CEA, Gif-sur-Yvette
An important issue for the new high power class ion linac projects is the preservation of the beam quality through the acceleration in the linac. An extremely low fraction of the beam (from 10-4 down to 10-7) is sufficient to complicate the hands on maintenance in such accelerator. This paper reviews the theory and the codes for the design and simulation of MW ion linacs. Basics rules for the definition of their architecture and the results applied to existing machines and projects are covered.
UFPel, Pelotas
UDESC, Joinville
IF-UFRGS, Porto Alegre
Although undesired in many applications, the intrinsic spurious spatial inhomogeneity that permeates real systems is the forerunner instability which leads high-intensity charged particle beams to its equilibrium. In general, this equilibrium is reached in a particular way, by the development of a tenuous particle population around the original beam, conventionally known as the halo. In this way, the purpose of this work is to analyze the influence of the magnitude of initial inhomogeneity over the dynamics and over the equilibrium characteristics of initially quasi-homogeneous mismatched beams. For that, all beam constituent particles, which are initially disposed in an equidistant form, suffer a progressive perturbation through random noise with a variable amplitude. Dynamical and equilibrium quantities are quantified as functions of the noise amplitude, which indirectly is a consistent measure of the initial beam inhomogeneity. The results have been obtained by the means of full self-consistent N-particle beam numerical simulations and seem to be an important complement to the investigations already carried out in prior works.
UFPel, Pelotas
IF-UFRGS, Porto Alegre
Beams of charged particles usually reach their stationary state by the development of a halo. Halo formation in charged beams is in fact a macroscopic transcription of microscopic instabilities acting inside the beam and upon its constituent particles. In previous works, investigations have been carried out to understand the role of the initial envelope mismatch and of magnitude of inhomogeneity in the beam route to the equilibrium. Although in that works the action of the mentioned instabilities has been studied individually, it is clear that in real implemented beams both act together. In this sense, the main purpose of this work is to generalize previous models, considering now concomitantly the effects of the envelope mismatch and of the inhomogeneity. As a final product of the investigation, a particle-core model for beam constituent particles is presented. The agreement with full self-consistent N-particle beam numerical simulations is satisfactory and the results provided by the model seem to be more compatible with that would be expected experimentally.
Hiroshima University, Higashi-Hiroshima
Osaka University, Graduate School of Science, Osaka
HU/AdSM, Higashi-Hiroshima
Space charge effects due to the strong Coulomb interactions expected in high intensity accelerator beams result in undesirable beam degradation and radio-activation of the vacuum tubes through halo formations. Various space charge effects have been studied intensively with particle simulations. This is partly because the analytical formulation of the nonlinear evolution in high intensity beams is not possible in general cases. And the systematic study of space charge effects with the real accelerators is not feasible. Although the development of computation environment is outstanding, some approximations are still necessary so far. Thus, it was proposed to use solenoid traps and linear Paul traps for investigating some properties of space charge dominated beams. The key idea is that the charged particles in these traps are physically equivalent with a beam in a FODO lattice. Some experimental results have been reported with the use of Paul traps. Here, a solenoid trap with a beam imaging system composed of a charge coupled device camera and a phosphor screen was employed to study the mismatch induced oscillations of a space charge dominated beams.
HU/AdSM, Higashi-Hiroshima
Hiroshima University, Higashi-Hiroshima
LLNL, Livermore, California
S-POD (Simulator for Particle Orbit Dynamics) is a tabletop, non-neutral plasma trap system developed at Hiroshima University for fundamental beam physics studies. The main components of S-POD include a compact radio-frequency quadrupole trap, various AC and DC power supplies, a vacuum system, a laser cooler, several diagnostics, and a comprehensive computer control system. A large number of ions, produced through the electron bombardment process, are captured and confined in the RFQ trap to emulate collective phenomena in space-charge-dominated beams traveling in periodic linear focusing lattices. This unique experiment is based on the isomorphism between a one-component plasma in the laboratory frame and a charged-particle beam in the center-of-mass frame. We here employ S-POD to explore the coherent betatron resonance instability which is an important issue in modern high-power accelerators. Ion loss behaviors and transverse plasma profiles are measured under various conditions to identify the parameter-dependence of resonance stopbands. Experimental observations are compared with PIC simulation results obtained with the WARP code.
KEK, Ibaraki
One of the most important issues for ERL injectors is to generate electron beams with ultra-low emittance and to accelerate the beams through the injector without emittance growth. For this purpose, we have developed an efficient simulation code to investigate the mechanism of emittance growth due to space charge effect and to exploit its suppression method. In this code, the longitudinal motion is treated by the one-dimensional difference equations for macro-particles, while the radial motion is solved by the envelope equations for the pieces of sliced bunch. We find that the total emittance takes a minimum when all ellipses of sliced envelope have the same direction on the a-a' plane, where a is the amplitude of sliced envelope and a' its derivative along the longitudinal direction. The parameters of a 5 MeV injector were optimized by this code, assuming that the voltage of the DC electron gun is 330 kV and the initial particle distribution at the exit of the gun has a uniform ellipse. Even for such a low voltage gun, we obtained a minimum value of the rms normalized emittance, 0.10 mm, and the rms bunch length, 0.83 mm, the values of which were calculated by using PARMELA.
MEPhI, Moscow
The undulator linear accelerator using electrostatic undulator (UNDULAC-E) is suggested as an initial part of high intensity ion linac*. In UNDULAC ion beam accelerating and focusing are realized by of the combined field of two non-synchronous harmonics. Indeed, the main factor limiting beam intensity in ion accelerator is a space charge force. There exist, at least, two ways to increase ion beam intensity: to enlarge the beam cross section and to use the space charge neutralization. The ribbon ion beam dynamics in UNDULAC-E was discussed in**. Accelerating force value in UNDULAC is proportional to squared particle charge and oppositely charged ions with the identical charge-to-mass ratio can be accelerated simultaneously within the same bunch and the beam space charge neutralization can be realized. These methods will be studied analytically and verified by numerical simulation for UNDULAC-RF in this paper.
*E.S. Masunov, Sov. Phys. ' Tech. Phys., 1990, v. 35 (8), pp. 962-965. **Masunov, S.M. Polozov. NIM A, 558 (2006), pp. 184-187.
MEPhI, Moscow
The beam loading effect is one of main problems limiting the beam current. Usually this effect takes into account only in high energy electron linacs. Due to low energy electron and, especially, ion linacs nowadays current increasing the beam loading effect should be considered here. Self consistent beam dynamics simulation methods with Coulomb field and beam loading effect are discussed. The simulation results are in good agreement with experiment which have been carried out on NRNU MEPhI electron linac.
St. Petersburg State University, St. Petersburg
New approach to define geometry of the radial matching section in RFQ accelerator is suggested. Approach is based on the application methods of the control theory. In paper special functionals are introduced which allow optimize radial section parameters with taking into account space charge. This approach gives wider opportunities for the design of the radial matching section because it does not have certain prescribed laws of variation of focusing strength along the section.
CIEMAT, Madrid
CEA, Gif-sur-Yvette
The design of the future IFMIF accelerators will be validated with the 9 MeV, 125 mA deuteron accelerator IFMIF-EVEDA. For this validation phase, a High Energy Beam Transport line (HEBT) is designed to drive the beam toward a beam dump with the required expansion, under the hands-on maintenance constraint. It consists of eight quadrupoles and one dipole. Given the very high space charge regime and the very high power (1.1 MW), any small deviation from the nominal conditions could seriously compromise the HEBT objective. That is why possible misalignments and rotations of those magnets as well as power supply errors have been thoroughly studied. The error budget is fairly distributed among the tolerances for the different components, and effects of those errors on loss distribution and beam profile at the beam dump entrance carefully analysed.
NTHU, Hsinchu
Tohoku University, School of Scinece, Sendai
NSRRC, Hsinchu
Design of an intense but tightly focused ultrashort electron beam for production of sub-hundred femtosecond x-ray pulses that based on head-on inverse Compton scattering (ICS) has been studied. The three dimensional (3D) space charge dynamics has been tracked and optimized throughout the whole beamline. It is found that the focusing ultrashort electron pulses as short as 67 fs can be produced by compressing the energy-chirped beam from a thermionic cathode rf gun with an alpha magnet and linac operating at injection phase near zero crossing. This multi-bunch electron beam has an intensity of 30 pC per bunch and is accelerated to 27 MeV with an S-band linac structure. The compressed electron beam is focused to 64 μm for scattering with an 800 nm, 3.75 mJ laser in the laser-beam interaction chamber. With this method, total peak flux of back-scattered x-ray photons exceeds 1018 photons/sec is achievable with the shortest wavelength of 0.7 Å.
NSC/KIPT, Kharkov
The electron injector for a storage ring is one of numerous applications of the rf linacs of intensive short-pulsed beams with duration about 100 ns, current about 1 A and energy of particles in a few ten MeB. Since acceleration of intensive short-pulsed beams takes place in transient mode, then the energy spread is determined by both intro- and multi- bunch spread. Getting the energy spread less than 1% is the actual problem. In this work we simulate numerically unsteady self-consistent dynamics of charged particles in an rf linac that consist of a low-voltage (25 keV) thermionic gun, a compact evanescent wave buncher, a traveling wave accelerating structure. For transient beam loading compensation a method of delay of a beam relatively rf pulse are applied. The simulation takes into account influence on the beam dynamic of such factors as: initial energy and phase spread; sliding of particles in relation to a wave in the initial part of accelerating section; temporal dependence of phase and energy of bunches at the enter of section; space charge field.
NSC/KIPT, Kharkov
Acceleration of intensive electron beams in transient mode with energy spread less than 1% is the actual problem for rf linacs. The transient beam loading phenomenon, consisting in coherent radiation of sequence of charged bunches, results in time dependence of electron energy loss within a beam pulse. In this work a method of delay of a beam relatively rf pulse for energy compensation at accelerating intense short-pulsed electron beams is discussed. An efficiency of the given method in depending on dispersion of group speed, phase advance per cell of an rf structure, an envelope profile of pulses both current and input rf field is studied. Contribution of non-resonant counter waves in the beam energy spread is estimated.
NSC/KIPT, Kharkov
At present work the results of numeral simulation of electron dynamics in an unhomogeneous disk-loaded waveguide which is used in the S-band linac are presented. Two approaches taking into account the self-fields of beam radiation are considered: the first method estimative based on the power diffusion equation; the second one based on of self-consistent equations of field excitation and particles motion. The self-consistent approach showed the presence of substantial phase slipping of particles in the homogeneous part of the rf structure, conditioned by the reactive beam loading.
Fermilab, Batavia
University of Delhi, Delhi
Results of analysis are presented for the longitudinal and transverse effects of High-Order Mode (HOM) excitation in the acceleration RF system of the CW proton linac of the Project X facility. Necessity of HOM dampers in the SC cavities of the linac is discussed.
PPPL, Princeton, New Jersey
To achieve high focal spot intensities in ion-beam-driven high energy density physics and heavy ion fusion applications, the ion beam must be compressed longitudinally by factors of ten to one hundred before it is focused onto the target. The longitudinal compression is achieved by imposing an initial velocity profile tilt on the drifting beam, and allowing the beam to compress longitudinally until the space-charge force or the internal thermal pressure stops the longitudinal compression of the charge bunch. In this paper, the problem of longitudinal drift compression of intense charged particle beams is analyzed analytically and numerically for the two important cases corresponding to a cold beam, and a pressure-dominated beam, using a one-dimensional warm-fluid model describing the longitudinal beam dynamics. The hodograph transformation is used to transform the nonlinear fluid equations into a single, second-order, linear partial differential equation (PDE). The general solution of this equation describing the intense beam system with stagnation point is analyzed and illustrated with several examples.
BNL, Upton, Long Island, New York
A prototype ampere-class superconducting energy recovery linac (ERL) is under advanced construction at BNL. The ERL facility is comprised of a five-cell SC Linac plus a half-cell SC photo-injector RF electron gun, both operating at 703.75 MHz. The facility is designed for either a high-current mode of operation up to 0.5 A at 703.75 MHz or a high-bunch-charge mode of 5 nC at 10 MHz bunch frequency. The R&D facility serves a test bed for an envisioned electron-hadron collider, eRHIC. The high-current, high-charge operating parameters make effective higher-order-mode (HOM) damping mandatory, and requires to determination of HOM tolerances for a cavity upgrade. The niobium cavity has been tested at superconducting temperatures and has provided measured dipole shunt impedances for the estimate of a beam breakup instability. The facility will be assembled with a highly flexible lattice covering a vast operational parameter space for verification of the estimates and to serve as a test bed for the concepts directed at future projects.