Frank Tsung

Generating multi-GeV electron bunches using single stage laser wakefield acceleration in a 3D nonlinear regime

The extraordinary ability of space-charge waves in plasmas to accelerate charged particles at gradients that are orders of magnitude greater than in current accelerators has been well documented. We develop a phenomenological framework for laser wakefield acceleration (LWFA) in the 3D nonlinear regime, in which the plasma electrons are expelled by the radiation pressure of a short pulse laser, leading to nearly complete blowout. Our theory provides a recipe for designing a LWFA for given laser and plasma parameters and estimates the number and the energy of the accelerated electrons whether self-injected or externally injected. These formulas apply for self-guided as well as externally guided pulses (e.g. by plasma channels). We demonstrate our results by presenting a sample particle-in-cell (PIC) simulation of a

A nonlinear theory for multidimensional relativistic plasma wave wakefields

30\text{ }\mathrm{fs}

Simulation of monoenergetic electron generation via laser wakefield accelerators for 5–25TW lasersa)

, 200 TW laser interacting with a 0.75 cm long plasma with density

Beam loading by electrons in nonlinear plasma wakes

1.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}

Exploiting multi-scale parallelism for large scale numerical modelling of laser wakefield accelerators

to produce an ultrashort (10 fs) monoenergetic bunch of self-injected electrons at 1.5 GeV with 0.3 nC of charge. For future higher-energy accelerator applications, we propose a parameter space, which is distinct from that described by Gordienko and Pukhov [Phys. Plasmas 12, 043109 (2005)] in that it involves lower plasma densities and wider spot sizes while keeping the intensity relatively constant. We find that this helps increase the output electron beam energy while keeping the efficiency high.

Numerical instability due to relativistic plasma drift in EM-PIC simulations

A nonlinear kinetic theory for multidimensional plasma wave wakes with phase velocities near the speed of light is presented. This theory is appropriate for describing plasma wakes excited in the so-called blowout regime by either electron beams or laser pulses where the plasma electrons move predominantly in the transverse direction. The theory assumes that all electrons within a blowout radius are completely expelled. These radially expelled electrons form a narrow sheath just beyond the blowout radius which is surrounded by a region which responds weakly (linearly). This assumption is reasonable when the spot size of the electron beam and laser are substantially less than the blowout radius. By using this theory one can predict the wakefield amplitudes and blowout radius in terms of the electron beam or laser beam parameters, as well as predict the nonlinear modifications to the wake’s wavelength and wave form. For the laser case, the laser spot size must also be properly matched in order for a narrow sheath to form. The requirements for forming a spherical wave form, i.e., “bubble,” are also discussed. The theory is also used to show when linear fluid theory breaks down and how this leads to a saturation of the logarithmic divergence in the linear Green’s function.

Self-modulated wakefield and forced laser wakefield acceleration of electrons

In 2004, using a 3D particle-in-cell (PIC) model [F. S. Tsung et al., Phys. Rev. Lett. 93, 185004 (2004)], it was predicted that a 16.5TW, 50fs laser propagating through nearly 0.5cm of 3×1018cm−3 preformed plasma channel would generate a monoenergetic bunch of electrons with a central energy of 240MeV after 0.5cm of propagation. In addition, electrons out to 840MeV were seen if the laser propagated through 0.8cm of the same plasma. The simulations showed that self-injection occurs after the laser intensity increases due to a combination of photon deceleration, group velocity dispersion, and self-focusing. The monoenergetic beam is produced because the injection process is clamped by beam loading and the rotation in phase space that results as the beam dephases. Nearly simultaneously [S. P. D. Mangles et al., Nature 431, 535 (2004); C. G. R. Geddes et al., ibid. 431, 538 (2004); J. Faure et al., ibid. 431, 541 (2004)] three experimental groups from around the world reported the generation of near nano-Cou...

A global simulation for laser-driven MeV electrons in 50-μm-diameter fast ignition targetsa)

An analytical theory for the interaction of an electron bunch with a nonlinear plasma wave is developed to make it possible to design efficient laser- and/or beam-driven accelerators that generate high quality monoenergetic electron beams. This theory shows how to choose the charge, the shape, and the placing of the bunch so that the conversion efficiency from the fields of the bubble to the accelerating electrons reaches nearly 100% and the beam quality is optimized. For intense drivers the nonlinear wake is described by the shape of the bubble and beam loading arises when the radial space-charge force of the beam acts back on the electron sheath surrounding the ion channel. The modification of the wake due to the presence of flat-top electron bunches is studied and it is shown that the energy spread of an externally injected flat-top electron bunch can be kept low. The bunch profile that leads to zero energy spread is also derived.

A Vlasov-Fokker-Planck code for high energy density physics

A new generation of laser wakefield accelerators (LWFA), supported by the extreme accelerating fields generated in the interaction of PW-Class lasers and underdense targets, promises the production of high quality electron beams in short distances for multiple applications. Achieving this goal will rely heavily on numerical modelling to further understand the underlying physics and identify optimal regimes, but large scale modelling of these scenarios is computationally heavy and requires the efficient use of state-of-the-art petascale supercomputing systems. We discuss the main difficulties involved in running these simulations and the new developments implemented in the OSIRIS framework to address these issues, ranging from multi-dimensional dynamic load balancing and hybrid distributed/shared memory parallelism to the vectorization of the PIC algorithm. We present the results of the OASCR Joule Metric program on the issue of large scale modelling of LWFA, demonstrating speedups of over 1 order of magnitude on the same hardware. Finally, scalability to over ∼106 cores and sustained performance over ∼2 P Flops is demonstrated, opening the way for large scale modelling of LWFA scenarios.

Role of direct laser acceleration in energy gained by electrons in a laser wakefield accelerator with ionization injection

Plasma based accelerators offer great potential for constructing compact accelerators that have numerous applications. The physical process in plasma based acceleration involves small spatial scale (\({\sim }\) \(\upmu \)m) and ultra-fast time scale (\({\sim }\mathrm {fs}\)), and they are very difficult to diagnose directly in experiments. Therefore high fidelity numerical simulations play a critical role in the development of plasma based accelerators. With the rapid progress of plasma based accelerators, peoples begin to show great interests of high energy (\({\sim } 10\,\mathrm {GeV}\) or higher) and high quality electrons acceleration as drivers for compact light sources and the building blocks for a future linear collider.