Collaborative Research: Preformed Laser-driven Plasma Waveguides for Multi-GeV Laser-Plasma Electron Acceleration

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This research project aims to demonstrate the physical principles underlying a new type of electron accelerator that would be thousands of times smaller and less expensive than the best modern accelerators. Since the 1930s, scientists have built ever bigger and more expensive machines, called accelerators, to accelerate electrons to more than 99% of the speed of light, which Einstein discovered to be the speed limit of the universe. At such enormous speeds, electrons can probe into nature's deepest subatomic secrets, irradiate cancerous tumors, and generate powerful x-rays that measure the structure of molecules essential to life. One of America's biggest electron accelerators, a 2-mile-long machine called "SLAC" (which originally stood for "Stanford Linear Accelerator Center") accelerates electrons to 99.99999999% of the speed of light. The energy carried by such an electron, at 30 giga-electronvolts (GeV), is less than a mosquito uses to flap its wings once, but it's a lot of energy for one electron. The goal of this research project is to enable tabletop electron accelerators that are thousands of times smaller and less expensive than SLAC, but which can nevertheless accelerate electrons to the same energy as SLAC does. The new technology is a long, narrow pipe made of plasma, or ionized gas (the same state of matter one finds inside fluorescent light bulbs and stars). This pipe is a "racetrack" that is intended ultimately to confine and guide electrons, and a powerful laser pulse that fuels their acceleration, until they reach 30 GeV. A separate powerful laser will be used to shape the fluid-like plasma into a pipe. Computer calculations will be used to understand how the plasma pipe forms, and a model version of the pipe will be demonstrated in the laboratory. This two-year project will elucidate the science underlying the formation of cylindrical plasma waveguides with axial electron density in the range between 1 and 3 times 10^17 particles per cm^3 and radius of ~50 µm. Such waveguides can ultimately guide 100 J, 150 fs drive pulses from the Texas Petawatt (PW) Laser in a low-order mode at relativistic intensity up to the pump depletion limit, in order to extend the performance of a single-stage 2 GeV laser-plasma electron accelerator to the tens-of-GeV level. The channel formation method is based upon physical principles developed during the 1990s, but is being extended to ~20x lower plasma density. Formation of short (~1 cm) channels in tenuous He plasma will be demonstrated, using 2J, 80-300 ps drive pulses available in the laboratory at the University of Texas at Austin. This prototype setup will enable the duration, energy and focus of channel-forming pulse, and the pre-ionization and doping conditions that optimize plasma heating and channel formation at low plasma density to be discovered. Simulations of channel formation in tenuous helium, and channeled propagation of relativistic laser pulses, by the research group at the University of Colorado Boulder will guide experiments. The intellectual merit lies in discovering laser-plasma conditions that optimize formation of high quality, single-mode plasma channels of lower density than ever previously demonstrated. The broader impacts include developing plasma waveguide technology that may ultimately extend single-stage energy gain of future laser-plasma accelerators to levels typical of SLAC; advancing the careers of three doctoral students from historically under-represented groups; and introducing an undergraduate student to professional research.