Session TI3: Particle Beams

Long-path-length experimental studies of longitudinal phenomena in intense beams

Intense charged particle beams are nonneutral plasmas and they can support a host of plasma waves and instabilities. For a long beam bunch, the longitudinal physics can often be reasonably described by a 1-D cold-fluid model, with a geometry factor to account for the transverse effects. The plasma physics of such beams has been extensively studied theoretically and computationally for decades, but until recently, the only experimental measurements were carried out on relatively short linacs. This work reviews experimental studies over the past 5 years on the U. Maryland Electron Ring, investigating longitudinal phenomena, for the first time, over time scales of hundreds and thousands of plasma periods. These results are in good agreement with theory and simulation. Topics that will be discussed are: \begin{itemize} \item Longitudinal confinement of a long bunch using barrier fields [1]. \item The generation of space charge waves from barrier field mismatches, their propagation along the bunch and reflection at the beam ends, as well as their long-term dissipation [1]. \item The characterization of solitary waves from density/velocity perturbations in the center of the bunch [2-3]. \item Compression of solitary wave trains with velocity ``tilts'' (head-to-tail gradient). \item Observation of a multi-stream instability driven by the longitudinal merging of bunches and the characterization of the onset of the instability with a PIC code [4]. \item The shock-wave compression of a bunch using rapidly-moving barrier fields [5]. \end{itemize} [1] B. Beaudoin, \textit{et al.,} \textit{Phys. Plasmas }\textbf{18}, 013104 (2011).\\[0pt] [2] B. Beaudoin, \textit{et al.,} \textit{NIM A} \textbf{733}, 178-181 (2014).\\[0pt] [3] Y.C. Mo, \textit{et al.,} \textit{Phys. Rev. Lett.} \textbf{110}, 084802 (2013).\\[0pt] [4] B.L. Beaudoin, \textit{et al.,} Proc. 2013 IPAC, 2044 (2013).\\[0pt] [5] B.L. Beaudoin, \textit{et al.,} \textit{``Barrier Shock Compression with Longitudinal Space-Charge,''} Proc. 2015 IPAC, to appear (2015).   

Novel aspects of direct laser acceleration of relativistic electrons

Production of energetic electrons is a keystone aspect of ultraintense laser-plasma interactions that underpins a variety of topics and applications, including fast ignition inertial confinement fusion and compact particle and radiation sources. There is a wide range of electron acceleration regimes that depend on the duration of the laser pulse and the plasma density. This talk focuses on the regime in which the plasma is significantly underdense and the laser pulse duration is longer than the electron response time, so that, in contrast to the wakefield acceleration regime, the pulse creates a quasi-static channel in the electron density. Such a regime is of particular interest, since it can naturally arise in experiments with solid density targets where the pre-pulse of an ultraintense laser produces an extended sub-critical pre-plasma. This talk examines the impact of several key factors on electron acceleration by the laser pulse and the resulting electron energy gain. A detailed consideration is given to the role played by: (1) the static longitudinal electric field [1], (2) the static transverse electric field [2,3], (3) the electron injection into the laser pulse [4], (4) the electromagnetic dispersion, and (5) the static longitudinal magnetic field [4]. It is shown that all of these factors lead, under conditions outlined in the talk, to a considerable electron energy gain that greatly exceeds the ponderomotive limit. The static fields do not directly transfer substantial energy to electrons. Instead, they alter the longitudinal dephasing between the electrons and the laser pulse, which then allows the electrons to gain extra energy from the pulse. The talk will also outline a time-resolution criterion that must be satisfied in order to correctly reproduce these effects in particle-in-cell simulations [5].\\[4pt] [1] A. Robinson, A. Arefiev, and D. Neely, Phys. Rev. Lett. {\bf{111}}, 065002 (2013);\\[0pt] [2] A. Arefiev, B. Breizman, M. Schollmeier, and V. Khudik, Phys. Rev. Lett. {\bf{108}}, 145004 (2012);\\[0pt] [3] A. Arefiev, V. Khudik, and M. Schollmeier, Phys. Plasmas {\bf{21}}, 033104 (2014);\\[0pt] [4] A. Arefiev, A. Robinson, and V. Khudik, J. Plasma Physics {\bf{81}}, 475810404 (2015);\\[0pt] [5] A. Arefiev, G. Cochran, D. Schumacher, A. Robinson, and G. Chen, Phys. Plasmas {\bf{22}}, 013103 (2015).   

Acceleration of plasma electrons by intense nonrelativistic ion and electron beams propagating in background plasma due to two-stream instability

In this paper we study the effects of the two-stream instability on the propagation of intense nonrelativistic ion and electron beams in background plasma. Development of the two-stream instability between the beam ions and plasma electrons leads to beam breakup, a slowing down of the beam particles, acceleration of the plasma particles, and transfer of the beam energy to the plasma particles and wave excitations. Making use of the particle-in-cell codes EDIPIC and LSP, and analytic theory we have simulated the effects of the two-stream instability on beam propagation over a wide range of beam and plasma parameters. Because of the two-stream instability the plasma electrons can be accelerated to velocities as high as twice the beam velocity. The resulting return current of the accelerated electrons may completely change the structure of the beam self - magnetic field, thereby changing its effect on the beam from focusing to defocusing. Therefore, previous theories of beam self-electromagnetic fields that did not take into account the effects of the two-stream instability must be significantly modified. This effect can be observed on the National Drift Compression Experiment-II (NDCX-II) facility by measuring the spot size of the extracted beamlet propagating through several meters of plasma. Particle-in-cell, fluid simulations, and analytical theory also reveal the rich complexity of beam- plasma interaction phenomena: intermittency and multiple regimes of the two-stream instability in dc discharges; band structure of the growth rate of the two-stream instability of an electron beam propagating in a bounded plasma and repeated acceleration of electrons in a finite system. In collaboration with E. Tokluoglu, D. Sydorenko, E. A. Startsev, J. Carlsson, and R. C. Davidson. Research supported by the U.S. Department of Energy.   

Enhancing proton acceleration by using composite targets

Radiation pressure acceleration (RPA) is a highly efficient mechanism of laser-driven ion acceleration, with the laser energy almost totally transferrable to the ions in the relativistic regime. However, there is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. The tightly focused laser pulses have group velocities smaller than the vacuum light speed, and, since they offer the high intensity needed for the RPA regime, it is plausible that group velocity effects would manifest themselves in the experiments involving tightly focused pulses and thin foils. However, in this case, finite spot size effects are important and another limiting factor, the transverse expansion of the target, comes into play that may dominate over the group velocity effect. As the laser pulse diffracts after passing the focus, the target expands accordingly due to the transverse intensity profile of the laser. Due to this expansion, the areal density of the target decreases, making it transparent for radiation and effectively terminating the acceleration. The utilization of external guiding may relax the constraints on maximum attainable ion energy. Such guiding can be provided by a composite target, a thin foil followed by a near critical density slab. This slab provides guiding of the laser pulse during the acceleration process. The comparison of a single foil RPA and a composite target RPA shows that, in the latter case, the ions have energy several times larger than in the former case, thus greatly increasing the effectiveness of the RPA regime of laser-driven ion acceleration. In such a configuration, the group velocity effects begin to dominate and determine the maximum achievable ion energy. Work supported by U.S. DOE under Contract No. DE-AC02-05CH11231.   

Plasma shaping in laser-plasma accelerators: injection, energy boost and beam collimation

The longitudinal density profile is a key parameter to optimize the properties of electron beams in laser-plasma accelerators. Tailored density profile can notably be used to control injection or increase the electron energy via density tapering. Here we present three different experiments illustrating the use of density tailoring for injecting, increasing the energy or focusing relativistic electron beams. First, we discuss results on shock-front injection in a gas mixture. We show that shocks allow to confine injection and hence to reduce significantly the beam energy spread, compared to pure ionization injection. Then we demonstrate that, with a different setup, shock fronts can also be used to rephase the electron beam with the wakefield. Using this setup we obtained an increase of the electron energy by almost 50 percent. Finally, we present the principle of the laser-plasma lens and show that this device can be used to reduce the electron beam divergence by a factor of almost 3. This last result is of particular importance for applications requiring beam transport; the divergence reduction should actually be sufficient to avoid transverse emittance growth in quadrupole triplets, provided that the energy spread is lower than 3 percent (chromatic emittance growth is due to the combination of large divergence and energy spread).   

Staging of laser-plasma accelerators

We present results of a probing experiment where two Laser-Plasma-Accelerator stages are coupled at a short distance by a plasma mirror. Stable electron beams were focused by a discharge capillary-based active plasma lens to a micrometer spot size in a H2 plasma, such that they interact with a dark-current-free, quasi-linear wakefield excited by the laser of the second stage. Changing the arrival time of the electron beam allowed localized reconstruction of the temporal field structure excited by the wake and determination of the on-axis plasma density. Injection into the wakefield of the second stage was verified by a momentum gain of the electron beam.