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We study the propagation of 0.05-1 TW power, ultrafast laser pulses in a 10 meter long rubidium vapor cell. The central wavelength of the laser is resonant with the $D_2$ line of rubidium and the peak intensity in the $10^{12}-10^{14} ~W/cm^2$ range, enough to create a plasma channel with single electron ionization. We observe the absorption of the laser pulse for low energy, a regime of transverse confinement of the laser beam by the strong resonant nonlinearity for higher energies and the transverse broadening of the output beam when the nonlinearity is saturated due to full medium ionization. We compare experimental observations of transmitted pulse energy and transverse fluence profile with the results of computer simulations modeling pulse propagation. We find a qualitative agreement between theory and experiment that corroborates the validity of our propagation model. While the quantitative differences are substantial, the results show that the model can be used to interpret the observed phenomena in terms of self-focusing and channeling of the laser pulses by the saturable nonlinearity and the transparency of the fully ionized medium along the propagation axis., Comment: 12 pages, 10 Figures

We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported in arXiv:2007.14894v2: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement., Comment: 13 pages, 11 figures

A precise characterization of the incoming proton bunch parameters is required to accurately simulate the self-modulation process in the Advanced Wakefield Experiment (AWAKE). This paper presents an analysis of the parameters of the incoming proton bunches used in the later stages of the AWAKE Run 1 data-taking period. The transverse structure of the bunch is observed at multiple positions along the beamline using scintillating or optical transition radiation screens. The parameters of a model that describes the bunch transverse dimensions and divergence are fitted to represent the observed data using Bayesian inference. The analysis is tested on simulated data and then applied to the experimental data. A precise characterization of the incoming proton bunch parameters is required to accurately simulate the self-modulation process in the Advanced Wakefield Experiment (AWAKE). This paper presents an analysis of the parameters of the incoming proton bunches used in the later stages of the AWAKE Run 1 data-taking period. The transverse structure of the bunch is observed at multiple positions along the beamline using scintillating or optical transition radiation screens. The parameters of a model that describes the bunch transverse dimensions and divergence are fitted to represent the observed data using Bayesian inference. The analysis is tested on simulated data and then applied to the experimental data.

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400\,GeV proton bunch. Results show that a positive/negative gradient in/decreases the number of micro-bunches and the relative charge per micro-bunch observed after 10\,m of plasma. The measured modulation frequency also in/decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradient adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory., Comment: 14 pages, 4 figures

We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields' growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simulations and validate our understanding of the development of self-modulation. Experiments were performed in the context of the Advanced Wakefield Experiment (AWAKE)., 4 figures

In this article, we briefly summarize the experiments performed during the first run of the Advanced Wakefield Experiment, AWAKE, at CERN (European Organization for Nuclear Research). The final goal of AWAKE Run 1 (2013-2018) was to demonstrate that 10-20 MeV electrons can be accelerated to GeV energies in a plasma wakefield driven by a highly relativistic self-modulated proton bunch. We describe the experiment, outline the measurement concept and present first results. Last, we outline our plans for the future. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

We report synthesis and characterization of a batch of three cesium potassium antimonide photocathodes that have been grown on pure copper substrates via a ternary co-deposition method whose procedure is described herein. A deposition system that was designed for synthesis of two-element photocathodes has been utilized for synthesis of the aforementioned three-element photocathodes with slope of the in situ photocurrent as the driver for the growth process. A variation of substrate temperature and deposition parameters among the three photocathodes during synthesis has yielded a maximum quantum efficiency of 6% for 140 ° C substrate temperature. Lifetime studies performed in a 65-kV DC electron gun on two of the photocathodes, but under oxidized states, at tens of μ A average currents (ampere-level peak currents) extracted utilizing a 532-nm wavelength, 1-kHz repetition rate laser, have resulted in charge-lifetimes of 6.13 C and 13.78 C, respectively. X-ray photoelectron spectroscopy analysis of the photocathode with the highest quantum efficiency has revealed a nearly impurity-free surface with stoichiometry Cs1.3K1.8Sb1.0 when pristine. Furthermore, it has been found that oxidation of the alkali surface atoms as well as carbon adsorption from hydrocarbons and minor fluorine uptake are the causes of quantum efficiency reduction during laser illumination in the utilized experimental set-up.

We explore nonlinear photoemission in cesium telluride (Cs$_2$Te) photocathodes where an ultrashort ($\sim 100$ fs full width at half max) 800-nm infrared laser is used as the drive-laser in lieu of the typical $\sim 266$-nm ultraviolet laser. An important figure of merit for photocathodes, the quantum efficiency, we define here for nonlinear photoemission processes in order to compare with linear photoemission. The charge against drive-laser (infrared) energy is studied for different laser energy and intensity values and cross-compared with previously performed similar studies on copper [P. Musumeci et al., \emph{Phys. Rev. Lett.}, \textbf{104}, 084801 (2010)], a metallic photocathode. We particularly observe two-photon photoemission in Cs$_2$Te using the infrared laser in contrast to the anticipated three-photon process as observed for metallic photocathodes., 5 pages, 4 figures, 1 table, submitted to Appl. Phys. Lett

A timely stable femto-second electron beam system is being constructed for pump-probe time-resolved experiments and applications. A pulsed S-band klystron is installed and fully commissioned with 5.5 MW peak power in a 2.5 µs pulse length and 1 Hz repetition rate. A single-cell RF photo-gun is designed to produce with 0.16 – 1.6 pC electron bunches in a photo-emission mode within a 600 fs ∓ 3 ps at 0.5 – 1 MeV. The measured RF system jitters are within ± 1 % in magnitude and ± 0.2° in phase, which would induce 3.4 keV and 0.25 keV of ΔE g , corresponding to 80 fs and 5 fs of Δt e , respectively. The beam brightness could be improved by replacing a conventional photo-cathode with field emission array or carbon nanotube tips as they limit the emission area, while providing high current emission. PIC simulations indicate that our designed bunch compressor reduces the TOA-jitter by about an order of magnitude. The transport and focusing optics of the designed beamline with the bunch compressor enables an energy spread within 10−4 and a bunch length (electron probe) within > 50 – 100 fs.

In this paper, we present recent results from bunch-shaping experiments using α-BBO crystal at the High Brightness Electron Beam Source Laboratory (HBESL). α-BBO crystals were used to shape the ultra-short laser pulse ( < 100fs) at HBESL to generate twin pulses at the cathode. The twin electron pulses were transported down the linac ( < 5 MeV) to a transverse deflecting mode cavity and the temporal profile recorded on the screen. The longitudinal time profile of the twin pulses show the difference between the effect of crystals and the longitudinal space charge effect. The electron beam was recorded for various position of the crystal and bunch charge.