Session WI1: Beams and Coherent Radiation II

Quantitative experiments with electrons in a positively-charged particle beam

Intense ion beams are an extreme example of non-neutral plasma. We use experiments and simulations to study the complex interactions between beam ions and (unwanted) electrons. (Such electron clouds limit the performance of many accelerators.) The detailed, self-consistent simulations use the 3-D Particle-In-Cell code WARP, with the addition of beam-transport fields, and electron and gas generation and transport, to compute unexpectedly rich behavior [1], much of which is confirmed experimentally. In magnetic-field-free regions, we observe a variety of beam-surface interaction phenomena: electron emission, gas desorption, ionization of gas, and virtual cathode fluctuations. In a quadrupole magnetic field, ion and dense electron plasmas interact to produce multi-kV oscillations in the electron plasma and distortions of the beam velocity space distribution, without becoming homogenous or locally neutral. We developed a variety of methods to measure and control electron and gas clouds in ion beams. Parameters we measure include: beam potential profiles and time dependence, total and local electron production and loss, electron line-charge density [2], gas pressure within the beam, electron accumulation, and electron trapping depth. Control methods include surface treatments to reduce electron and gas emission, and techniques to remove electrons from the beam. \newline \newline 1. R. H. Cohen, et al., Phys. Plasmas \textbf{12}, 056708 (2005). \newline 2. M. Kireeff Covo, et al., Accepted by Phys. Rev. Lett. May 2006. \newline \newline **Collaborators: M. Kireeff Covo, R. Cohen, J-L. Vay, D. Baca, F. Bieniosek, A. Friedman, C. Leister, P. Seidl, W. Sharp, and K. van den Bogert.   

Manipulating the Particle Phase Space with Nonadiabatic Ponderomotive Barriers

A ponderomotive potential is an effective potential seen by a particle in ac field on average over the fast oscillations. It is not a true potential though, and hence can be used for particle manipulations more advanced compared to those via static potentials. If the field scale is small enough, the particle motion in a ponderomotive barrier is essentially phase-dependent and resembles the dynamics of a quantum object in a conservative field. Probabilistic transmission is possible in this case [1, 2] and can produce attosecond electron bunches when a uniform relativistic electron beam is scattered off an intense laser wave in vacuum. For particles exhibiting natural oscillations (e.g., Larmor rotation or internal vibrations), nonadiabatic yet phase-independent ponderomotive manipulations by resonant ac fields are also available [3-5]. An approximate integral of particle motion is found for resonant nonlinear interactions, and a new ponderomotive potential is introduced accordingly [6]. Unlike static potentials, resonant barriers can produce a ratchet effect by asymmetrically transmitting thermal particles in a preferential direction [3, 4, 7]; techniques of selective separation and cooling of plasma species are also proposed [6].\\ \textrm{[1] I. Y. Dodin and N. J. Fisch, Phys. Rev. Lett. 95, 115001 (2005).}\\ \textrm{[2] I. Y. Dodin and N. J. Fisch, submitted to Phys. Rev. E.}\\ \textrm{[3] N. J. Fisch, J. M. Rax, and I. Y. Dodin, Phys. Rev. Lett. 91, 205004 (2003).}\\ \textrm{[4] I. Y. Dodin, N. J. Fisch, and J. M. Rax, Phys. Plasmas 11, 5046 (2004).}\\ \textrm{[5] I. Y. Dodin and N. J. Fisch, J. Plasma Phys. 71, 289 (2005).}\\ \textrm{[6] I. Y. Dodin and N. J. Fisch, Phys. Lett. A 349, 356 (2006).}\\ \textrm{[7] I. Y. Dodin and N. J. Fisch, Phys. Rev. E 72, 046602 (2005).}   

Collective Temperature Anisotropy Instabilities in Intense Charged Particle Beams

Periodic focusing accelerators, transport systems and storage rings have a wide range of applications ranging from basic scientific research in high energy and nuclear physics, to applications such as ion-beam-driven high energy density physics and fusion, and spallation neutron sources. Of particular importance at the high beam currents and charge densities of practical interest, are the effects of the intense self fields produced by the beam space charge and current on determining the detailed equilibrium, stability and transport properties. Charged particle beams confined by external focusing fields represent an example of nonneutral plasma. A characteristic feature of such plasmas is the non-uniformity of the equilibrium density profiles and the nonlinearity of the self fields, which makes detailed analytical investigation very difficult. The development and application of advanced numerical tools such as eigenmode codes [1] and Monte-Carlo particle simulation methods [2] are often the only tractable approach to understand the underlying physics of different instabilities familiar in electrically neutral plasmas which may cause a degradation in beam quality. Two such instabilities are the electrostatic Harris instability [2] and the electromagnetic Weibel instability [1], both driven by a large temperature anisotropy which develops naturally in accelerators. The beam acceleration causes a large reduction in the longitudinal temperature and provides the free energy to drive collective temperature anisotropy instabilities. Such instabilities may lead to an increase in the longitudinal velocity spread, which will make focusing the beam difficult, and may impose a limit on the beam luminosity and the minimum spot size achievable in focusing experiments. This paper reviews recent advances in the theory and simulation of collective instabilities in intense charged particle beams caused by temperature anisotropy. We also describe new simulation tools that have been developed to study these instabilities. The results of the investigations that identify the instability growth rates, levels of saturations, and conditions for quiescent beam propagation will also be discussed. \newline [1] E.A. Startsev and R.C. Davidson, Phys.Plasmas 10, 4829 (2003). \newline [2] E.A. Startsev, R.C. Davidson and H. Qin, Phys.Rev. ST Accel. Beams 8,124201 (2005).   

Relativistic dynamical bi-stability and adiabatic excitation of strong plasma waves

Resonant excitation of nonlinear dynamical systems is one of the most common uniting threads throughout plasma science. Best known examples are ECR rf heating of plasma and beat-wave excitation of electron plasma waves in a plasma beat-wave accelerator (PBWA). Beat-wave excitation mechanism is realized when the laser intensity is modulated with the temporal periodicity of the plasma wave. Despite being the oldest of the plasma-based theoretical and experimentally realized acceleration concepts, it continues attracting significant experimental and theoretical attention. It was recently realized that one can improve a PBWA by using a pair of long laser pulses with beat-wave amplitude exceeding a certain threshold and detuned from each other by a frequency less than the plasma frequency. The resulting plasma wake is essentially bi-stable as it can be either with certain large amplitude or near-zero. Its amplitude only weakly depends on the beat-wave amplitude and, because there are only two outcomes, can be reliably controlled by the beat-wave pulse duration. This phenomenon, referred to as Dynamical Bi-Stability (DBS), is caused by the relativistic nonlinearity of a high-amplitude plasma wave. We developed the description of strongly driven plasma wave whose phase and amplitude are described as a representative particle [1] moving according to a nonlinear Hamiltonian. The Hamiltonian, depending on the driver parameters, has a variable number of fixed points and always follows the same trajectory for a slow varying driver, regardless of whether the plasma is excited or left quiescent. Using the standard nonlinear dynamics concepts such as separatrix crossing, we analyze the evolution of the plasma wave and explain how a long adiabatic laser beat-wave pulse can leave behind a large wake. Also it is shown that the ``auto-resonant'' excitation by slowly varying (``chirping'') the driver frequency is described by the same Hamiltonian as DBS and, consequently, can be understood from the same standpoints as the latter. Further, it is demonstrated that the auto-resonance and dynamical bi-stability can be combined in one mixed DBS-auto resonant scheme [2] which relaxes the beat-wave conditions needed to produce a large plasma wake. \newline \newline [1] S. Kalmykov, O. Polomarov et. al,, Phil. Trans. Royal. Soc. 364 (1840), 725 (2006) \newline [2] O. Polomarov and G. Shvets, Phys. Plasmas 13, 054502 (2006)