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| TPAE038 | Particle-in-Cell Simulation of LWFA Using 50 fs Pulses in Guided and Unguided Plasmas | |
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Funding: Work supported by DOE and NSF. In 2004, we reported on 3D simulation results that using a modest laser, it was possible to generate a ~250 MeV monoenergetic beam with .5 nC of charge and to generate a few .8GeV electrons (Tsung et al, Phys. Rev. Lett., 93, 185002). We found that the self-injected electrons originated only after the laser distorted from a combination of photon deceleration and longitudinal group velocity dispersion and these electrons originated from the edge of the laser. We also observed that the mono-energetic nature arose due to phase space rotation and beam loading. In the September, 30, 2004 issue of Nature, many experimental groups have reported the observation of mono-energetic beams of electrons in the range of 100 MeV. These experiments have been performed for a range of plasma parameters. We have begun to systematically study (in 2 and 3D) the acceleration mechanisms for plasma conditions under which these experiments operated to verify that what we observed in our simulations is universal. Our 3D simulation of the experiment by Mangles et al produced excellent agreement in electron energy spectrum and we have begun to look at the other two experiments reported in Nature. |
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| TPAE040 | Nonlinear Theory in the Blowout Regime for Both Particle Beam and Laser Drivers | |
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Funding: DOE and NSF. Recent progress in both PWFA and LWFA has confirmed the promising characteristics of the blowout regime. So it is worthwhile to understand the wake excitation process and other relevant issues (e.g. self trapping mechanism, laser self guiding, scaling laws) in this regime. Because the plasma electrons always cross each other in the blowout regime, it is not possible to use a fluid model. Instead, we use a particle picture. Based on the analysis of the innermost particle trajectory, we have developed a self-consistent theory for this regime. For particle beam drivers, we explained why linear theory can be a good approximation in the weakly nonlinear blowout regime and also obtained formulas to predict the wake amplitude. In the strongly nonlinear blowout regime (relativistic or ultra-relativistic), the theory can predict the wake structures and amplitudes in terms of the particle beam or laser pulse intensity. The theory also provides a basis for a beam loading theory in the blowout regime as well a basis for finding optimum driver profile. We will also give some results on the energy gain and total charge scalings based on this theory. |
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| TPAE045 | Is it Possible To Generate nC, Mono-Energetic Electron Beams at 1GeV and Beyond Using Existing or Near Term Lasers via LWFA? | |
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Funding: Work supported by DOE and NSF. Recently, several groups around the world observed mono-energetic beams between 80 MeV and 170 MeV using ~15 TW lasers. We have begun a comprehensive study of the acceleration of electrons by the interaction of ultra-intense short and ultra-short laser pulses with underdense plasma. We concentrate our parameter space to existing and near term laser parameters, i.e., laser parameters between 30 and 100 TW. We use 3D particle in cell simulations using the code OSIRIS. The goal is to show that the generation of mono-energetic beams with energy beyond 1GeV with current and near future laser systems is possible without the need for any external injection. In this ultra-relativistic regime the laser blows out all the electrons forming an almost spherical cavity. Some electrons are self-injected in the blowout region and accelerated by the laser wakefield to ultrahigh energy. In order to maximize the energy gain, the beam charge and quality, we need an improved understanding of the wakefield generation as well as of processes such as self-injection and beam loading. We will provide theoretical estimates and verify their validity with 3D simulations. We will address possible limitations of particle acceleration in this regime. Tsung et al., Phys. Rev. Lett., 93, 185002. S.P.D. Mangles et al. Nature 431, 535 (2004). C.G.R. Geddes et al. Nature 431, 538 (2004). J. Faure et al. Nature 431, 541 (2004). A. Pukhov and J. Meyer-ter-vehn, Appl.Phys.B, 74, 355 (2002). |
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| TPAE024 | Determination of Longitudinal Phase Space in SLAC Main Accelerator Beams | 1856 |
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| In the E164 Experiment at that Stanford Linear Accelerator Center (SLAC), we seek to drive plasma wakes for electron acceleration using 28.5 GeV bunches from the main accelerator. These bunches can now be made with an RMS length of less than 20 microns, and direct measurement is not feasible. Instead, we use an indirect technique, measuring the energy spectrum at the end of the linac and comparing with detailed simulations of the entire machine. We simulate with LiTrack, a 2D code developed at SLAC which includes wakefields, synchrotron radiation and all second order optical aberrations. Understanding the longitudinal profile allows a better understanding of acceleration in the plasma wake, as well as investigation of possible destructive transverse effects. We present results from the July 2004 experimental run and show how this technique aids in data analysis. We also discuss accuracy and validation of phase space determinations. | ||
| TPAE025 | Field Ionization of Neutral Lithium Vapor using a 28.5 GeV Electron Beam | 1904 |
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| The E164/E164X plasma wakefield experiment studies beam-plasma interactions at the Stanford Linear Acceleration Center (SLAC). Due to SLAC recent ability to variably compress bunches longitudinally from 650 microns down to 20 microns, the incoming beam is sufficiently dense to field ionize the neutral Lithium vapor. The field ionization effects are characterized by the beam’s energy loss through the Lithium vapor column. Experimental results are presented. | ||
| TPAE041 | Modeling TeV Class Plasma Afterburners | 2666 |
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Funding: Work supported by DOE and NSF. Plasma wakefield acceleration can sustain acceleration gradients three orders of magnitude larger than conventional RF accelerator. In the recent E164X experiment, substantial energy gain of about 3Gev has been observed. Thus, a plasma afterburner, which has been proposed to double the incoming beam energy for a future linear collider, is now of great interest. In an afterburner, a particle beam drives a plasma wave and generates a strong wakefield which has a phase velocity equal to the velocity of the beam. This wakefield can then be used to accelerate part of the drive beam or a trailing beam. Several issues such as the efficient transfer of energy and the stable propagation of both the drive and trailing beams in the plasma are critical to the afterburner concept. We investigate the nonlinear beam-plasma interaction in such scenario using the 3D computer modeling code QuickPIC. We will report the latest simulation results of both 50 GeV and 1 TeV plasma afterburner stages for electrons including the beam-loading of a trailing beam. Analytic analysis of hosing instability in this regime will be presented. |
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| TPAE042 | Beam Matching to a Plasma Wake Field Accelerator Using a Ramped Density Profile at the Plasma Boundary | 2702 |
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Funding: DOE Grant No. DE-FG03-92ER40727. An important aspect of plasma wake field accelerators (PWFA) is stable propagation of the drive beam. In the under dense regime, the drive beam creates an ion channel which acts on the beam as a strong thick focusing lens. The ion channel causes the beam to undergo multiple betatron oscillations along the length of the plasma. There are several advantages if the beam size can be matched to a constant radius. First, simulations have shown that instabilities such as hosing are reduced when the beam is matched. Second, synchrotron radiation losses are minimized when the beam is matched. Third, an initially matched beam will propagate with no significant change in beam size in spite of large energy loss or gain. Coupling to the plasma with a matched radius can be difficult in some cases. This paper shows how an appropriate density ramp at the plasma entrance can be useful for achieving a matched beam. Additionally, the density ramp is helpful in bringing a misaligned trailing beam onto the drive beam axis. A plasma source with boundary profiles useful for matching has been created for the PWFA experiments at SLAC. |
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| TPAE046 | Modeling Self-Ionized Plasma Wakefield Acceleration for Afterburner Parameters Using QuickPIC | 2905 |
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Funding: DOE A plasma wakefield accelerator (PWFA) has been proposed as a way to double the energy of a future linear collider. This afterburner concept will require meter long uniform plasmas. For the parameters envisaged in possible afterburner stages, the self-fields of the particle beam are intense enough to tunnel ionize some neutral gases such as lithium. Tunnel ionization has been investigated as a way for the beam itself to create the plasma.* Furthermore, tunnel ionization in a neutral or partially pre-ionized gas may create new plasma electrons and alter the plasma wake.*,** Unfortunately, it is not possible to model a PWFA with afterburner parameters using the models described in Bruhwiler et al. and Deng et al. Here we describe the addition of a tunnel ionization package using the ADK model into QuickPIC, a highly efficient quasi-static particle in cell (PIC) code which can model a PWFA with afterburner parameters. There is excellent agreement between QuickPIC and OSIRIS(a full PIC code) for pre-ionized plasmas. Effects of self-ionization on hosing instability –one of the most critical issues to overcome to make an afterburner a reality – for a bunch propagating in a plasma hundreds of betatron oscillations long will be discussed. *D. L. Bruhwiler et al., Phys. Plasmas 10 (2003), p. 2022. **S. Deng et al., Phys. Rev. E, 68, 047401 (2003). |
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| TPAE058 | Plasma Dark Current in Self-ionized Plasma Wake Field Accelerators (PWFA) | 3444 |
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| Particle trapping is investigated with experiment, theory and simulations for conditions relevant to beam driven Plasma Wake Field Accelerators. Such trapping produces plasma dark current when the wakefield aplitude is above a threshold values and may place a limit on the maximum acceleration gradient in a PWFA. Trapping and dark current are enhanced when in an ionizing plasma, that is self-ionized by the beam as well as in gradual density gradients. In the E164X conducted at the Stanford Linear Accelerator Center by a collaboration of USC, UCLA and SLAC, evidence of trapping has been observed. Here we present experimental results and a simplified analytical model of the particle trapping threshold which is compared to simulations done with an object oriented fully parallel 3-D PIC code OSIRIS. | ||
| TOPA002 | Review of Beam-Driven Plasma Wakefield Experiments at SLAC | |
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Funding: Department of Energy contracts DE-AC02-76SF00515 (SLAC), DE-FG03-92ER40745, DE-FG03-98DP00211, DE-FG03-92ER40727, DE-AC-0376SF0098, and National Science Foundation grants No. ECS-9632735, DMS-9722121 and PHY-0078715. In the plasma wakefield accelerator, a short relativistic-electron bunch drives a large amplitude plasma wave or wake. In experiment E-164X, we use the 28.5 GeV, ultra-short (?80 femtosecond), high peak-current (?30 kiloamperes) bunch now available at the Stanford Linear Accelerator Center Final Focus Test Beam facility. The head of this bunch fieldionizes a lithium vapor and excites the wake, and the tail samples the accelerating field. The latter is accomplished by setting the plasma density to match the plasma wavelength to the bunch length. After the plasma, the bunch is dispersed in energy by an imaging magnetic-spectrometer. Preliminary analysis shows that gradients in excess of 15 GeV/m are excited over a plasma length of approximately 10 cm, leading to energy gain on the order of of 1.5 GeV, or about an order of magnitude larger than energy gains reported to date. This gradient is also three orders of magnitude larger than that in the three-kilometer long Stanford linear accelerator that produces the incoming beam. These results are obtained in a new regime for beam-driven plasma accelerators in which the electron bunch creates its own plasma. The current status of the experiment as well as future plans will be discussed. |
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| RPAE019 | Positron Source from Betatron X-Rays Emitted in a Plasma Wiggler | 1625 |
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| In the E-167 plasma wakefield accelerator (PWFA) experiments in the Final Focus Test Beam (FFTB) at the Stanford Linear Accelerator Center (SLAC), an ultra-short, 28.5 GeV electron beam field ionizes a neutral column of Lithium vapor. In the underdense regime, all plasma electrons are expelled creating an ion column. The beam electrons undergo multiple betatron oscillations leading to a large flux of broadband synchrotron radiation. With a plasma density of 3x1017 cm-3, the effective focusing gradient is near 9 MT/m with critical photon energies exceeding 50 MeV for on-axis radiation. A positron source is the initial application being explored for these X-rays, as photo-production of positrons eliminates many of the thermal stress and shock wave issues associated with traditional Bremsstrahlung sources. Photo-production of positrons has been well-studied; however, the brightness of plasma X-ray sources provides certain advantages. In this paper, we present results of the simulated radiation spectra for the E-167 experiments, and compute the expected positron yield. | ||
| RPAT079 | Resolution of Transverse Electron Beam Measurements Using Optical Transition Radiation | 4042 |
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Funding: Work supported by Department of Energy contracts DE-AC02-76SF00515 (SLAC), DE-FG03-92ER40745, DE-FG03-98DP00211, DE-FG03-92ER40727, DE-AC-0376SF0098, and National Science Foundation grants No. ECS-9632735, DMS-9722121 and PHY-0078715. In the plasma wakefield acceleration experiment E-167, optical transition radiation is used to measure the transverse profile of the electron bunches before and after the plasma acceleration. The distribution of the electric field from a single electron does not give a point-like distribution on the detector, but has a certain extension. Additionally, the resolution of the imaging system is affected by aberrations. The transverse profile of the bunch is thus convolved with a point spread function (PSF). Algorithms that deconvolve the image can help to improve the resolution. Imaged test patterns are used to determine the modulation transfer function of the lens. From this, the PSF can be reconstructed. The Lucy-Richardson algorithm is used to deconvolute this PSF from test images. |
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| RPAT078 | Bunch Length Measurements Using Coherent Radiation | 4027 |
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Funding: Work supported by Department of Energy contracts DE-AC02-76SF00515 (SLAC), DE-FG03-92ER40745, DE-FG03-98DP00211, DE-FG03-92ER40727, DE-AC-0376SF0098, and National Science Foundation grants No. ECS-9632735, DMS-9722121 and PHY-0078715. The accelerating field that can be obtained in a beam-driven plasma wakefield accelerator depends on the current of the electron beam that excites the wake. In the E-167 experiment, a peak current above 10kA will be delivered at a particle energy of 28GeV. The bunch has a length of a few ten micrometers and several methods are used to measure its longitudinal profile. Among these, autocorrelation of coherent transition radiation (CTR) is employed. The beam passes a thin metallic foil, where it emits transition radiation. For wavelengths greater than the bunch length, this transition radiation is emitted coherently. This amplifies the long-wavelength part of the spectrum. A scanning Michelson interferometer is used to autocorrelate the CTR. However, this method requires the contribution of many bunches to build an autocorrelation trace. The measurement is influenced by the transmission characteristics of the vacuum window and beam splitter. We present here an analysis of materials, as well as possible layouts for a single shot CTR autocorrelator. |