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All-optical Time-resolved Measurement of Laser Energy Modulation in a Relativistic Electron Beam

Description: Hamiltonian light-front theory has been proposed as a promising method for solving bound states problems in quantum field theory a long time ago, see, e.g., the review article[1] for its various advantages compared to the traditional instant-form theories. Recently the Basis Light-Front Quantization (BLFQ) approach [2, 3] has been developed as a nonperturbative approach to solve Hamiltonian light-front quantum field theory. Numerical efficiency is a key advantage of this approach. The basic idea of BLFQ is to represent the theory in an optimal basis which respects many symmetries of the theory and thus minimizes the dimensionality of the Hamiltonian for a fixed precision. Specifically, the BLFQ approach employs a plane wave basis in the light-front longitudinal direction and a 2D harmonic oscillator basis in the transverse directions. In previous work [3] this approach has been applied to evaluate the anomalous magnetic moment of electrons which are confined in an external trap with an extrapolation to the zero trap limit. In this work we extend and improve this approach in several aspects including the direct evaluation of a free electron system. This article is organized as follows: In Sec. 2 we discuss the key extensions and improvements made in this work over Ref [3]; in Sec. 3 we present the numerical results for the electron anomalous magnetic moment evaluated in different harmonic oscillator bases and compare to the perturbation theory result. Finally we conclude and give an outline for future works in Sec. 4.
Date: February 15, 2012
Creator: Xiang, D.; Colby, E.; Dunning, M.; Gilevich, S.; Hast, C.; Jobe, K. et al.
Partner: UNT Libraries Government Documents Department

Evidence of High Harmonics from Echo-Enabled Harmonic Generation for Seeding X-ray Free Electron Lasers

Description: Echo-enabled harmonic generation free electron lasers hold great promise for the generation of fully coherent radiation in x-ray wavelengths. Here we report the first evidence of high harmonics from the echo-enabled harmonic generation technique in the realistic scenario where the laser energy modulation is comparable to the beam slice energy spread. In this experiment, coherent radiation at the seventh harmonic of the second seed laser is generated when the energy modulation amplitude is about 2-3 times the slice energy spread. The experiment confirms the underlying physics of echo-enabled harmonic generation and may have a strong impact on emerging seeded x-ray free electron lasers that are capable of generating laserlike x rays which will advance many areas of science.
Date: February 15, 2012
Creator: Xiang, D.; Colby, E.; Dunning, M.; Gilevich, S.; Hast, C.; Jobe, K. et al.
Partner: UNT Libraries Government Documents Department

Observation of Beam ION Instability in Spear3

Description: Weak vertical coupled bunch instability with oscillation amplitude at {mu}m level has been observed in SPEAR3. The instability becomes stronger when there is a vacuum pressure rise by partially turning off vacuum pumps and it becomes weaker when the vertical beam emittance is increased by turning off the skew quadrupole magnets. These confirmed that the instability was driven by ions in the vacuum. The threshold of the beam ion instability when running with a single bunch train is just under 200 mA. This paper presents the comprehensive observations of the beam ion instability in SPEAR3. The effects of vacuum pressure, beam current, beam filling pattern, chromaticity, beam emittance and bunch-by-bunch feedback are investigated in great detail. In an electron accelerator, ions generated from the residual gas molecules can be trapped by the beam. Then these trapped ions interact resonantly with the beam and cause beam instability and emittance blow-up. Most existing light sources use a long single bunch train filling pattern, followed by a long gap to avoid multi-turn ion trapping. However, such a gap does not preclude ions from accumulating during one passage of the single bunch train beam, and those ions can still cause a Fast Ion Instability (FII) as predicted by Raubenheimer and Zimmermann. FII has been observed in ALS, and PLS by artificially increasing the vacuum pressure by injecting helium gas into the vacuum chamber or by turning off the ion pumps in order to observe the beam ion instability. In some existing rings, for instance B factory, the beam ion instability was observed at the beginning of the machine operation after a long period of shutdown and then it automatically disappeared when the vacuum was better. However, when the beam emittance becomes smaller, the FII can occur at nominal conditions as observed in PLS, SOLEIL ...
Date: December 14, 2011
Creator: Teytelman, D.; Cai, Y.; Corbett, W. J.; Raubenheimer, T. O.; Safranek, J. A.; Schmerge, J. F. et al.
Partner: UNT Libraries Government Documents Department

Beam Dynamics Study of X-Band Linac Driven X-Ray FELS

Description: Several linac driven X-ray Free Electron Lasers (XFELs) are being developed to provide high brightness photon beams with very short, tunable wavelengths. In this paper, three XFEL configurations are proposed that achieve LCLS-like performance using X-band linac drivers. These linacs are more versatile, efficient and compact than ones using S-band or C-band rf technology. For each of the designs, the overall accelerator layout and the shaping of the bunch longitudinal phase space are described briefly. During the last 40 years, the photon wavelengths from linac driven FELs have been pushed shorter by increasing the electron beam energy and adopting shorter period undulators. Recently, the wavelengths have reached the X-ray range, with FLASH (Free-Electron Laser in Hamburg) and LCLS (Linac Coherent Light Source) successfully providing users with soft and hard X-rays, respectively. FLASH uses a 1.2 GeV L-band (1.3 GHz) superconducting linac driver and can deliver 10-70 fs FWHM long photon pulses in a wavelength range of 44 nm to 4.1 nm. LCLS uses the last third of the SLAC 3 km S-band (2.856 GHz) normal-conducting linac to produce 3.5 GeV to 15 GeV bunches to generate soft and hard X-rays with good spatial coherence at wavelengths from 2.2 nm to 0.12 nm. Newer XFELs (at Spring8 and PSI) use C-band (5.7 GHz) normal-conducting linac drivers, which can sustain higher acceleration gradients, and hence shorten the linac length, and are more efficient at converting rf energy to bunch energy. The X-band (11.4 GHz) rf technology developed for NLC/GLC offers even higher gradients and efficiencies, and the shorter rf wavelength allows more versatility in longitudinal bunch phase space compression and manipulation. In the following sections, three different configurations of X-band linac driven XFELs are described that operate from 6 to 14 GeV. The first (LOW CHARGE DESIGN) has an electron bunch charge ...
Date: December 13, 2011
Creator: Adolphsen, C.; Limborg-Deprey, C.; Raubenheimer, T.O.; Wu, J.; /SLAC; Sun, Y. et al.
Partner: UNT Libraries Government Documents Department

A Study of Single Pass Ion Effects at the ALS

Description: We report the results of experiments on a 'fast beam-ion instability' at the Advanced Light Source (ALS). This ion instability, which can arise even when the ions are not trapped over multiple beam passages, will likely be important for many future accelerators. In our experiments, we filled the ALS storage ring with helium gas, raising the pressure approximately two orders of magnitude above the nominal pressure. With gaps in the bunch train large enough to avoid conventional (multi-turn) ion trapping, we observed a factor of 2-3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. Ion trapping has long been recognized as a potential limitation in electron storage rings. The ions, generated by beam-gas collisions, become trapped in the negative potential of the beam and accumulate over multiple beam passages. The trapped ions are then observed to cause a number of deleterious effects such as an increasing beam phase space, a broadening and shifting of the beam transverse oscillation frequencies (tunes), collective beam instabilities, and beam lifetime reductions. All of these effects are of concern for the next generation of accelerators, such as the B-factories or damping rings for future linear colliders, which will store high beam currents with closely spaced bunches and ultra-low beam emittances. One of the standard solutions used to prevent ion trapping is to include a gap in the bunch train which is long compared to the bunch spacing. In this case, the ions are first strongly-focused by the passing electron bunches and then over-focused in the gap. With a sufficiently large gap, the ions can be driven to large amplitudes where they form a diffuse halo and do not affect the beam. In this paper, we describe experiments that study a new regime of transient ion instabilities ...
Date: September 13, 2011
Creator: Byrd, J.M.; Thomson, J.; /LBL, Berkeley; Chao, A.W.; Heifets, S.; Minty, M.G. et al.
Partner: UNT Libraries Government Documents Department

Observation of Magnetic Resonances in Electron Clouds in a Positron Storage Ring

Description: The first experimental observation of magnetic resonances in electron clouds is reported. The resonance was observed as a modulation in cloud intensity for uncoated as well as TiN-coated aluminum surfaces in the positron storage ring of the PEP-II collider at SLAC. Electron clouds frequently arise in accelerators of positively charged particles, and severely impact the machines performance. The TiN coating was found to be an effective remedy, reducing the cloud intensity by three orders of magnitude.
Date: August 24, 2011
Creator: Pivi, M.T.F.; Ng, J.S.T.; Cooper, F.; Kharakh, D.; King, F.; Kirby, R.E. et al.
Partner: UNT Libraries Government Documents Department

Suppression of Beam-Ion Instability in Electron Rings with Multi-Bunch Train Beam Fillings

Description: The ion-caused beam instability in the future light sources and electron damping rings can be serious due to the high beam current and ultra-small emittance of picometer level. One simple and effective mitigation of the instability is a multi-bunch train beam filling pattern which can significantly reduce the ion density near the beam, and therefore reduce the instability growth rate up to two orders of magnitude. The suppression is more effective for high intensity beams with low emittance. The distribution and the field of trapped ions are benchmarked to validate the model used in the paper. The wake field of ion-cloud and the beam-ion instability is investigated both analytically and numerically. We derived a simple formula for the build-up of ion-cloud and instability growth rate with the multi-bunch-train filling pattern. The ion instabilities in ILC damping ring, SuperKEKB and SPEAR3 are used to compare with our analyses. The analyses in this paper agree well with simulations.
Date: August 18, 2011
Creator: Wang, L.; Cai, Y.; Raubenheimer, T.O.; /SLAC; Fukuma, H. & /KEK, Tsukuba
Partner: UNT Libraries Government Documents Department

Commissioning the Echo-Seeding Experiment Echo-7 at SLAC

Description: ECHO-7 is a proof-of-principle echo-enabled harmonic generation (EEHG) FEL experiment in the Next Linear Collider Test Accelerator (NLCTA) at SLAC. The experiment is intended to test the EEHG principle at low electron beam energy, 120 MeV, and determine the sensitivities and limitations to understand the expected performance at the higher energy scales and harmonic numbers required for x-ray FELs. In this paper we present the experimental results from the commissioning run of the completed experimental setup which started in April 2010.
Date: June 2, 2011
Creator: Dunning, M.; Weathersby, S.; Colby, E.; Gilevich, S.; Hast, C.; Jobe, K. et al.
Partner: UNT Libraries Government Documents Department

FACET Emittance Growth

Description: FACET, the Facility for Advanced Accelerator and Experimental Tests, is a new facility being constructed in sector 20 of the SLAC linac primarily to study beam driven plasma wakefield acceleration. The FACET beamline consists of a chicane and final focus system to compress the 23 GeV, 3.2 nC electron bunches to {approx}20 {micro}m long and {approx}10 {micro}m wide. Simulations of the FACET beamline indicate the short-duration and large, 1.5% rms energy spread beams may suffer a factor of four emittance growth from a combination of chromaticity, incoherent synchrotron radiation (ISR), and coherent synchrotron radiation (CSR). Emittance growth is directly correlated to head erosion in plasma wakefield acceleration and is a limiting factor in single stage performance. Studies of the geometric, CSR, and ISR components are presented. Numerical calculation of the rms emittance can be overwhelmed by long tails in the simulated phase space distributions; more useful definitions of emittance are given. A complete simulation of the beamline is presented as well, which agrees with design specifications.
Date: April 5, 2011
Creator: Frederico, J; Hogan, M.J.; Nosochkov, Y.; Litos, M.D.; Raubenheimer, T. & /SLAC
Partner: UNT Libraries Government Documents Department

Experimental Measurements of the Secondary Electron Yield in the Experimental Measurement of the Secondary Electron Yield in the PEP-II Particle Accelerator Beam Line

Description: Beam instability caused by the electron cloud has been observed in positron and proton storage rings and it is expected to be a limiting factor in the performance of the positron Damping Ring (DR) of future Linear Colliders (LC) such as ILC and CLIC. To test a series of promising possible electron cloud mitigation techniques as surface coatings and grooves, in the Positron Low Energy Ring (LER) of the PEP-II accelerator, we have installed several test vacuum chambers including (i) a special chamber to monitor the variation of the secondary electron yield of technical surface materials and coatings under the effect of ion, electron and photon conditioning in situ in the beam line; (ii) chambers with grooves in a straight magnetic-free section; and (iii) coated chambers in a dedicated newly installed 4-magnet chicane to study mitigations in a magnetic field region. In this paper, we describe the ongoing R&D effort to mitigate the electron cloud effect for the LC damping ring, focusing on the first experimental area and on results of the reduction of the secondary electron yield due to in situ conditioning.
Date: August 25, 2010
Creator: Pivi, M.T.F.; Collet, G.; King, F.; Kirby, R.E.; Markiewicz, T.; Raubenheimer, T.O. et al.
Partner: UNT Libraries Government Documents Department

First Demonstration of the Echo-Enabled Harmonic Generation Technique for Short-Wavelength Seeded Free Electron Lasers

Description: We report the first experimental demonstration of the echo-enabled harmonic generation (EEHG) technique which holds great promise for generation of high power, fully coherent short-wavelength radiation. In this experiment, coherent radiation at the 3rd and 4th harmonic of the second seed laser is generated from the so-called beam echo effect. The experiment confirms the physics behind this technique and paves the way for applying the EEHG technique for seeded x-ray free electron lasers.
Date: August 25, 2010
Creator: Xiang, D.; Colby, E.; Dunning, M.; Gilevich, S.; Hast, C.; Jobe, K. et al.
Partner: UNT Libraries Government Documents Department

Preliminary Results of the Echo-Seeding Experiment ECHO-7 at SLAC

Description: ECHO-7 is a proof-of-principle echo-enabled harmonic generation FEL experiment in the Next Linear Collider Test Accelerator (NLCTA) at SLAC. The experiment aims to generate coherent radiation at 318 nm and 227 nm, which are the 5th and 7th harmonic of the infrared seed laser. In this paper we present the preliminary results from the commissioning run of the completed experimental setup which started in April 2010.
Date: June 15, 2010
Creator: Xiang, D.; Colby, E.; Ding, Y.; Dunning, M.; Frederico, J.; Gilevich, S. et al.
Partner: UNT Libraries Government Documents Department

Preliminary results of the echo-seeding experiment at SLAC

Description: ECHO-7 is a proof-of-principle echo-enabled harmonic generation FEL experiment in the Next Linear Collider Test Accelerator (NLCTA) at SLAC. The experiment aims to generate coherent radiation at 318 nm and 227 nm, which are the 5th and 7th harmonic of the infrared seed laser. In this paper we present the preliminary results from the commissioning run of the completed experimental setup which started in April 2010.
Date: May 23, 2010
Creator: Xiang, D.; Colby, E.; Ding, Y.; Dunning, M.; Frederico, J.; Gilevich, S. et al.
Partner: UNT Libraries Government Documents Department