Search Results (5 total)

Fundamental advances in experimental nuclear physics will require ion beams with orders of magnitude luminosity increase and temperature reduction. One of the most promising particle accelerator techniques for achieving these goals is electron cooling, where the ion beam repeatedly transfers thermal energy to a copropagating electron beam. The dynamical friction force on a fully ionized gold ion moving through magnetized and unmagnetized electron distributions has been simulated, using molecular dynamics techniques that resolve close binary collisions. We present a comprehensive examination of theoretical models in use by the electron cooling community. Differences in these models are clarified, enabling the accurate design of future electron cooling systems for relativistic ion accelerators.

Capillary channels of ≈3  cm in length and with plasma densities ≈10¹⁸  cm^-3 are a promising alternative to the much shorter, higher-density gas jets for GeV-scale laser wakefield acceleration of electrons. However, the large discrepancy between length scales of the plasma and the laser presents a major computational challenge for particle-in-cell (PIC) simulations. Methods are therefore sought that relax the need to concurrently resolve both length scales. For example, the commonly used “moving window” algorithm enables a reduction of the computational domain to a few plasma wavelengths, which is orders of magnitude smaller than the full length of the laser-plasma interaction. In addition, ponderomotive guiding center methods enable relaxation of the constraint to resolve the laser wavelength. These averaging methods split the laser-induced current into a rapidly varying part and a slowly varying envelope. The average over fast time scales is performed in a semianalytic way, leaving the evolution of the laser envelope and the plasma response to be modeled numerically. Here, we present a ponderomotive guiding center algorithm and demonstrate its applicability to model capillary channels by comparing it with fully kinetic PIC simulations.

We present 2D simulations of both beam-driven and laser-driven plasma wakefield accelerators, using the object-oriented particle-in-cell code XOOPIC, which is time explicit, fully electromagnetic, and capable of running on massively parallel supercomputers. Simulations of laser-driven wakefields with low \(∼10¹⁶ W/cm²\) and high \(∼10¹⁸ W/cm²\) peak intensity laser pulses are conducted in slab geometry, showing agreement with theory and fluid simulations. Simulations of the E-157 beam wakefield experiment at the Stanford Linear Accelerator Center, in which a 30 GeV electron beam passes through 1 m of preionized lithium plasma, are conducted in cylindrical geometry, obtaining good agreement with previous work. We briefly describe some of the more significant modifications to XOOPIC required by this work, and summarize the issues relevant to modeling relativistic electron-neutral collisions in a particle-in-cell code.

Particles interacting resonantly with large-amplitude coherent one-dimensional wave packets can trap and subsequently detrap or even reflect. Many resonant particles are strongly scattered in the process, and the long-time dynamics of such particles is stochastic throughout a large region of phase space when repeated wave-particle interactions occur. We apply adiabatic invariance theory and separatrix crossing theory to this Hamiltonian system, which is beyond the realm of quasilinear theory. We calculate the adiabatic invariant through first order in the (small) slowness parameter ɛ for all particle trajectories. Because the trajectories of resonant particles cross a separatrix, the adiabatic invariant is broken and separatrix-crossing theory must be used. Our Hamiltonian provides a simple model for the fundamental physics of narrow-spectrum plasma turbulence, for strong rf current drive in a tokamak, and for electron dynamics in a recirculating free-electron laser.

A new adiabatic theory permits the understanding of one-dimensional dynamics of particles interacting with a large-amplitude wave packet for bounce time short compared with the transit time. This theory differs from previous ones in that the Hamiltonian varies slowly not with the time, but with the coordinate. The resulting adiabatic invariant is not equal, even in lowest order, to the usual action. This theory predicts the basic features of the interaction observed in previous numerical studies.