Jefferson Lab is developing a 10 MeV injector to provide an electron beam for a high-power free-electron laser (FEL). To characterize the transverse phase space of the spacecharged-dominated beam produced by this injector, we designed an interceptive multislit emittance diagnostic. It incorporates an algorithm for phase-space reconstruction and subsequent calculation of the Twiss parameters and emittance for both transverse directions at an update rate exceeding 1 H z , a speed that will facilitate the transverse-phasespace matching between the injector and the FEL's accelerator that is critical for proper operation. This paper describes issues pertaining to the diagnostic's design. It also discusses the acquisition system, as well as the software algorithm and its implementation in the FEL control system. First results obtained from testing this diagnostic in Jefferson Lab's Injector Test Stand are also included.
CEBAF produces a continuous electron beam with an emittance of 2-3 nm-Rad. Transverse low frequency magnetic oscillations act to dilute this emittance. These fields are typically associated with AC line conductors. The CEBAF injector is approximately 40 m long. To locate the source(s) of the beam motion, measured offsets were back propagated along the beamline using the DIMAD model. Field measurements were then made at the calculated field source positions and correlated with the measured offsets.Corrections and final beam measurements were made to ver@ the corrections. I. CALCULATION OF SOURCE POSITIONThe beam motion at different points along the beam line can be related by the equation:Where d l and d2 are the measured beam motions at points 1 and 2 along the beamline. p is the calculated betatron function, y is the measureh relativistic term for the beam and Y is the calculated betatron phase advance at the points of measurement along the machine.K is the variation in the normalized emittance. When k is greater than one it indicates that the obsemed transverse motion is greater than that to be expected from a motion being simply propagated by the lattice along the beam line. Where k equals one the beam motion is simply propagating along the beam line according to the lattice function. Where k is less than one there is the suggestion of an unknown damping function or experimental error. The magnitude of the observed motions, the energy and the calculated 0's are tabulated in Table 1 for the three measurement points, 0.5, 5 and 25 MeV. The betatron functions along the beamline were calculated usinga DIMAD model of this particular machine configuration.This model uses the experimental values to backpropagate betatron functions upstream of the diagnostic instrumentation used.The 60 Hz motion at the A4 aperture in front of the quarter cryounit was measured by positioning the beam partially on the aperture, desync'ing the pulser from the 60 I-Jz line frequency and observing the oscillations in intercepted current on the aperture. This technique assumes that the spot is round and that, because of the emittance defining aperture upstream, the current density is uniform across the spot. Given these assumptions, this technique is very sensitive, allowing motions of only .02 mm to be m&ured. The measurements at the 5 and 25 Table 2. The last column in Table 2 plots the k from the first equation above: The values are very close to one.These results suggest a source before the A4 aperture which is being propagated down the machine or a distributed effect with the bulk of the disturbance before the 5 MeV point. The region of the injector most sensitive to stray low frequency magnetic fields is the 15 cni long acceleration region between the cathode and anode in the gun. The momentum goes from 0 to 0.34 MeV/c in this distance and is difficult to shield because the acceleration is electrostatic and the structures are dielectrics. To quantitize the problem low frequency field measurements were made using a milliGauss meter, ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.