We report macroparticle simulations for comparison with measured results from a proton beam halo experiment in a 52-quadrupole periodic-focusing channel. An important issue is that the input phasespace distribution is not experimentally known. Three different initial distributions with different shapes predict different beam profiles in the transport system. Simulations have been fairly successful in reproducing the core of the measured matched-beam profiles and the trend of emittance growth as a function of the mismatch factor, but underestimate the growth rate of halo and emittance for mismatched beams. In this study, we find that knowledge of the Courant-Snyder parameters and emittances of the input beam is not sufficient for reliable prediction of the halo. Input distributions with greater population in the tails produce larger rates of emittance growth, a result that is qualitatively consistent with the particle-core model of halo formation in mismatched beams.
We report measurements of transverse beam halo in mismatched proton beams in a 52-quadrupole FODO transport channel following the 6.7-MeV LEDA RFQ. Beam profiles in both transverse planes are measured using beam-profile diagnostic devices that consist of a movable carbon filament for measurement of the dense beam core, and scraper plates for measurement of the halo. The gradients of the first four quadrupoles can be independently adjusted to mismatch the RFQ output beam into the beam-transport channel.
An experimental effort has been undertaken to investigate the production of halo particles in a proton beam having significant space charge forces. The LEDA RFQ was used to inject a pulsed 6.7 MeV 15-75 mA beam into a linear FODO channel.Four matching quads at the input of this 52-quadrupole transport line were used to generate specific mismatch oscillations, believed to be a key mechanism in the generation of beam halo. A suite of diagnostics that provide beam profile measurements over a wide dynamic range enabled a detailed comparison of measurements with theoretical models.
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...
Powerful cw proton linear accelerators (100 mA at 0.5–1.0 GeV) are being proposed for spallation neutron-source applications. A 75-keV, 110-mA dc proton injector using a microwave ion source is being tested for these applications. It has achieved 80-keV, 110-mA hydrogen-ion-beam operation. Video and dc beam-current toroid diagnostics are operational, and an EPICS control system is also operational on the 75-keV injector. A technical base development program has also been carried out on a 50-keV injector obtained from Chalk River Laboratories, and it includes low-energy beam transport studies, ion source lifetime tests, and proton-fraction enhancement studies. Technical base results and the present status of the 75-keV injector will be presented.
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