Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session NI01: Invited: Laser Wakefield Acceleration (LWFA)Live
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Chair: Michael Downer, University of Texas at Austin |
Wednesday, November 11, 2020 9:30AM - 10:00AM Live |
NI01.00001: Ion Motion, Hosing Suppression, and Beam Quality Preservation in Plasma-Based Accelerators Invited Speaker: Carlo Benedetti Plasma accelerators can produce extremely large fields, enabling compact accelerators, and application of plasma accelerators to the next generation of colliders has attracted considerable interest. Acceleration of ultra-low emittance beams with high efficiency is critical to realizing this application. Low emittance beams are strongly focused and pinched in the plasma reaching beam densities sufficiently high to move background ions, inducing beam emittance growth. High efficiency requires large longitudinal wakefield excitation by the accelerated beam, and this has an associated large transverse wakefield that will drive the hosing instability (growth in head-to-tail beam centroid misalignment). In this talk, the nonlinear response of the ions to a dense beam will be described, including the coupling to the hosing instability. It is shown that the head-to-tail variation in the average plasma wakefield induced by ion motion results in the suppression of hosing. A class of initial beam distributions are identified that are equilibrium solutions in the plasma wakefield including ion motion. Using these beam distributions enables ion motion without emittance growth. A plasma-based method to generate the matched equilibrium beam distribution is proposed. Hence, it is shown that stable acceleration in plasma-based accelerators is possible and, by proper bunch shaping, both the low emittance may be preserved and the energy spread minimized. [Preview Abstract] |
Wednesday, November 11, 2020 10:00AM - 10:30AM Live |
NI01.00002: Optimization of Electron Injection and Stable Long-Term Operation of a kHz Laser-Plasma Accelerator Invited Speaker: Jerome Faure High-repetition rate laser-plasma accelerators (LPA) have great potential for applications in femtosecond irradiation, X-ray generation and electron diffraction. As relativistic laser intensities are now available at kilohertz, important progress has been achieved in high-repetition rate LPA in the past few years [1] Here, we will review recent experimental results and developments on a kHz LPA. Our laser system is equipped with a post-compression stage which gives us a unique opportunity to tune the spectral bandwidth, thereby generating chirp-free laser pulses ranging from 3.5-fs to 25-fs. This allowed us to study the effect of the number of optical cycles in the acceleration process. In the few cycle regime, we observe a transition from resonant to self-modulated laser wakefield acceleration when the number of cycles N is increased from N$=$1 to N$=$3. We observe that the best beam quality is obtained for near single-cycle laser pulses in the resonant regime. In order to further optimize our laser-plasma accelerator, several injection mechanisms were studied as well as their impact on the performance and stability of the accelerator. Density gradient injection was found to yield the most stable electron beams and we demonstrated stable continuous operation for a period of 5 hours [2]. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetric shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. References: [1] D. Guenot et al., Nat. Photonics \textbf{11}, 293 (2017). F. Salehi et al., Opt. Lett. \textbf{42}, 2015 (2017) [2] L. Rovige et al., ArXiV~:2005.06929 (2020) [Preview Abstract] |
Wednesday, November 11, 2020 10:30AM - 11:00AM Live |
NI01.00003: Polarization-Dependent Self-Injection and Electron Beam Dynamics in a Laser Wakefield Accelerator Invited Speaker: Yong Ma Laser wakefield acceleration is a very promising alternative novel accelerator technology for scientific applications such as compact TeV electron-positron colliders, X-ray free electron lasers, experimental high-field quantum electrodynamics studies and many more. All these applications place highly restrictive requirements on the beam quality, which critically depends on control of the injection process and subsequent beam dynamics. In the so-called ``self-injection'' process of LWFA, the wakefield is driven by the ponderomotive force, which is determined by the intensity gradient and thus has no dependence on laser polarization. However, the laser pulse front ionize the gas medium to generate the plasma. As we know, ionization depends on laser polarization due to the well-known above threshold ionization process. Therefore, we presents an experimental investigation on the role of laser polarization on electron beam self-injection in laser-wakefield acceleration. The experimental results revealed that the self-injection threshold can be decreased by using laser pulses with circular polarization in laser-wakefield acceleration experiments, compared to the usually-employed linear polarization. A significantly higher electron beam charge was also observed for circular polarization compared to linear polarization over a wide range of parameters. Theoretical analysis and quasi-3D particle-in-cell simulations revealed a different injection mechanism for circularly polarized laser pulses, originating from the larger momentum gain during above-threshold-ionization, in which electrons gain residual momenta due to the conservation of transverse canonical momentum. This enables electrons to fulfill the trapping condition more easily. The expected resulting higher plasma temperature was confirmed via spectroscopy of the XUV plasma emission. [Preview Abstract] |
Wednesday, November 11, 2020 11:00AM - 11:30AM Live |
NI01.00004: Measuring Attosecond Electron Pulses with Coherent Nonlinear Thomson Scattering Invited Speaker: Colton Fruhling Attosecond-duration electron pulses can potentially enable new scientific discoveries on the frontier research area of ultrafast dynamics. However, current conventional methods for measuring attosecond-duration of electron pulses, such as coherent transition radiation and Smith-Purcell radiation, are inapplicable in the important low energy region. We show that nonlinear Thomson scattering is a robust solution to this problem, and can in principle provide even zeptosecond resolution (10$^{\mathrm{-21}}$ s). The pulse duration is revealed by features of the radiation spectrum due to coherent scattering by tightly bunched electrons. The limits and robustness of this method, as well as considerations for its application in realistic experimental conditions, will be discussed in detail. [Preview Abstract] |
Wednesday, November 11, 2020 11:30AM - 12:00PM Live |
NI01.00005: Ultra-short radiation generation from Mid IR-THz range using plasma wakes and relativistic ionization fronts Invited Speaker: Zan Nie In this talk we will discuss two different concepts for frequency downshifting and upshifting of an IR laser to cover the entire bandwidth from 1 to 300 $\mu $m using two different plasma techniques. Recently we have demonstrated a new scheme that utilizes frequency downshifting of a Ti-saphhire laser using a wake produced in a tailored plasma structure to generate multi-millijoule energy, single-cycle, long-wavelength IR pulses [1,2]. Extending this idea, sub-joule, single-cycle terahertz pulses can be generated by using a picosecond 10 $\mu $m CO$_{\mathrm{2}}$ driving laser. On the other hand, such a CO$_{\mathrm{2}}$ laser can be frequency upshifted by colliding it with an underdense but relativistic ionization front [3]. In this case the wavelength can be tuned from 1-10 $\mu $m by simply tuning the gas density. These plasma techniques seem extremely promising to covering the entire molecular fingerprint region. References: [1] Z. Nie, et. al., Nat. Photon. 12, 489-494 (2018). [2] Z. Nie, et. al., Nat. Comm. 11, 2787 (2020). [3] W. B.~Mori,~Phys.~Rev. A~44,~5118~(1991). [Preview Abstract] |
Wednesday, November 11, 2020 12:00PM - 12:30PM Live |
NI01.00006: Narrow energy spread electron beams from a laser plasma accelerator Invited Speaker: Manuel Kirchen The demonstration of a free-electron laser presents one of today’s main challenges in the field of plasma acceleration. Driving the FEL process with laser-plasma accelerated electron beams requires low transverse emittances and high spectral charge densities. Here we present our recent progress on the generation of high-quality electron beams at the LUX beamline. Few-percent relative energy spread beams at several hundred MeV with tens of pC charge are produced from controlled injection in tailored plasma targets. The impact of laser and target parameter variations on the injection and acceleration dynamics are discussed and validated by Particle-In-Cell simulations. [Preview Abstract] |
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