Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session GI2: Beams and Radiation |
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Chair: Eric Esarey, Lawrence Berkeley National Laboratory Room: Plaza E |
Tuesday, November 12, 2013 9:30AM - 10:00AM |
GI2.00001: Wakefield excitation and electron injection using multiple laser pulses in plasmas Invited Speaker: Carlo Benedetti Efficient excitation of large amplitude, relativistic plasma waves and the production of high quality (low energy spread and low emittance) electron bunches is of fundamental importance to plasma-based accelerators and their applications. In this talk, several methods for wakefield excitation and laser-triggered injection of high quality electron bunches that rely on one or more laser pulses will be discussed. Novel techniques for exciting and controlling the laser-induced wakefields using multiple, multi-color pulses and pulse trains will be presented. In addition, an electron injection method that relies on two-color ionization injection will be discussed. Here a low frequency, high normalized vector potential pulse is used to drive the wakefield, and a trailing high frequency, low normalized vector potential (but high electric field) pulse is used for ionization injection. This injection mechanism is compared to self-trapping in the nonlinear bubble regime, in which the trapping threshold dependence on laser intensity and wake phase velocity has been numerically explored. [Preview Abstract] |
Tuesday, November 12, 2013 10:00AM - 10:30AM |
GI2.00002: First experimental results on the self modulation instability (SMI) of long electron bunches in dense plasmas Invited Speaker: Yun Fang We demonstrate experimentally for the first time the seeding of the self-modulation instability (SMI) by a relativistic electron bunch in a plasma. The long ($\approx3.2~ps$) bunch with a square current profile available at BNL-ATF drives wakefields with periods one to one seventh of the bunch length in plasmas in the $10^{15}\sim10^{16}~cm^{-3}$ density range. The effect of these $MV/m$ range seed wakefields on the long bunch is observed as a periodic modulation of the bunch correlated energy spectrum after propagation along the $2~cm$ plasma. While it is the transverse wakefields that seed the SMI, the longitudinal wakefields are always accompanied by corresponding transverse wakefields. The seeding of the SMI by the sharp ($<\lambda_{pe}$) rising edge of the bunch is confirmed by the observation that the position of the first bunch in the modulated energy spectra does not change when the plasma density is varied. In an accelerator experiment this is a necessary condition to deterministically inject a witness bunch into the accelerating and focusing phase of the wakefields. Simulations [1] and experimental results confirm that the SMI does not grow significantly over the plasma length with the chosen $50~pC$ bunch charge. Simulation results also show that, with a $1~nC$ bunch charge, the SMI grows and saturates over the same length. However, due to dephasing between the bunch particles and the wakefields, the actual energy gain/loss is significantly lower than estimated from the peak accelerating field. We also observe in simulations that the finite radial plasma size and the radial plasma density profile expected in the capillary discharge do not significantly affect the development of the SMI. A number of SMI experiments are planned at major facilities (i.e. AWAKE at CERN, E209 at SLAC-FACET etc.). All of them will rely on seeding to observe the instability, using it to externally inject electrons in the wakefields or to mitigate the occurrence of the hose instability. The results presented here are an important seed for these major experiments. Detailed experimental and simulation results will be presented.\\[4pt] [1] R. A. Fonseca \emph{et al.}, Lect. Notes Comp. Sci. vol. 2331/2002, (Springer Berlin/Heidelberg, (2002). [Preview Abstract] |
Tuesday, November 12, 2013 10:30AM - 11:00AM |
GI2.00003: Ion motion and hosing suppression in self-modulated plasma wakefield acceleration Invited Speaker: Jorge Vieira With more than 10 kJ, currently available proton bunches at CERN were proposed as drivers for large amplitude wakefields capable to accelerate e-s to the TeV scale in Km long plasmas - proton driven plasma wakefield acceleration (PDPWFA). Unlike typical LWFA or PWFA experiments, which use ultra-short, 10-100 micrometer long drivers, proof-of-principle PDPWFA experiments will employ 10 cm long proton bunches, encompassing tens-hundreds of plasma wavelengths, and operating through the self-modulation instability (SMI). In this talk we explore and give solutions to some of the challenges posed by future PDPWFA experiments. We find that background plasma ion motion can occur in low Z plasmas (e.g. H, He). In this scenario, the motion of background plasma ions can lead to the suppression of focusing/accelerating wakefields, resulting in the early saturation of SMI and preventing particle acceleration. Ion motion is mostly due to the transverse plasma ponderomotive force, but can be fully avoided in experiments by using high Z plasmas (e.g. Ar, Rb). We address the competition between hosing instability (HI) and SMI and provide the conditions for stable, hosing-free self-modulated plasma acceleration. Despite having similar growth rates to SMI, we find that hosing damps after the SMI saturation. Damping occurs due to betatron frequency detuning along the self-modulated bunch due to wakefield secular growth. The potential of the PDPWFA motivated several self-modulation experiments using electrons and positrons. The approved E209 experiment at SLAC FACET for instance will explore key physics of future PDPWFA experiments with currently available lepton bunches. We show that SMI of electrons and positrons can differ at SLAC FACET. We find that SMI of 20 GeV e-/e$+$ SLAC FACET bunches can occur in less than 1m, leading to 10-20 GV/m wakefields, and to multi GeV e-/e$+$ energy gain/loss. [Preview Abstract] |
Tuesday, November 12, 2013 11:00AM - 11:30AM |
GI2.00004: Short-pulse, high-energy radiation generation from laser-wakefield accelerated electron beams Invited Speaker: Will Schumaker Recent experimental results of laser wakefield acceleration (LWFA) of $\sim GeV$ electrons driven by the 200TW HERCULES and the 400TW ASTRA-GEMINI laser systems and their subsequent generation of photons, positrons, and neutrons are presented. In LWFA, high-intensity ($I > 10^{19}$ $W/cm^2$), ultra-short ($\tau_L < 1/(2\pi \omega_{pe})$) laser pulses drive highly nonlinear plasma waves which can trap $\sim nC$ of electrons and accelerate them to $\sim GeV$ energies over $\sim cm$ lengths. These electron beams can then be converted by a high-Z target via bremsstrahlung into low-divergence ($<$ 20 $mrad$) beams of high-energy ($<$ 600 $MeV$) photons and subsequently into positrons via the Bethe-Heitler process. By increasing the material thickness and Z, the resulting $N_{e^+}/N_{e^-}$ ratio can approach unity, resulting in a near neutral density plasma jet. These quasi-neutral beams are presumed to retain the short-pulse ($\tau_L$ $<$ 40 $fs$) characteristic of the electron beam, resulting in a high peak density of $n_{e^-/e^+}\sim$ $10^{16}$ $cm^{-3}$, making the source an excellent candidate for laboratory study of astrophysical leptonic jets. Alternatively, the electron beam can be interacted with a counter-propagating, ultra-high intensity ($I > 10^{21}$ $W/cm^2$) laser pulse to undergo inverse Compton scattering and emit a high-peak brightness beam of high-energy photons. Preliminary results and experimental sensitivities of the electron-laser beam overlap are presented. The high-energy photon beams can be spectrally resolved using a forward Compton scattering spectrometer. Moreover, the photon flux can be characterized by a pixelated scintillator array and by nuclear activation and ($\gamma$,n) neutron measurements from the photons interacting with a secondary solid target. Monte-Carlo simulations were performed using FLUKA to support the yield estimates. [Preview Abstract] |
Tuesday, November 12, 2013 11:30AM - 12:00PM |
GI2.00005: Dense Plasma Focus Z-Pinch Fully Kinetic Modeling and Ion Probe-Beam Experiments Invited Speaker: Andrea Schmidt The Z-pinch phase of a dense plasma focus (DPF) emits multiple-MeV ions on a cm-scale length, even for kJ-scale devices. The mechanisms through which these physically simple devices generate such high energy beams in a relatively short distance are not fully understood. We are exploring the mechanisms behind these large gradients using the first fully kinetic simulations of a DPF Z-pinch as well as an ion probe beam experiment in which a 4 MeV deuteron beam is injected along the z-axis of a DPF Z-pinch plasma and accelerated. Our table-top DPF has demonstrated \textgreater~50 MV/m acceleration gradients during 800 J operation using a fast capacitive driver [1]. We have now directly measured the DPF gradients and demonstrated acceleration of an injected ion beam for the first time. Our particle-in-cell simulations have successfully predicted observed DPF ion beams and neutron yield, which past fluid simulations have not reproduced [2]. We have now experimentally measured and observed in the simulations for the first time, electric field oscillations near the lower hybrid frequency. This is suggestive that the lower hybrid drift instability, long speculated to be the cause of the anomalous plasma resistivity that produces large DPF gradients, is playing an important role. Direct comparisons between the experiment and simulations enhance our understanding of these plasmas and provide predictive design capability for accelerator and neutron source applications. This work 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 (11-ERD-063) at LLNL.\\[4pt] [1] J. Ellsworth et al., ``Design and Initial Results from a Kilojoule Level Dense Plasma Focus with Hollow Anode and Cylindrically Symmetric Gas Puff,'' RSI, 2012 (submitted).\\[0pt] [2] A. Schmidt et al., ``Fully Kinetic Simulations of Dense Plasma Focus Z-Pinch Devices,'' PRL, 109(20), 205003, 2012. [Preview Abstract] |
Tuesday, November 12, 2013 12:00PM - 12:30PM |
GI2.00006: Relativistically Induced Transparency Acceleration (RITA) - laser-plasma accelerated quasi-monoenergetic GeV ion-beams with existing lasers? Invited Speaker: Aakash A. Sahai Laser-plasma ion accelerators have the potential to produce beams with unprecedented characteristics of ultra-short bunch lengths (100s of fs) and high bunch-charge ($10^{10}$ particles) over acceleration length of about 100 microns. However, creating and controlling mono-energetic bunches while accelerating to high-energies has been a challenge. If high-energy mono-energetic beams can be demonstrated with minimal post-processing, laser ($\omega_0$)-plasma ($\omega_{pe}$) ion accelerators may be used in a wide-range of applications such as cancer hadron-therapy, medical isotope production, neutron generation, radiography and high-energy density science. Here we demonstrate using analysis and simulations that using relativistic intensity laser-pulses and heavy-ion ($M_i \times m_e$) targets doped with a proton (or light-ion) species ($m_p \times m_e$) of trace density (at least an order of magnitude below the cold critical density) we can scale up the energy of quasi-mono-energetically accelerated proton (or light-ion) beams while controlling their energy, charge and energy spectrum. This is achieved by controlling the laser propagation into an overdense ($\omega_0 < \omega_{pe}^{\gamma\simeq 1}$) increasing plasma density gradient by incrementally inducing relativistic electron quiver and thereby rendering them transparent to the laser while the heavy-ions are immobile. Ions do not directly interact with ultra-short laser that is much shorter in duration than their characteristic time-scale ($\tau_p\ll \sqrt{m_p}/\omega_0\ll\sqrt{M_i}/\omega_0$). For a rising laser intensity envelope, increasing relativistic quiver controls laser propagation beyond the cold critical density. For increasing plasma density ($\omega^2_{pe}(x)$), laser penetrates into higher density and is shielded, stopped and reflected where $\omega^2_{pe}(x)/\gamma(x,t)=\omega^2_0$. In addition to the laser quivering the electrons, it also ponderomotively drives ($F_p\propto\frac{1}{\gamma}\nabla_za^2$) them forward longitudinally, creating a constriction of snowplowed $e^-$s. The resulting longitudinal $e^-$-displacement from laser's push is controlled by the electrostatic space-charge pull by the immobile background ions. In the rest-frame of the laser, the electrostatic-potential that the ions create to balance the ponderomotive force on $e^-$s, scales as the effective vector potential, $a_{plasma}$. This potential hill, due to snowplowed $e^-$s, co-propagating with the rising laser can reflect protons and light-ions (Relativistically Induced Transparency Acceleration, RITA). Desired proton or light-ion energies can be achieved by controlling the velocity of the snowplow, which is shown to scale inversely with the rise-time of the laser (higher energies for shorter pulses) and directly with the scale-length of the plasma density gradient. Similar acceleration can be produced by controlling the increase of the laser frequency (Chirp Induced Transparency Acceleration, ChITA). References - ChITA - A. A. Sahai, T. C. Katsouleas, et. al., Proton acceleration by a relativistic laser frequency-chirp driven plasma snowplow, WEPPD059, Proceedings of IPAC 2012, May 2012, New Orleans, Louisiana, USA. RITA - A. A. Sahai, T. C. Katsouleas, et. al., Proton acceleration by trapping in a relativistic laser driven uphill plasma snowplow, MOP081, Proceedings of 2011 Particle Accelerator Conference, March 2011, New York, NY, USA. [Preview Abstract] |
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