Subnanosecond Chopper-Buncher for a 14UD pelletron

2014 A novel chopper-buncher system is under construction for the Rehovot 14UD Pelletron. Beam dynamics calculations of that system are described. The ultimate performance for nickel ions is 100 picoseconds pulse, at a repetition rate of 4 MHz, and transmission of 15 % of the d.c. beam current. REVUE DE PHYSIQUE APPLIQUÉE TOME 12, OCTOBRE 1977, PAGE The beam dynamics of a chopper-buncher system is discussed. This system was designed to operate in the 14UD Pelletron electrostatic accelerator. Its ultimate goal is to bunch ion beams from protons to about nickel to better than 100 picoseconds pulse width. The performance of the system was calculated numerically using a computer code named TLBD. The pulsing system comprises of three units. A chopper and a prebuncher preceding the low energy tube, and a superconducting cavity buncher at the target area. The beam dynamics program TLBD simulates the ion optical system from the ion source up to the target area. All the optical components are introduced in the thin lensapproximation. The trajectory of a particle crossing a# optical element is replaced by a fictitious one, in which the whole change in its path (in longitudinal and both transverse planes) occurs in the median plane. The elements accepted by the program are: drift space, quadrupole singlet, deflection magnet, accelerating tube, gas or foil stripper, aperture, rf-deflector and acceleration gap. The program generates a set of particles uniformly distributed in a six dimensional phase space volume. This volume is bounded by an ellipse in (x, x’), a rectangle in (E, t) and another ellipse in (y, y’). The new coordinates of each particle, as it goes through an optical element, are calculated individually. This procedure allows the inclusion of nonlinear terms in the description of the various elements, as well as coupling between any two subspaces of phase space. (*) Work supported by the Unitcd States-Israel Binational Science Foundation under Grant 926. The set of particles is moved through the machine element by element, and its distribution in phase space can be plotted at any point. To simulate the beam of the 14UD Pelletron at Rehovot, we ran particles through the structure composed. of the elements given in table I. The first four elements in table 1 constitute the chopper. This deflector-redeflector chopper [1] ] corrects to first order energy and deflection modulations of the beam. The two deflectors require the same rffield strength, and are in phase relative to a standard particle. The thin lens at the chopper’s center has a focal length equal to one quarter of the total length of the chopper. Therefore this lens images the beam from the deflector to the redeflector with a negative unity magnification. That is, transverse position and angle of a particle at the deflector, repeat with inverted signs at the redeflector. This together with the phasing of the two deflectors, results in a cancellation of their contribution to the energy and angular deflection of each particle. The energy spread which the beam has in transit between the two deflectors, produces a small phase mismatch, which leads to a residual energy and deflection modulations after the redeflector. However, these residual errors are much smaller than those made by a conventional single deflector chopper. As can be seen in table I, the prebuncher is located only 50 cm above the low energy tube entrance, since the final pulse width decreases with this distance. The prebuncher is made of two subunits: the main accelerating gap and a third harmonic unit. By a proper choice of the voltages applied to the two gaps, the velocity modulation waveform approaches linearity in a given phase range. It was found that over a range of ± 35° (at the prebuncher’s frequency) the non-lineaArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197700120100157500