Optimizing High Harmonic Generation in Ionized Plasma for Seeding of X-Ray Free-Electron Lasers

Derrek Drachenberg (15-FS-007)

Project Description

Next-generation x-ray free-electron light sources require seeding with high-energy kiloelectronvolt photons to increase efficiency and coherence. Several proposed seeding schemes exist, including high harmonic generation via high-intensity laser–plasma interactions. High harmonic generation can be accomplished by the interaction of ultrashort laser pulses with pressurized noble gases. At sufficient intensity, the interaction between laser and gas atoms causes tunnel ionization of an outer-shell electron. The ionized electron is accelerated by the laser electric field outside of the ion, ultimately recombining and emitting a high harmonic photon. While seeding of x-ray free-electron light sources by high harmonic generation has been demonstrated and is a contender in the planned Linac Coherent Light Source upgrade at the SLAC National Accelerator Laboratory at Stanford, significant developments in both the drive laser and the high harmonic-generation process are required. We are exploring the feasibility of an improved model of the high harmonic-generation process and creating a set of physics requirements and a design for a high-repetition-rate, 1-keV coherent x-ray source. The x-ray free-electron light sources such as the Linac Coherent Light Source upgrade are seeded by spontaneous emission, but would benefit from direct seeding. A new model will inform the design of a coherent x-ray source with significantly improved efficiency, and therefore lower the required pump-laser power to generate desired x-ray photons. We will develop our improved model by applying the existing LLNL nonequilibrium atomic physics code CRETIN to high harmonic generation in a pressurized noble gas.

We expect to determine the feasibility of modifying existing atomic models by including high harmonic-generation theory that describes the multiple-photon and tunnel ionization processes and generation of high harmonics. Modifications will include the addition of atomic rates to established atomic codes to enable high harmonic-generation modeling. With an improved model of the process, we expect to create the capability to exploit new insights into x-ray and pump phase matching, which will lead to increased efficiency, and potentially make high harmonic-generation seeding of x-ray free-electron light sources practical. In parallel, we will identify a set of physics requirements and proof-of-principle experiments to show the feasibility of high harmonic-generation seeding of advanced light sources such as the upgrade to the Linac Coherent Light Source.

Mission Relevance

Variation in the simulated x-ray pulse with gas pressure.
Variation in the simulated x-ray pulse with gas pressure.

This feasibility study will broaden and extend the Laboratory's core competency in lasers and optical science and technology by providing enabling technology for a future laser source of high-brightness, high-flux x-rays for new physics experiments in support of Laboratory missions in stockpile stewardship and remote-target laser sensing for defense. In addition, this technology will have commercial applications in the medical field. It also offers the potential for new collaborations with the SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory in lasers for accelerator systems.

FY16 Accomplishments and Results

In FY16 we (1) completed modification of the CRETIN code, which follows the time evolution of atomic populations and photon distributions as radiation interacts with a plasma, to include a description of multi-photon and tunnel ionization, allowing time-dependent simulations of the initial stages of laser pulse propagation; (2) developed the post-processing tools necessary to describe the generation of the high harmonic-generation pulse and phase matching in the plasma target (see figure); and (3) performed limited time-dependent simulations of existing, published high harmonic-generation data as verification of the work.