CUOS Noon Seminar: Spin Polarization, Plasma Density Gradient, Electron Injection, Two color X-ray, and Pizza!

Qian Qian: Spin and polarization effects in QED cascades

The fast development of high-power laser technology enables studies of plasma physics under extreme field strength in the lab. When the fields are strong enough and the plasma is energetic enough, quantum effects could modify the plasma’s collective behavior substantially. This system is defined in some literature as a ‘QED plasma’. Most of the studies for QED plasma use a PIC code with spin and polarization averaged QED rates, in which the QED processes only depend on the momentum of the particles and the EM field they experience. However, the strong field QED (SFQED) processes are fundamentally spin and polarization dependent. Here, we present our study of the QED cascade process using our newly developed spin and polarization-resolved QED module based on the particle-in-cell code OSIRIS. The QED cascade is one of the most famous predictions of SFQED, in which the energy of the laser field transfers to electrons, positrons, and high-energy photons, causing exponential growth in particle number and creating a hot dense pair plasma. Including spin and polarization allows us to simulate this complicated multistage SFQED process more accurately. We explore how spin and polarization effects in SFQED could possibly influence the evolution of the cascade process as well as correlations in the pair plasma’s momentum and spin distribution. The emitted dense, high-energy photons are expected to be polarized, which potentially can be a source of intense polarized gamma rays.

Hongmei Tang: The Effect of Plasma Density Gradient on the Direct Laser Acceleration of Electrons

Direct laser acceleration (DLA) is capable of generating superponderomotive energy electrons to hundreds of MeV, as well as secondary particles and radiation from high-intensity picosecond laser pulses interacting with underdense plasma. Experiments performed on the OMEGA EP facility using apodized beams and supersonic gas nozzle targets demonstrates the sensitivity of the complex process of DLA to the gradient of the plasma density ramps. 2D particle-in-cell OSIRIS simulations mimic the interaction using different plasma density profiles and provides insight into the laser channel creation, laser fields evolution, as well as the significant effect of the sheath fields on the corresponding electron dynamics. Our results show an optimal plasma density gradient and a path towards optimizing DLA conditions.

Nicholas Ernst: Controlled electron injection from wake shaping using co-propagating laser pulses

We introduce a novel method of controlled electron injection for Laser Wakefield Acceleration (LWFA) operating in the high-intensity bubble regime. In this scheme, the plasma acts to couple a high-intensity “driver” pulse to a phase controlled, low power “satellite” pulse co-propagating off-axis. The satellite is tightly focused such that it perturbs and drives a transient, asymmetric plasma wave before depleting. Doing so allows for spatio-temporal manipulation or “shaping” of the wakefield to create a trigger for overcoming the wave-breaking threshold and leads to efficient particle trapping and acceleration. Supported by 2D and 3D Particle-in-Cell simulations using OSIRIS, we demonstrate systematic investigation of the two-beam parameter space (e.g. temporal delay, beam displacement, etc.) leads to control over beam pointing, charge, and emittance. Results indicate this technique could be used to induce self-injection at plasma densities and laser intensities well below theoretical predictions using satellites of less than 1% the driver energy. Further scaling to additional co-propagating pulses proves to distort the initial plasma wave formation in a predictable manner for near arbitrary wake-shaping. This allows for an ad hoc spatiotemporal setup to control the momentum space of injected electrons, leading to a route for enhanced and polarized betatron oscillations. The results show promise for an all-optical knob to transition between a high charge, mono-energetic, GeV accelerator and an enhanced x-ray source from betatron radiation through independent tuning of the satellite.

André Antoine: Characterization of Non-Thermal Phase Transitions in Ionic Compounds with Two-color X-ray Pulses

High resolution crystallography has benefited from the availability of x-ray Free Electron Lasers (FEL). It has been possible to resolve hydrogen atoms and water molecules. [2] Intense x-ray FEL pulses interact with samples changing their electronic and atomic structure. To date, the experiments studying the x-ray FEL-matter interaction have predominantly examined semiconductors, such as diamond and silicon [4-5]. There is little known about how x-ray induced bond breaking occurs in a solid with more than one element or in a solid with ionic bonding[HP1] . A recent calculation has predicted a crystalline to disordered phase transition in the case of the high-intensity x-ray interaction with sodium chloride, an ionic solid [6]. With FEL x-ray pump and x-ray probe pulses, non-thermal phase transitions are predicted and new material phases can be detected. We have investigated the time-dependent intensity of diffraction peaks in sodium chloride (NaCl) and magnesium oxide (MgO). We will discuss the observed ultrafast responses of the materials through analysis of the observed diffraction peak intensities.

Fast-Neutron Generation with Ultrafast Spatially and Temporally Coherently Combined Fiber Laser Drive

To date the architecture of most laser driven neutron sources are designed around high pulse energy (>100mJ) laser systems; however, the neutron flux of these systems is limited by their repetition rate of 1-10Hz. We are developing a new ultrashort fiber laser based driver architecture which delivers high pulse energies at kHz repetition rates, suitable for driving both high flux particle sources and allowing for real time feedback control to achieve optimized system performance. Our demonstration system currently achieves pulses with energies of tens of millijoules at a 2kHz repetition rate by simultaneously incorporating spatial and temporal coherent combining techniques. As a practical demonstration of this unique architecture for scientific applications, we show coincident isotropic fast neutron generation via D(d,n)³He fusion reactions in a free-flowing microscale deuterated liquid jet target. To our knowledge, this is the first fast neutron source driven by a fiber laser system. This proof-of-principle experimental demonstration with near-relativistic pulses highlights the potential for increased scaling of the fluxes of particle accelerators and secondary radiation sources as this fiber technology matures towards multiple joule pulses at several kilohertz. This work was funded by DOE Advanced Accelerator Stewardship Grant FP00013287 and DOE Grant DE-SC0016804.