Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session YI3: Plasma Acceleration |
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Chair: Bernhard Hidding, University of Strathclyde Room: 103ABC |
Friday, October 27, 2017 9:30AM - 10:00AM |
YI3.00001: Particle acceleration and exotic light emission in structured plasma wakefields Invited Speaker: Jorge Vieira Twisted Laguerre Gaussian lasers, with orbital angular momentum (OAM), are characterised by twisted wavefronts and doughnut intensity profiles. These lasers provide a transformative set of research directions in a growing range of fields, particularly at intensities below damage thresholds. Here, we explore how the interaction between ultra-intense twisted light and matter could enhance plasma accelerators and light sources. We support our findings with theory and massively parallel three-dimensional particle-in-cell Osiris simulations [R.A. Fonseca et al, PPCF, 55 124011 (2013)]. Providing a solution to a long-lasting challenge in plasma acceleration, we show that twisted light can excite a nonlinear doughnut blowout suitable for electron and positron focusing and acceleration [J. Vieira et al PRL 112 215001 (2014)]. This is a new type of wakefield that contrasts with the nonlinear spherical blowout, characterised by positron defocusing forces. Despite being driven by an OAM laser, doughnut plasma waves contain no OAM. This picture changes dramatically, when the laser driver contain multiple OAM levels characterised by distinct frequencies. The corresponding beating pattern results in a spiralling laser intensity profile, known as a light spring [G. Pariente et al Optics Lett. 40, 2037 (2015)], which can excite a twisted plasma wave with OAM. The twisted wakefields accelerate particles both longitudinally and azimuthally. This feature can be exploited towards the generation of relativistic bunches with similar longitudinal and transverse momenta, which changes radiation emission processes. Structured lasers have been produced in the laboratory, using conventional spiral phase plates and plasma based holograms [A. Denoeud et al PRL 118 033902 (2017)]. Here, we demonstrate how to create and amplify ultra-intense OAM lasers in the plasma, through stimulated Raman backscattering [J. Vieira et al Nat. Comms. 7 10371 (2016); J. Vieira et al. PRL 117, 265001 (2016)]. [Preview Abstract] |
Friday, October 27, 2017 10:00AM - 10:30AM |
YI3.00002: Single-shot measurements of low emittance beams from laser-plasma accelerators comparing two triggered injection methods. Invited Speaker: Jeroen van Tilborg The success of laser plasma accelerator (LPA) based applications, such as a compact x-ray free electron laser (FEL), relies on the ability to produce electron beams with excellent 6D brightness, where brightness is defined as the ratio of charge to the product of the three normalized emittances. As such, parametric studies of the emittance of LPA generated electron beams are essential. Profiting from a stable and tunable LPA setup, combined with a carefully designed single-shot energy-dispersed emittance diagnostic, we present a direct comparison of charge dependent emittance measurements of electron beams generated by two different injection mechanisms: ionization injection and shock-induced density down-ramp injection. Both injection mechanisms have gained in popularity in recent years due to their demonstrated stable LPA performance. For the down-ramp injection configuration, normalized emittances a factor of two lower were recorded: less than 1 micron at spectral charge densities up to 2 pC/MeV. For both injection mechanisms, a contributing correlation of space charge to the emittance was identified. This measurement technique in general, and these results specifically, are critical to the evaluation of LPA injection methods and development of high-quality LPA beam lines worldwide. [Preview Abstract] |
Friday, October 27, 2017 10:30AM - 11:00AM |
YI3.00003: Frontiers of Beam Diagnostics in Plasma Accelerators: Measuring the Ultra-fast and Ultra-cold Invited Speaker: Alessandro Cianchi Advanced diagnostics are essential tools in the development of plasma-based accelerators. The accurate measurement of the quality of beams at the exit of the plasma channel is crucial to optimize the parameters of the plasma accelerator. 6D electron beam diagnostics will be reviewed with emphasis on emittance measurement, which is particularly complex due to large energy spread and divergence of the emerging beams, and on femtoseconds bunch length measurements. [Preview Abstract] |
Friday, October 27, 2017 11:00AM - 11:30AM |
YI3.00004: Mechanisms for the mitigation of the hose instability in plasma-wakefield accelerators Invited Speaker: Timon Mehrling The \emph{hose instability} is a long standing challenge for plasma wakefield accelerators (PWFAs). It results from a coherent coupling between transverse phase space asymmetries of beam particles and plasma electrons. According to current models, the beam centroid displacement is amplified exponentially during the beam propagation in the plasma, resulting in an unstable acceleration process or beam-breakup. However, particle-in-cell (PIC) simulations indicate that these models overestimate the hosing growth rates, suggesting that PWFAs intrinsically provide saturation mechanisms for the hose instability [T.J.~Mehrling, \textit{et al.}~Phys.~Rev.~Lett.~\textbf{118}, 174801 (2017)]. In this work we review the theory for the hose instability in order to identify and describe diverse mitigation mechanisms. By means of a self-consistent theoretical model that includes the energy exchange between beam and plasma, we show that the beam energy evolution can significantly mitigate the hose instability. We also discuss other mechanisms which disrupt the coherent coupling between beam and plasma, and thereby lead to a saturation or damping of the beam centroid oscillations. In addition, we examine how the transverse beam asymmetries, which seed hosing, can be reduced. Hence, the presented work reveals crucial mechanisms allowing for stable beam acceleration over long distances in PWFAs. [Preview Abstract] |
Friday, October 27, 2017 11:30AM - 12:00PM |
YI3.00005: Experimental realization of underdense plasma photocathode wakefield acceleration at FACET. Invited Speaker: Paul Scherkl Novel electron beam sources from compact plasma accelerator concepts currently mature into the driving technology for next generation high-energy physics and light source facilities. ~Particularly electron beams of ultra-high brightness could pave the way for major advances for both scientific and commercial applications, but their generation remains tremendously challenging. The presentation outlines the experimental demonstration of the world's first bright electron beam source from spatiotemporally synchronized laser pulses [1] injecting electrons into particle-driven plasma wakefields at FACET. Two distinctive types of operation - laser-triggered density downramp injection (``Plasma Torch'') and underdense plasma photocathode acceleration (``Trojan Horse'') [2] -- and their intermediate transitions are characterized and contrasted. Extensive particle-in-cell simulations substantiate the presentation of experimental results. In combination with novel techniques to minimize the beam energy spread [3], the acceleration scheme presented here promises ultra-high beam quality and brightness. [1] A. Knetsch, P. Scherkl, T. Heinemann, et al., to be submitted [2] A. Deng, O. Karger, P. Scherkl, et al., to be submitted [3] G. G. Manahan, A.F. Habib, P. Scherkl,~et al. Single-stage plasma-based correlated energy spread compensation for ultrahigh 6D brightness electron beams. Nat. Comm$.$8:15705, Jun 2017 [Preview Abstract] |
Friday, October 27, 2017 12:00PM - 12:30PM |
YI3.00006: First direct observation of runaway electron-driven whistler waves in tokamaks Invited Speaker: Donald A. Spong Whistlers are electromagnetic waves that can be driven unstable by energetic electrons and are observed in natural plasmas, such as the ionosphere and Van Allen belts. Recent DIII-D experiments at low density demonstrate the first direct observation of whistlers in tokamaks, with 100-200 MHz waves excited by runaway electrons (REs) in the multi-MeV range. Whistler activity is correlated with RE intensity and the frequencies scale with magnetic field strength and electron density consistent with a whistler dispersion relation. Fluctuations occur in discrete frequency bands, and not a continuum as would be expected from plane wave analysis, suggesting the important role of toroidicity. An MHD model including the bounded/periodic nature of the plasma identifies multiple eigenmode branches. For a toroidal mode number $n =$\textit{ 10}, the predicted frequencies and spacing are similar to observations. The instabilities are stabilized with increasing magnetic field, as expected from the anomalous Doppler resonance. The whistler amplitudes show intermittent time variations. Predator-prey cycles with electron cyclotron emission (ECE) signals are observed, which can be interpreted as wave-induced pitch angle scattering of moderate energy REs. Such nonlinear dynamics are supported by quasi-linear simulations indicating that REs are scattered both by whistlers and high frequency magnetized plasma waves. The whistler wave predominantly scatters the high energy REs, while the magnetized plasma wave scatters the low energy REs, abruptly enhancing the ECE signal. Amplitude variations are also associated with sawtooth activity, indicating that the REs sample the $q =$\textit{ 1} surface. These features of the RE-driven whistler have connections to ionospheric plasmas and open up new directions for the modeling and active control of tokamak REs. [Preview Abstract] |
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