A. Irman

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150u2009TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20u2009MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5u2009μm) and planar (20u2009μmu2009×u20092u2009μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions.

Optical free-electron lasers with Traveling-Wave Thomson-Scattering

We present a fully analytic model of an all-optical free electron laser (OFEL) undulator based on the Traveling-Wave Thomson-Scattering (TWTS) scheme. The TWTS undulator provides for sub-mm undulator wavelengths, does not require any material or plasma to generate or contain the undulator field and allows for sub-meter saturation lengths. Starting from a fully analytic description of the three-dimensional TWTS field we derive the OFEL pendulum equation for electrons in the TWTS field and discuss the constraints on laser and electron pulse parameters that have to be fulfilled for OFEL operation. We conclude in applying the TWTS OFEL to the realization of compact free electron laser sources at 13.5 nm and 0.2 nm using laser and electron sources in reach of present day technologies.

First results with the novel petawatt laser acceleration facility in Dresden

We report on first commissioning results of the DRACO Petawatt ultra-short pulse laser system implemented at the ELBE center for high power radiation sources of Helmholtz-Zentrum Dresden-Rossendorf. Key parameters of the laser system essential for efficient and reproducible performance of plasma accelerators are presented and discussed with the demonstration of 40 MeV proton acceleration under TNSA conditions as well as peaked electron spectra with unprecedented bunch charge in the 0.5 nC range.

Design and simulation of laser wakefield acceleration with external electron bunch injection in front of the laser pulse

In this article we present a theoretical investigation on an experimental design of a laser wakefield accelerator in which electron bunches from a photocathode radio frequency linac are injected into a capillary discharge plasma channel just in front of a few tens of terawatt drive laser pulse. The electron bunch, with a kinetic energy of 2.9 MeV and an energy chirp imposed by the linac, is magnetically compressed by a factor of 8 to a duration of 250 fs, and is magnetically focused into the plasma channel where it matches the spot size of the drive laser ([approximate]30 µm). The dynamics of the bunch, starting from the photocathode, through the linac, along the beam transportation line, through the magnetic compressor, and its focusing into the plasma channel are comprehensively simulated with the general particle tracer code. Further, we use our three-dimensional numerical codes to calculate the laser wakefield and to determine and optimize the trapping and acceleration of the injected bunch in the wakefield. We show that, injecting a 5 pC electron bunch of 250 fs duration, the experiment should deliver an electron bunch of approximately 744 MeV energy, with 1.1% relative energy spread, and with an extremely short duration (6 fs), after acceleration in a 5.4 cm long plasma channel

Demonstration of a beam loaded nanocoulomb-class laser wakefield accelerator

Laser-plasma wakefield accelerators have seen tremendous progress, now capable of producing quasi-monoenergetic electron beams in the GeV energy range with few-femtoseconds bunch duration. Scaling these accelerators to the nanocoulomb range would yield hundreds of kiloamperes peak current and stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and γ-ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. However, accelerators generating such currents operate in the beam loading regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. Here we demonstrate that, if appropriately controlled, the beam loading effect can be employed to improve the accelerator’s performance. Self-truncated ionization injection enables loading of unprecedented charges of ∼0.5u2009nC within a mono-energetic peak. As the energy balance is reached, we show that the accelerator operates at the theoretically predicted optimal loading condition and the final energy spread is minimized.Higher beam quality and stability are desired in laser-plasma accelerators for their applications in compact light sources. Here the authors demonstrate in laser plasma wakefield electron acceleration that the beam loading effect can be employed to improve beam quality by controlling the beam charge.

Calibration and cross-laboratory implementation of scintillating screens for electron bunch charge determination

We revise the calibration of scintillating screens commonly used to detect relativistic electron beams with low average current, e.g., from laser-plasma accelerators, based on new and expanded measurements that include higher charge density and different types of screens than previous work [Buck et al., Rev. Sci. Instrum. 81, 033301 (2010)]. Electron peak charge densities up to 10 nC/mm2 were provided by focused picosecond-long electron beams delivered by the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf. At low charge densities, a linear scintillation response was found, followed by the onset of saturation in the range of nC/mm2. The absolute calibration factor (photons/sr/pC) in this linear regime was measured to be almost a factor of 2 lower than that reported by Buck et al. retrospectively implying a higher charge in the charge measurements performed with the former calibration. A good agreement was found with the results provided by Glinec et al. [Rev. Sci. Instrum. 77, 103301 (2006)]. Furthermore long-term irradiation tests with an integrated dose of approximately 50 nC/mm2 indicate a significant decrease of the scintillation efficiency over time. Finally, in order to enable the transfer of the absolute calibration between laboratories, a new constant reference light source has been developed.

Investigation of electron dynamics in an ionization-injection laser-wakefield accelerator via betatron radiation (Conference Presentation)

The injection process of electrons into the plasma cavity in laser-wakefield accelerators is a nonlinear process that strongly influences the property of the accelerated electrons. During the acceleration electrons perform transverse (betatron) oscillations around the axis. This results in the emission of hard x-ray radiation (betatron radiation) whose characteristics depend directly on the dynamic of the accelerated electrons. Thus, betatron radiation can be utilized as a powerful diagnostic tool to investigate the acceleration process inside the wakefield. Here we describe our recent LWFA experiments deploying ionization induced injection technique carried out with the Draco Ti:Sapphire laser. We focused 30 fs short pulses down to a FWHM spot size of 19 μm resulting in a normalized vacuum laser intensity a0 = 3.3 on a gas target. The target, which was a supersonic gas jet, provided a flat plasma profile of 3mm length. By varying the plasma density from 2x10^18 cm^-3 to 5x10^18 cm^-3 and the laser pulse energy from 1.6 J to 3.4 J we were able to tune the electron bunch and betatron parameters. Electron spectra were obtained by acquiring an energy resolved and charge calibrated electron profile after detection from the beam axis by a permanent magnetic dipole. Simultaneously, a back-illuminated and deep-depleted CCD placed on axis recorded the emitted x-ray photons with energies up to 20keV. Equipped with an 2D spectroscopy technique based on single pixel absorption events, we reconstructed the corresponding energy resolved x-ray spectrum for every shot and deduced the betatron source size at the plasma exit. Combining the data of the electron and betatron spectrum, we compare the characteristics of the betatron spectra for different electron bunches. In our experiments we recorded a total number of 25x10^4 photons per shot within a divergence angle of 1 mrad and betatron radii in the order of 1 μm. Finally, we compare our results with simulated spectra from the parallel classical radiation calculator Clara2 that is based on the Liénard-Wiechert potentials.

Simulate what is measured: next steps towards predictive simulations (Conference Presentation)

Simulations of laser matter interaction at extreme intensities that have predictive power are nowadays in reach when considering codes that make optimum use of high performance compute architectures. Nevertheless, this is mostly true for very specific settings where model parameters are very well known from experiment and the underlying plasma dynamics is governed by Maxwells equations solely. When including atomic effects, prepulse influences, radiation reaction and other physical phenomena things look different. Not only is it harder to evaluate the sensitivity of the simulation result on the variation of the various model parameters but numerical models are less well tested and their combination can lead to subtle side effects that influence the simulation outcome. We propose to make optimum use of future compute hardware to compute statistical and systematic errors rather than just find the mots optimum set of parameters fitting an experiment. This requires to include experimental uncertainties which is a challenge to current state of the art techniques. Moreover, it demands better comparison to experiments as inclusion of simulating the diagnostics response becomes important. We strongly advocate the use of open standards for finding interoperability between codes for comparison studies, building complete tool chains for simulating laser matter experiments from start to end.