Abstract: The interaction of light with the quantum-vacuum is predicted to give rise to some of the most fundamental and exotic processes in modern physics, which remain untested in the laboratory to date. Electron-positron pair production from a pure vacuum target, which has yet to be observed experimentally, is possibly the most iconic. The advent of ultra-intense lasers and laser accelerated GeV electron beams provide an ideal platform for the experimental realisation. Collisions of high energy gamma-ray photons derived from the GeV electrons and intense laser fields result in detectable pair production rates at field strengths that approach and exceed the Schwinger limit in the centre-of-momentum frame. A detailed experiment has been designed to be implemented at the ATLAS laser at the centre of advanced laser applications. We show full calculations of the expected backgrounds and beam parameters which suggest that single pair events can be reliably generated and detected.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)
Abstract: This work explores the experimental observation of the Breit-Wheeler process, first described by Gregory Breit and John A. Wheeler in 1934 , where two photons collide to form an electron positron pair from the quantum vacuum. The specific challenge thereby is the low cross section of a few 10e 29 m2 or 0.1 b combined with the requirement of photon energies in the range of mega electronvolt. Such beams can be provided by particle accelerators, for instance LCLS at SLAC or the European XFEL at DESY. Experiments exploring photon photon collisions with conventional accelerators were done in the past, for example E144 at SLAC in 1997 , however the two photon process described by Breit and Wheeler has not yet been observed. Over the last few decades, novel laser driven plasma based particle accelerators (LWFA) made significant progress [3, 4, 5, 6], allowing the production of the required photon beams to study the Breit-Wheeler process at pure laser facilities [7, 8, 9]. The work in hand explores the challenges related to such an experiment specifically at high power laser facilities using the example of Astra Gemini, a multi 100TW dual beam system at the CLF in England. In an experiment, multi 100MeV γ-rays from LWFA electron bremsstrahlung and 1-2 keV x-rays from Germanium M-L shell transition radiation are collided to produce pairs through the Breit-Wheeler process. A detection system to measure those pairs composed of a permanent magnet beam line and shielded single particle detectors is developed and tested within this thesis. The acquired data allows an estimate of the requirements for future experiments to measure the two-photon Breit-Wheeler process.
Abstract: We present experimental evidence of relativistic electron-cyclotron resonances (RECRs) in the vicinity of the relativistically intense pump laser of a laser wakefield accelerator (LWFA). The effects of the RECRs are visualized by imaging the driven plasma wave with a few-cycle, optical probe in transverse geometry. The probe experiences strong, spectrally dependent and relativistically modified birefringence in the vicinity of the pump that arises due to the plasma electrons’ relativistic motion in the pump’s electromagnetic fields. The spectral birefringence is strongly dependent on the local magnetic field distribution of the pump laser. Analysis and comparison to both 2D and 3D particle-in-cell simulations confirm the origin of the RECR effect and its appearance in experimental and simulated shadowgrams of the laser-plasma interaction. The RECR effect is relevant for any relativistic, magnetized plasma and in the case of LWFA could provide a nondestructive, in situ diagnostic for tracking the evolution of the pump’s intensity distribution with propagation through tenuous plasma.
Abstract: Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as ∼50 μm and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.
Abstract: 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. 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.. 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.
Abstract: We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.