Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session NI3: Wakefield Acceleration and MFE: Energetic Particles and RFInvited
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Chair: Sven Steinke, Lawrence Berkeley National Laboratory Room: 210 ABEF |
Wednesday, November 2, 2016 9:30AM - 10:00AM |
NI3.00001: Kilohertz laser wakefield accelerator using near critical density plasmas and millijoule-level drive pulses Invited Speaker: Andy Goers Laser wakefield accelerators operating in the so-called bubble or blowout regime are typically driven by Joule-class femtosecond laser systems driving plasma waves in highly underdense plasmas ($10^{17}-10^{19} cm^{-3}$). While these accelerators are very promising for accelerating GeV scale, low emittance electron beams, the large energy requirements of the laser systems have so far limited them to repetition rates below 10 Hz. However, there are a variety of applications, such as ultrafast electron diffraction or high repetition rate gamma ray sources for materials characterization or medical radiography, which would benefit from lower energy (1-10 MeV) but higher repetition rate ($\sim$1 kHz) sources of relativistic electrons. This talk will describe relativistic wakefield acceleration of electron bunches in the range 1-10 MeV, driven by a 1 kHz, 30 fs, 1-12 mJ laser system. Our results are made possible by the use of very high density cryogenic $H_2$ and $He$ gas jet targets yielding electron densities $>10^{21} cm^{-3}$ in thin $\sim$100 $\mu$m gas flows. At these high densities the critical power for relativistic self-focusing and the plasma wave phase velocity are greatly reduced, leading to pulse collapse and self-injection even with $\sim$1 mJ drive laser pulses. Applications of this source to ultrafast electron diffraction and gamma ray radiography will be discussed. [Preview Abstract] |
Wednesday, November 2, 2016 10:00AM - 10:30AM |
NI3.00002: Synergistic Direct/Wakefield Acceleration of Plasma Electrons In the Plasma Bubble Regime Using Tailored Laser Pulses Invited Speaker: Gennady Shvets The integration of direct laser acceleration (DLA) and laser wakefield acceleration (LWFA) is a new approach to plasma-based acceleration that confers several benefits over both schemes taken separately. Such integration requires a significant portion [1] of the laser energy (e.g., a separate laser pulse [2,3]) to trail the main bubble-producing laser pulse, and resonantly interact with the trapped accelerated electrons undergoing betatron motion inside the plasma bubble. I will demonstrate how electron dephasing from the accelerating wakefield, which is one of the key limitations of LWFA, is reduced by their growing undulating motion. Moreover, the distinct energy gains from wake and the laser pulse are compounding, thereby increasing the total energy gain. Even more significant increases of the overall acceleration can be obtained by moving away from single-frequency laser format toward combining mid-infrared laser pulses for plasma bubble generation with short-wavelength trailing pulses for DLA. Various injection mechanisms, such as ionization injection, external injection, self-injection, and their advantages will also be discussed. Translating these new concepts into specific experiments will take advantage of recent technological advances in synchronizing laser and electron beams, and using multiple beamlines for producing sophisticated laser pulse formats.\\ \\$[1]$ J. L. Shaw et. al., Plasma Phys. Cont. Fusion \textbf{56,} 084006 (2014) \newline [2] X. Zhang, V. N. Khudik, and G. Shvets, Phys. Rev. Lett. \textbf{114,} 184801 (2015) \newline [3] X. Zhang, V. N. Khudik, A. Pukhov, and G. Shvets, Plasma Phys. Cont. Fusion \textbf{58,} 034011 (2016). [Preview Abstract] |
Wednesday, November 2, 2016 10:30AM - 11:00AM |
NI3.00003: Ultrafast science using Laser Wakefield Accelerators Invited Speaker: Alec G. R. Thomas Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have considerable benefits for ultrafast science. Laser wakefield acceleration provides radiation pulses that have femtosecond duration and intrinsic synchronisation with the laser source, allowing for pump-probe measurements with unprecedented temporal resolution. These pulses can be used to study ultrafast dynamical phenomena in plasma and dense material, such as transient magnetic fields, rapidly evolving plasma dynamics and crystal lattice oscillations. In this talk, I will review recent experiments in laser wakefield acceleration and energetic photon generation using the laser systems HERCULES and Lambda-Cubed at the University of Michigan and their use for capturing the dynamics of laser-pumped samples. Studies of the electron beam hosing instability and the generation of annular phase space distributions increase X-ray flux while maintaining its femtosecond duration. Single-shot, spectrally resolved absorption measurements in laser pumped foils can be made on ultrafast timescales using this broadband photon source. Ultrafast electron radiography is able to temporally resolve relativistically expanding magnetic fields in high-intensity laser-solid interactions and the evolution of electric fields in low density plasma. Time-resolved electron diffraction captures structural dynamics in crystalline silicon. I will also discuss the technological needs for and potential impact of such revolutionary compact radiation sources for ultrafast science in the future. [Preview Abstract] |
Wednesday, November 2, 2016 11:00AM - 11:30AM |
NI3.00004: Betatron x-rays from laser plasma accelerators: a new probe for warm dense matter at LCLS Invited Speaker: Felicie Albert Betatron x-ray radiation, driven by electrons from laser-wakefield acceleration, has unique properties to probe high energy density (HED) plasmas and warm dense matter. Betatron radiation is produced when relativistic electrons oscillate in the plasma wake of a laser pulse. Its properties are similar to those of synchrotron radiation, with a 1000 fold shorter pulse. This presentation will focus on the experimental challenges and results related to the development of betatron radiation for x-ray absorption spectroscopy of HED matter at large-scale laser facilities. A detailed presentation of the source mechanisms and characteristics in the blowout regime of laser-wakefield acceleration will be followed by a description of recent experiments performed at the Linac Coherent Light Source (LCLS). At LCLS, we have recently commissioned the betatron x-ray source driven by the MEC short pulse laser (1 J, 40 fs). The source is used as a probe for investigating the X-ray absorption near edge structure (XANES) spectrum at the K- or L-edge of iron and silicon oxide driven to a warm dense matter state (temperature of a few eV and solid densities). The driver is either LCLS itself or an optical laser. These experiments demonstrate the capability to study the electron-ion equilibration mechanisms in warm dense matter with sub-picosecond resolution. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and supported by the Laboratory Directed research and development program under tracking codes 13-LW-076, 16-ERD-041 and by the Office of Fusion Energy Sciences under SCW1476 and SCW1569. [Preview Abstract] |
Wednesday, November 2, 2016 11:30AM - 12:00PM |
NI3.00005: Plasma heating and generation of energetic ions with novel three-ion ICRF scenarios on Alcator C-Mod and JET tokamak facilities Invited Speaker: Yevgen Kazakov This talk will report the first experimental results of novel three-ion ICRF scenarios (two or more majority ion species and one minority) for plasma heating and generating energetic ions in fusion facilities [1]. The key feature of these scenarios is strong absorption of RF power possible at lower concentrations of minority ions than in two-ion plasmas. Effective plasma heating by injecting a small amount of $^{3}{\rm He}$ ions into H-D plasma mixtures with $n_{\rm H}/n_{e} \sim 70\%$ has been successfully demonstrated in Alcator C-Mod and JET tokamaks. In C-Mod, efficient plasma heating was observed for $^{3}{\rm He}$ concentrations from 0.4-2\%. During the discharges, a strong increase in Alfv\'{e}n eigenmode activity was found to coincide with the addition of $^{3}{\rm He}$ to the H-D plasmas [2]. Even lower $^{3}{\rm He}$ concentrations $(\sim 0.2\%)$ were utilized in recent JET experiments. The potential of the D-$(^{3}{\rm He})$-H scenario for plasma heating and generating MeV-range ions in JET plasmas was confirmed by a set of independent measurements, including stabilization of sawteeth, characteristic $\gamma$-ray emission, fast-ion loss detector. Furthermore, toroidal Alfv\'{e}n eigenmodes with a range of toroidal mode numbers $n$ were detected, which is another indication for the presence of significant population of high-energy $^{3}{\rm He}$ ions in a plasma. The discussed mechanism of resonant wave-particle interaction opens up various unexplored opportunities for ICRF system, including new scenarios for plasma heating. Three-ion ICRF scenarios are also relevant for the experimental programme of ITER. The possibility of using intrinsic $^{9}{\rm Be}$ impurities as the minority (instead of $^{3}{\rm He}$) was suggested for heating bulk ions in D-T plasmas of JET and ITER [3], as well as heating trace amounts of $^{3}{\rm He}$ and $^{4}{\rm He}$ ions in H majority plasmas of ITER. The latest results and simulation comparisons will be presented. \newline [1] Y. Kazakov et al., \textit{Nucl. Fusion} \textbf{55}, 032001 (2015) \newline [2] J. Wright et al. and Y. Lin et al., \textit{this conference} \newline [3] Y. Kazakov et al., \textit{Phys. Plasmas} \textbf{22}, 082511 (2015) [Preview Abstract] |
Wednesday, November 2, 2016 12:00PM - 12:30PM |
NI3.00006: A critical gradient model for energetic particle transport from Alfven eigenmodes: GYRO verification, DIII-D validation, and ITER projection Invited Speaker: R.E. Waltz Local nonlinear gyrokinetic code GYRO simulations of energetic particle driven low-n Alfven eigenmodes embedded in high-n microturbulence motivate a local critical gradient model (CGM) for stiff energetic particle (EP) transport from Alfven eigenmodes (AEs). The simulations show unbounded EP transport when the local linear low-n AE growth rate exceeds the ion temperature gradient and trapped electron mode (ITG/TEM) rate at the same low-n[1]. This linear rate condition for the critical EP density gradient is again verified by new nonlinear GYRO simulations of a well-studied neutral beam injected (NBI) DIII-D discharge (146102) where about half the fast ions are lost from the inner half to the outer half radius by AE induced transport. The CGM is revised to accounted for the small effect of ExB shear stabilization. This CGM incorporated in the ALPHA EP density transport code, used in a previous ITER projection of AE fusion alpha loses[2], is validated by the EP pressure profile in good agreement with the DIII-D experimental fast ion pressure profile[3]. A beam-like slowing down EP distribution in GYRO was used to find the AE linear rates. Non-local EP drift orbit broadening of the local critical gradient profile was found to be important in the DIII-D validation (but not in ITER projections)[4]. A two-EP-species CGM to include simultaneous AE drive from (and transport of) fusion alphas and 1 Mev NBI EPs is used for a revised projection of ITER EP losses.\par \vskip3pt \noindent [1] E.M. Bass and R.E. Waltz, Phys. Plasmas 17, 112319 (2010)\par \noindent [2] R.E. Waltz and E.M. Bass, Nucl. Fusion 54, 104006 (2014)\par \noindent [3] R.E. Waltz, E.M. Bass, W.W. Heidbrink, and M.A, VanZeeland, Nucl. Fusion 55, 123002 (2015)\par \noindent [4] H. Sheng and R.E. Waltz, Nucl. Fusion 56, 056004 (2016) [Preview Abstract] |
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