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To develop plasma wakefield acceleration into a compact and affordable replacement for conventional accelerators, beams of charged particles must be accelerated at high efficiency in a high electric field; here this is demonstrated for a bunch of charged electrons ‘surfing’ on a previously excited plasma wave. Particle colliders that operate at the high-energy frontier using electric fields generated by radio waves are approaching the limits of feasibility in terms of size and cost, but there are other acceleration techniques that could make less expensive and more compact devices. The plasma wakefield accelerator, in which an electron bunch is accelerated by making it 'surf' on a plasma wave excited by another electron bunch, promises an energy gain in the gigaelectron-volt regime over just a few centimetres — an energy gain that requires hundreds of metres using traditional accelerators. Previously, this technique had only been used to accelerate a very small number of electrons at a time. Now, researchers working at FACET, the Facility for Advanced Accelerator Experimental Tests at SLAC National Accelerator Laboratory, USA, have achieved acceleration of about half a billion electrons at once with an unprecedented efficiency for a plasma accelerator. This achievement could be a milestone in the development of affordable and compact accelerators for applications ranging from high energy physics to medical and industrial applications. High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept1,2,3 being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma4,5,6. If a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy. Previous experiments using just a single 42-gigaelectronvolt drive bunch have accelerated electrons with a continuous energy spectrum and a maximum energy of up to 85 gigaelectronvolts from the tail of the same bunch in less than a metre of plasma7. However, the total charge of these accelerated electrons was insufficient to extract a substantial amount of energy from the wake. Here we report high-efficiency acceleration of a discrete trailing bunch of electrons that contains sufficient charge to extract a substantial amount of energy from the high-gradient, nonlinear plasma wakefield accelerator. Specifically, we show the acceleration of about 74 picocoulombs of charge contained in the core of the trailing bunch in an accelerating gradient of about 4.4 gigavolts per metre. These core particles gain about 1.6 gigaelectronvolts of energy per particle, with a final energy spread as low as 0.7 per cent (2.0 per cent on average), and an energy-transfer efficiency from the wake to the bunch that can exceed 30 per cent (17.7 per cent on average). This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology.

The recent experimental progress in plasma wakefield accelerator (PWFA) experiments [I. Blumenfeld et al., Nature 445, 741 (15 February 2007)] make it possible to contemplate the application of the PWFA to a future electron positron linear collider. One of the main parameter of such a collider is the luminosity. The luminosity determines the number of occurrence of a given process with a given cross section, per unit time. For example, current designs call for luminosities in the1034 cm−2s−1. To reach such luminosities, particle beams with average powers in the 10 MW range are necessary. In a plasma wakefield accelerator, a plasma works as an energy transformer to convert a relatively high-current, low-energy bunch train to a proportionately lower-current, higher-energy bunch train. This transformation is continued until the desired particle energy is reached to do physics. Microwave powered structures have a demonstrated ability to produce bunch trains with parameters of interest to the PWFA with high efficiency. Such structures operate most efficiently when the bunches are spaced tightly together, e.g. by a few ns. Consequently, the plasma source must be stable and reproducible on the ns time scale. Assuming an energy transfer efficiency from a PWFA drive bunch to the accelerated witness bunch of 35% implies that about 6.5 MW of beam power is left in the plasma. For a beam with an initial energy of 25 GeV, a final energy of 250 GeV and an accelerating gradient of 25 GeV/m, the total plasma length is 9 m, corresponding to an energy deposition of ≈720 kW/m. The energy will be deposited in a very small volume of plasma, typically a radius of about 100 µm for densities of interest. Therefore, while heat evacuation at a large radius may not be a serious issue, thermal shock waves can be expected. These waves will modify the gas and plasma density for the following bunches. The beam and plasma source parameters will be presented, and the main issues with the plasma source discussed.

Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. They rely on the excitation of a plasma wave in the wake of a drive beam. To generate the plasma, a neutral gas can be field-ionized by the head of the drive beam, in which case the distance of acceleration and energy gain can be strongly limited by head erosion. Here we overcome this limit and demonstrate that electrons in the tail of a drive beam can be accelerated by up to 27 GeV in a high-ionization-potential gas (argon), boosting their initial 20.35 GeV energy by 130%. Particle-in-cell simulations show that the argon plasma is sustaining very high electric fields, of ∼150 GV m−1, over ∼20 cm. The results open new possibilities for the design of particle beam drivers and plasma sources., Plasma accelerators driven by particle beams are a promising technology, but the acceleration distance and energy gain are strongly limited by head erosion in a high-ionization-potential gas. Here the authors observe up to 130% energy boost in a self-focused electron beam, with limited head erosion.

We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here we report on characteristics these three unusual CR events, which develop nearly horizontally, 20-30~km above the surface of the Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible $\tau$-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the Standard Model ${\tau}$-neutrino cross section., Comment: 5pp,4 figures, referees' comments addressed, this version accepted to Phys. Rev. Letters. Supplemental PDF material available; see source files

Lasing in a hard-X-ray free-electron laser is typically seeded from noise due to the self-amplification of spontaneous emission, which limits temporal coherence and spectral characteristics. Researchers now demonstrate self-seeding using X-rays from the first half of the magnetic undulator to seed the second half via a diamond-based monochromator at angstrom wavelengths.

Localized injection of electrons within a relativistic plasma wake can potentially produce an ultrashort, monoenergetic electron bunch. Recent experiments at the FACET facility at SLAC explored the injection of helium electrons at the helium-lithium interface of a lithium heat pipe oven and the subsequent acceleration in the beam-produced plasma wake. Electrons accelerated to over 10 GeV in 30 cm of plasma were observed as a distinct charge bunch.

We have investigated the viability of using plasmas formed by ionization of high Z, low ionization potential element rubidium (Rb) for beam-driven plasma wakefield acceleration. The Rb vapor column confined by argon (Ar) buffer gas was used to reduce the expected limitation on the beam propagation length due to head erosion that was observed previously when a lower Z but higher ionization potential lithium vapor was used. However, injection of electrons into the wakefield due to ionization of Ar buffer gas and nonuniform ionization of ${\mathrm{Rb}}^{1+}$ to ${\mathrm{Rb}}^{2+}$ was a possible concern. In this paper we describe experimental results and the supporting simulations which indicate that such ionization of Ar and ${\mathrm{Rb}}^{1+}$ in the presence of combined fields of the beam and the wakefield inside the wake does indeed occur. Some of this charge accumulates in the accelerating region of the wake leading to the reduction of the electric field---an effect known as beam loading. The beam-loading effect is quantified by determining the average transformer ratio $⟨R⟩$ which is the maximum energy gained divided by the maximum energy lost by the electrons in the bunch used to produce the wake. $⟨R⟩$ is shown to depend on the propagation length and the quantity of the accumulated charge, indicating that the distributed injection of secondary Rb electrons is the main cause of beam loading in this experiment. The average transformer ratio is reduced from 1.5 to less than 1 as the excess charge from secondary ionization increased from 100 to 700 pC. The simulations show that while the decelerating field remains constant, the accelerating field is reduced from its unloaded value of 82 to $46\text{ }\text{ }\mathrm{GeV}/\mathrm{m}$ due to this distributed injection of dark current into the wake.

Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron-positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas. Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered--'self-loaded'--so that about a billion positrons gain five gigaelectronvolts of energy with a narrow energy spread over a distance of just 1.3 metres. They extract about 30 per cent of the wake's energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8 per cent. This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the PWFA scheme very attractive as an energy booster to an electron-positron collider.

The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000–5000, wavelength stability of 10$^{−4}$, and an increase in peak brightness by a factor of 2–5 across the photon energy range of 500–1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

In the E-157 experiment now being conducted at the Stanford Linear Accelerator Center, a 30 GeV electron beam of 2×1010 electrons in a 0.65-mm-long bunch is propagated through a 1.4-m-long lithium plasma of density up to 2×1014 e−/cm3. The initial beam density is greater than the plasma density, and the head of the bunch expels the plasma electrons leaving behind a uniform ion channel with transverse focusing fields of up to several thousand tesla per meter. The initial transverse beam size with σ=50–100 μm is larger than the matched size of 5 μm resulting in up to three beam envelope oscillations within the plasma. Time integrated optical transition radiation is used to study the transverse beam profile immediately before and after the plasma and to characterize the transverse beam dynamics as a function of plasma density. The head of the bunch deposits energy into plasma wakes, resulting in longitudinal accelerating fields which are witnessed by the tail of the same bunch. A time-resolved Cherenkov imag...

Acceleration of relativistic electrons in a dielectric laser accelerator at high electric field gradients is reported, setting the stage for the development of future multi-staged accelerators of this type. Conventional particle accelerators, based on radio-frequency technology, are large-scale installations that are expensive to run. Micro-fabricated dielectric laser accelerators (DLAs) offer an attractive alternative, as they are able to support much larger accelerating fields than current accelerators, while being compact, economical and simple to manufacture using lithographic techniques. This paper presents the first experimental demonstration of a DLA capable of sustained, high-gradient (beyond 250 MeV m−1) acceleration of relativistic electrons. The results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems, which would enable compact table-top MeV–GeV-scale accelerators. Applications include security scanners and medical therapy, X-ray light sources for biological and materials research, and portable medical imaging devices. The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today’s accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently1,2,3,4, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m−1) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10–30 MeV m−1, and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m−1 (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV–GeV (106–109 eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (1012 eV) scale.

Head erosion is one of the limiting factors in plasma wakefield acceleration (PWFA). We present a study of head erosion with emittance growth in field-ionized plasma from the PWFA experiments performed at the FACET user facility at SLAC. At FACET, a 20.3 GeV bunch with 1.8×1010 electrons is optimized in beam transverse size and combined with a high density lithium plasma for beam-driven plasma wakefield acceleration experiments. A target foil is inserted upstream of the plasma source to increase the bunch emittance through multiple scattering. Its effect on beamplasma interaction is observed with an energy spectrometer after a vertical bend magnet. Results from the first experiments show that increasing the emittance has suppressed vapor field-ionization and plasma wakefields excitation. Plans for the future are presented.

We report generation of density modulation at terahertz (THz) frequencies in a relativistic electron beam through laser modulation of the beam longitudinal phase space. We show that by modulating the energy distribution of the beam with two lasers, density modulation at the difference frequency of the two lasers can be generated after the beam passes through a chicane. In this experiment, density modulation around 10 THz was generated by down-converting the frequencies of an 800 nm laser and a 1550 nm laser. The central frequency of the density modulation can be tuned by varying the laser wavelengths, beam energy chirp, or momentum compaction of the chicane. This technique can be applied to accelerator-based light sources for generation of coherent THz radiation and marks a significant advance toward tunable narrow band THz sources.

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.

The southern Pierre Auger Observatory has been detecting cosmic rays above 1018eV since 2004, exploiting a hybrid air shower detection technique, with 1660 water Cherenkov detectors together with 24 air fluorescence telescopes on a 3000 km2 site. As low energy enhancements to the observatory, 3 additional telescopes with elevated fields of view were built (HEAT). The detector density was increased in the HEAT fields of view by a factor of four in an area of about 25 km2. This setup enables unbiased hybrid data taking above 1017eV. The infilled area is also being equipped with large underground scintillator muon detectors (AMIGA). Finally, a prototype array of radio antenna stations (AERA), working from 30 to 80 MHz, has been installed in a part of the infill. Properties and status of AMIGA, HEAT, and AERA are presented.

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.

The scaling of the two important figures of merit, the transformer ratio $T$ and the longitudinal electric field ${E}_{z}$, with the peak drive-bunch current ${I}_{p}$, in a nonlinear plasma wakefield accelerator is presented for the first time. The longitudinal field scales as ${I}_{P}^{0.623\ifmmode\pm\else\textpm\fi{}0.007}$, in good agreement with nonlinear wakefield theory ($\ensuremath{\sim}{I}_{P}^{0.5}$), while the unloaded transformer ratio is shown to be greater than unity and scales weakly with the bunch current. The effect of bunch head erosion on both parameters is also discussed.

We report the first experimental demonstration of the echo-enabled harmonic generation 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 harmonics 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 echo-enabled harmonic generation technique for seeded x-ray free electron lasers. © 2010 The American Physical Society.

Short, relativistic particle bunches propagating in plasmas drive large amplitude wakefields. These wakefields have longitudinal, accelerating and decelerating components, as well as transverse, focusing and defocusing components. When the particle bunch length is comparable to the electron plasma wavelength of the relativistic plasma wake, only plasma electrons respond to the bunch fields, and the plasma ions are immobile. Plasma electrons are expelled from an electron bunch, while they are attracted toward a positron bunch. This asymmetry leads to important differences in the focusing of these bunches in the plasma. In particular, in the case of bunches with densities larger than the plasma density, an electron bunch creates a pure, uniform density ion column with focusing properties free of geometric aberrations. Therefore, the incoming emittance of electron bunch with a narrow energy spread can be preserved upon acceleration by the plasma wake. On the contrary, the plasma exerts a non-uniform focusing force on a positron bunch. The focusing force is dependent on the incoming bunch parameters, and the plasma electron density inside the positron bunch can exceed the bunch density. As a result, the positron bunch develops a charge halo, and its emittance grows along the plasma. These differences are observed in experiments as well as in numerical simulations, and could have important implications for the applications of plasma-based acceleration to a future electron/positron linear collider. Experimental and simulation results will be presented.

Multi-GeV trapped electron bunches in a plasma wakefield accelerator (PWFA) are observed with normalized transverse emittance divided by peak current, {epsilon}{sub N,x}/I{sub t}, below the level of 0.2 {micro}m/kA. A theoretical model of the trapped electron emittance, developed here, indicates that emittance scales inversely with the square root of the plasma density in the nonlinear 'bubble' regime of the PWFA. This model and simulations indicate that the observed values of {epsilon}{sub N,x}/I{sub t} result from multi-GeV trapped electron bunches with emittances of a few {micro}m and multi-kA peak currents.

Recent electron beam driven plasma wakefield accelerator experiments carried out at SLAC showed trapping of plasma electrons. These trapped electrons appeared on an energy spectrometer with smaller transverse size than the beam driving the wake. A connection is made between transverse size and emittance; due to the spectrometer's resolution, this connection allows for placing an upper limit on the trapped electron emittance. The upper limit for the lowest normalized emittance measured in the experiment is 1 mm-mrad.

This paper presents the initial results in laser electron acceleration from the newly commissioned E163 program at SLAC and an outline of the initial wave of experiments. These results include an inverse free electron laser (IFEL) interaction with 800 nm light from a Ti: sapphire laser which will be used along with a chicane to produce optically spaced electron microbunches. The microbunching is independently diagnosed via coherent optical transition radiation (COTR) at the second harmonic (400 nm) in order to avoid large background at the fundamental due to the laser. We will also discuss experiments that take the microbunch train formed by the IFEL/chicane and perform net acceleration using a second stage: an inverse transition radiation accelerator (ITR). This discussion includes the experiment layout and hardware as well as simulation of expected results.

We report on a recent set of measurements of the transverse wakefields from longitudinally tapered collimators. The measurements were performed with a low-emittance 1.19 GeV beam in the SLAC linac by inserting a collimator aperture into the beam path and reconstructing the vertical deflection of the beam as a function of the vertical position of the aperture. Each collimator in the experiment was designed to present a relatively large transverse impedance and to minimize the impedance from other contributions such as resistivity. In addition, the collimator parameters were chosen to provide some insight into the scaling of the transverse geometric wakefield as a function of the collimator’s geometry. A description of the experimental apparatus and the aperture design, the method of data collection and analysis, and a comparison to theoretical and numerical predictions are presented.

Evidence of particle trapping has been observed in a beam driven Plasma Wake Field Accelerator (PWFA) experiment, E164X, conducted at the Stanford Linear Accelerator Center by a collaboration which includes USC, UCLA and SLAC. Such trapping produces plasma dark current when the wakefield amplitude is above a threshold value 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. Here we present experimental results.

In the E164 Experiment at the Stanford Linear Accelerator Center (SLAC), we 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 12 microns, and accurate direct measurement of their lengths is not feasible shot by shot. 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 particle tracking code developed at SLAC. Understanding the longitudinal profile allows a better understanding of acceleration in the plasma wake, as well as investigation of related effects. We discuss the method and validation of our phase space determinations.

The E164/E164X plasma wakefield experiment studies beam-plasma interactions at the Stanford Linear Acceleration Center (SLAC). Due to SLAC’s recent ability to variably compress bunches longitudinally from 650 μm down to 20 μm, the incoming beam is sufficiently dense to field ionize the neutral lithium (Li) vapor. The field ionization effects are characterized by the beams energy loss through the Li vapor column. Experiment results are presented.

An important aspect of plasma wake field accelerators (PWFA) is stable propagation of the drive beam. In the under dense plasma 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 [1]. 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 E-164X PWFA experiments at SLAC.

An experiment is planned to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range. This new UCLA/SLAC/USC collaboration will take advantage of the unique SLAC FFTB electron beam and its demonstrated ultra-short pulse lengths and high currents (e.g., σ z = 20 μm at Q = 3 nC). The electron beam will be focused down and sent through varying lengths of fused silica capillary tubing with two different sizes: ID = 200 μm / OD = 325 μm and ID = 100 μm / OD = 325 μm. The pulse length of the electron beam will be varied in order to alter the accelerating gradient and probe the breakdown threshold of the dielectric structures. In addition to breakdown studies, we plan to collect and measure coherent Cerenkov radiation emitted from the capillary tube to gain information about the strength of the accelerating fields.

Coherent transition radiation is used to measure the length of the ultra-short electron bunches available at the Stanford Linear Accelerator Center. The results and the limitations of the method are described.

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.

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 10 kA will be delivered at a particle energy of 28 GeV. 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.

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 3 × 1017cm-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.

For the parameters envisaged in possible afterburner stages[1] of a plasma wakefield accelerator (PWFA), the self-fields of the particle beam can be intense enough to tunnel ionize some neutral gases. Tunnel ionization has been investigated as a way for the beam itself to create the plasma, and the wakes generated may differ from those generated in pre-ionized plasmas[2],[3]. However, it is not practical to model the whole stage of PWFA with afterburner parameters using the models described in [2] and [3]. 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. Comparison between results from OSIRIS (a full PIC code with ionization) and from QuickPIC with the ionization package shows good agreement. Preliminary results using parameters relevant to the E164X experiment and the upcoming E167 experiment at SLAC are shown.

Ultra-high gradient dielectric wakefield accelerators are a potential option for a linear collider afterburner since they are immune to the ion collapse and electron/positron asymmetry problems implicit in a plasma based afterburner. The first phase of an experiment to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range has been completed. The experiment took advantage of the unique SLAC FFTB electron beam and its ultra-short pulse lengths and high currents (e.g., s z = 20 m m at Q = 3 nC). The FFTB electron beam was successfully focused down and sent through short lengths of fused silica capillary tubing (ID = 200 m m / OD = 325 m m). The pulse length of the electron beam was varied to produce a range of electric fields between 2 and 20 GV/m at the inner surface of the dielectric tubes. We observed a sharp increase in optical emissions from the capillaries in the middle part of this surface field range which we believe indicates the transition between sustainable field levels and breakdown. If this initial interpretation is correct, the surfaced fields that were sustained equate to on axis accelerating field of several GV/m. In future experiments being developed for the SLAC SABER and BNL ATF we plan to use the coherent Cerenkov radiation emitted from the capillary tube as a field strength diagnostic and demonstrate GV/m range particle energy gain.

A new method for generating positrons has been proposed that uses betatron X-rays emitted by an electron beam in a high-K plasma wiggler. The plasma wiggler is an ion column produced by the head of the beam when the peak beam density exceeds the plasma density. The radial electric field of the beam blows out the plasma electrons transversely, creating an ion column. The focusing electric field of the ion column causes the beam electrons to execute betatron oscillations about the ion column axis. If the beam energy and the plasma density are high enough, these oscillations lead to synchrotron radiation in the 1-50 MeV range. A significant amount of electron energy can be lost to these radiated X-ray photons. These photons strike a thin (.5Xo), high- Z target and create e + /e − pairs. The experiment was performed at the Stanford Linear Accelerator Center (SLAC) where a 28.5 GeV electron beam with σr ≈ 10μm and σz ≈ 25μm was propagated through a neutral Lithium vapor (Li). The radial electric field of the dense beam was large enough to field ionize the Li vapor to form a plasma. The positron yield was measured as a function of plasma density, ion column length and electron beam pulse length. A computational model was written to match the experimental data with theory. The measured positron spectra are in excellent agreement with those expected from the calculated X-ray spectral yield from the plasma wiggler. After matching the model with the experimental results, it was used to design a more efficient positron source, giving positron yields of 0.44 e + /e − , a number that is close to the target goal of 1-2 e + /e − for future positron sources.

Summary form only given. A highly relativistic electron beam passes through an oven filled with the particular gas used in the experiment creating a plasma and a large amplitude wake field which causes the beam to lose and gain energy. The energy dumped into the plasma is dissipated through recombination and thermalization. Intensity of the plasma light is proportional to the wakefield amplitude. As the only non-beam diagnostic, study of plasma light can be used to characterize the plasma beam interaction to get the highest acceleration gradient. Moreover the unique spectrum of the gas can be used to as a reliable tool to measure the density vial the theory of Stark Broadening as an alternative to the other plasma density diagnostic tools which may not be available at the higher densities. Application of Plasma Light diagnostic to the past and future plasma experiments will be presented.

Summary form only given. A nearly light-speed electron plasma wave (EPW); i.e., an EPW with the Lorentz factor associated with its phase velocity /spl Gt/1, may become a useful particle accelerating structure for science/industry. The accelerating fields E/sub z/ can exceed the /spl sim/200 MeV/m breakdown limit in /spl sim/10 GHz copper structures by orders of magnitude. As one goes up in EPW frequency /spl nu//sub p/ (by increasing the plasma density n/sub e/), the wavelength /spl lambda//sub p/ /spl ap/c//spl nu//sub p/ becomes smaller and above 10/sup 18/ cm/sup -3/ is

When considering intense particle or laser beams propagating in dense plasma or gas, ionization plays an important role. Impact ionization and tunnel ionization may create new plasma electrons, altering the physics of wakefield accelerators, creating and modifying instabilities, etc. Here we describe the addition of an ionization package into the 3-D object-oriented fully parallel PIC code OSIRIS [R.G Hemker, F.S. Tsung, V.K. Decyk, W.B. Mori, S. Lee, and T. Katsouleas, Development of a parallel code for modeling plasma based accelerators, IEEE Particle Accelerator Conference 5, 3672-3674 (1999).]. Using intense beams to tunnel-ionize neutral gas may become a new source of plasma. For the beams whose electrical fields are right above threshold, the optimal gas density for maximize electrical field is about 7 n/sub 0/(n/sub 0/ is the optimal density according to linear theory /spl omega//sub p//spl sigma//sub z//c=2/sup 1/2/ [E164 proposal, unpublished.]). We apply the simulation tool to the parameters of the current E164 [R. Keinigs and M.E. Jones, Phys. Fluids 30, 252 (1987)] Plasma Wakefield Accelerator experiment at the Stanford Linear Accelerator Center (SLAC). We find that tunnel ionization affects the wakefield and energy gain of E-164 experiment.

The E162 experiment at the Stanford Linear Accelerator Center was the first experiment in which a positron beam gained energy in a plasma wakefield accelerator [B. Blue et al., Accepted for Publication in Physical Review Letters.]. A single positron bunch both excited (gave energy to) and witnessed (extracted energy from) the plasma wakefield. The energy dynamics within the single positron bunch were measured in a dispersive section of the beamline with both time-resolved (1 ps streak camera) and time-integrated (CCD camera) diagnostics. This paper will correlate the energy gain and loss measurements from both diagnostics.

Recent experiments and simulations have shown that positron beams propagating in plasmas can be focused and also create wakes with large accelerating gradients. For similar parameters, the wakes driven by positron beams are somewhat smaller compared to the case of an electron beam. Simulations have shown that the wake amplitude can be increased if the positron beam is propagated in a hollow plasma channel. This paper, compares experimentally, the propagation and beam dynamics of a positron beam in a meter scale homogeneous plasma, to a positron beam hollow channel plasma. The results show that positron beams in hollow channels are less prone to distortions and deflections. Hollow channels were observed to guide the positron beam onto the channel axis. Beam energy loss was also observed implying the formation of a large wake amplitude. The experiments were carried out as part of the E-162 plasma wakefield experiments at SLAC.

The plasma afterburner has been proposed as a possible advanced acceleration scheme for a future linear collider. In this concept, a high energy electron(or positron) drive beam from an existing linac such as the SLC will propagate in a plasma section of density about one order of magnitude lower than the peak beam density. The particle beam generates a strong plasma wave wakefield which has a phase velocity equal to the velocity of the beam and this wakefield can 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 the particle beam in the plasma are critical to the afterburner concept. We investigate the nonlinear beam‐plasma interactions in such scenario using a new 3D particle‐in‐cell code called QuickPIC. Preliminary simulation results for electron acceleration, beam‐loading and hosing instability will be presented.

An expression for the maximum accelerating field in the narrow bunch limit (kpσr<

Summary form only given, as follows. Large gradient accelerators are necessary to reach the very high energies required at the collision point of future electron/positron colliders In the plasma wakefield accelerator (PWFA), a short electron or positron bunch drives a large amplitude plasma wave or wake. The transverse component of the wake leads to focusing of the particle bunch, while longitudinal components of the wake lead to energy loss and energy gain by particles. The PWFA is an energy transformer in which the energy is transferred from the particles in the core of the bunch in a single bunch scheme, or from a driver bunch in a two bunch scheme, to the particles in the back of the same bunch, or to a trailing witness bunch In the experiments described here, the 28.5 GeV electron or positron beam of the Stanford Linear Accelerator Center Final Focus Test Beam line is sent in a long lithium plasma. The bunch charge density is density is larger than the plasma density and the plasma wake is driven in the non-linear regime. In the case of an electron bunch, the bunch space charge field expels all the plasma electrons from the beam volume. The pure plasma ion column left behind the bunch head acts as an aberration-free plasma lens on the bunch core.

Summary form only given, as follows. A plasma wakefield acceleration (PWFA) experiment is presently being conducted at the Stanford linear Accelerator Center (SLAC). In this experiment 28.5 GeV, 2 ps long electron (e/sup -/) or positron (e/sup +/) bunches with /spl ap/2/spl times/10/sup 10/ particles are sent in a 1.4 m long plasma with a density in the 0-2/spl times/10/sup 14/ cm/sup -3/ range. The plasma is obtained by single-photon ionization of a lithium vapor. The focusing of the bunches is observed by imaging on a CCD camera the optical transition radiation emitted by the bunches when traversing thin titanium foils placed /spl ap/1 m upstream and downstream from the plasma. The time resolved dynamics is studied by imaging onto a streak camera the light emitted by the beam when traversing a thin piece of aerogel located /spl ap/25 m downstream from the plasma, after a dispersive magnet. Quadrupoles located between the plasma and the aerogel image the particle beam at the plasma exit onto the aerogel. At the aerogel location, the beam image in the dispersive (vertical) plane is dominated by the beam energy, whereas in the perpendicular, non-dispersive plane it is dominated by the beam spot size. The incoming bunches have a correlated energy spread, and time integrated images of the beam after dispersion in energy also give an insight into the beam transverse dynamics. The beam density is larger than the plasma density, and the experiment can access the non-linear or blow-out regime of the PWFA. The plasma acts as a thick focusing element. Both e/sup -/ and e/sup +/ bunches are focused by the plasma. The focusing dynamics is observed with single bunches. The bunch particles work on the plasma electron to expel them from (e/sup -/), or attract them into (e/sup +/) the bunch volume, and energy loss of the particles in the bunch core is observed. Some of the energy is transferred back from the plasma electrons to the particles in the back of the bunch, and plasma acceleration is observed with e/sup -/ bunches. The experimental set up as well as experimental results will be presented.

The design of the NLC beam delivery system requires a firmer understanding of the effects of collimators on short, intense bunches than is presently available. We describe an experiment to directly measure these effects through use of a dedicated apparatus located at the 1.19 GeV point in the SLAC linac. The apparatus consists of an outer vacuum vessel and an interchangeable insertion containing up to 5 distinct collimator apertures. The insertion is capable of remote-controlled translation, allowing the collimator apertures to be misaligned relative to the electron beam without changing the incoming beam orbit; the wakefield deflection is then measured by observing the change in the outgoing orbit on 32 beam position monitors. The parameters of the apertures have been selected to allow confirmation of the scaling laws for collimator wakefields, and to strongly enhance either the geometric or resistive wall contribution of each aperture. Details of the apparatus design, the aperture parameters, and the experimental program are discussed.

A plasma-based wakefield acceleration (PWFA) experiment is proposed that will accelerate parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the one meter long plasma and to witness the resulting beam acceleration. The proposed experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large scale accelerators. The one meter length of the experiment is about two orders of magnitude larger than other high gradient PWFA experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the proposed experiment will allow the study of high gradient acceleration at the forefront of advanced accelerator research.

Our approach to describe the tuning states of a linear collider by a Markov model looks very promising. In the future, the model presented here should be extended so as to include information about other beam parameters, e.g., emittances, energy and bunch lengths, and it should be further developed to understand the impact of multiple subsystem failures.

We have observed the focusing of high energy electron beams by self-induced gaseous plasmas. This work is being done in the Final Focus Test Beam line (FFTB) at the Stanford Linear Accelerator Center (SLAC). The level of impact ionization of the neutral nitrogen gas target by the 29 GeV electron beam is adequate to produce clear focusing action. A 40% reduction in the size of each transverse beam dimension is observed for plasma lengths of only 3 mm. Experimental data is integrated over the electron bunch length of 700 microns in which we have 1.5/spl times/10/sup 10/ particles (per bunch). The incident transverse beam size of 7 microns (x dimension) by 3 microns (y dimension) corresponds to an incident beam density of about 7/spl times/10/sup 16/ cm/sup -3/. Normalized transverse beam emittances are 5/spl times/10/sup -5/ and 6/spl times/10/sup -6/ m.rad. For x and y directions respectively. Neutral molecular nitrogen is injected into the vacuum beamline with a pulsed solenoid valve. Gas target densities up to 4/spl times/10/sup 18/ cm/sup -3/ were used. Beam sizes are determined by a scanning technique. With this method a fine carbon wire is placed on the beam centroid. The electron beam is then controllably dithered about the wire position while monitoring the generated bremsstrahlung signal downstream. Because the beam operates at a 10 Hz rate while the gas is introduced at only 0.5 Hz each scan provides transverse profile data with and without the plasma lens. These results describe a small plasma element with a focusing strength which exceeds that of conventional magnets by about two orders of magnitude.

A series of plasma wakefield acceleration (PWFA) experiments are being conducted with a 30 GeV drive beam from the Stanford Linear Accelerator Center (SLAC). These experiments continue to address the application of meter-scale plasmas to focus and accelerate electrons and positrons in the context of future applications to high-energy accelerators.