LINAC 2004 - Classification:Theory, Codes, Simulations/High Current Beam Dynamics, Instabilities

Theory, Codes, Simulations

High Current Beam Dynamics, Instabilities

Paper	Title	Page
TUP15	Space Charge Compensation in Low Energy Proton Beams	324
	A.B. Ismail, U.D. Uriot CEA/DSM/DAPNIA, Gif-sur-Yvette R. Duperrier CEA/DAPNIA-SACM, Gif-sur-Yvette Cedex N. Pichoff CEA/DAM, Bruyères-le-Châtel
	High power accelerators are being studied for several projects including accelerator driven neutron or neutrino sources. The low energy part of these facilities has to be carefully optimized to match the beam requirements of the higher energy parts. In this low energy part, the space charge self force, induced by a high intensity beam, has to be carefully managed. This nonlinear force can generate a high irreversible emittance growth of the beam. To reduce space charge effects, neutralization of the beam charge can be done by capturing some particles of the ionised residual gas in the vacuum chamber. This space charge compensation (SCC) regime complicates the dynamic study. Modelling the beam behaviour in such regime would be a significant contribution to the development of high intensity accelerators. Numerical and experimental study of SCC is in progress on the Saclay High Intensity Proton Injector. Experimental measurements and 2D/3D simulations of proton beam SCC will be presented.
TUP16	Investigation on Beam Dynamics Design of High-Intensity RFQs	327
	C. Zhang, A. Schempp IAP, Frankfurt-am-Main J. Chen, J. Fang, Z.Y. Guo PKU/IHIP, Beijing
	Recently various potential uses of high-intensity beams bring new opportunities as well as challenges to RFQ accelerator research because of the new problems arising from the strong space-charge effects. Unconventional concepts of beam dynamics design, which surround the choice of basic parameters and the optimization of main dynamics parameters’ variation along the machine, are illustrated by the designing Peking University (PKU) Deuteron RFQ. An efficient tool of LANL RFQ Design Codes for beam dynamics simulation and analysis, RFQBAT, is introduced. Some quality criterions are also presented for evaluating design results.
TUP18	Beam Dynamics Issues of SPES-1 Linac	330
	E. Fagotti INFN Milano, Milano M. Comunian, A. Palmieri, A. Pisent INFN/LNL, Legnaro, Padova
	An Independent Superconducting Cavity Linac able to accelerate 10 mA CW proton beam up to 20 MeV has been studied for the SPES-1 project. This paper presents the results of beam dynamics studies through SPES linac including mapped fields effects on cavities and magnets.
TUP19	Characterization of Beam Parameter and Halo for a High Intensity RFQ Output under Different Current Regimes	333
	E. Fagotti INFN Milano, Milano M. Comunian, A. Palmieri, A. Pisent INFN/LNL, Legnaro, Padova
	The characterization of the beam distribution at the exit of a high intensity RFQ is a crucial point in view of a correct simulation of beam behavior in the following linac structure. At this scope we need to know the beam halo quantification as a function of the input beam and RFQ parameters. In this paper, the description of Beam halo based upon moments of the particle distribution at the exit of the TRASCO-RFQ is given.
TUP21	Beam Dynamics Design of J-PARC Linac High Energy Section	339
	M. Ikegami, T. Kato, S. Noguchi KEK, Ibaraki H. Ao, Y. Yamazaki JAERI/LINAC, Ibaraki-ken K. Hasegawa, T. Ohkawa, A. Ueno JAERI, Ibaraki-ken N. Hayashizaki TIT, Tokyo V.V. Paramonov RAS/INR, Moscow
	J-PARC linac consists of a 3 MeV RFQ linac, a 50 MeV DTL (Drift Tube Linac), a 190 MeV SDTL (Separate-type DTL), and a 400 MeV ACS (Annular-Coupled Structure) linac. Recently, the beam dynamics design of the ACS part has been slightly modified to reduce construction cost. Namely, the number of klystron modules are reduced from 23 to 21, and the number of accelerating cells in one klystron module is increased from 30 to 34 to maintain the total energy gain. This design change curtails the margin for RF power by around 5 %, and the total length of the ACS section is nearly unchanged. The beam matching section between SDTL and ACS is also revised correspondingly. These modifications of the design are described in this paper together with 3D particle simulation results for the new design.
TUP22	A Simulation Study on Chopper Transient Effects in J-PARC Linac	342
	M. Ikegami KEK, Ibaraki Y. Kondo, T. Ohkawa, A. Ueno JAERI, Ibaraki-ken
	J-PARC linac has an RF chopper system to reduce uncontrolled beam loss in the succeeding ring injection. The chopper system is located in MEBT (Medium Energy Beam Transport line) between a 3 MeV RFQ and a 50 MeV DTL, and consists of two RFD (Radio-Frequency Deflection) cavities and a beam collector. During the rising- and falling-times of the RFD cavities, the beams are half-kicked and cause excess beam loss downstream. In this paper, the behavior of these half-kicked beams is examined with 3D PARMILA simulations, and resulting beam loss is estimated.
	Transparencies
TUP23	A Simulation Study on Error Effects in J-PARC Linac	345
	M. Ikegami KEK, Ibaraki Y. Kondo, T. Ohkawa, A. Ueno JAERI, Ibaraki-ken
	In high-current proton linacs, prevention of excess beam loss is essentially important to enable hands-on maintenance. In addition, requirements on the momentum spread and transverse emittance are quite severe for J-PARC linac to realize effective injection to the succeeding RCS (Rapid Cycling Synchrotron). As losses and beam-quality deterioration are believed to be mainly caused by various errors, such as misalignment, RF mistuning, etc, it is essentially important to perform particle simulations for J-PARC linac with as realistic errors as possible to estimate their effects. In this paper, effects of realistic errors on beam loss and beam-quality deterioration in J-PARC linac are examined with a systematic 3D simulations with PARMILA. Necessity of transverse collimation is also discussed.
TUP49	Simulations of the Ion-Hose Instability for DARHT-II Long-Pulse Experiments	381
	K. C. D. Chan, C. Ekdahl LANL, Los Alamos, New Mexico C. Genoni, P. Hughes MRC, Albuquerque, NM
	Ion-hose effect has been described extensively in literatures. Computer simulations of the effect typically use particle-in-cell (PIC) computer codes or codes using the spread-mass formulation [1]. PIC simulations, though offering more reliable results, will require extended running time in large computers To support commissioning experiments in the DARHT-II induction linac in Los Alamos National Laboratory, we have modified a spread-mass code so that we can survey quickly the parameter space for the experiment. It can also be used to provide quick answers during experiment. The code was originally written by Genoni from Mission Research Corporation (MRC) for constant linac parameters. We have modified it so that parameters can have dependence along the length of the linac. In this paper, we will describe simulation results using this code for the DARHT-II commissioning experiment and also our benchmarking results comparing to LSP, a PIC code from MRC. [1] T. C. Genoni and T. P. Hughes, "Ion-hose instability in a long-pulselinear induction accelerator", PRST-AB, 6, 030401 (2003)
TUP50	Cumulative Beam Breakup with Time-Dependent Parameters	384
	J. R. Delayen Jefferson Lab, Newport News, Virginia
	A general analytical formalism developed recently for cumulative beam breakup (BBU) in linear accelerators with arbitrary beam current profile and misalignments [1] is extended to include time-dependent parameters such as energy chirp or rf focusing in order to reduce BBU-induced instabilities and emittance growth. Analytical results are presented and applied to practical accelerator configurations. [1] J. R. Delayen, Phys. Rev. ST Accel. Beams 6, 084402 (2003)
TUP52	Methods for Measuring and Controlling Beam Breakup in High Current ERLs	387
	C. Tennant, K. Jordan, E. Pozdeyev, R.A. Rimmer, H. Wang Jefferson Lab, Newport News, Virginia S. Simrock DESY, Hamburg
	It is well known that high current Energy Recovery Linacs (ERL) utilizing superconducting cavities are susceptible to a regenerative type of beam breakup (BBU). The BBU instability is caused by the transverse deflecting higher-order modes (HOMs) of the cavities which can have high impedance. We present MATLab simulation results for the BBU stability using the analysis tools of control theory. In this framework, methods of experimentally determining the threshold current and the means of suppressing the onset of the instability become more transparent. A scheme was developed to determine the threshold current due to a particular HOM by measuring the decay and rise times of the mode's field in response to an amplitude modulated beam as a function of the average electron beam current. To combat the harmful effects of a particularly dangerous mode, two methods of directly damping HOMs through the cavity HOM couplers were demonstrated. In an effort to suppress the BBU in the presence of multiple, dangerous HOMs, a conceptual design for a bunch-by-bunch transverse feedback system has been developed. By implementing beam feedback, the threshold for instability can be increased substantially.
TUP53	Temporal Profile of the LCLS Photocathode Ultraviolet Drive Laser Tolerated by the Microbunching Instability	390
	J. Wu, Z. Huang SLAC, Menlo Park, California M. Borland ANL, Argonne, Illinois P. Emma SLAC/ARDA, Menlo Park, California C. Limborg SLAC/SSRL, Menlo Park, California
	The high quality LCLS electron beam generated in the photoinjector is subject to all possible instabilities in the downstream acceleration and compression. The instability can be initiated by any possible density modulation of the electron beam when it is generated at the photocathode. In this note, we prescribe the tolerance on the initial electron beam density modulation possibly introduced by the ultraviolet (uv) laser at the cathode. Our study shows that the initial rms density modulation of the electron beam at the photocathode shall be less than 5 % to ensure the FEL lasing and saturation.
TUP54	Resistive-Wall Wake Effect in the Beam Delivery System	393
	J. Wu SLAC, Menlo Park, California J. R. Delayen Jefferson Lab, Newport News, Virginia T.O. Raubenheimer SLAC/NLC, Menlo Park, California J.-M. Wang BNL/NSLS, Upton, Long Island, New York
	The resistive wall instability is investigated in the context of the final beam delivery system of linear colliders. The emittance growth is calculated analytically and compared against the results of full numerical simulations. Criteria for the design of final beam delivery systems are developed.