Yuri Batygin

Analytical treatment of particle–core interaction

Particle-core interaction is the well-developed model of halo formation in high-intensity beams. In present paper an analytical solution for averaged single particle dynamics around uniformly charged beam core is obtained. The problem is analyzed through sequence of canonical transformations of Hamiltonian describing nonlinear particle oscillations. An analytical expression for maximum particle deviation from the axis is obtained. Results of the study are in good agreement with numerical simulations and with previously achieved data.

A Particle-in-cell scheme of the RFQ in the SSC-Linac

A 52 MHz Radio Frequency Quadrupole (RFQ) linear accelerator (linac) is designed to serve as an initial structure for the SSC-Linac system (injector into Separated Sector Cyclotron). The designed injection and output energy are 3.5 keV/u and 143 keV/u, respectively. The beam dynamics in this RFQ have been studied using a three-dimensional Particle-In-Cell (PIC) code BEAMPATH. Simulation results show that this RFQ structure is characterized by stable values of beam transmission efficiency (at least 95%) for both zero-current mode and the space charge dominated regime. The beam accelerated in the RFQ has good quality in both transverse and longitudinal directions, and could easily be accepted by Drift Tube Linac (DTL). The effect of the vane error and that of the space charge on the beam parameters have been studied as well to define the engineering tolerance for RFQ vane machining and alignment.

Space charge effects in cyclotron gas stopper

The cyclotron gas stopper is a newly proposed device to stop energetic rare isotope ions from projectile fragmentation reactions in a helium-filled chamber. The radioactive ions are slowed down by collisions with a buffer gas inside a cyclotron-type magnet and are extracted via interactions with a Radio Frequency (RF) field applied to a sequence of concentric electrodes (RF carpet). The present study focuses on a detailed understanding of space charge effects in the ion extraction region. The space charge is generated by the ionized helium gas created by the stopping of the ions and eventually limits the beam rate. Particle-in-cell simulations of a two-component (electron-helium) plasma interacting via Coulomb forces were performed in the space charge field created by the stopping beam.

Experimental improvement of 750-keV H− beam transport at LANSCE Accelerator Facility

The 750-keV low-energy beam transport of the Los Alamos Neutron Science Center (LANSCE) linac consists of two independent beam lines that facilitate simultaneous injection of H+ and H- beams into the linear accelerator. The efficiency of beam transport within this structure is largely controlled by the effects of space-charge neutralization. In this paper, we report a series of experiments that were performed to determine the level, as well as the required timescale for the onset of beam space charge neutralization. Measurements performed along the structure indicate significant variation of neutralized space charge beam dynamics in the beamline, and the corresponding results have been used to substantially improve beam transport with diminished beam losses. The analysis of experimental and theoretical data employed in optimization of beam performance in the transport line is discussed.

Comparison of profile measurements and TRANSPORT beam envelope predictions along the 80-m LANSCE pRad beamline

Here we report a comparison between the simulation of beam phase-space and profile distributions with diagnostic measurements. TRANSPORT, a particle transport code, was used for the prediction of a 800 MeV proton beam envelope from the end of the linac to the proton radiography (pRad) facility (a total length of 80 meters). The beam profile was measured at key positions along the beamline using wire scanners and gated CCD camera systems. These measurements were compared to their respective points along the simulated beamline. The predicted beam envelope and measured data correspond within expected errors.

Suppression of space charge induced beam halo in nonlinear focusing channel

Abstract An intense non-uniform particle beam exhibits strong emittance growth and halo formation in focusing channels due to nonlinear space charge forces of the beam. This phenomenon limits beam brightness and results in particle losses. The problem is connected with irreversible distortion of phase space volume of the beam in conventional focusing structures due to filamentation in phase space. Emittance growth is accompanied by halo formation in real space, which results in inevitable particle losses. A new approach for solving a self-consistent problem for a matched non-uniform beam in two-dimensional geometry is discussed. The resulting solution is applied to the problem of beam transport, while avoiding emittance growth and halo formation by the use of nonlinear focusing field. Conservation of a beam distribution function is demonstrated analytically and by particle-in-cell simulation for a beam with a realistic beam distribution.

Operation of LANSCE Linear Accelerator with Double Pulse Rate and Low Beam Losses

In 2014 the LANSCE accelerator facility returned to 120 Hz pulse rate operation after a long period of operation at a 60 Hz pulse rate. Increased capabilities required careful tuning of all components of the linear accelerator. Transformation to the double pulse rate resulted in re-evaluation of tuning procedures in order to meet new challenges in beam operation. This paper summarizes experimental activity on sustaining high productivity of the accelerator facility while keeping beam losses along accelerator low. LANSCE ACCELERATOR FACILITY The LANSCE Accelerator facility has been in operation for more than 40 years. Currently it operates with four 800 MeV H beams and one 100 MeV proton beam (see Table 1). The accelerator facility is equipped with two independent injectors for H and H beams, merging at the entrance of a 201.25 MHz Drift Tube Linac (DTL). The DTL performs acceleration up to the energy of 100 MeV. After the DTL, the Transition Region (TR) beamline directs 100 MeV proton beam to the Isotope Production Facility (IPF), while H beam is accelerated up to the final energy of 800 MeV in an 805 MHz Coupled Cavity Linac (CCL). The H beams, created by different time structure of a low-energy chopper, are distributed in the Switch Yard to four experimental areas. The most powerful H beam, average current 100 μA, is accumulated in the Proton Storage Ring (PSR) and is extracted to the Lujan Neutron Scattering Center facility for production of moderated neutrons with meV-keV energy. Another H beam, as a sequence of short pulses, is delivered to the Weapon Neutron Research (WNR) facility to create un-moderated neutrons in the keVMeV energy range. The third H beam is shared between the Proton Radiography Facility (pRad) and the Ultra-Cold Neutron (UCN) facility. Between 2006 – 2014, the LANSCE accelerator was in operation at 60 Hz pulse rate to prevent excessive cathode emission and ceramic cracking in 201.25 MHz amplifiers feeding the DTL. The LANSCE Risk Mitigation Project [1] was initiated to replace three out of four 201.25 MHz amplifiers with newly developed RF power systems based on TH628L Diacrodes [2]. The first RF power system was replaced in 2014 enabling restoration of 120 Hz operation. The replacement of 201.25 MHz RF system will be completed in 2016. In addition, a new low-level RF control system was installed, and end-of-life CCL klystrons were replaced to insure further stable beam operation. ________________________ *Work supported by US DOE under contract DE-AC52-06NA25396 #batygin@lanl.gov Table 1: Beam Parameters at 120 Hz LANSCE Accelerator (number in brackets are given for previous 60 Hz operation) Area Rep. Rate (Hz) Pulse Length (μs) Current / bunch (mA) Average current

Dynamics of intense particle beam in axial-symmetric magnetic field

Axial-symmetric magnetic field is often used in focusing of particle beams. Most existing ion Low Energy Beam Transport lines are based on solenoid focusing. Modern accelerator projects utilize superconducting solenoids in combination with superconducting accelerating cavities for acceleration of high-intensity particle beams. Present paper discusses conditions for matched beam in axial-symmetric magnetic field. Analysis allows us to minimize power consumption of solenoids and beam emittance growth due to nonlinear space charge, lens aberrations, and maximize acceptance of the channel. Expressions for maximum beam current in focusing structure, beam emittance growth due to spherical aberrations and non-linear space charge forces are derived. 1. LATTICE OF PERIODIC SOLENOID CHANNEL Consider a focusing lattice consisting of a periodic sequence of focusing solenoids of length D, field Bo, distance between lenses l, and period L= l + D (see Fig. 1). A matched beam reaches its maximum size in the center of the solenoids, and minimum size in the middle of drift space (see Fig. 2). The transformation matrix in a rotating frame through a period of the structure between centers of solenoids is given by [1]

Space-charge Neutralization of 750-keV H⁻ Beam at LANSCE

The injector part of Los Alamos Neutron Science Center (LANSCE) includes a 750-keV Hbeam transport located upstream of the Drift Tube Linac. Space charge effects play an important role in the beam transport therein [1]. A series of experiments were performed to determine the level of beam space charge neutralization, and time required for neutralization. Measurements performed at different places along the structure indicate significant variation of neutralized space charge beam dynamics along the beamline. Results of measurements were compared with numerical simulations using macroparticle method and envelope equations to determine values of the effective beam current after neutralization, and effective beam emittance, required for beam tuning. 750 KEV LANSCE BEAM TRANSPORT The H beam injector includes a cesiated, multicuspfield, surface –production ion source and two-stage lowenergy beam transport line. In the first stage, extracted beam is accelerated up to 80 keV, and then is transported through a solenoid, electrostatic deflector, a 4.5 bending magnet, and a second solenoid. The 670 kV CockroftWalton column accelerates beam up to an energy of 750 keV. The 750 keV LEBT (see Fig. 1) consists of a quadrupole lattice, 81 and 9 bending magnets, slowwave chopper, RF bunchers, an electrostatic deflector, diagnostics and steering magnets to prepare beam for injection into the Drift Tube Linac (DTL). Slit-collector beam emittance measurements at 750 keV are performed at five locations: 1) TBEM1 (just after the Cockroft Walton column), 2) TBEM2 (downstream of the chopper), 3) TBEM3 (downstream of the 81 bend before RF pre-buncher), 4) TBEM4 (between the first RF (pre)buncher and second (main) buncher), and 5) TDEM1 (before the entrance to the DTL). BEAM EMITTANCE SCANS Ionization of residual gas by transported particles is an important factor of low-energy beam transport. Fig. 2 illustrates a typical spectrum of residual gas in the 750 keV H transport channel obtained from a Residual Gas Analyzer installed in the middle of the channel. Main components are H2 (48%), H2O (38%) and N2 (9%). Fractions of other components are significantly smaller. Typical total pressure measured by ion gauges along the transport channel range from 5 10 Torr to 10 Torr. _______________________ *Work supported by US DOE under contract DE-AC52-06NA25396 batygin@lanl.gov Figure 1: Layout of 750-keV H Low Energy Beam Transport of LANSCE. Figure 2: Residual gas analyzer scan. A series of beam emittance scans along 750 keV H beam transport were performed to determine time and level of space charge neutralization of the beam, value of effective beam current under space–charge neutralization, and the value of effective beam emittance. Measurements were done as pair measurements between each pair of emittance stations TBEM1–TBEM2, TBEM2-TBEM3, TBEM3–TBEM4, TBEM4–TDEM1. Measurements were performed with an ion source pulse length of 825 μs. The emittance was sampled within the last 50 s of the ion source pulse. The beam pulse start time was varied between  = 10 – 575 s before the emittance sampling through delay in the 80 kV electrostatic deflector. Typical value of H beam current at 750 keV was 14 – 17 mA. 10 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPRO097