Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session CI2: Beams and Coherent Radiation |
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Chair: Eric Esarey, Lawrence Berkeley National Laboratory Room: Ballroom DE |
Monday, October 29, 2012 2:00PM - 2:30PM |
CI2.00001: Ping-Pong Modes and Higher-Periodicity Multipactor Invited Speaker: Rami Kishek Multipactor is a vacuum discharge based on secondary electron emission. Multiple period multipactors have long been known to exist but have been studied less extensively. In a period-n multipactor, electrons undergo multiple impacts in one rf period, with the synchronous phase alternating periodically between multiple values. A novel resonant form is proposed that combines one- and two-surface impacts within a single period, provided the total transit time is an odd number of rf half-periods, and the product of secondary yields exceeds unity. For low fD products, the simplest such mode is shown to significantly increase the upper electric field boundary of the multipacting region, and lead to overlap of higher-order bands. The results agree nicely with 3-D particle-in-cell code simulations. Practical implications of the findings are discussed, including consequences for multipactor suppression strategies using a DC magnetic field. [Preview Abstract] |
Monday, October 29, 2012 2:30PM - 3:00PM |
CI2.00002: Modeling of ultrafast laser induced electron emission from a sharp tip Invited Speaker: Lay Kee (Ricky) Ang The emission mechanism of ultrafast laser induced electron emission from a sharp metallic tip has attracted considerable interest in recent years due to its applications, such as ultrafast electron-beam based imaging at nanometer scale, and also as high brightness short electron bunches for applications in future light sources such as x-ray free-electron lasers. The underlying electron emission mechanism is difficult to pin down as it occurs in the region of Keldysh parameter $\gamma \sim $ 1, which is between the multiphoton ($\gamma >>$ 1) and tunneling $\gamma <<$ 1) regimes. In this paper, we will present a consistent time-dependent quantum model that is able to combine the effects of (a) time-dependent tunneling, (2) ultrafast laser non-equilibrium excitation on metal, and (3) field gradient on the tip. It is found that the onset of the tunneling regime is given by a universal formula, depends only on the work function over a wide range of laser parameters. More interestingly, the classical concept of photoelectric effect for electron emission by absorption $N$ number of photons is no longer valid at very short time scale for which the required $N$ may be reduced by a photon by using a sub-10 fs ultrafast laser. The non-equilibrium electron distribution due to ultrafast laser excitation is also self-consistently included with good agreement with experimental findings. Due to the close correlation between the amount of electron emission and the phase of the ultrafast laser pulse, this model may provide a new way measure the phase of the laser. While the model is initially focused on metallic tip, it is ready to extend to novel materials such as single-layer graphene, for which a relativistic quantum model has been created to include the effect of Klein tunneling. These results show that traditional equilibrium models in the electron emission process will require a revision in the limit of ultrafast time scale, when the laser pulse length is comparable or shorter to the tunneling time, and also to the electron relaxation time. Dependence of laser parameters (wavelength, pulse length, phase) and material properties will be studied thoroughly. [Preview Abstract] |
Monday, October 29, 2012 3:00PM - 3:30PM |
CI2.00003: Self-modulated plasma wakefield accelerators Invited Speaker: Carl B. Schroeder A long, relativistic, particle beam propagating in an overdense plasma is subject to the self-modulation instability. This instability modulates the beam radius and density at the plasma wavelength, exciting a large plasma wave that potentially could be used for high-gradient acceleration of particle beams. Self-modulation of proton beams (such as those available at CERN) are actively being considered to drive compact plasma accelerators. The beam self-modulation instability is analyzed in homogeneous and inhomogeneous plasma. While undergoing modulation, the phase velocity of the plasma wave is significantly less than the beam velocity, which severely limits the energy gain of the accelerated electron beam. Tapering (i.e., a plasma density that increases along the direction of beam propagation) offers the possibility to compensate for the slow wave phase velocity, improving the efficiency of the accelerator, and the optimal form of the taper is presented. Small plasma density inhomogeneities may result in decoherence effects that will suppress the instability, making experimental realization challenging. The transverse stability of the drive beam (e.g., the growth of beam hosing) is a critical concern for beam-driven plasma wakefield accelerators, and, in particular, for long beams. Coupling of the beam envelope self-modulation to the beam centroid displacements (hosing) is described. Methods to mitigate hosing by seeding the self-modulation will be presented. Implications for proton-beam-driven plasma accelerator experiments will be discussed. [Preview Abstract] |
Monday, October 29, 2012 3:30PM - 4:00PM |
CI2.00004: Dense Electron-Positron Plasmas Generated by 10PW Lasers in the QED-Plasma Regime Invited Speaker: Christopher Ridgers Electron-positron plasmas are a prominent feature of the high-energy Universe. In the relativistic winds from pulsars and black holes it is thought that non-linear quantum electrodynamics (QED) processes cause electromagnetic energy to cascade into an e$^{- }$e$^{+}$ plasma [1]. We show that next-generation 10PW lasers, available in the next few years, will generate such a high density of pairs that they create a micro-laboratory for the first experimental study of a similarly generated e$^{- }$e$^{+}$ plasma [2]. In the first simulations of a 10PW laser striking a solid we demonstrate the production of a pure electron-positron plasma of density 10$^{26}$m$^{-3}$. This is seven orders of magnitude denser than currently achievable in the laboratory [3] and is comparable to the critical density for commonly used lasers, marking a step change to collective e$^{-}$e$^{+}$ plasma behavior. Furthermore, a new ultra-efficient laser-absorption mechanism converts 35{\%} of the laser energy to a burst of gamma-rays of intensity 10$^{22}$Wcm$^{-2}$, potentially the most intense gamma-ray source available in the laboratory. This absorption results in a strong feedback between both pair and $\gamma $-ray production and classical plasma physics leading to a new physical regime of QED-plasma physics [2]. In this new regime the standard particle-in-cell (PIC) simulation approach, which has been one of the most important kinetic simulation tools in plasma physics for 50 years, is inadequate. We have developed a new approach (QED-PIC) which will provide a powerful new modeling tool essential to the future advancement of the field of high intensity laser-plasma interactions.\\[4pt] [1] P. Goldreich {\&} W.H. Julian, Astrophys. J. 157, 869 (1969)\\[0pt] [2] C.P. Ridgers et al, Phys. Rev. Lett. 108, 165006 (2012)\\[0pt] [3] H. Chen et al, Phys. Rev. Lett. 105, 015003 (2010) [Preview Abstract] |
Monday, October 29, 2012 4:00PM - 4:30PM |
CI2.00005: Dynamics of Relativistic Transparency and Optical Shuttering in Expanding Overdense plasma Invited Speaker: Sasi Palaniyappan Overdense plasmas are usually opaque to incident laser light. But when the light is of sufficient intensity to drive electrons in the plasma to near light speeds, the plasma becomes transparent. In the physical picture, as the electrons reach near light speeds their mass increases due to relativistic effect. The increase in electron mass in turn slows their motion such that they can no-longer shield the plasma from the incident laser, making the plasma subsequently transparent to the incident laser. This process -- known as relativistic transparency (RT) -- takes just a tenth of a picosecond. Yet all studies of RT to date have been restricted to measurements collected over time-scales much longer than this, limiting our understanding of the dynamics of this process. Here we present optical signatures of relativistic transparency by measuring the time-resolved electric fields and temporal phases (with temporal resolution $\sim $50 fs) of the light, initially reflected from, and subsequently transmitted through, an expanding overdense plasma due to temporal evolution of RT. These measurements are done using a single-shot Frequency-Resolved-Optical-Gating (FROG) technique. The measured electric fields show the temporal chopping nature of RT in expanding overdense plasma from nanofoils. In addition the temporal phases of the corresponding electric fields record the plasma critical surface movement via Doppler-shift in reflection and plasma refractive index in transmission. Our result provides insight into the dynamics of the transparent-overdense-regime (TOR) of relativistic plasmas, which should be useful in the development of laser-driven particle accelerators, x-ray sources, and techniques for controlling the shape and contrast of intense laser pulses. [Preview Abstract] |
Monday, October 29, 2012 4:30PM - 5:00PM |
CI2.00006: Bright MeV-energy x-ray beams from a compact all-laser-driven inverse-Compton-scattering source Invited Speaker: Donald Umstadter Bright MeV energy x-ray beams produced by conventional inverse-Compton-scattering sources are used for nuclear physics research, but their large size ($>$100-m) restricts accessibility and utilization for real-world radiological applications. By developing a method to integrate a compact laser-driven accelerator with Compton scattering, we have developed a source that produces MeV energy x-rays, but with a four orders-of-magnitude increase in peak brightness, and yet has a size ($<$ 10 m) small enough to fit in a hospital laboratory, or even on a portable platform. Our design employs two independently adjustable laser pulses---one to accelerate electrons by means of a high-gradient laser wakefield, and one to Compton scatter. The use of two separate pulses from the same high-peak-power laser system allowed us to independently optimize the electron accelerator and the Compton scattering process. It also allowed the electron bunch and scattering laser pulse to be spatially overlapped on the micron scale, and be synchronized with femtosecond timing accuracy. The resulting x-ray photon energy was peaked at 1 MeV, and reached up to 4 MeV, which is twenty times higher than from an earlier all-laser-driven Compton source with a different design [K. Ta Phuoc \textit{et al}., \textit{Nature Photonics}\textbf{ 6}, 308 (2012)]. The total photon number was measured to be 2$\times $10$^{7}$; the source size was 5 $\mu $m; and the beam divergence angle was $\sim $10 mrad. The measurements were found to be consistent with a theoretical model that included realistic beams. We also discuss the results of the first application of the source, namely, the diagnosis---with micron resolution---of both the radiation source size and the emittance of a laser-wakefield-accelerated electron beam. Ultrafast nuclear science can also be enabled by MeV x-ray energy combined with ultrashort pulse duration (fs). [Preview Abstract] |
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