Hiroshi Nishimura

Feasibility of a synchrotron storage ring for neutral polar molecules

Using calculations and mathematical modeling, we demonstrate the feasibility of constructing a synchrotron storage ring for neutral polar molecules. The lattice is a racetrack type 3.6 m in circumference consisting of two 180° arcs, six bunchers, and two long straight sections. Each straight section contains two triplet focusing lenses and space for beam injection and experiments. The design also includes a matched injector and a linear decelerator. Up to 60 bunches can be loaded and simultaneously stored in the ring. The molecules are injected at 90 m/s but the velocity of the circulating beam can be decelerated to 60 m/s after injection. The modeling uses deuterated ammonia (14N2H3) molecules in a weak-field seeking state. A beam that survives 400 turns (15 s), has horizontal and vertical acceptances of 35 and 70 mmu200amrad, respectively, and an energy acceptance of ±2%.

Orbit stability of the ALS storage ring

The Advanced Light Source (ALS) storage ring, a synchrotron light source of the third generation, is specified to maintain its electron orbit stable within one tenth of the rms beam size. In the absence of a dedicated orbit feed-back system, several orbit-distorting effects were investigated, aided by a new interactive simulation tool, the code TRACY V. The effort has led to a better understanding of the behavior of a variety of accelerator subsystems and in consequence produced a substantial improvement in day-to-day orbit stability.

Accelerator physics challenges of the fs-slicing upgrade at the ALS

The goal of the Femtoslicing project at the ALS is to provide 100-200 fs long pulses of soft and hard x-rays with moderate flux and with a repetition rate of 10-40 kHz for experiments concerning ultrafast dynamics in solid state physics, chemistry and biology. The femtoslicing principle employs a femtosecond laser beam to interact resonantly (inverse FEL interaction) with the electron beam in the ALS. The induced energy spread over the femtosecond duration is converted to a transverse displacement by exploiting the dispersion of the storage ring. The displaced femtosecond electron pulse then radiates and produces femtosecond synchrotron radiation. To achieve the necessary spatial separation of the energy modulated slice from the rest of the bunch, a sizeable local vertical dispersion bump in the undulator used as radiator is required. This presents challenges in terms of the nonlinear dynamics and control of the vertical emittance.

Symplectic models for general insertion devices

A variety of insertion devices (IDs), wigglers and undulators, linearly or elliptically polarized, are widely used as high brightness radiation sources at the modern light source rings. Long and high-field wigglers have also been proposed as the main source of radiation damping at next generation damping rings. As a result, it becomes increasingly important to understand the impact of IDs on the charged particle dynamics in the storage ring. In this paper, we report our recent development of a general explicit symplectic model for IDs with the paraxial ray approximation. High-order explicit symplectic integrators are developed to study real-world insertion devices with a number of wiggler harmonics and arbitrary polarizations.

The effects of insertion devices on beam dynamics in the ALS

It is shown that introducing undulators into the ALS (Advanced Light Source) causes a significant reduction in dynamic aperture. However, the effect is similar to that which is expected from the storage ring when realistic magnetic imperfections, magnet misalignments, and a corrected closed orbit are included. Moreover, when undulators are added into the imperfect machine there is little further degradation of the dynamic aperture. In all cases the machine acceptance will be determined by the physical (vacuum aperture), rather than the diminished, dynamic aperture. Thus, the insertion devices are not expected to impact the elastic gas scattering lifetime of the ALS. The momentum acceptance of the storage ring is seriously affected by the undulators; this will impact the Touschek lifetime of the ALS. However, at the levels predicted by the present study, the resulting overall beam lifetimes are still acceptable.<<ETX>>

Global beta-beating compensation of the ALS W16 wiggler

The W16 wiggler is the first wiggler and highest field insertion device to be installed in the ALS storage ring. When the gaps of the W16 wiggler are closed, the vertical tune increases by 0.065 and the vertical beta function is distorted by up to /spl plusmn/37%. There are 48 quadrupoles in the ring whose fields can be adjusted individually to restore the tunes and partially compensate the beta-beating. In order to adjust the quadrupole field strengths to accurately compensate the focusing, it is necessary to have a method to precisely determine the beta-beating. In this paper we compare measurements of the induced beta-beating using two methods: measuring the tune dependence on quadrupole field strength and fitting a lattice model with measured response matrices. The fitted model also allows us to predict quadrupole field strengths that will best compensate the beta beating. These quadrupole field strengths are then applied and the resultant beta-beating is measured.

Vertically integrated simulation tools for self-consistent tracking and analysis

A modeling, simulation, and analysis code complex, the Gemini package, was developed for the study of single-particle dynamics in the Advanced Light Source (ALS), a 1-2 GeV synchrotron radiation source now being built at Lawrence Berkeley Laboratory. The philosophy behind the package is described, with special emphasis on the vertical approach used. Vertical integration is an approach based on a well-defined model of the storage ring that is preserved at all of the hierarchical and sequential stages of manipulation and aimed at various goals. Thus, all the codes inside the package are consistent with this model.<<ETX>>

Feasibility of a storage ring for polar molecules in strong-field-seeking states

Abstract.We show, through modeling and simulation, that it is feasible to construct a storage ring that will store dense bunches of strong-field-seeking polar molecules at 30 m/s (kinetic energy of 2 K) and hold them, for several minutes, against losses due to defocusing, oscillations, and diffusion. The ring, 3 m in diameter, has straight sections that afford access to the stored molecules and a lattice structure that may be adapted for evaporative cooling. Simulation is done using a newly-developed code that tracks the particles, in time, through 400 turns; it accounts for longitudinal velocity changes as a function of external electric field, focusing and deflection nonlinearities, and the effects of gravity. An injector, decelerator, and source are included and intensities are calculated.

Als top-off simulation studies for radiation safety

We plan to commission top-off injection[1,2] at the advanced light source (ALS[3]) in the near future. In order to guarantee radiation safety, we need to exclude the possibility of injecting electrons into the users photon beam lines because of the very high radiation doses involved in case of such an event. This issue must carefully investigated and experimental tests cannot be easily performed. The only reliable way is through simulation. We have developed a scheme based in exhaustive simulations that accounts for all possible dangerous scenarios and that at the same time requires a reasonable amount of computing time. This paper describes such a method and presents a summary of the studies performed for the ALS at the present time.

ID modeling at the ALS

At the Advanced Light Source there are several projects being proposed that will require high field insertion devices. It is important that these devices do not significantly impact the performance of the machine. In particular they should not degrade the beam lifetime or injection efficiency. It is known that high field devices with large field roll off can impact the beam lifetime. It is therefore important to model the effect of the insertion devices including both transverse and longitudinal field roll off. In this paper we present the result of tracking studies using an explicit symplectic integrator with both transverse and longitudinal field roll off. The simulations show where sufficiently large field roll off will impact the beam lifetime.