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The European physical journal / D 76(11), 205 (2022). doi:10.1140/epjd/s10053-022-00535-y, The petawatt (PW) laser facility of the Berkeley Lab Laser Accelerator (BELLA) Center has recently commissioned its second laser pulse transport line. This new beamline can be operated in parallel with the first beamline and enables strong-field quantum electrodynamics (SF-QED) experiments at BELLA. In this paper, we present an overview of the upgraded BELLA PW facility with a SF-QED experimental layout in which intense laser pulses collide with GeV-class laser-wakefield-accelerated electron beams. We present simulation results showing that experiments will allow the study of laser-particle interactions from the classical to the SF-QED regime with a nonlinear quantum parameter of up to χ∼2. In addition, we show that experiments will enable the study and production of GeV-class, mrad-divergence positron beams via the Breit–Wheeler process., Published by Springer, Heidelberg

Nanostructured surfaces enhance ion yields in matrix-assisted laser desorption–ionization mass spectrometry (MALDI-MS). The spike protein complex, S1, is one fingerprint signature of Sars-CoV-2 with a mass of 75 kDa. Here, we show that MALDI-MS yields of Sars-CoV-2 spike protein ions in the 100 kDa range are enhanced 50-fold when the matrix–analyte solution is placed on substrates that are coated with a dense forest of multi-walled carbon nanotubes, compared to yields from uncoated substrates. Nanostructured substrates can support the development of mass spectrometry techniques for sensitive pathogen detection and environmental monitoring.

Ultrafast lasers play an increasingly critical role in the generation, manipulation, and acceleration of electron beams for High Energy Physics applications. Laser plasma accelerators enable order of magnitude improvements in accelerating gradient and promise compact tunable GeV electron beam sources, while novel photocathode systems permit fundamental advances in electron beam manipulation for accelerator and radiation applications Advances in fast feedback systems are required to stabilize laser performance at kHz repetition rate operation against environmental fluctuations. A field programmable gate array (FPGA) based digital control system, coupled with responsive optics, can provide rapid and precise stabilization of ultrafast lasers. A collaboration between RadiaSoft and the Lawrence Berkeley National Laboratory BELLA Center to develop, test, and deploy these systems across a range of beamlines operating at >1 Hz repetition rate, including 1 kHz systems, was created.

A reliable, specific, selective and robust liquid chromatography-tandem mass spectrometry (LC–MS/MS) method was developed for the determination of ribociclib in both dried blood spot (DBS) samples and potassium EDTA plasma. DBS samples were obtained simultaneously with a plasma sample in advanced breast cancer patients treated with ribociclib. A 6 mm disk from the central part of the dried blood spot sample was punched, followed by extraction of ribociclib using liquid-liquid extraction spiked with ribociclib-d6 as internal standard. Concentrations of ribociclib in DBS samples were correlated with corresponding plasma concentrations. From the blood sample also hematocrit was determined. The method was validated for selectivity, sensitivity, precision, lower limit of detection, linearity, stability and accuracy according to the food and drug administration (FDA) guideline. The within- and between-run precisions were ≤10.6 and ≤1.07 %, respectively; while the average accuracy ranged from 100 to 103 %. The influence of hematocrit on validation parameters was tested in the range of 0.20 – 0.40 L/L. No influence of hematocrit on validation parameters was observed. Regression analysis and a Bland-Altman plot indicated correlation between the results obtained from DBS and plasma samples. A strong correlation (R2 >0.97) between DBS samples and plasma concentration from 17 breast cancer patients was found. A number of 12 out of 17 processed DBS samples (71 %) fell inside the acceptable range of 20 % difference of simultaneously obtained plasma samples. The lower limit of quantification in DBS is 10.0 ng/mL and linearity was demonstrated up to 1000 ng/mL. In conclusion, the newly developed assay met the required standard for validation. The methods were used to study ribociclib disposition in patients with advanced breast cancer.

We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$ 10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $ % and their average on-axis plasma electron density to $ %. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

Background & aims: The level of adherence to the updated guidelines of The European Societies for Clinical Nutrition and Metabolism (ESPEN) and for Blood and Marrow Transplantation (EBMT) on nutrition in intensively treated adult acute myeloid leukemia (AML) patients in clinical practice is unknown. The aim of this nationwide survey was to investigate ESPEN/EBMT nutritional guideline adherence during intensive AML treatment, variation in nutrition support practices among hospitals and whether these practices changed after guideline publication.Methods: All 22 Dutch hospitals providing (aftercare following) high-dose chemotherapy and/or hematopoietic stem cell transplantation for adult AML patients were surveyed on nutrition support practices during these intensive AML treatments. We used an online questionnaire in 2015 and semi structured telephone interviews in 2018-2019. Both surveys were completed by registered dieticians and addressed the use of enteral (EN) and parenteral (PN) nutrition. The ESPEN/EBMT nutritional guideline adherence was investigated through the telephone interviews.Results: High-level ESPEN/EBMT guideline adherence and/or uniformity among hospitals regarding nutrition support practices during intensive AML treatment were observed for nutritional screening,-aims, safe food handling and exercise training. Adherence to ESPEN/EBMT recommendations that were not implemented into national guidelines, including nutritional assessment and use of medical nutrition, was poor. All hospitals assessed nutritional intake,-impact symptoms and body weight, but muscle mass, physical performance and degree of systemic inflammation were rarely and variably monitored. Although the number of hospitals using EN as first-choice nutritional intervention increased from 3 hospitals in 2015 to 8 in 2019, PN remained the preferred method of nutrition support. Furthermore, the timing of medical nutrition varied.Conclusions: Although the use of EN increased after publication of the updated ESPEN/EBMT nutritional guidelines, adherence to these standards was limited and there was heterogeneity in nutrition support practices during intensive AML treatment among hospitals. Incorporating international nutritional standards into national guidelines by nutrition expert groups immediately upon publication may improve adherence. (c) 2020 European Society for Clinical Nutrition and Metabolism. Published by Elsevier Ltd. All rights reserved.

Physics of plasmas 27(9), 093101 (2020). doi:10.1063/5.0014961, Laser-heated capillary discharge waveguides are novel, low plasma density guiding structures able to guide intense laser pulses over many diffraction lengths and have recently enabled the acceleration of electrons to 7.8 GeV by using a laser-plasma accelerator (LPA). These devices represent an improvement over conventional capillary discharge waveguides, as the channel matched spot size and plasma density can be tuned independently of the capillary radius. This has allowed the guiding of petawatt-scale pulses focused to small spot sizes within large diameter capillaries, preventing laser damage of the capillary structure. High performance channel-guided LPAs require control of matched spot size and density, which experiments and simulations reported here show can be tuned over a wide range via initial discharge and laser parameters. In this paper, measurements of the matched spot size and plasma density in laser-heated capillary discharges are presented, which are found to be in excellent agreement with simulations performed using the MHD code MARPLE. Strategies for optimizing accelerator performance are identified based on these results., Published by American Institute of Physics, [S.l.]

Physics of plasmas 27(5), 053102 (2020). doi:10.1063/5.0002769, A plasma channel created by the combination of a capillary discharge and inverse Bremsstrahlung laser heating enabled the generation of electron bunches with energy up to 7.8 GeV in a laser-driven plasma accelerator. The capillary discharge created an initial plasma channel and was used to tune the plasma temperature, which optimized laser heating. Although optimized colder initial plasma temperatures reduced the ionization degree, subsequent ionization from the heater pulse created a fully ionized plasma on-axis. The heater pulse duration was chosen to be longer than the hydrodynamic timescale of ���1 ns, such that later temporal slices were more efficiently guided by the channel created by the front of the pulse. Simulations are presented which show that this thermal self-guiding of the heater pulse enabled channel formation over 20 cm. The post-heated channel had lower on-axis density and increased focusing strength compared to relying on the discharge alone, which allowed for guiding of relativistically intense laser pulses with a peak power of 0.85 PW and wakefield acceleration over 15 diffraction lengths. Electrons were injected into the wake in multiple buckets and times, leading to several electron bunches with different peak energies. To create single electron bunches with low energy spread, experiments using localized ionization injection inside a capillary discharge waveguide were performed. A single injected bunch with energy 1.6 GeV, charge 38 pC, divergence 1 mrad, and relative energy spread below 2% full-width half-maximum was produced in a 3.3 cm-long capillary discharge waveguide. This development shows promise for mitigation of energy spread and future high efficiency staged acceleration experiments., Published by American Institute of Physics, [S.l.]

Physics of fluids 32(6), 066108 (2020). doi:10.1063/5.0005888, Density transition injection is an effective technique for controllably loading electrons into a trapped phase for laser plasma accelerators. One common technique to achieve the required fluid structure is to impinge a thin blade on the plume of a supersonic nozzle. Density transitions induced in this way are often assumed to be bow shocks and therefore sharp, but simulations and fluorescence measurements presented in this work show that in many cases of interest, the density transition accessible to a laser propagating transverse to the shock is an intercepting shock, and therefore, shock thickness and density vary with pressure, laser height, and blade position. The fluid dynamics of a supersonic nozzle impinged on by a thin, flat object are explored through simulations and relevant features are verified via planar laser-induced fluorescence measurements. The implications of the results for tuning electron beam injectors in laser plasma accelerators are discussed., Published by American Institute of Physics, [S.l.]

Physical review accelerators and beams 22(3), 032401 (2019). doi:10.1103/PhysRevAccelBeams.22.032401, A study of the optimization of iron dominated dipole magnets with pole face widths comparable or less than the gap size, i.e., low aspect ratio (AR), is conducted using both theoretical and computational approaches. This regime of magnet design is particularly relevant in the context of laser plasma accelerators (LPA) due to unique beam parameters and geometric constraints, namely large energy spreads and the requirement for large apertures to accommodate drive laser passage. The breakdown of commonly employed approximations and rules of thumb in typical AR≫1 magnet design is examined. A library of generalized, optimized pole face geometries is provided to expedite optimization of future magnets. Finally, this methodology is used to design an electromagnetic chicane which has been fabricated, validated, and is currently in use in an x-ray free electron laser driven LPA experiment at LBNL., Published by American Physical Society, College Park, MD

Applied physics letters 116(23), 234108 (1-5) (2020). doi:10.1063/5.0005114, High-resolution (sub-%), single-shot energy distribution, and emittance measurements of GeV-class electron beams generated by laser plasma accelerators (LPAs) have been enabled through the use of an active plasma lens combined with relatively short dipole magnets. This setup, with only 60 cm between the LPA source and the final diagnostic screen, was facilitated through the use of a replenishable ���20���40 nm thick liquid crystal plasma mirror to separate remnant laser radiation from the electron beam without emittance degradation. As LPAs capable of generating GeV-class electron beams at cm-scale distances become increasingly ubiquitous, there is a need to supplement these compact accelerators with equally compact diagnostics., Published by American Inst. of Physics, Melville, NY

Precise measurements of the plasma density in ionized gas cells and discharged capillaries are critical to the design and operation of plasma-based accelerators, active plasma lenses, and plasma-based radiation sources. In this manuscript, a spectral-domain common-path second-harmonic interferometer is upgraded with the simultaneous measurement of the group and phase velocity, allowing for high-sensitivity density characterization (from the phase velocity advance) without the need for phase tracking from zero-density (enabled by the group velocity delay). The technique is applied to 1.5-cm-long plasma structures, without density ambiguity in parameter scans with >2π phase jumps. The single-shot sensitivity in phase retrieval is demonstrated at 63 mrad, equivalent to a density-length product of 1.8·1015 cm -2 . This is an improvement of ×45 compared to group velocity analysis alone.

© 2019 American Physical Society. Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

In this report, we demonstrate precise control of a laser plasma accelerator (LPA) through characterization of a shock-induced density downramp injector. Experiments systematically varied the shock injector profile, including shock angle, shock position, up-ramp width, and acceleration length. As a result, an electron beam that was highly tunable from 30 to 300 MeV with 8.5% energy spread, 1.5 mrad divergence and 0.35 mrad pointing fluctuation was produced. We also report the progress of a project focusing on demonstrating the crucial enablers of a compact LPA based MeV Thomson photon source in an integrated experiment and a dedicated 100 TW laser facility built at the Berkeley Lab Laser Accelerator (BELLA) Center, Lawrence Berkeley National Laboratory (LBNL).

Physical review accelerators and beams 21(12), 122801 (2018). doi:10.1103/PhysRevAccelBeams.21.122801, Active plasma lenses have the potential to enable broad-ranging applications of plasma-basedaccelerators owing to their compact design and radially symmetric kT/m-level focusing fields, facilitatingbeam-quality preservation and compact beam transport. We report on the direct measurement of magneticfield gradients in active plasma lenses and demonstrate their impact on the emittance of a charged particlebeam. This is made possible by the use of a well-characterized electron beam with 1.4 mm mrad normalizedemittance from a conventional accelerator. Field gradients of up to 823 T/m are investigated. The observedemittance evolution is supported by numerical simulations, which suggests the potential for conservationof the core beam emittance in such a plasma lens setup., Published by American Physical Society, College Park, MD

99Mo photonuclear yield was measured using high-energy electrons from Laser Plasma Accelerators and natural molybdenum. Spectroscopically resolved electron beams allow comparisons to Monte Carlo calculations using known 100Mo(γ,n)99Mo cross sections. Yields are consistent with published low-energy data, and higher energy data are well predicted from the calculations. The measured yield is (15±2)×10−5 atoms/electron (0.92±0.11 GBq/μA) for 25 mm targets at 33.7 MeV, rising to (1391±20)×10−5 atoms/electron (87±2 GBq/μA) for 54 mm/ 1.7 GeV, with peak power-normalized yield at 150 MeV.

Author(s): Swanson, KK; Tsai, HE; Barber, SK; Lehe, R; Mao, HS; Steinke, S; Van Tilborg, J; Nakamura, K; Geddes, CGR; Schroeder, CB; Esarey, E; Leemans, WP | Abstract: Control of the properties of laser-plasma-accelerated electron beams that were injected along a shock-induced density downramp through precision tailoring of the density profile was demonstrated using a 1.8 J, 45 fs laser interacting with a mm-scale gas jet. The effects on the beam spatial profile, steering, and absolute energy spread of the density region before the shock and tilt of the shock were investigated experimentally and with particle-in-cell simulations. By adjusting these density parameters, the electron beam quality was controlled and improved while the energy (30-180 MeV) and energy spread (2-11 MeV) were independently tuned. Simple models that are in good agreement with the experimental results are proposed to explain these relationships, advancing the understanding of downramp injection. This technique allows for high-quality electron beams with percent-level energy spread to be tailored based on the application.

Laser plasma acceleration has been intensely investigated for its ability to produce energetic, ultrashort electron bunches in a compact distance. A high intensity laser pulse propagating through a plasma expels the electrons from the optical axis via the ponderomotive force, leaving behind a column of ions and driving a density wake. The accelerating electric fields present in the wake can reach several orders of magnitude greater than those found in radio-frequency cavities, allowing for compact systems much smaller than those using conventional accelerators. This compact source can provide electrons for various applications including stages for a high energy collider or for production of x-ray pulses from coherent undulator radiation. However, these applications require tunable, stable and high-quality electron beams. We report on a study of controlled injection along a shock-induced density downramp of laser-plasma- accelerated electrons through precision tailoring of the density profile produced from a mm-scale gas jet. Using BELLA Center’s TREX Ti:Sapphire laser, the effects of the plasma density profile and the tilt of the shock front on the beam spatial profile, steering, and energy were investigated experimentally. To explain these rela- tionships, we propose simple models which agree well with experimental results. Using this technique, electron beam quality was tailored, allowing for the production of high-quality electron beams with percent-level energy spreads over a range of energies.

Single-shot, charge-dependent emittance measurements of electron beams generated by a laser plasma accelerator (LPA) reveal that shock-induced density down-ramp injection produces beams with normalized emittances a factor of 2 smaller than beams produced via ionization injection. Such a comparison is made possible by the tunable LPA setup, which allows electron beams with nearly identical central energy and peak spectral charge density to be produced using the two distinct injection mechanisms. Parametric measurements of this type are essential for the development of LPA-based applications which ultimately require high charge density and low emittance.

Author(s): Van Tilborg, J; Barber, SK; Tsai, HE; Swanson, KK; Steinke, S; Geddes, CGR; Gonsalves, AJ; Schroeder, CB; Esarey, E; Bulanov, SS; Bobrova, NA; Sasorov, PV; Leemans, WP | Abstract: Active plasma lenses have attracted interest in novel accelerator applications due to their ability to provide large-field-gradient (short focal length), tunable, and radially symmetric focusing for charged particle beams. However, if the discharge current is not flowing uniformly as a function of radius, one can expect a radially varying field gradient as well as potential emittance degradation. We have investigated this experimentally for a 1-mm-diameter active plasma lens. The measured near-axis field gradient is approximately 35% larger than expected for a uniform current distribution, and at overfocusing currents ring-shaped electron beams are observed. These observations are explained by simulations.

We present results of an experiment where two independent Laser-Plasma-Accelerator (LPA) stages are coupled at a short distance by a plasma mirror. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed reconstruction of the temporal field structure and determination of the plasma density. Injection into the wakefield of the second stage was verified by a 100 MeV energy gain of the electron beam.

To advance the current state-of-the-art of capillary-based laser plasma accelerators (LPAs), the tunability of capillary discharge plasma channels needs to be improved. We present the techniques used to determine critical properties of the plasma density distribution. Independent tailoring of plasma channel width and on-axis density are required to produce higher energy electron beams with existing facilities. A scheme involving an additional, nanosecond laser pulse to locally heat the channel has been proposed previously. We discuss recent progress on the implementation of this scheme, demonstrating a heating effect on the plasma channel as evidenced from nanosecond-resolved spectroscopy on transversely emitted plasma light. PIC simulations indicate the possibility of accelerating high charge beams up to 8.4 GeV average energy if other technique advances are made as well. These include the need for longer plasma channels of 10s of centimeters, low plasma density and an ionization injection scheme to inje...

Density transition injection is an effective technique for controllably loading electrons into a trapped phase for laser-plasma accelerators. One common technique to achieve this fluid phenomenon is to impinge a thin blade on the plume of a supersonic nozzle. 2-D simulations show that the density transition accessible to a transverse laser is produced by a rapid re-expansion of the high pressure region behind the initial bow shock, and not by the bow shock produced by the blade, as is commonly thought. This pressure mismatched re-expansion generates compression waves that could coalesce into shock-fronts as they interact with the surrounding ambient gas. This has consequences when interpreting the electron injection mechanism. In the simulations presented here, the fluid dynamics of a supersonic nozzle impinged on by a thin, flat object is explored, along with the implications for electron beam injectors in laser-plasma accelerators.

Laser-plasma accelerators (LPAs) have the potential to drive compact free-electron lasers (FELs). Even with LPA energy spreads typically at the percent level, the e-beam brightness can be excellent, due to the low normalized emittance (

Typical parameters for an electron beam generated by a laser plasma accelerator pose significant challenges for conventional transport. Numerous factors related to divergence, energy spread and space charge can lead to intolerable emittance growth. We discuss here a novel beamline configuration utilizing an active plasma lens that will be implemented in upcoming experiments aimed at demonstrating the delivery of 100–300 MeV beams which can be matched to external focusing channels with minimal emittance growth.

In this paper, we demonstrate a highly tunable, controlled-injection laser-plasma accelerator (LPA) through systematically varying parameters of a density shock injector. Beam energy, energy spread, charge and pointing can be controlled in the range of 50−300 MeV, with

Laser-driven particle accelerators have the potential to be much cheaper than conventional accelerators. But so far, the reliability and energy spread of the beams they produce has been poor. A technique that decouples the particle-injection and acceleration stages of these devices could improve their performance.

Electrically discharged plasma structures, typically several centimeters in length and sub-millimeter in diameter, have been applied to guide laser pulses in laser plasma accelerators and to focus ion and relativistic electron beams in compact, radially symmetric transport configurations. Knowledge of the on-axis plasma density is critical. Traditional density interferometry has been ineffective for these laser-machined structures, while group velocity delay (GVD) techniques involve combining two laser paths with corresponding alignment complexities and stability sensitivities. Here the GVD technique is advanced to a common-path two-color interferometer configuration performed in the spectral domain of a broad-bandwidth femtosecond laser. Multi-shot tracking of the phase is not required, and the common path assures improved stability. This in situ technique was validated on 15 mm long plasma structures, measuring electron densities of 1017-1018 cm-3 for various fill pressures.

Electrically discharged active plasma lenses (APLs) are actively pursued in compact high-brightness plasma-based accelerators due to their high-gradient, tunable, and radially symmetric focusing properties. In this manuscript, the APL is experimentally compared with a conventional quadrupole triplet, highlighting the favorable reduction in the energy dependence (chromaticity) in the transport line. Through transport simulations, it is explored how the non-uniform radial discharge current distribution leads to beam-integrated emittance degradation and a charge density reduction at focus. However, positioning an aperture at the APL entrance will significantly reduce emittance degradation without additional loss of charge in the high-quality core of the beam. An analytical model is presented that estimates the emittance degradation from a short beam driving a longitudinally varying wakefield in the APL. Optimizing laser plasma accelerator operation is discussed where emittance degradation from the non-unifor...

Author(s): Barber, SK; Van Tilborg, J; Schroeder, CB; Lehe, R; Tsai, HE; Swanson, KK; Steinke, S; Nakamura, K; Geddes, CGR; Benedetti, C; Esarey, E; Leemans, WP | Abstract: Laser plasma accelerators (LPA) offer an exciting possibility to deliver high energy, high brightness electrons beams in drastically smaller distance scales than is typical for conventional accelerators. As such, LPAs draw considerable attention as potential drivers for next generation light sources and for a compact linear collider. In order to asses the viability of an LPA source for a particular application, the brightness of the source should be properly characterized. In this paper, we present charge dependent transverse emittance measurements of LPA sources using both ionization injection and shock induced density down ramp injection, with the latter delivering smaller transverse emittances by a factor of two when controlling for charge density. The single shot emittance method is described in detail with a discussion on limitations related to second order transport effects. The direct role of space charge is explored through a series of simulations and found to be consistent with experimental observations.

Ultrafast X-ray science is an exciting frontier that promises the visualization of electronic, atomic and molecular dynamics on atomic time and length scales. A largely unexplored area of ultrafast X-ray science is the use of light to control how X-rays interact with matter. To extend control concepts established for long-wavelength probes to the X-ray regime, the optical control field must drive a coherent electronic response on a timescale comparable to femtosecond core-hole lifetimes. An intense field is required to achieve this rapid response. Here, an intense optical control pulse is observed to efficiently modulate photoelectric absorption for X-rays and to create an ultrafast transparency window. We demonstrate an application of X-ray transparency relevant to ultrafast X-ray sources: an all-photonic temporal cross-correlation measurement of a femtosecond X-ray pulse. The ability to control X-ray–matter interactions with light will create new opportunities for present and next-generation X-ray light sources. Intense optical beams can alter the way that a material interacts with X-ray radiation. This is now demonstrated by experiments that use femtosecond laser pulses to affect inner-shell processes in neon atoms, increasing the transmission of X-rays. This could allow imprinting of optical pulse trains onto much longer X-ray pulses.

High-harmonic generation (HHG) from replenishing tape surfaces allows for multi-Hz applications. Unlike conventional gas-based HHG, solid-surface HHG (SHHG) uses peak laser intensities upwards of 1018 - 1019 W/cm2. This allows a fs-class 100 mJ pulse to be focused down tighter, enabling a more compact harmonic generating setup. Operating in the sub-relativistic Coherent Wake Emission (CWE) regime, we have experimentally investigated MW-level XUV-domain photons produced from different tape materials. The tapes are thin and can be used as beam combining optics for LPA electron beams and HHG seed radiation.

We present results of an experiment where two independent Laser-Plasma-Accelerator (LPA) stages are coupled at a short distance by a plasma mirror. Stable electron beams were focused by a discharge capillary-based active plasma lens [2] to a 10-micron-level beam size in a Hydrogen plasma, such that they interact with a dark-current-free, quasi-linear wakefield excited by the laser of the second stage. Changing the arrival time of the electron beam with respect to the second stage laser pulse allowed reconstruction of the temporal field structure and determination of the plasma density. Injection into the wakefield of the second stage was verified by a 100 MeV energy gain of the electron beam.

The laser group velocity in discharged capillary channels plays a critical role in laser-plasma accelerators. The laser travel time (and thus the group velocity) was measured through two-pulse frequency-domain interferometry and was found to depend on the on-axis plasma density and laser spot size. Experiments and multi-mode simulations were found to be in agreement.

The plasma properties of a plasma waveguide are critical to the performance of laser-plasma accelerators (LPAs). By measuring the group velocity in plasma channels through spectral interferometry, the density in the channel is retrieved. In this paper, experimental methods and results are presented for the plasma density in LPA-relevant plasma channels of various lengths.

We present a study of the effects and uses of gas dynamics within targets for laser-plasma accelerators (LPAs) based on capillary-discharge waveguides with embedded gas jets. The temporal evolution of the longitudinally-structured density profile was determined by using helium in the jet and hydrogen in the capillary and resolving the spatial distribution of the two gases as a function of the timing between the jet and the discharge. The gas distributions were captured by imaging the associated emission lines resulting from the plasma recombination. The results of the study guide the choice of jet and capillary pressures required to achieve the desired functionality of the target and also to offer possibilities for tuning of the target based on timing.

Ongoing efforts towards a compact free electron laser based on laser-plasma acceleration (LPA) are presented. Recent results [van Tilborg (PRL 2015) and Steinke (Nature, accepted)] demonstrate compact transport and support LPA-based FEL design and simulations. Article not available.

We present initial experiments on staging of two separately driven laser plasma accelerators (LPAs) towards high energy physics and beam deceleration experiments. A study establishing long term stability of electron beams accelerated by an LPA in density downramp configuration [1] is presented, demonstrating the appropriateness of these beams as an injector for staged acceleration. Subsequently, these injector beams are used to longitudinally probe the fully characterized wakefield [2] excited in a discharge-capillary-based second stage accelerator.

© 2015 American Physical Society. © 2015 American Physical Society. Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius--by a discharge capillary-based active plasma lens--into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.

The electric field in the temporal and spectral domain of coherent diffraction-limited transition radiation is studied. An electron bunch, with arbitrary longitudinal momentum distribution, propagating at normal incidence to a sharp metal-vacuum boundary with finite transverse dimension is considered. A general expression for the spatiotemporal electric field of the transition radiation is derived, and closed-form solutions for several special cases are given. The influence of parameters such as radial boundary size, electron momentum distribution, and angle of observation on the waveform (e.g., radiation pulse length and amplitude) are discussed. For a Gaussian electron bunch, the coherent radiation waveform is shown to have a single-cycle profile. Application to a novel THz source based on a laser-driven accelerator is discussed.