DOE, Office of Defense Programs, and the Office of Nuclear Energy, Science and Technology.
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 6 . The analog signal is digitized to 14 bits plus sign, and the equivalent input noise is nominally 30fC. WIRE-SCANNER/HALO-SCRAPER SIGNALSThe 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. ELECTRONIC INTERFACEA 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 provid...
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